CID MINE DRAINAGE TREATMENT OPTIONS USING ... Kaur_Thesis...Acid mine drainage water, characterised as having a low pH, and high metal and sulphate concentrations, has a detrimental

ACID MINE DRAINAGE TREATMENT OPTIONS

USING RECYCLED BAYER LIQUOR

By

Gurkiran Kaur

Submitted in fulfilment of the requirements for the degree of

Doctor of Philosophy

School of Chemistry, Physics and Mechanical Engineering

Faculty of Science and Engineering

Queensland University of Technology

2018

i

STATEMENT OF ORIGINAL AUTHORSHIP

The work contained in this thesis has not been previously submitted to meet requirements

for an award at this or any other higher education institution. To the best of my knowledge and

belief, the thesis contains no material previously published or written by another person except

where due reference is made.

QUT Verified Signature

ii

ACKNOWLEDGMENTS

I would like to thank the following people and organisations without whom this thesis could

not have been completed.

They include:

1. My QUT supervisors: Dr. Sara Couperthwaite and Prof. Graeme Millar for providing a

challenging research project, guidance and assisting in editing the Thesis.

2. I would like to extend my gratitude to Dr. Sara Couperthwaite for the emotional and

encouragement throughout the project and past years.

3. Mr. Tony Raftery and Dr Henry Spratt for advice and technical support with the XRD

instrument, preparation and analysis.

4. Dr. Wayde Martens for his advice and technical assistance with thermogravimetric

analysis

5. Dr. Llew Rintoul for his assistance with the vibrational spectroscopy instruments and

training.

6. Mr. John Outram and Mr. Mitchell de Bruyn for their advice and technical assistance

with the operation of ICP-OES.

7. Dr. Josh Lipton-Duffin for assistance and guidance with the X-ray photoelectron

spectroscopy, sample preparation and analysis.

8. Ms Elizabeth Graham for her assistance with surface analysis techniques.

9. The entire Couperthwaite and Millar group, with special thanks to Mitch Kenneth

Nuttall and Dominic Alexander for training and assistance in numerous instrumental

techniques.

Finally, I would like to give special thanks to my family and friends for their love, support and

encouragement given throughout these years. Also, special thanks go to my husband, Mr.

Shamsher Singh Sidhu for his never-ending faith and support.

iii

This work is dedicated to

Bhangu and Sidhu family

iv

ABSTRACT

Acid mine drainage water, characterised as having a low pH, and high metal and sulphate

concentrations, has a detrimental effect on ecosystems in overflow events. Commercial alkali

such as lime, caustic and limestone are used to raise the pH of AMD waters, which in turn

facilitates the precipitation of metals and formation of gypsum (CaSO4.2H2O). This research

has assessed the viability of using Bayer precipitates, formed by the seawater neutralisation of

Bayer liquor residues produced by the alumina industry, as an alternative neutralisation

reagent. Bayer precipitates formed by the seawater neutralisation of 1-10 g/L Bayer liquors

have been characterised by XRD, XPS, IR and TGA, and then assessed for their acid

neutralising capacity. The 10 Bayer precipitates were found to satisfy ANZECC requirements

when used for the treatment of AMD water from the open pit at Mt Morgan mine.

Neutralisation of the acidity was due to the dissolution of Bayer hydrotalcite

(Mg6Al2(OH)16CO3.xH2O) and calcium carbonate (CaCO3), while heavy metal removal was

found to be a complex process involving hydroxide precipitation, and reactions involving the

regeneration of Bayer hydrotalcite (surface precipitation, isomorphic substitution and surface

complexation reactions). Bayer precipitates out-performed Bayer liquor and commercially

used alkali in the removal of heavy metals, as well as in the neutralisation of acidity by a

weight-by-weight basis. Manganese was the only heavy metal that did not meet ANZECC

guidelines at a pH of 8.5, and therefore thermal activation (320-440 °C) of Bayer precipitates

was investigated to increase their metal uptake capacity. This research has found that Bayer

precipitates are an attractive alternative for the treatment of AMD waters, showing robustness

and versatility with AMD compositions, and therefore has the potential to reduce the footprint

of both industries.

v

LIST OF PUBLICATIONS

1. Gurkiran Kaur, Sara Couperthwaite, Bradley W. Hatton-Jones and Graeme Millar,

Alternative neutralisation materials for acid mine drainage treatment. Journal of

Water Process Engineering, 22 (2018) 46-58.

2. Gurkiran Kaur, Sara Couperthwaite and Graeme Millar, Acid mine drainage

treatment using Bayer precipitates obtained from seawater neutralisation of

Bayer liquor. Submitted to Science of the Total Environment, (2018).

3. Gurkiran Kaur, Sara Couperthwaite and Graeme Millar, Enhanced removal of Mn

(II) from solution by thermally activated Bayer precipitates. Submitted to

Minerals Engineering, (2018).

4. Gurkiran Kaur, Sara Couperthwaite and Graeme Millar, Performance of Bauxite

Refinery Residues for Treating Acid Mine Drainage. Under review Journal of

Water Process Engineering, (2018).

5. Wentao Liang, Sara J. Couperthwaite, Gurkiran Kaur, Cheng Yan, Dean W. Johnstone

and Graeme J. Millar, Effect of strong acids on red mud structural and fluoride

adsorption properties. Journal of Colloid and Interface Science, 423 (2014) 158-

165.

6. Sara J. Couperthwaite, Sujung Han, Talitha Santini, Gurkiran Kaur, Dean W.

Johnstone, Graeme J. Millar and Ray L. Frost, Bauxite residue neutralisation

precipitate stability in acidic environments. Environmental chemistrty, 10 (2013)

455-464.

vi

Papers presented at conferences:

1. A poster was prepared and presented at the International Mining and resources

Conference, Melbourne, Australia, 12-15 November 2015 on the use of Bauxite

refinery residues for the treatment of acid mine drainage from an abandoned

open pit.

vii

KEYWORDS

• Bauxite refinery residues

• Bayer liquor

• Seawater neutralisation

• Layered double hydroxides

• Bayer precipitates

• Hydrotalcite

• Calcium Carbonate

• Acid mine drainage (AMD)

• Heavy metals

• Active AMD treatment methods

• Lime

• Sodium hydroxide

• Sodium carbonate

• Neutralisation

• Chemical precipitation

• Inductively couple plasma optical emission spectroscopy

• Infrared spectroscopy

• X-ray diffraction

• Thermal analysis

• X-ray photoelectron spectroscopy

• Thermal activation

• Thermogravimetric anlaysis

• Brunauer, Emmett and Teller (BET) Surface analysis

viii

LIST OF ABBREVIATIONS

AMD Acid mine drainage

ARD Acid rock drainage

ANC Acid neutralising capacity

ANZECC Australian and New Zealand Environment and

Conservation Council

B.PPT Bayer precipitates

BL Bayer liquor

BET Brunauer, Emmett and Teller (BET) Surface analysis

CO32- Carbonate

FTIR Fourier transform infrared spectroscopy

HT Hydrotalcite

ICP-OES Inductively coupled plasma optical emission spectroscopy

LDH Layered double hydroxides

OH- Hydroxide ions

Ppt Precipitates

QUT Queensland University of Technology

SEL Strong evaporation liquor

SEM Scanning electron microscopy

SNL Supernatant liquor

SWN Seawater neutralised

TA Thermally activated

TA B.PPT Thermally activated Bayer precipitates

TGA Thermogravimetric analysis

XRD X-ray diffraction

XPS X-ray photoelectron spectroscopy

ix

TABLE OF CONTENTS

Statement of Original Authorship ................................................................................. i

Acknowledgments........................................................................................................ ii

Abstract ....................................................................................................................... iv

List of Publications .......................................................................................................v

List of Abbreviations ................................................................................................ viii

Table of Contents ........................................................................................................ ix

List of Figures ............................................................................................................ xii

List of Tables ............................................................................................................ xiv

CHAPTER 1: INTRODUCTION 1

CHAPTER 2: LITERATURE REVIEW 11

2.1 Acid mine drainage (AMD)...............................................................................11

2.1.1 Pyrite oxidation ........................................................................................11

2.2 Effects of acid mine drainage (AMD) ...............................................................13

2.2.1 Effects of low pH .....................................................................................13

2.2.2 Effects of heavy metals ............................................................................14

2.3 Control and treatment of acid mine drainage ....................................................16

2.3.1 Source control ..........................................................................................16

2.3.2 Migration control .....................................................................................17

2.4 Bauxite refinery residues ...................................................................................24

2.4.1 Bauxite .....................................................................................................24

2.4.2 Bauxite refining - extraction of alumina from bauxite ore ......................24

2.4.3 Red mud and Bayer liquor .......................................................................26

2.4.4 Disposal of bauxite residue ......................................................................26

2.4.5 Neutralisation of bauxite refinery wastes .................................................27

2.4.6 Bauxite residue utilisation options ...........................................................29

2.5 Layered double hydroxides (LDHs) ..................................................................35

2.5.1 Structure ...................................................................................................35

2.5.2 Preparation ...............................................................................................37

2.6 Removal of heavy metals ..................................................................................39

2.6.1 Chemical precipitation .............................................................................39

2.6.2 Heavy metal removal using layered double hydroxides ..........................41

2.6.3 Reaction kinetics of heavy metal removal by LDH .................................47

2.6.4 Removal affinities ....................................................................................48

2.7 Factors affecting heavy metal removal..............................................................48

2.7.1 pH ……………………………………………………………………...48

2.7.2 Temperature .............................................................................................49

2.7.3 Ionic strength............................................................................................50

2.8 Conclusion .........................................................................................................50

x

CHAPTER 3: CHARACTERISATION TECHNIQUES AND

EXPERIMENTAL METHOD ................................................................................. 53

3.1 Characterisation techniques ...............................................................................53

3.1.1 Inductively coupled plasma optical emission spectroscopy (ICP-OES) ..53

3.1.2 X-ray diffraction (XRD) ..........................................................................53

3.1.3 Infrared Spectroscopy ..............................................................................54

3.1.4 Thermalgravimetric analysis ....................................................................54

3.1.5 Optical imaging ........................................................................................54

3.1.6 Freeze Drying of Mt Morgan Lime Neutralisation Sludge ......................55

3.1.7 X-Ray Fluorescence .................................................................................55

3.1.8 Process Simulation using AqMB Software ..............................................56

3.1.9 Surface analysis........................................................................................56

3.1.10 X-ray Photoelectron Spectroscopy (XPS) .............................................56

3.1.11 Water Quality Standards ........................................................................57

3.2 Experimental methods .......................................................................................58

3.2.1 Effect of Bayer liquor concentrations on the synthesis of Bayer

precipitates .........................................................................................................58

3.2.2 Assessing the effectiveness of Bauxite refinery residues with

conventionally used alkali .................................................................................62

3.2.3 Heavy metal removal efficiencies of thermally activated Bayer

precipitates .........................................................................................................66

3.2.4 AMD water treatment with Bayer precipitates and thermally activated

Bayer precipitates ..............................................................................................69

CHAPTER 4: EFFECT OF BAYER LIQUOR COMPOSITION ON THE

FORMATION OF BAYER PRECIPITATES ....................................................... 75

4.1 Introduction .......................................................................................................76

4.2 Impact of Bayer Liquor Composition on Seawater Neutralisation Precipitates 78

4.3 Impact of Bayer liquor composition on neutralisation efficiency .....................92

4.4 Acid Mine Drainage Treatment with Bayer Precipitates ..................................97

4.5 Conclusion .......................................................................................................107

CHAPTER 5: ASSESSING THE EFFECTIVENESS OF BAUXITE REFINERY

RESIDUES WITH CONVENTIONALLY USED ALKALI .............................. 109

5.1 Introduction .....................................................................................................110

5.2 Characterisation of Bayer Precipitate ..............................................................112

5.3 AMD Characteristics .......................................................................................112

5.4 AMD neutralisation using various alkalis .......................................................115

5.4.1 Iron removal ...........................................................................................118

5.4.2 Aluminium removal ...............................................................................120

5.4.3 Copper removal ......................................................................................121

5.4.4 Zinc removal ..........................................................................................122

5.4.5 Nickel removal .......................................................................................123

5.4.6 Manganese removal ...............................................................................124

5.5 Precipitate Analysis .........................................................................................125

5.5.1 Elemental composition ...........................................................................125

xi

5.5.2 X-ray diffraction ....................................................................................126

5.5.3 Particle size analysis ..............................................................................130

5.6 Performance versus Operational Considerations .............................................132

5.7 Conclusions .....................................................................................................137

CHAPTER 6: ENHANCED REMOVAL OF MN (II) BY BAYER

PRECIPITATES AND THERMALLY ACTIVATED BAYER PRECIPITATES

................................................................................................................................ 139

6.1 Introduction .....................................................................................................140

6.2 Effect of Thermal Activation of Bayer Precipitates ........................................141

6.3 Impact of Bayer and Thermally Activated Bayer Precipitates upon Manganese

Concentration in Solution .........................................................................................147

6.4 Examination of Precipitates after Removal of Manganese .............................153

6.5 Conclusion .......................................................................................................164

CHAPTER 7: EVALUATE THE PERFORMANCE OF BAYER MATERIALS

FOR TREATING DIFFERENT MINE WASTEWATER ................................. 166

7.1 Introduction .....................................................................................................167

7.2 Variations in Mine Water Composition at Mt Morgan Mine Site ...................168

7.3 Performance of Bayer Precipitates with Variable AMD Water Composition 169

7.4 Produced Sludge Composition and Stability ...................................................184

7.5 Conclusions .....................................................................................................190

CHAPTER 8: CONCLUSIONS ........................................................................... 192

REFERENCES ........................................................................................................ 194

APPENDIX .............................................................................................................. 222

Appendix 1: Supplementary information for Chapter 4 ...........................................222




xii

LIST OF FIGURES

Figure 1: Biological and abiotic strategies for remediating AMD (adapted from [13]) ............... 18

Figure 2: Schematic representation of the 3:1 hydroxide layers of hydrotalcite [33] ................... 36

Figure 5: Schematic representation of the hydrotalcite structure (modified from [198]) ............ 36

Figure 6: Adsorption mechanism of cation removal using LDHs .................................................. 42

Figure 7: Precipitation mechanism of cation removal using LDHs ............................................... 44

Figure 8: Isomorphic substitution mechanism of cation removal using LDHs ............................. 45

Figure 9: Chelation mechanism of cation removal using LDHs ..................................................... 46

Figure 10: XRD patterns for Bayer precipitates formed at pH 9.25 (a) 1-5 g/L Al2O3 b) 6-10 g/L Al2O3

..................................................................................................................................................... 80

Figure 11: Infrared spectra (3800 – 2800 cm-1) for (a) 1-5 g/L (b) 6-10 g/L Al2O3 samples ......... 85

Figure 12: Infrared spectra (1650 – 1000 cm-1) for (a) 1-5 g/L (b) 6-10 g/L Al2O3 samples ........ 87

Figure 13: DTG curves of the Bayer precipitates in the dehydroxylation/decarbonation region (a) 1-5 g/L

(b) 6-10 g/L Bayer liquor ........................................................................................................... 91

Figure 14: pH variation as a function of seawater addition to Bayer liquor with different alumina

concentrations ............................................................................................................................ 93

Figure 15: ICP-OES of remaining Mg2+ in seawater neutralised Bayer liquor supernatants .... 94

Figure 16: ICP-OES of remaining Al3+ in seawater neutralised Bayer liquor supernatants ...... 95

Figure 17: ICP-OES of remaining Ca2+ in seawater neutralised Bayer liquor supernatants ...... 97

Figure 18: Neutralisation curve obtained by the addition of Bayer precipitates to AMD .......... 98

Figure 19: Variation of iron concentration in AMD as a function of solution pH when 1-10 g/L Bayer

precipitates were added ........................................................................................................... 101

Figure 20: Variation of aluminium concentration in AMD as a function of solution pH when 1-10 g/L Bayer


Figure 21: Variation of concentration of dissolved components in AMD as a function of solution pH when

Bayer precipitates were added (a) Copper (b) Zinc ............................................................. 103

Figure 22: Variation of manganese concentration in AMD as a function of solution pH when 1-10 g/L Bayer


Figure 23: Variation of concentration of dissolved components in AMD as a function of solution pH when

Bayer precipitates were added (a) Magnesium (b) Calcium ............................................... 106

Figure 24: XRD pattern of Bayer precipitate formed by the seawater neutralisation of Bayer liquor 114

Figure 25: Neutralisation curves for different alkaline materials as indicated ........................... 116

Figure 26: Trends for Al, Mg, Mn, Cu, Zn, Si, Fe and Ni removal from AMD solution when using different

alkaline materials ..................................................................................................................... 119

Figure 27: XRD pattern of precipitates obtained from treatment of AMD with different alkaline materials

................................................................................................................................................... 129

Figure 28: Image J particle size analysis of optical images of precipitates formed during AMD treatment a)

with lime at pH 6.75 b) sodium hydroxide at pH 6.80 c) sodium carbonate at pH 6.26 d) Bayer liquor

at pH 6.49 and e) Bayer precipitates at pH 6.50 .................................................................... 132

Figure 29: XRD pattern of Bayer precipitates and thermally activated Bayer precipitates ...... 142

Figure 30: IR spectra of Bayer precipitates and thermally activated Bayer precipitates (a) high

wavenumber region (b) low wavenumber region .................................................................. 146

xiii

Figure 31: Adsorption-desorption isotherm: (a) Bayer precipitates (b) Thermally activated Bayer

precipitates 320 °C (c) Thermally activated Bayer precipitates 380 °C (d) Thermally activated Bayer

precipitates 440 °C ................................................................................................................... 147

Figure 32: Concentration of Mn,Mg and Ca after addition of Bayer precipitates ..................... 148

Figure 33: Concentration of Mn, Mg and Ca after addition of TA B.PPT at 320 °C ................. 149

Figure 34: Concentration of Mn,Mg and Ca after addition of TA B.PPT at 380 °C .................. 149

Figure 35: Concentration of Mn, Mg and Ca after addition of TA B.PPT at 440 °C ................. 150

Figure 36: XRD pattern of precipitates obtained after treatment with Mn solution ................. 155

Figure 37: Wide scan XPS spectra .................................................................................................. 158

Figure 38: Deconvolution of Mg 2p peak (a) B.PPT (b) B.PPT-Mn (c) TA B.PPT and (d) TA B.PPT-Mn

................................................................................................................................................... 159

Figure 39: Al 2p (a) B.PPT (b) B.PPT-Mn (c) TA B.PPT and (d) TA B.PPT-M ........................ 160

Figure 40: Deconvolution of O1s peak (a) B.PPT (b) B.PPT-Mn (c) TA B.PPT and (d) TA B.PPT-Mn 161

Figure 41: Mn 2p peak after treatment with (a) Bayer precipitate (b) Thermally activated Bayer precipitate

(380 oC) ..................................................................................................................................... 162

Figure 42: XRD pattern of Bayer precipitates and thermally activated Bayer precipitates ...... 171

Figure 43: Neutralisation curves for Open Pit, Airfield, and Mundic West AMD water treated with (a)

Bayer precipitates and (b) thermally activated Bayer precipitates: Comparison curve representing

addition of Bayer precipitates and thermally activated Bayer precipitates to acidified water shown in

(c) ............................................................................................................................................... 173

Figure 44: Heavy metal removal curves for a) open pit b) airfield sump and c) mundic west treated with

Bayer precipitates .................................................................................................................... 176

Figure 45: Heavy metal removal curves for a) open pit b) airfield sump and c) mundic west treated with

thermally activated Bayer precipitates .................................................................................. 177

Figure 46: Metal removal mechanism using LDH ......................................................................... 183

Figure 47: XRD patterns of precipitates obtained after AMD water treated with (a) Bayer precipitates (b)

thermally activated Bayer precipitates .................................................................................. 185

xiv

LIST OF TABLES

Table 1:Effects of pH on aquatic life [49, 52, 53] ............................................................................. 14

Table 2: Effects of heavy metals on plants, animals and aquatic animals [55-62] ........................ 15

Table 3: ANZECC water quality guidelines relevant to Mount Morgan AMD water ................. 57

Table 4: Masses and volumes required to prepare different Bayer liquors .................................. 58

Table 5: Ions concentration (mg/L) of Bayer liquor and Seawater ................................................ 59

Table 6: Volume of seawater added to attain pH (13-9.25) ............................................................. 60

Table 7: Composition of mine pit water from Mount Morgan, June 2017 .................................... 61

Table 8: Mass of Bayer precipitates added to treat AMD ............................................................... 61

Table 9: Mass of Bayer precipitates used for acid digestion ........................................................... 62

Table 10: Composition of open pit water from Mount Morgan, August 2014 .............................. 63

Table 11: Amount of alkali added to 25 mL of AMD water ........................................................... 65

Table 12: Mass of precipitates (obtained after AMD treatment) used for acid digestion ............ 66

Table 13: Mass of Bayer precipitates and thermally activated Bayer precipitates added to 40 mL Mn

solution ........................................................................................................................................ 68

Table 14: Mass of precipitates used for acid digestion .................................................................... 69

Table 15: Water compositions found at Mt Morgan mine site, August 2014 ................................ 70

Table 16: Mass loss during thermal activation ................................................................................ 71

Table 17: Mass of Bayer precipitates and thermally activated Bayer precipitates added to treat AMD

waters .......................................................................................................................................... 73

Table 18: Mass of precipitates (obtained after AMD treatment) used for leaching experiment . 74

Table 19: Mass of precipitates (obtained after AMD treatment) used for acid digestion ............ 74

Table 20: Phase composition of Bayer precipitates formed from seawater neutralisation of 1-10 g/L Bayer

liquor ........................................................................................................................................... 79

Table 21: Infrared peaks for Bayer precipitates obtained by seawater neutralisation of 1-10 g/L Al2O3 [205,

294] .............................................................................................................................................. 83

Table 22: Concentration of elements in precipitates obtained at pH 9.25 by seawater neutralisation of Bayer

Liquor ......................................................................................................................................... 88

Table 23: Mount Morgan mine pit water treatment using various alkaline materials and their metal

removal capacity in mg/L ........................................................................................................ 117

Table 24: Metal concentrations (mg/g) in precipitates between pH 6.5 and 7.5.......................... 126

Table 25: Possible phases precipitated at pH 7.5 using lime and sodium hydroxide based on AqMB

simulations ................................................................................................................................ 127

Table 26: XRF data of freeze dried tailings beach and lime neutralisation plant sludge from Mt Morgan

................................................................................................................................................... 134

Table 27: Metals leached in DI water from precipitates obtained after treatment of AMD water with

different alkali .......................................................................................................................... 135

Table 28: Metals concentration in AMD water from precipitates obtained after treatment of AMD water

with different alkali ................................................................................................................. 135

Table 29: Concentration (mg/g) of Mg, Al and Mn in precipitates before and after treatment with Mn

solution. ..................................................................................................................................... 154

xv

Table 30: XPS results of Bayer precipitates and thermally activated Bayer precipitates before and after

treatment with Mn solution ..................................................................................................... 163

Table 31: Concentrations of heavy metals in AMD water from three sites at Mt. Morgan ....... 170

Table 32: Heavy metal concentration (mg/L) in AMD waters after treatment with Bayer precipitates 179

Table 33: Heavy metal concentration in AMD waters after treatment with thermally activated Bayer

precipitates ............................................................................................................................... 180

Table 34: Metal concentrations (mg/g) in precipitates obtained by AMD treatment with Bayer precipitates

between pH 7.0 to 8.0 ............................................................................................................... 187

Table 35: Metal concentrations (mg/g) in precipitates obtained by AMD treatment with thermally activated

Bayer precipitates between pH 7.0 to 8.0 ............................................................................... 187

Table 36: Metals leached from precipitates obtained after treatment of AMD waters with Bayer precipitates

and thermally activated Bayer precipitates ........................................................................... 189

1

Chapter 1: Introduction

The Department of Natural Resources and Mines (Queensland Government) states that there

are over 15000 abandoned mine sites across Queensland [1]. The greatest environmental issue

associated with abandoned mines are the large waste rock (produced during mining) and tailing

dumps (waste residue produced after processing) that form acid and metalliferous drainage and

seepage, commonly known as acid mine drainage (AMD) [2, 3]. AMD is a phenomenon that

occurs when sulphide minerals, like pyrite, in waste rock and tailings are exposed to the

atmosphere, which leads to a series of geochemical and microbial reactions that result in the

formation of sulphuric acid [4]. The acidic conditions then have the ability to leach heavy

metals from surrounding rocks [5].

AMD waters with high acidity and dissolved metal content can have severe environmental

impacts on all forms of life [6-8]. The Dee River in central Queensland is a representative

example of effects of the release of AMD water from the abandoned mine (Mount Morgan

mine) on aquatic plant and animal life [9]. It has been reported that effects of AMD are more

intense in the surrounding area of mine and may have a negative impact on aquatic organisms

for up to 50 kilometres [9]. In January 2013, AMD from the open pit (Mount Morgan mine

site) overflowed into the Dee River due to excessive rainfall from ex-tropical cyclone Oswald,

decreasing the pH of the river and increasing the heavy metal concentrations [10]. The

Queensland government placed warning signs along the Dee River to advice people not to use

the river water for drinking, swimming and agriculture purposes [11]. In addition to the

environmental impact, economic impacts include the remediation, treatment, clean-up of

overflow events, as well as loss of primary industries that relied on natural resources.

2

AMD can be prevented and controlled by removing one or more steps of sulphide mineral

oxidation. AMD can be regulated by reducing oxygen supply and excluding the water needed

for oxidation and transport of pollutants [4, 12]. Various methods involved to prevent and

control of AMD are:

1. pyritic mine wastes can be encapsulated within layers of neutralising material

2. addition of limestone limits the activity of iron oxidising bacteria

3. bactericides can be used to control bacterial growth which causes oxidation of sulphide

minerals

4. carbonates are also used to neutralise the acid produced during pyrite oxidation

Over the last few decades, several different technologies have been developed to remediate

AMD from mine sites, however due to its complexity and variability no treatment method suits

all AMD water [13, 14]. In general, the treatment of AMD can be classified as active or passive

systems based on their requirements for chemical additions, infrastructure, maintenance and

monitoring. In an active system, AMD is treated by the addition of alkaline materials such as

lime, magnesia or sodium hydroxide [15]. This encourages the precipitation of iron

oxyhydroxides and metals hydroxides that can then be removed by solid-liquid separation

techniques [16, 17]. A variety of passive treatment systems such as aerobic wetlands, anaerobic

or compost wetlands [18], vertical flow wetlands, AMD treatment ponds, bioreactors and

permeable reactive barriers have been applied widely for the treatment of AMD [14]. Sulphate

reducing bacteria can also be used for the treatment of AMD by removing metals and sulphate

[12]. Under anaerobic environments sulphate reducing bacteria oxidise simple organic

3

mixtures and produce bicarbonate ions and hydrogen sulphide, which can remove acidity and

heavy metals from AMD water [19-22]. This research will focus on developing materials, from

the waste residues of the alumina industry, for an alternative to current materials used in active

treatments of AMD water.

The largest source of aluminium is found in bauxite ore, in the form of gibbsite (Al(OH)3),

boehmite (γ-AlO(OH)) and diaspore (α-AlO(OH)) [23]. The Bayer process, developed by Karl

Joseph Bayer 110 years ago, is the cornerstone of alumina production [24]. However, the large

volumes of waste residue, known as bauxite refinery residue (more commonly known as red

mud), cause dust pollution in arid regions, pollution to water and land via leachates. Economic

concerns include the availability of land, and high costs associated with the large area of land

needed for storage [25, 26]. The waste residue produced by the Bayer process is strongly

alkaline (pH ranging from 10 to 13) and contains a variety of heavy metal species [27] . Due

to these hazardous characteristics, bauxite refinery residues typically require treatment prior to

disposal. Several methods have been reported including: infiltration of seawater and

atmospheric CO2, treatment with strong acids, gypsum addition and seawater neutralisation

[24, 27-29]. However, the type of treatment depends on factors such as the location of alumina

refineries, volume and cost of reagents used for bauxite residue neutralisation. For example,

seawater neutralisation for instance is a possible option but only for alumina refineries on the

coast as pumping seawater inland would be very costly, while the cost associated with using

mineral acids (HCl, H2SO4 or HNO3) or gypsum is relatively high due to large volumes of

reagent required for neutralization to be successful [30]. This research is interested in the

by-product of refineries that use seawater to neutralise the caustic supernatant fraction of the

bauxite residue produced.

4

This research will investigate the potential of combining the wastes of two mining industries;

it is proposed that the caustic nature and ion composition of bauxite refinery residue will be

able to neutralise AMD waters. Numerous studies have investigated the use of red mud as a

neutralising agent for AMD water [31, 32], however this results in large volumes of solid waste

that requires subsequent disposal and capping to prevent metals leaching. This investigation

will focus on using the seawater derivatives (Bayer precipitates) formed from the treatment of

Bayer liquor (liquor component of red mud or red mud leachates) with seawater. It is

envisioned that the use of these “cleaner” wastes from bauxite residues will produce similar

volumes of waste to traditional neutralisation agents, such as lime, with similar or improved

metal removal capacities due to the presence of hydrotalcite in the Bayer precipitate (a known

adsorbent material) [33].

Objectives

The main objective of this research project is to compare the effectiveness of recycled bauxite

refinery residues (Bayer liquor, Bayer precipitates and thermally activated Bayer precipitates)

and commercially used alkali (lime, sodium hydroxide and sodium carbonate) in the treatment

of AMD water. This research will focus primarily on the neutralisation of AMD water produced

at the abandoned Mount Morgan mine. To compare the neutralisation capacity of Bayer

precipitates with commercial alkali the following aims will need to be achieved:

1. Obtain an in-depth understanding of the seawater neutralisation process of Bayer liquor

and acid neutralising capacity (ANC) of the resultant precipitates; is there an ideal Bayer

precipitate composition?

5

2. Compare the performance of Bayer materials with commercial alkali and elucidate the

cause for differences in heavy metal removal; do they provide any advantages over

commercial alkali?

3. Assess the advantages of using thermally activated Bayer precipitates in the removal of

Mn from aqueous solution; can significant improvements in heavy metal removal be

achieved?

4. Evaluate the performance of Bayer materials for treating different mine wastewater; are

they versatile?

Obtain an in-depth understanding of the seawater neutralisation process of Bayer liquor

and acid neutralising capacity of the resultant precipitates

The initial stages of this research require an understanding of how the formation of Bayer

materials can benefit both the alumina and mines generating AMD water. For the alumina

industry, it is essential that Bayer precipitates formed from all compositions of Bayer liquor

(ranging from liquor in red mud slurries and leachates from red mud disposal sites) can be used

in the neutralisation of AMD water due to variations in liquor compositions from refineries and

the source of liquor. Therefore, the composition and ANC of Bayer precipitates formed from

multiple concentrations of Bayer liquor have been assessed. In the case of mines generating

AMD water, an assessment of the ANC of each Bayer precipitate will allow predictions to be

made on the quantity of Bayer precipitate required to achieve a pH between 6.5 and 8.0.

Overall, a Bayer liquor that produces a Bayer precipitate with a high hydrotalcite portion and

high ANC will be used in subsequent test work. This phase of research will also give an insight

into the potential dissolution species produced during the treatment of AMD water from Mount

Morgan mine’s open pit.

6

Compare the performance of Bayer materials with commercial alkali and elucidate the

cause for differences in heavy metal removal

The ANC and heavy metals removal capacity of recycled bauxite refinery residues, Bayer

liquor and Bayer precipitates, will be compared with commercially used alkali, lime, sodium

hydroxide and sodium carbonate. The performance of each alkali will be based on the metal

removal percentage and the pH when removal occurs (important for meeting discharge water

qualities). Mechanisms involved in the removal of heavy metals from AMD water with the

various alkali will be elucidated. In addition, an assessment of the suitability for each

neutralising agent will be made in terms of the amounts of each material required, supernatant

chemistry and impacts on environmental ecosystems, solid residue composition and disposal,

and safety issues like handling and transportation of each material.

Assess the advantages of using thermally activated Bayer precipitates in the removal of

Mn from aqueous solution

The use of thermally activated hydrotalcites in neutral to alkaline solutions have previously

shown superior uptake capacities compared to hydrotalcite. However, little research has looked

at the performance of thermally activated hydrotalcites under acidic conditions; they dissociate

which is not favourable in typical adsorption applications. In the case of AMD water, the

dissociation of hydrotalcite releases hydroxides that neutralise the acid component

(favourable), while the regeneration of hydrotalcite can remove heavy metals. This phase of

research will compare the performance of Bayer precipitates that have been unactivated and

thermally activated to determine if the activation process improves the removal of Mn from

acidic solution. As the hydrotalcite structures dissociate and reform as the pH increases, this

7

phase of work will also elucidate the mechanisms involved in the removal of metals during the

treatment of AMD water.

Evaluate the performance of Bayer materials for treating different mine wastewater

For Bayer precipitates to be a viable option in the treatment of AMD waters, it is important to

know how they perform with different feed water compositions. Therefore, this phase of

research will assess the performance of Bayer precipitates that have been unactivated and

thermally activated on three compositions of AMD water found at Mount Morgan (Open pit,

Mundic West and Airfield Dump). These AMD waters, formed from different waste rock and

tailings sources, have varying quantities of elements (Al (1233-1703 mg/L), Fe (17-241 mg/L),

Mn (162-186 mg/L), Cu (77-101 mg/L), Zn (49-73 mg/L), and Ni (1.23-1.54 mg/L)) and

acidity ( pH 2.70 Airfield dump, 2.85 Mundic West and 3.75 Open pit), which is proposed to

influence the performance of Bayer precipitate materials. The final phase of this research will

involve stress testing the Bayer precipitates with different solution chemistry and to find what

causes variations in treatment efficiency and limitations with regards to feed water quality.

Weathering/stability studies of the sludge produced will provide insights into potential

storage/disposal options.

Significance

AMD is one of the most serious problem created by mining and can badly disturb surrounding

environments due to its low pH and high levels of sulphates and heavy metals. On the other

hand, storage of the huge amount of highly alkaline Bauxite refinery residue has become a

significant issue for alumina refineries. Therefore, this research attempts to addresses both

issues 1) to neutralise acidity and reduce metal concentrations in AMD, and 2) lessening the

8

environmental and economic concerns associated with disposal of bauxite refinery residues.

The approach of using recycled bauxite refinery residues could be used as a gateway to the

development of alternative materials for the neutralisation of AMD for discharge into

neighbouring creeks, as well as the removal of heavy metals to meet environmental regulations.

This approach will reduce the disposal and management costs associated with alumina

production, whereas mining industries would benefit through the use of an inexpensive material

in the treatment of AMD.

Thesis outline

Chapter Two presents a review of current literature on the formation of AMD, and various

methods used for the treatment of AMD and heavy metal removal worldwide. It will be divided

into three main parts 1) AMD causes, effects and treatment, and 2) bauxite refinery residues;

origin, disposal, reuse options and neutralisation techniques, and 3) heavy metal removal by

conventional techniques and by using layered double hydroxides.

Chapter Three highlights the experimental procedures that were used and describe the

characterisation techniques used in this study such as:

▪ Inductively coupled plasma optical emission spectrometry (ICP-OES)

▪ Fourier transform infrared spectroscopy (FTIR)

▪ X-ray diffraction (XRD)

▪ Thermogravimetric analysis (TGA)

▪ X-ray fluorescence spectroscopy (XRF)

9

▪ X-ray photoelectron spectroscopy (XPS)

▪ Brunauer, Emmett and Teller (BET) Surface Analysis

Chapter Four provides a better understanding of how the Bayer liquor composition influences

the seawater neutralisation process and composition of the resultant precipitates formed. The

combination of X-ray diffraction, infrared spectroscopy and thermogravimetric analysis have

been used to characterise the precipitates formed by the seawater neutralisation of Bayer liquor

of different concentrations. ICP-OES has been used to determine the concentrations of

aluminium, magnesium, calcium and sodium in the supernatants of seawater neutralisation

process to determine the effect of Bayer liquor composition on neutralisation efficiency of

seawater. The acid neutralising capacity of the obtained precipitates have been assessed by

using them to treat the AMD water from Mount Morgan mine.

Chapter Five presents results and discussion on the comparison of performance of recycled

bauxite refinery residues (Bayer liquor and Bayer precipitates) with conventionally used alkali

(lime, sodium hydroxide and sodium carbonate) for the treatment of AMD from Mt.Morgan

mine’s open pit. The performance of each alkali was assessed by measuring the acid

neutralising capacity, metal uptake capacity the stability of produced sludge. ICP-OES has

been used to govern the metal concentration changes with each addition of alkali and the results

were compared with ANZECC guidelines to see if discharge water quality was obtained. X-

ray diffraction and AqMB process simulation software has been used to determine the species

present in the produced sludge after treatment with different alkali.

10

Chapter six has focussed on the effect of thermal activation on Bayer precipitates and the

impact compositional changes had on the uptake capacity of manganese. X-ray diffraction,

infrared spectroscopy, X-ray photoelectron spectroscopy (XPS) and BET surface analysis were

used to characterise the thermally activated samples and identify mechanisms for manganese

removal. ICP-OES has been used to determine the metal concentrations in the solution and in

precipitates obtained after treatment with unactivated and thermally activated Bayer

precipitates.

Chapter seven sheds light on the robustness and versatility of Bayer precipitates for different

AMD water quality types. AMD samples collected from 10 different locations at Mt Morgan

were characterised and found to have significantly different compositions based on whether

there was tailings or waste rock in the dam walls. The efficiency of Bayer precipitates and

thermally activated Bayer precipitates for treating different mine waters was based on 3 water

qualities from Mt Morgan. Performance has been assessed using neutralisation curves, heavy

metal removal curves, and sludge stability. Bayer materials used in this study have been

characterised by X-ray diffraction and infrared spectroscopy. ICP-OES has been used to

determine the concentration of heavy metals in solution and produced sludge (via acid

digestion), while the stability of produced sludge from the treatment of different mine waters

was studied by weathering studies.

Chapter eight then presents a conclusion of the work performed and suggestions are given for

further study from the results and discussion of experimental work.

11

Chapter 2: Literature review

2.1 Acid mine drainage (AMD)

Acid mine drainage is an accelerated form of the naturally occurring acid rock drainage (ARD)

at mine sites and is one of the most critical problems caused by mining [34]. The oxidation of

sulphides in rock is a natural process (ARD), which normally occurs slowly as the surface of

earth erodes and the underlying rock oxidizes [35, 36]. Production of waste rocks and tailings

by mining operations lead to the exposure of pyrite and other sulphide bearing minerals to

environmental oxygen and water [37]. In the presence of oxygen, water and/or oxidising

bacteria, sulphide minerals oxidise and form sulphuric acid that can cause liberation of metals

from affected soils and rocks [15]. Water passing through this waste pile becomes acidic and

metal rich, and is referred to as AMD [4].

2.1.1 Pyrite oxidation

On exposure to air and water the oxidation of pyrite and other sulphide bearing minerals in

mine wastes occur, which leads to the formation of acid mine/rock drainage [34]. At mine sites,

this can occur in tailing dams, waste rock dumps, pit walls and in underground workings. The

pyrite oxidation process is complex and involves several chemical, biological and

electrochemical reactions [38, 39]. The chemical reactions that characterise various stages of

pyrite oxidation and formation of AMD are as follows [4]:

1. Iron sulphide oxidation: pyrite is oxidised when exposed to oxygen and water resulting

in hydrogen ion released acidity, sulphate ions and soluble metal ions as shown in

Equation 1.

12

Equation 1: 𝐹𝑒𝑆2 + 7

2𝑂2 + 𝐻2𝑂 → 𝐹𝑒2+ + 2 𝑆𝑂4

2− + 2𝐻+

2. Ferrous ion oxidation: oxidation of ferrous (Fe2+) produced in Equation 2 to ferric

(Fe3+) occurs very slowly at pH of about 3, when sufficient oxygen is dissolved in water.

However, oxidation of ferrous ions can be accelerated by a factor of 106 in the presence

of iron-oxidising bacteria, such as thiobacillus ferrooxidans [40, 41].

Equation 2: 𝐹𝑒2+ + 1

4𝑂2 + 𝐻+ → 𝐹𝑒3+ +

1

2𝐻2𝑂

3. Ferric ion hydrolysis: hydrolysis of ferric ion occurs at pH values of above 3.5 and

forms ferric hydroxide Equation 3, which is seen as a red-orange precipitate in waters

affected by AMD.

Equation 3: 𝐹𝑒3+ + 𝐻2𝑂 → 𝐹𝑒(𝑂𝐻)3 + 3 𝐻+

4. Enhance oxidation of ferric sulphide ions: ferric ions (Fe3+) produced by ferrous ion

(Fe2+) oxidation can react directly with pyrite to form more ferrous ions and acidity as

shown in Equation 4. In the presence of sufficient oxygen and ferrous ions, the cycle

of reactions 2 and 3 becomes continuous.

Equation 4: 𝐹𝑒𝑆2 + 14 𝐹𝑒3+ + 8 𝐻2𝑂 → 15 𝐹𝑒2+ + 2 𝑆𝑂42− + 16 𝐻+

In the above reactions, ferric ions and oxygen are the major pyrite oxidants. The rate of pyrite

oxidation depends on various factors such as surface morphology of pyrite, the oxygen

concentration, pH, presence of bacteria and acid consuming materials (calcium carbonate for

example) [39, 42]. In addition to pyrite, oxidation and hydrolysis of chalcopyrite (CuFeS2),

13

sphalerite ((Zn,Fe)S), pyrrohite ((Fe(1-x)S), x=0-0.17), galena (PbS), pentandite (Fe, Ni)9S8 and

millerite (NiS) are other sulphide minerals associated with the release of metals such as copper

(Cu), zinc (Zn), lead (Pb) and nickel (Ni) during the formation of AMD waters [41, 43]. Once

started, the release of pollutants by oxidation of sulphide minerals as AMD is persistent and a

main source of pollution on mine sites.

2.2 Effects of AMD

As discussed in previous sections, due to its low pH and high dissolved heavy metal

concentrations, AMD can cause serious environmental problems and is poisonous to aquatic

animals, abolishes ecosystems, destroys infrastructure and taints water [45-47]. The effects of

AMD on human health, plant and aquatic life are discussed below.

2.2.1 Effects of low pH

Low pH of AMD affects all forms of life including plants, animals and humans. Soil pH

influences organism communities, which are important in the uptake of nutrients that are

essential to plant health [48]. A low pH, in soil causes an increase in soil toxicity due to

increased levels of leached aluminium, iron and manganese from soil particles and/or a

deficiency in nutrients such as phosphorous, nitrogen and potassium [48].

The pH of water is important to aquatic plants and animals as it affects their growth,

development and reproduction, as well as regulating other physiological functions like

respiration and osmoregulation in some species [49]. A low pH has a devastating impact on

crustaceans, as their exoskeletons are calcium carbonate based and will deteriorate in acidic

waters [50]. Change in pH not only affects the aquatic life but it also alters the food supply for

14

upper fauna in the food chain [51]. Most of the water bodies (freshwater lakes, ponds and

streams) have a natural pH in the range of 6-8 [49]. If the pH of water increases or decreases

from a normal pH range, it can result in numerous sub-lethal effects (like retarded growth) and

even mortality [51]. The main effects of pH on aquatic life are shown in Table 1 [49, 52, 53].

Table 1:Effects of pH on aquatic life [49, 52, 53]

pH Effect

3.0-4.0

• Toxic and lethal to most fish

• Some plants and invertebrates can survive such as the water bug, water

boatmen and white mosses

4.0-5.0 • Harmful to salmonoids eggs, goldfish, roach, fry and common carp

• All fish stock disappears because embryos fail to mature at this level

5.0-6.0 • Restricted fish population, molluscs rare, growth rate of carp reduced

• Bacterial species decreased, yeasts and algae will grow

6.5-9.0 • Harmless to most aquatic species

9.0-10.0 • Harmful to salmonoids

• Some typical stoneflies and mayflies survive with reduce emergence

10.0-11.0 • Lethal to all fish including salmonoids, carp, trench, goldfish and pike

2.2.2 Effects of heavy metals

Heavy metals with atomic density greater than 6g/cm3 are one of the most persistent pollutants

in ecosystems [54, 55]. Unlike other pollutants, heavy metals are difficult to degrade and have

the potential to persist in natural ecosystems for longer periods. Secondly, they can accumulate

throughout the food chain producing potential health risks and ecological disturbances to

numerous species [56]. In humans, heavy metal poisoning results either from their

accumulation in vital organs and glands (heart, brain, kidney, liver and bones) or by inhibiting

the adsorption of vital nutrients in various biological functions [55].

15

The introduction of heavy metals to human is typically due to their presence in food sources,

such as plants, animals and aquatic species [55]. The effects of heavy metals on plants, animals

and aquatic species have been described in Table 2 [55-62]. In summary, high concentrations

of heavy metals in plants cause cellular level damage that disrupt their physiology and

morphology [63], while in aquatic species, heavy metals can kill aquatic organisms or cause

severe oxidative stress [64] and other non-lethal effects such as stunted growth, reduced

reproduction and deformities [65]. The impact of heavy metals on animals ranges from nausea,

dizziness, mental retardation, organ damage (liver, kidney, lungs and gastrointestinal damage)

to retarded growth [66].

Table 2: Effects of heavy metals on plants, animals and aquatic animals [55-62]

METALS EFFECTS ON

PLANTS

EFFECTS ON

ANIMALS

EFFECTS ON

AQUATIC

ANIMALS

Cadmium

(Cd)

decreases seed

germination, lipid

content, and plant

growth; induces

phytochelatins

production

kidney disease, lung

damage,

fragile bones.

Skeletal

deformities,

reduced kidney

functioning,

reduced growth

Chromium

(Cr)

decreases enzyme

activity and plant growth;

produces membrane

damage, chlorosis and

root damage

damage to liver, kidney

circulatory and nerve

tissues, as well as skin

irritation, skin ulcers,

nose ulcers

Anaemia,

lymphocytosis,

bronchial and

renal lesions

Copper (Cu) inhibits photosynthesis,

plant growth and

reproductive process;

decreases thylakoid

surface area

nausea, dizziness,

diarrhoea, intentionally

high intakes of copper

can cause liver and

kidney damage and even

death

Infertility, frayed

gills,

hyperactivity,

reduced olfaction

(sense of smell),

Nickel (Ni) reduces seed

germination, dry mass

accumulation, protein

production, chlorophylls

and enzymes; increases

free amino acids

lung cancer, larynx

cancer and prostate

cancer respiratory

failure, birth defects,

asthma, heart disorders

Retarded growth,

convulsions, loss

of equilibrium,

hypoxia,

16

Zinc (Zn) reduces seed

germination; increases

plant growth and

ATP/chlorophyll ratio

Damage to nervous

membrane nausea,

diarrhoea, fatigue

Disruption of gill

tissue, hypoxia,

structural

damage, retarded

growth,

Mercury (Hg) decreases photosynthetic

activity, water uptake

and antioxidant enzymes

damage to nervous

system, spontaneous

abortion, gingivitis,

acrodynia, protoplasm

poisoning

Reduced growth,

decreased

hatching rate,

Lead (Pb) reduces chlorophyll

production and plant

growth

mental retardation,

developmental delay,

liver, kidney and

gastrointestinal damage

Impaired larval

development,

spinal curvatures,

poor immunity,

2.3 Control and treatment of AMD

2.3.1 Source control

Due to its low pH and high levels of sulphates and metals (Cr, Fe, Zn, Cu, Ni. As, Cd), AMD

can cause serious water and soil pollution [67-69]. Various methods used to prevent and control

AMD at the source of formation are known as source control measures. AMD can be regulated

by reducing oxygen supply and excluding the water needed for oxidation and transport of

pollutants [4, 12, 70]. Various source control techniques used to prevent or minimise the

generation of AMD are discussed below [71]:

1. AMD production can be prevented by flooding/sealing abandoned underground mines.

However, this is only effective where the location of all shafts and adits are known and

where influx of oxygen-containing water does not occur.

2. Underwater storage has been used for disposing acid producing tailing waste and for

this shallow water covers can be used [72]. The effectiveness of this method can be

improved by covering the tailings with a layer of sediment or organic material to

prevent the contact between the minerals and dissolved oxygen [72].

17

3. Pyrite oxidation can be temporarily subdued by blending acid generating and acid

consuming material such as solid-phase phosphates and carbonates [73].

4. Anionic surfactants such as sodium dodecyl sulphate have been used as biocides to

inhibit the activity of sulphur-oxidising bacteria. However, this technique requires

continuous application of the chemical [19, 74].

5. Dry covers, grouts, plastic liners or seals can be used to slow down the movement of

water and oxygen into areas containing sulphide bearing waste [75]. But this technique

is ineffectual in acute wet and dry seasons because of cracking of the cover [72].

2.3.2 Migration control

Due to the difficulties associated with “source control” techniques, “migration control”

measures are the primary options used to minimise the impact of AMD water on the

environment (in particular, neighbouring streams and rivers). Migration control measures can

be classified as active or passive systems based on their requirements for chemical additions,

infrastructure, and maintenance and monitoring. Remediation of AMD is primarily categorised

based on the biological activity (abiotic or biological) of the technology and then further

classified into active and passive systems (Figure 1) [13]. Abiotic passive and biological

remediation is beyond the scope of this study and thus has not been discussed.

18

Figure 1: Biological and abiotic strategies for remediating AMD (adapted from [13])

2.3.2.1 Abiotic remediation

2.3.2.1.1 Active treatment of AMD

In an abiotic active system, AMD is treated by the addition of alkaline materials such as lime,

limestone, soda ash, magnesia or sodium hydroxide. Addition of alkaline material increases

the pH of water and encourages the precipitation of iron oxyhydroxides and metal

oxyhydroxides that can be removed by solid-liquid separation techniques [16, 17]. Heavy

metals can also be removed through coprecipitation [16, 17]. Generally, active AMD treatment

systems consist of an inflow pipe, a chemical storage tank, a means of controlling its

application rate, a settling pond to capture precipitated metal oxyhydroxides and a discharge

point [70, 76] . Pre-treatment of AMD through sedimentation techniques may be required

when high concentrations of total dissolved solids are present, as they can cause clogging of

piping and damage to pumps [76].

19

In active treatment systems, selection of alkali to treat AMD depends on economic and

technical factors. The economic factors related to the selection of alkali are costs of reagent,

labour, machinery, collection and disposal of sludge, while technical factors essential to be

measured are such as pH, total suspended solids, flow rate of AMD, availability of electric

power, distance from chemical pond, and the Fe and Mn concentrations [76].

Trumm [76], developed a flow chart that consider these factors when designing an active

treatment system. After the addition of alkali to AMD water, the rate and degree of metal

precipitation depends upon their concentration, identity and complex interaction between

dissolved species in water [77]. For example, in an oxygen rich environment the recommended

pH for iron (Fe3+) precipitation is 3.5 to 4.0, while in oxygen poor environments a pH of 8.5 is

required [78]. In the case of divalent metal ions such as Ni2+ and Zn2+ precipitation at pH

values ranging from 8 to 9 occurs [79].

Manganese is one of the most challenging species to treat in AMD waters. Precipitation of Mn

is dependent upon the oxidation state present, but will generally precipitate at pH 9, a value

which is typically higher than recommended water discharge limits [78]. If the iron

concentration in water is significantly higher than manganese, it may be removed with iron at

pH 8 due to co-precipitation [78]. Aluminium (Al) generally precipitates at pH ≥ 5 however,

enters back into solution at pH 9.0 as soluble aluminate ions (Al(OH)4-) [80]. Therefore,

increasing the pH to 9 to remove Mn can result in the dissolution of aluminium precipitates.

2.3.2.1.2 Chemicals used for treating AMD

Pros and cons of various chemicals used for AMD treatment are discussed below:

20

Calcium carbonate (limestone): is the cheapest, safest and easiest chemicals to handle in the

treatment of AMD [5]. The sludge produced after treatment is also dense and easy to handle

and collect [81]. Calcium carbonate can be used to treat AMD in anaerobic (anoxic limestone

drain) [82] and aerobic (open limestone channel) environments [83]. Unfortunately, its

application is limited due to its low solubility and tendency to develop an external coating of

ferric hydroxide it is generally only used when Fe concentrations are below 5 mg/L and total

acidity is less than 50 mg/L [84, 85]. Typically limestone is used in the form of channels or

drains that AMD runoff flows through prior to entering waterways [81]. Zurburch et al. [86]

reported that in western Virginia (USA) the introduction of limestone into the stream by water

powered rotating drums improved 22 km of drainage and kept the pH above 6.0. Another study

has reported that instream limestone neutralisation has effectively restored waterways polluted

by AMD [87]. Limestone (CaCO3) was employed to treat AMD in laboratory experiments and

reported to remove 90 % of heavy metals such as Cd, Pb, Zn, Ni, Cu and Cr(III) from solutions

at pH 8.5 [88].

Calcium hydroxide (hydrated lime): is the most commonly used alkali due to its low

chemical cost and ability to be used for treating large flow and high acidity AMD waters [37].

Approximately 0.76 kg of hydrated lime is required for neutralising 1 kg of sulphuric acid [89].

However, due to its hydrophobic nature, extensive mixing is required to make it soluble in

water, which leads to increased capital costs for a system [84, 90]. Depending on the amount

of flow that needs to be treated, the installation cost can be approximately $58,000-$200,000

[37]. Another key drawback when using hydrated lime is the large amounts of sludge that are

produced when metal oxyhydroxides precipitate. Therefore, a large area is required at the

treatment site, to build a large retention pond to capture and retain the metal oxyhydroxides

precipitates before the effluent discharges into a water body.

21

Khorasanipour et al. [91] has conducted laboratory and field tests to assess the effectiveness of

lime to treat AMD at the Sarcheshmeh Porphyry Copper Mine, Iran. It was reported that field

treatment tests supported lab tests and the mean treatment efficiency is ≤ 99.5 % for Co, Cu,

Ni and Zn, 99.4 % for Al and Cd and 98.5 % for Mn at pH 10 [91]. Another study conducted

in lab reported decrease in arsenic (As), cadmium (Cd) and chromium (Cr) concentration after

30 minutes using 2 g of hydrated lime for 1 L of AMD water collected from tin tailings located

in Pengkalon Hulu [92].

Lime neutralisation is currently being used at Mount Morgan to control the volume of AMD in

the open pit to avoid overflow events [93]. This process involves the neutralisation of AMD

water using slaked lime to increase the pH to between 6.5 and 8.5 to facilitate the precipitation

of metals as metal hydroxides. After approximately 2 hrs of residence time in the neutralisation

tanks, the slurry is dosed with a flocculating agent prior to clarification and disposal [93]. The

treated water is ultimately discharged into the adjoining Dee River if water quality

requirements are met [93].

Calcium oxide (quick lime): is the second least expensive chemical used, however to avoid

hydration and formation of hydrated lime it must be stored in water tight containers. It is

typically used for periodic flows of high acidity [76]. Quicklime is sold in a dehydrated form,

which makes it very reactive and decreases the amount of the reagent to be used. It has been

reported that 1 kg of sulfuric acid requires 0.57 kg of quick lime for neutralisation [89]. Under

high sulphate concentrations quick lime can react with sulphate to form anhydrite or insoluble

gypsum, which may clog/block the pipes used to transfer the treated water to receiving stream

[89]. Initially quicklime is used in combination with aqua fix water system for periodic flows

22

of acidity, however it has been reported that in case of high flow of acidity, waterwheels have

been used in conjunction with large bins containing quick lime [94]. Skousen et al. [85] has

reported approximately 75 and 40 % cost savings over systems using sodium hydroxide and

ammonia, respectively. However, special attention is needed while using calcium oxide to treat

AMD as it is tremendously caustic and can cause severe damage to the skin, eyes, or respiratory

tract [84].

Sodium hydroxide (caustic soda): is the most expensive and hazardous chemical used in

AMD treatment, however it has a low capital cost [76]. It is typically used in remote locations

(where electricity is not possible) and in low flow high acidity sites [76]. It is highly soluble in

water and can raise the pH of water quickly [37]. The caustic is added to the water by gravity

using a dripping system from a tank and the release of caustic applied is regulated by a gate

valve at the bottom of tank. It has been reported that 20 % caustic solution is prepared by

dissolving 1.8 lb of solid caustic in one gallon of water [37]. Preparing a 20 % caustic solution

from solid caustic is not cost-effective when liquid caustic is available, but the use of solid

caustic for treating AMD is cost-effective compared to soda ash briquettes [37].

The caustic soda is liquid and susceptible to freezing during the winter. Therefore, in colder

regions the tank containing caustic must have a heater installed or have a freeze prevention

solution such as potassium hydroxide (KOH), or use a more diluted solution of caustic from

50 % to 20 % (lowers the freezing point from 0 °C to about-37 °C) [37]. Some of the drawbacks

of caustic soda application for AMD treatment include: difficulty of handling due to toxicity,

lack of sludge stability, and its high costs [76].

23

Sodium carbonate (soda ash): It is typically used for the treatment of AMD water with

periodic flow and low amounts of acidity and low concentrations of metals (particularly iron)

[84]. The soda ash comes in solid briquettes and 1 kg of sulphuric acid needs 0.82 kg or 1.51

L of 20 % solution of sodium carbonate for neutralisation [89]. It can be added as slurry to

AMD water, but is generally added by dissolving soda ash briquettes. The quantity of briquettes

required is subjective to the flow rate and quality of the AMD water. A huge disadvantage of

using soda ash is the high reagent cost (as compared to limestone) and the poor settling

properties of the sludge. The sludge is very unstable because it does not form a dense enough

precipitate to allow for easy removal. For this reason, the metal content of the water must be

low so that high volumes of hard-to-remove sludge do not accumulate in the stream. Moreover,

briquettes must be added at an adequate rate to maintain an alkaline pH to prevent metals within

the sludge from re-entering into the water if it becomes acidic.

Ammonia: is the second most expensive chemical used in the treatment of AMD with high

ferrous and/or manganese concentrations as it acts as a strong base and can quickly raise the

pH of water to 9.2. The pH can be raised by injecting ammonia into AMD at the entrance of

pond to ensure good mixing because it is lighter than water [95]. Skousen et al. [37, 96] has

reported that ammonia is very hazardous to handle and thus operators have to monitor the

amount of ammonia added to treat AMD as excess of it in water has off-site impacts such as

toxicity to fish and other aquatic life, eutrophication, and nitrification. The various factors such

as pH, temperature, and dissolved oxygen can affect the ammonia related toxicity [97].

Therefore, it is mandatory to monitor the downstream effluent for metals, pH and nitrate

content to ensure a productive stream for aquatic species [98]. Skousen et al. [84] has reported

a reduction of 50-70 % in cost by changing NaOH to 20 % by ammonia.

24

2.4 Bauxite refinery residues

2.4.1 Bauxite

Bauxite is formed as the result of extreme chemical weathering of alumina rich rocks in tropical

climates [99]. The four major mineral components of bauxite are Al2O3, Fe2O3, SiO2 and

TiO2 [27]. Alumina is present in varying concentrations from 20-70 %; gibbsite (γ-Al(OH)3),

boehmite (γ-Al(O)OH) and diaspore (α-Al(O)OH) are the primary aluminium minerals found in

bauxite ore. Bauxite contains 10-25 % iron (Fe2O3), as geothite (α-FeO(OH)) and hematite (α-

Fe2O3) [27]. Silica in bauxites can be described as reactive when contained in clay minerals, while

it is considered as non-reactive when present as quartz. Titanium is the least soluble component of

bauxite and is mainly present as anatase (TiO2) [100]. Bauxite ores also contain organic carbon

which is derived mainly from decomposed vegetation and roots [101].

2.4.2 Bauxite refining - extraction of alumina from bauxite ore

The Bayer process is used for refining bauxite into smelting grade alumina (Al2O3); the

precursor of aluminium. The process was developed and patented by Karl Josef Bayer 110

years ago and has become the cornerstone of aluminium production [24]. Bauxite used in the

Bayer process is generally composed of 50 % alumina (Al2O3), 25 % water and 25 % of other

constituents (mineral oxides and organic matter). The Bayer process can be summarised in

three main steps:

1. Extraction or digestion:

Crushed bauxite is digested in concentrated caustic (NaOH) as shown in Equation 5 and

Equation 6 at elevated temperatures and pressures. The quantity of gibbsite (γ-Al(OH)3),

boehmite (γ-Al(O)OH), and diaspore (α-Al(O)OH) present in the bauxite ore decides the

25

digestion temperature used. Bauxites containing predominantly gibbsite require lower

digestion temperatures (145–175 °C), while those with high boehmite and diaspore require

stronger caustic concentrations and temperatures (245–275 °C) [102]. The process results in

the dissolution of gibbsite and boehmite as sodium aluminate (NaAl(OH)4), while the

remaining insoluble residue (45 % liquor and 55 % solid mud), known widely as red mud, is

removed by means of flocculation and decantation [103].

For gibbsite:

Equation 5: 𝐴𝑙(𝑂𝐻)3 (𝑠) + 𝑁𝑎𝑂𝐻 (𝑎𝑞) → 𝑁𝑎+𝐴𝑙(𝑂𝐻)4− (𝑎𝑞)

For boehmite and diaspore:

Equation 6: 𝐴𝑙𝑂(𝑂𝐻)(𝑠) + 𝑁𝑎𝑂𝐻 (𝑎𝑞) + 𝐻2𝑂 → 𝑁𝑎+𝐴𝑙(𝑂𝐻)4− (𝑎𝑞)

2. Precipitation:

The precipitation of aluminium hydroxide (Al(OH)3) is basically the reverse of the extraction

process, except that size of the hydrate formed is carefully controlled by the temperature,

cooling rate, and seeding [24]. During precipitation, supersaturated liquor is cooled to 50-60

°C and aluminium hydroxide precipitates as a white, fluffy solid [104].

Equation 7: 𝑁𝑎+𝐴𝑙(𝑂𝐻)4− (𝑎𝑞) → 𝐴𝑙(𝑂𝐻)3(𝑠) + 𝑁𝑎𝑂𝐻 (𝑎𝑞)

3. Calcination:

26

In the final stage, hydrate is calcined at around 1050 °C in rotary kilns to form alumina for the

aluminium smelting process [45].

Equation 8: 2 𝐴𝑙(𝑂𝐻)3(𝑠) → 𝐴𝑙2𝑂3 + 3 𝐻2𝑂 (𝑔)

2.4.3 Red mud and Bayer liquor

As mentioned earlier, red mud is derived from the extraction step of the Bayer process and is

essentially all the metal oxides/hydroxides not digested in bauxite ore. Annually millions of

tonnes of red mud is produced [102], with 1.0–1.5 tonnes of red mud residue produced for

every tonne of alumina produced [105]. The exact composition of the fine textured residue

depends on the initial type of bauxite ore and the digestion conditions used at the refinery [106].

The liquor is strongly alkaline (pH ranging from 10 to 13) [29, 101, 107] and requires

neutralisation to a pH below 9, with an optimum pH value of 8.5–8.9 [27], before becoming

environmentally benign. The liquor contains relatively high concentrations of aluminium

carbonate and caustic, while the solid contains, iron (hematite (Fe2O3), and goethite (FeOOH),

boehmite (AlO(OH)), other aluminium hydroxides, calcium and titanium oxides, and

aluminosilicate minerals (such as sodalite and cancrinite) [27, 103, 108, 109]. Charged lime

species may also be present in the form of calcium carbonate (CaCO3), as well as various forms

of calcium phosphate (carbonate or hydroxyapatite) and titanium oxides (perovskite (CaTiO3)

and/or kassite (CaTi2O4(OH)2)). These minerals are the chemically stable end products of

bauxite formation and refining, and are the components responsible for the high surface

reactivity of red muds [27, 102, 103, 108].

2.4.4 Disposal of bauxite residue

27

Before 1970, most alumina refineries utilised two key wet disposal techniques namely marine

discharge and lagooning [110]. For many years, the marine disposal method was used and

involved the direct release of bauxite residue into the deep sea [111]. However, refineries

established after 1970 don’t employ the marine disposal method and additional methods have

been implemented [23, 112]; lagooning is normally used for dumping bauxite residue [111].

Lagooning involves the dumping of refinery resiudes into land based impoundments known as

red mud ponds [23]. To minimise the hazard of leakage of caustic or alkaline water into the

soil and water, this practice involves increased engineering input and, in some cases, additional

neutralisation methods. Since 1960, around 103 major tailings dam failures have been recorded

globally, leading to at least 1838 human deaths and untold environmental degradation [113].

Dam failures and limited land areas have led to an increase in dry stacking and dry cake

disposal methods [114], however runoff from these disposal sites still possess environmental

risks [115].

2.4.5 Neutralisation of bauxite refinery wastes

Several methods of red mud neutralisation have been reported including: infiltration of

rainwater and atmospheric CO2, treatment with strong acids, gypsum addition, and seawater

neutralisation [27, 116, 117]. Many alumina refineries located in coastal areas have

implemented the neutralisation of bauxite refinery residue with seawater, which provides a

reduction in both pH and dissolved metal concentrations. Implementation of the seawater

neutralisation process at the Queensland Alumina Ltd. (QAL) refinery at Gladstone, QLD,

began as a fresh water conservation measure but led to many benefits [27]:

1. reduced alkalinity and sodicity in the solid wastes and entrained liquor

2. increased acid neutralisation capacity

28

3. improved soil properties after rehabilitation

Seawater neutralisation results in the neutralisation of alkalinity through the precipitation of

Mg, Ca, and Al hydroxides and carbonate minerals [33]. Electrostatic bridges are formed by

multivalent exchange cations, like Ca and Mg, which then act as nucleation sites for the

precipitation of magnesium and calcium hydroxides [118]. Formation of these hydroxides

reduces the concentration of hydroxide ions in solution, therefore reducing the pH of the

solution [119]. As the electrostatic conditions of the surface changes and pH decreases, the

elements that exhibited colloidal behaviour initially at high pH lose stability [118]. Further

decreases in pH cause the precipitation of hydroxycarbonates of aluminium, calcium, and

magnesium, resulting in the precipitation of hydrotalcite-like (M1−x2+ Mx

3+(OH)2x+Ax/m

m-.nH2O)

compounds [27]. Seawater neutralised red mud would consist of a range of hydrotalcite

structures, due to the large pH range that they form in and the variety of divalent/trivalent

cations [33]. However, the predominate reaction involved in the seawater neutralisation process

is as follows:

Equation 9:

6𝑀𝑔𝐶𝑙2(𝑎𝑞) + 2𝑁𝑎𝐴𝑙(𝑂𝐻)4(𝑎𝑞) + 8𝑁𝑎𝑂𝐻(𝑎𝑞) + 𝑁𝑎2𝐶𝑂3(𝑎𝑞)

→ 𝑀𝑔6𝐴𝑙2(𝑂𝐻)16(𝐶𝑂3) · 𝑥𝐻2𝑂 + 12𝑁𝑎𝐶𝑙(𝑠)

As discussed in section 2.4.3, bauxite refinery wastes are composed of 40-45 % liquor known

as Bayer liquor. Palmer et al. [120] has reported that seawater neutralisation of Bayer liquor

causes a decrease in pH (initial pH 13 to pH 9.25) and dissolved metal concentration which

results in the precipitation of stable alkaline products known as Bayer precipitates. It has also

been reported that Bayer precipitates obtained at pH 9.5 are composed of three main

29

mineralogical phases known as (1) hydrotalcite, (2) calcite, and (3) aragonite and that the

composition of hydrotalcite found in Bayer precipitates is dependent on both the pH and initial

Bayer liquor composition [120]. Johnstone et al. [121] has studied the neutralisation of Bayer

liquor with alternative seawater sources such as nanofiltered seawater and reverse osmosis

brine and suggested that the neutralisation mechanism is very much like seawater

neutralisation. It has been reported that Bayer precipitates formed at high pH may exhibit

additional phases such as Mg2Al(OH)7 or brucite (Mg(OH)2) [122].

2.4.6 Bauxite residue utilisation options

The high production rate and the alkaline nature of bauxite residues means significant land area is

required for impoundments, while the chemical composition of the residue poses a significant

environmental risk. Therefore, many technologies have been developed to reuse bauxite residues

in the hopes of reducing the amount of land required for storage.

Environmental treatment

There have been numerous investigations on the use of bauxite residues in the removal of heavy

metals and metalloids, inorganic anions such as nitrate, fluoride and phosphate, organics including

dyes, phenolic compounds and bacteria from water due to its chemical composition, surface

properties and oxidising potential [123-128]. Red mud and its derivatives (obtained by seawater

neutralisation or thermal activation) have been used as adsorbents for metals such as arsenic (As)

[129-131], cadmium (Cd) [132-134], boron [135], copper (Cu) [123, 132, 134, 136], lead (Pb)

[132], zinc (Zn) [123, 134, 136], and nickel (Ni) [123, 136]. It has been found that the treatment of

30

seawater neutralised red mud (Bauxol) with acid (20 % HCl) resulted in an increase in arsenic (As)

sorption from 89 to 95 %, while treatment of Bauxol with combined acid and heat treatment

increased the arsenic (As) sorption to approximately 100 %, compared to 89 % for Bauxol [137].

Apak et al. [132] reported that the high metal uptake capacity for metals by red mud is essentially

irreversible as KDdes. value were 3-4 orders of magnitude higher than KD

ads.; the study found that

minimal leaching occurred when exposed to carbonic acid and bicarbonate buffered solutions.

Thermally activated red mud (500 °C for 3h) used for removal of Cd (1.78 x 10-5 M to 1.78 x 10-

3 M) and Zn (3.06 x 10-5 M to 3.06 x 10-3 M) from aqueous solution has shown complete removal

of both metals at low concentrations, while 60-65 % removal was achieved for higher

concentrations [138].

Metal removal by red mud is dependent on pH; at low pH, red mud retains a positive charge that

facilitates the sorption of oxyanions, while at high pH red mud holds a net negative charge that

facilitates cation sorption [139]. Palmer et al. [140] have studied the removal of arsenate, vanadate

and molybdate from solution using thermally activated seawater neutralised red mud. It has been

reported that due to the formation of Bayer hydrotalcite during neutralisation process, thermally

activated seawater neutralised red mud removes at least twice the concentration of anionic species

(such as arsenate, vanadate and molybdate from solution) compared to thermally treated

red mud [140].

Red mud has also been used to remove phosphate from wastewater and the result showed that up

to 70 % of phosphate was removed in the pH range 6.5 to 7.5 [141], while the use of acid (0.25 M

HCl) and heat treated red mud (700 °C for 2 hrs) has shown 99 % phosphate removal [142].

Cengeloghu et al. [143] conducted an experiment to compare the nitrate adsorption between red

31

mud and acid treated red mud (20 % HCl) and reported that adsorption capacity of acid treated

material was five times higher than untreated red mud.

Research has also been done for the treatment of AMD using raw bauxite residue and amended

bauxite residues; hydroxides (OH-), carbonates (CO32-) and aluminate (Al(OH)4

-) were found

to neutralise acid, while heavy metals were removed via precipitation or adsorption [99, 144,

145]. AMD with an initial pH 3 was treated by Doye and Duchesne [144] using 10 and 50 wt.

% of bauxite residue to raise the pH to 6 and 9, respectively. The study reported a significant

reduction in concentration of toxic metals like Al, Cu, Fe and Zn (below the instrument

detection limit) [144]. The comparison of life cycle assessment of seawater neutralised bauxite

residues (SNRBs) against CaCO3 (processed to form quicklime) to neutralise AMD at a site in

Queensland (Mount Morgan mine) has shown that SNRBs emit only 20 % of the CO2 and

consume only 44 % of electricity as compared to CaCO3 [32]. Douglas et al. [146-148] studied

the use of sodium aluminate (sourced from Bayer liquor) in combination with NaOH or

Ca(OH)2, to neutralise acidity and remove trace metals from acid water at Ranger mine by in

situ formation of hydrotalcite. The addition of sodium aluminate to Mg-rich mine water raised

the pH to form hydrotalcite, with carbonates as interlayer anions [147]. Hydrotalcite formed

from the mine water also had the potential to remove cations such as Mn and Fe [147]. Douglas

[146], also reports that calcined magnesia can be used at mine sites with excess aluminium (Al)

as a source of alkalinity to form hydrotalcite to neutralise acidic mine water.

It has been reported that damages caused by AMD in Australia costs $60 million per year, and

since 1997 more than $900 million has been spent in Australia for AMD remediation [149].

Certainly, there is substantial market for AMD neutralisation technology using some alternate

cheap materials.

32

Improving soil quality

Bauxite residues have been used as effective soil amendments in treating acidic sulphate soils by

neutralising the pH and removal of heavy metals [150]. A study conducted by Lombi et al. [151,

152] showed a remarkable decrease in the concentration of heavy metals (Cd, Cu, Ni, Pb and Zn)

in the soil pore water, reduced metal uptake into oilseed, pea, wheat and lettuce and significant

increase in the soil microbial biomass after treatment with 2 % red mud. Similar results have been

reported by another study which showed a reduction in the uptake of heavy metals in lettuce (such

as Cd, Pb and Zn has been reduced to 86, 58 and 73 % respectively) after the application of bauxite

residue to contaminated soil [153]. Friesl et al. [154] reported that compared to untreated soil, red

mud applied at a rate of 10 % (w/w) considerably reduced Cd, Zn, and Ni uptake in fescue (grass)

and Amaranthus (perennial plants) by up to 87, 81, and 87 %, respectively. Pot trials were

conducted using different neutralising agents such as hydrated lime, red mud, biosolids, fertilizers

and zeolite to investigate the effects of various soil treatments on the growth of vetiver grass and

the results reported that treatment of soil (pH 2.76) with red mud and hydrated lime had positive

effect on the growth of grass, whereas the application of other used alkali were negative on the

grass growth [155]. It has also been reported that acidic sandy soils with low nutrient content and

poor water holding capacity can be treated with the slow addition of bauxite residue [156]. Zobel

et al. [157] claimed that the application of bauxite residues treated with sodium silicate and

Ca(OH)2 increased the water retention capacity of sandy soils from 21.7 to 38.9 %.

The application of bauxite residue in agricultural land with sandy soils characterised by low

phosphate and nutrient holding capacities, has also been found to improve the phosphorous content

by reducing the phosphate leaching into ground and surface water and thus creating a phosphate

pool available to plants [107]. In past, Western Australian Department of Agriculture has

33

conducted a series of research studies and reported that addition of bauxite residues is useful for

improving phosphate content in soil, which further increased the grass growth by 25 % and in well

controlled area grass growth was increased up to 200 % [156, 158-160].

Despite large number of laboratory studies showing the benefits of using bauxite residues to

improve soil quality, further research is required to investigate the speciation of heavy metals and

naturally occurring radioactive materials (NORMs) in bauxite residue, to understand their potential

leaching and uptake by plants under different conditions for various agronomic applications.

Chemical applications

Recent studies have used bauxite residue as a catalyst in hydrogenation of coal, biomass, oil shale

and petroleum products [161-163], hydro-dechlorination [164, 165], exhaust gas clean-up [166,

167] and in other areas (such as degradation of poly (vinyl chloride) containing polymer mixture

into fuel oil [168], conversion of waste oil and waste plastic to fuel [169], heavy crude oil

hydrotreating [170], hydrodemetallization of hydrocarbons containing metallic compounds as

impurities and hydro-treating such hydrocarbons [171], ammonia decomposition in presence of

sulphur compounds [172] and nitrile synthesis from aldehydes and hydroxyamine[173] due to the

presence of Fe2O3, TiO2 and its high surface area [24, 104]. Sushil et al. [172] have concluded that

the performance of untreated bauxite residue was poor compared to conventionally used catalysts.

Therefore, for most applications some prior treatment (such as heat treatment at 400 ˚C, size

reduction, sulphidization and acid addition) of the residue is required to enhance its catalytic

efficiency [167, 172, 174-176]. It is unlikely that red mud as a catalyst will consume sufficient

amounts of residue to have a measurable impact on reducing storage facilities. It should be noted

34

however, that the spent catalytic material will most likely be more toxic than red mud and will

require disposal.

In the ceramic industry, bauxite residue can be used as a barrier for radiation shielding and performs

better compared to normal Portland cement-based shielding materials [177]. However, like

catalysts, this application consumes low volumes of bauxite residue and thus cannot play a

significant role in reducing the amount of residue stored globally.

Construction material

Early research conducted in 1936 reported that due to its iron and aluminium content, bauxite

residue could be used as an additive in Portland cement [178]. It has also been reported that the

replacement of soda with calcia enhanced the performance of residue as an additive [178]. Special

cements (namely aluminoferrite (C4AF)-belite (β-C2S), aluminoferrite-ferrite (C2F)-aluminates

(C3A and C12A7) and sulfoaluminate (C4A3S̄)-aluminoferrite-ferrite) have been prepared by using

a mixture of gypsum, lime, fly ash and bauxite residues [100, 179]. Singh et al. [100, 179] reported

that due to the low silica content and high aluminium and iron content, bauxite residues perform

better than fly ash, while titanium in the residue was found to increase the concrete’s strength. Back

in 1999, it was reported that the global usage of bauxite residue in construction materials was

approximately 1.0 to 1.2Mtpa, which equates to 1 % of the total bauxite residue produced in 2001

[180].

Bauxite refinery residue has also been used for manufacturing fire bricks [181, 182]. However, it

has been reported that radon levels were higher in bricks formed from bauxite residue as compared

to conventional concrete [183, 184]. Radon has adverse implications on human health due to its

35

carcinogenic nature and thus the possible use of bauxite residue as brick materials become

limited [184].

2.5 Layered double hydroxides (LDHs)

The formation of LDH in the seawater neutralisation of bauxite residues is the primary means

of reducing the alkalinity of Bayer liquors [120]. The formation of hydrotalcite is believed to

be the main precipitate formed during this process, however the anion chemistry,

stoichiometric ratio and even other LDHs are possible [120]. In the past decade, hydrotalcite

has been broadly investigated due to its many potential applications in water treatment [185-

190]. It is for this reason; Bayer precipitates are being considered as a potential neutralisation

agent for AMD water treatment.

2.5.1 Structure

The chemical composition of LDHs is represented by the general formula

[M1−x2+ Mx

3+(OH)2]x+Ax/m

m-.nH2O, where M2+ is a divalent cation, M3+ is a trivalent cation and

A an interlamellar anion with charge m- and value of x is equal to molar ratio of M2+/ (M2++

M3+) and generally lies in the range of 0.20-0.33. Values outside the specified x range will

form: (i) boehmite (AlOOH) for x > 0.337, (ii) hydromagnesite (4MgCO3·Mg(OH)2·4H2O) for

0.105 < x < 0.201, and (iii) a mixture of hydromagnesite and Mg(OH)2 for x < 0.105 [191-194].

Hydrotalcite is produced when M2+=Mg2+ and M3+=Al3+ giving the general formula

Mg6Al2(OH)16CO3·4H2O [195].

LDHs have a brucite like structures consisting of layers of metal cations (M2+ and M3+) of

similar radii, which are randomly distributed in the octahedral positions (Figure 2). The

36

enthalpy of bond formation within the layers is responsible for the thermodynamic stability of

these layered materials [196].

Figure 2: Schematic representation of the 3:1 hydroxide layers of hydrotalcite

(adapted from [33])

The positively charged brucite-type layers are formed by the substitution of divalent cations

with trivalent cations, are stacked on top of each other and are held together by weak

interactions through the hydrogen atoms [33, 194]. The degree to which the framework is

positively charged is determined by the ratio of M2+ to M3+ cations, where a low M2+:M3+ ratio

(2:1) will result in highly positively charged layers. To maintain electroneutrality, the interlayer

region must be occupied by an adequate number of anions (Figure 3) [120, 191, 197]. Layered

double hydroxides are quite stable as charge neutrality is not confined to the interlayer region,

but also to the external surfaces of the LDH structure [197].

Figure 3: Schematic representation of the hydrotalcite structure (modified from [198])

37

Various studies have reported that different types of layered double hydroxides are formed by

different combinations of divalent and monovalent cations and different interlayer

anions [193, 199, 200]. Formation of LDHs with a divalent anion (CO32− > SO4

2−) is more

favourable over one containing monovalent anions (OH− >F− >Cl− >Br− >NO3− > I−), because

an increase in anionic charge increases the electrostatic interactions between the positively

charged hydroxide layer and the anion. [199, 201].

As discussed earlier, seawater neutralisation of Bayer liquor does not remove hydroxide from

solution but convert it into weakly alkaline solids in the form of hydrotalcite [120].

Smith et al. reported that during the seawater neutralisation of Bayer liquor, hydrotalcite

formed at high pH had Mg:Al (2:1) and microcrystalline carbonate hydrotalcite (Mg4Al2

(CO3)(OH)12.xH2O) is formed due to the formation of saturated carbonate solution by

adsorption of atmospheric CO2, whereas at low pH (<9.5) amorphous hydrotalcite (Mg8Al2

Cl(CO3)0.5(OH)20.xH2O) is formed due to a decrease in carbonate in solution [202].

2.5.2 Preparation

LDHs are an unusual class of layered materials with positively charged layers that are

neutralised by weakly bound, often exchangeable, charge-balancing anions located in the

interlayer region [203]. This is unusual in solid state chemistry because most materials have

negatively charged layers with cations in the interlayer spaces. LDHs are sometimes referred

to as anionic or hydrotalcite like clays, and are based on the brucite structure, Mg(OH)2. These

layered structures are prepared by a variety of methods like co-precipitation, urea reduction,

salt oxide method, hydrothermal, electrochemical and sol-gel [33]. The seawater neutralisation

of bauxite residues resembles the coprecipitation method [33].

38

Coprecipitation

Coprecipitation is the simplest and most commonly used preparative technique for the

synthesis of layered double hydroxides [197]. Coprecipitation is carried out at either low or

high saturation [204]. Low saturation involves the preparation of separate metal salt and

alkaline solutions, which are then added simultaneously to a solution containing the desired

interlayer anion at constant pH [204]. The seawater neutralisation of bauxite residues is a low

saturation coprecipitation. High saturation requires the mixture of desired anion and base

before the slow addition of the metal salt solution [33]. Coprecipitation methods produce

layered double hydroxides with a wide range of particle sizes and morphologies, therefore,

synthesis conditions must be optimised to obtain desired products with well organised layered

double hydroxide structures [205]. Amorphous hydrotalcite like compounds is obtained at pH

range between 7 and 10, while brucite (Mg(OH)2) crystallises with the layered double

hydroxide phase at higher pH [205]. Another study reported that layered double hydroxide with

higher crystallinity, surface area and average pore diameter are formed if constant pH is

obtained throughout the reaction [197].

Thermal Activation

Calcination of layered double hydroxides between 300 and 500 °C generates mixed metal

oxides (Mg-Al oxides) with large surface area [206]. These mixed metal oxides form due to

the removal of interlayer water, interlayer anions and hydroxyl groups in the hydrotalcite

structure [206]. The decomposition steps of thermally activated hydrotalcites are (1)

dehydration (Equation 10), (2) dehydroxylation and decarbonation (Equation 11) are:

Equation 10:

39

𝑀𝑔6𝐴𝑙2(𝑂𝐻)16𝐶𝑂3. 𝑥𝐻2𝑂(𝑠) → 𝑀𝑔6𝐴𝑙2(𝑂𝐻)16𝐶𝑂3(𝑠) + 𝑥𝐻2𝑂(𝑔)

Equation 11:

𝑀𝑔6𝐴𝑙2(𝑂𝐻)16𝐶𝑂3. 𝑥𝐻2𝑂(𝑠)

→ 𝑀𝑔𝐴𝑙2𝑂4(𝑠) + 5𝑀𝑔𝑂(𝑠) + 𝐶𝑂2(𝑔) + 𝑥𝐻2𝑂(𝑔) + 221𝑂2(𝑔)

When thermally activated LDH is exposed to aqueous solution containing metal cations and

anions, the layered structure regenerates and incorporates cations in layer (by isomorphic

substitution) and anions in the interlayer region [206, 207]. This conversion of metal oxides

into layered double hydroxides has been referred to as regeneration, restoration, structural

memory effect or simply memory effect [195]. Thermal activation is typically used to remove

carbonate anions in LDH structures; carbonate has the highest affinity for LDH structures and

limits their use in water treatment applications [208].

2.6 Removal of heavy metals

It is clear from Table 1 that heavy metals have adverse effects on the environment and are

becoming one of the most serious environmental problems. Current technologies employed for

the treatment of heavy metals include various physicochemical treatments [209] such as ion

exchange [210, 211], chemical precipitation [212-214], coagulation- flocculation [215-217],

flotation [218, 219], membrane filtration [220, 221], biosorption [58], and electrochemical

techniques such as electrocoagulation [222-224], electro-flotation [225-227], and

electrodeposition [228-230].

2.6.1 Chemical precipitation

Chemical precipitation is the most widely and effective method used for the removal of heavy

metals from wastewaters and polluted water sources [231]. Precipitation requires the addition

of chemicals that will react with heavy metal ions to form insoluble precipitates that can be

40

filtered/separated from the treated water source. The chemical precipitation process includes

two types of methods:

1. Hydroxide precipitation: is the most applied chemical precipitation technique due to its

relative simplicity, low cost and ease of pH control [232]. A variety of alkali such as

(hydrated lime (Ca(OH)2), lime (CaO), sodium hydroxide (NaOH), magnesium hydroxide

(Mg(OH)2)) have been used to precipitate metals from wastewater [233]. The mechanism

of heavy metal removal by hydroxide precipitation is presented below in Equation 12.

Equation 12: 𝑀2+(𝑎𝑞) + 2(𝑂𝐻)− → 𝑀(𝑂𝐻)2 ↓

Here M2+, OH- and M(OH2) represents the dissolved metal ions, precipitant and insoluble

metal hydroxide, respectively. Even though hydroxide precipitation is widely used, it has

some limitations due to production of large volumes of low density sludge (which can

present dewatering and disposal problems), and difficulty in the removal of amphoteric

metal hydroxides [234]. Finding an ideal pH for the removal of an amorphous metal may

put another metal back into solution [234].

2. Sulphide precipitation: is also an effective process for the removal of toxic heavy metals

ions. Pyrite and synthetic iron sulphide have been used to remove Cu2+, Cd2+ and Pb2+ from

an aqueous solution [235]. The mechanism governing the metal removal processes is shown

in Equation 13 & Equation 14.

Equation 13: 𝐹𝑒𝑆 (𝑠) + 2𝐻+(𝑎𝑞) → 𝐻2𝑆 (𝑔) + 𝐹𝑒2+(𝑎𝑞)

41

Equation 14: 𝑀2+ (𝑎𝑞) + 𝐻2𝑆 (𝑔) → 𝑀𝑆 (𝑠) ↓ +2𝐻+ (𝑎𝑞)

Where M2+, MS represent the dissolved heavy metal ions and the insoluble metal sulphides

precipitants, respectively [236, 237]. The main advantages of using sulphides is the lower

solubilities of the metal sulphide precipitates compared to hydroxide precipitates and

sulphide precipitates are not amphoteric in nature, hence, high degree of metal removal can

be achieved over a broad pH range [236].

This research will compare commercial chemicals used in the treatment of AMD water and

Bayer precipitates (LDH primary component).

2.6.2 Heavy metal removal using layered double hydroxides

The removal of heavy metal ions using LDHs is believed to be due to one of the following

mechanisms: 1) surface precipitation of metal hydroxides onto LDH surfaces, 2) adsorption to

the surface hydroxyl groups, 3) isomorphic substitution, or 4) chelation with a functional ligand

in the interlayer region [238].

Adsorption

Adsorption reactions are normally considered as intermolecular interactions between solute

and solid phases. Adsorption can be described by one of the following complexation surface

reactions: 1) chemical binding between metal cation and surface functional groups (inner-

sphere surface complex), and 2) electrostatic binding metal cations and oppositely charged

functional groups (outer-sphere surface complexes). Inner-sphere complexes (specific

adsorption) are more selective and less reversible than outer-sphere complexes (non-specific

42

adsorption) [239]. Adsorption reactions are pH dependent, whereby the structural metal (Mg

or Al in the LDH layer) acts as a Lewis acid and exchanges cations or anions (H+, OH-, M2+

and MOH+) [240]. The surface of LDHs have hydroxyl groups as functional groups that are

able to bind with metal cations to form inner-sphere complexes, while deprotonated hydroxyl

groups (Surface-O-) are able to form outer-sphere complexes (Figure 4).

Figure 4: Adsorption mechanism of cation removal using LDHs

Metal cation adsorption reactions of LDHs is believed to involve all or some of the reactions

below [238]:

Surface-OH + M2+ → Surface-O-M+ + H+

Surface-OH + MOH+ → Surface-O-M+ + H2O

Surface-OH + OH- → Surface-O- + H2O

followed by

Surface-O- + M2+/MOH+ → Surface-O-M- + H+/ H2O

43

A study by Ulibarri et al. [241] determined that outer-sphere complexes contributed to metal

cation adsorption at a pH < 7, while inner-sphere complexes occurred at pH > 7.

Zhang et al. [242] reported the Pb (II) removal mehanism for uncalcined LDH to be surface

precipitation and electrostatic binding adsorption, while a calcined LDH removed Pb(II) by

surface precipitation and chemical binding adsorption. Liang et al. [243] studied the removal

of Pb(II) using Mg-Fe layered double hydroxides from aqueous solution and also reported the

removal mechanism to be the surface-induced precipitation and chemical binding adsorption.

The study also found that surface-induced precipitation removed more Pb(II) than chemical

binding adsorption [243].

Precipitation

Precipitation can be defined by the formation of a new solid phase that exhibits its own crystal

structure (Figure 5). LDHs have a large amount of hydroxide associated with its structure

(layers and interlayer) which can lead to pH increases that favour precipitation. The instability

of LDHs in acidic environments (divalent cations are selectively dissolved due to their lower

stability than trivalent cation) can also lead to the release of hydroxide ions that facilitate

precipitation [238]. Precipitate formation can be affected by the following: 1) layer

composition (determines stability), 2) the solubility constant of the heavy metal hydroxide, 3)

co-existing species that form a coprecipitate that consists of chemical species derived from

both the aqueous solution and disolution of LDH, and 4) overall LDH stability [244]. Typically,

precipitates that form in these reactions are metal hydroxides, metal hydroxide chlorides and

metal hydroxide carbonates [245].

44

Figure 5: Precipitation mechanism of cation removal using LDHs

The reactions of Cu2+ and Pb2+ with layered double hydroxide (LDH), has been investigated in

aqueous solutions to elucidate their removal behaviors by LDH [246]. The reaction of aqueous

CuCl2 and PbCl2 solution with Mg-Al LDH, resulted in the precipitation of copper hydroxide

chloride (Cu7Cl4(OH)10·H2O), lead carbonate chloride (PbCO3PbCl2) and lead hydroxide

chloride (Pb(OH)Cl) [246]. Hydrocalumite (Ca-Al LDH) has been shown to effectively remove

Zn2+ (>95 %) from aqueous solutions at initial concentrations <6.3 mmol/L [247]. It has been

reported that removal of Zn2+ was reported to be due to hydrocalumite dissolution and selective

precipitation of ZnAl-LDHs [247].

Isomorphic substitution

Isomorphic substitution is the replacement of similar sized atoms or ions in the LDH layer that

does not cause any alterations to the crystal structure (Figure 6). Replacement ions need to

have the same total ionic charge as those replaced, otherwise the substitution of larger atoms

45

or ions will cause steric hindrance which will destabilise the structure [248]. An example of

isomorphic substitution is provided below in Equation 15.

Equation 15:

𝑀𝑔2𝐴𝑙(𝑂𝐻)6𝐶𝑙. 𝑥𝐻2𝑂 + 𝑀2+ → [𝑀𝑥2+, 𝑀𝑔1−𝑥]2𝐴𝑙(𝑂𝐻)6𝐶𝑙. 𝑥𝐻2𝑂 + (1 − 𝑥)𝑀𝑔2+

Figure 6: Isomorphic substitution mechanism of cation removal using LDHs

Stanimirova et al. [249] reported that treatment of aqueous solutions containing divalent

cations (such as Zn2+, Ni2+, Co2+ and Cu2+ ) with mixed metal oxides (obtained by heating 3:1,

2:1 and 3.7:1 Mg-Al LDH at 600 ˚C for 2h) resulted in the regeneration of layered double

hydroxides with high M2+/Al ratio (maximum value 3.8). In this study, it was foud that divalent

cations such Zn2+, Ni2+, Co2+ and Cu2+ were incorporated into the layered structure instead of

Mg2+ (detected in solution) thus indicating that isomorphic substitution took place [249].

46

Chelation

Chelation involves heavy metals to react with intercalated functional ligands, such as

ethylenediaminetetraacetate (EDTA) (Figure 7). However, it has been reported that the

intercalation of polydentate ligands can produce partial erosion of the layers due to the

chelation of metals from the LDH layer rather than the desired metal cation [250]. The uptake

of metal cations is influenced by the ligand in the interlayer and the stability of the metal-

complex that forms (increased stability results in increased uptake) [251, 252].

Figure 7: Chelation mechanism of cation removal using LDHs

Perez et al. [253] reports the uptake of Cu2+, Cd2+ and Pb2+ using Zn-Al-EDTA LDH with the

removal of metals being due to the formation of metal-EDTA complexes within the interlayer

without apparent deformation of the layered double hydroxide structure. Mg-Al LDHs

containing citrate, malate and tartrate in the interlayer region have also been used to remove

Cu2+ and Cd2+ rapidly from an aqueous solution at a constant pH of 5.0 due to the formation of

47

citrate–metal, malate–metal, and tartrate–metal complexes in the interlayers of the Mg-Al

LDHs [254]. Another study has also shown that the removal of Sr radionuclides using Mg-Al-

EDTA LDH was due to the complexation of Sr2+ with [H2(EDTA)]2 [255]. The sorption of

Pb2+ by Mg-Al-DTPA and Mg-Al-Cl LDH prepared by coprecipitation were also

examined and the mechanism of Pb2+ adsorption on Mg-Al-DTPA LDH was due to the

[Pb(DTPA)]3−chelation, while in case of Mg-Al-Cl LDH surface-induced precipitation was the

main mechanism for Pb2+ removal [256].

2.6.3 Reaction kinetics of heavy metal removal by LDH

Reaction kinetics of heavy metal removal using LDHs is limited, however models typically

used for the cation adsorption on layered double hydroxides are second (sorption of Cu (II) and

Pb (II) on Mg-Al-Cl LDH [246], Pb (II) removal by Mg-Fe-Cl LDH [243] and Mg-Al-DTPA

LDH [256]) and first order kinetcs (sorption of Hg (II) 2-mercaptobenzimidazole intercalated

Mg-Al LDH [257], Cu (II) removal by Mg-Al-EDTA LDH [258]). Time-dependent adsorption

studies generally report the initial adsorption as being fast followed by a slower process to

reach equilibrium [242, 259] . The fast removal rate has been proposed to be attributed to the

rapid diffusion from solution to the external surfaces, while the subsequent slow step is

attributed to the longer diffusion range to the internal surfaces [238, 259]. Liang et al. [238]

reports that pseudo first or second order kinetic models are unable to identfy the diffusion

mechanism, and thus explained the process using diffusion models. It was determined that the

following processes are involved in cation adsorption to LDHs: 1) boundary layer diffusion

between the external surface of the sorbent particles and solute, 2) intraparticle transport within

the particle, and 3) chemisorption (reaction at phase boundary typically controlled by bond

formation) [238, 260, 261].

48

2.6.4 Removal affinities

There are a number of studies on the removal of heavy metals using layered double hydroxides

[250, 253, 262, 263], however the mechanism of removal and the method of treatment varies

significantly. Based on the studies conducted to date, the affinity of heavy metals for Mg-Al-

LDH structures can be summarised by the following ranges: 0.49-0.93 mmol g-1 for Cr (VI)

(adsorption) [264]; 0.01-0.15 mmol g-1 for Cd2+ (chelation) [258], 0.33-0.52 mmol g-1 for Co2+

[265, 266] (isomorphic substitution), 0.03-2.50 mmol g-1 for Cu2+ (surface

complexation/precipitation, isomorphic substitution, chelation and hydroxide precipitation)

[258, 262, 267], 0.73 mmol g-1 for Ni2+ (isomorphic substitution) [266] and 1.02-4.76 mmol g-

1 for Zn2+ (isomorphic substitution and hydroxide precipitation) [266, 267]. The actual affinity

of the cations is dependent on the pH, temperature, ionic strength and how the experiment is

conducted (batch or flow) [268]. Typically the adsorption process improves with increasing

pH and temperature, while the ionic strength has a much weaker effect.

2.7 Factors affecting heavy metal removal

2.7.1 pH

The effect of pH on cation adsorption using LDHs can sometimes show no real changes in

affinity due to the pH buffering effect of LDHs [269]. However, the point of zero charge (PZC)

can provide insights to the effects of pH on the removal capacity of LDHs. LDHs generally

have a PZC of around 10.7 [270], which means at pH<10.7 the surfaces are more positively

charged and electrostatic repulsion prevents cations adsorption, while at pH>10.7 the surface

is negatively charged allowing cation adsorption via electrostatic forces of attraction [242]. The

further away the pH is from the PZC the greater the density of positively or negatively charged

surface sites, whereby the more negative sites the better the adsorption affinity for cations

49

[242]. It should be noted that very low pH effects the stability of the LDH structure (acidic

solutions will cause the dissolution of LDH), while at very high pH the precipitation of

hydroxides is favoured [271-273]. For example, Kameda et al. [258] found that Mg2+ was

leached from Mg-Al-EDTA LDH structures during adsorption experiments at a pH of 5, when

trying to avoid the precipitation of hydroxides of Cu2+ and Cd2+. Few other studies also

reported the decomposition of layered structure during the removal of heavy metals from

aqueous solution at low pH [246, 274].

2.7.2 Temperature

The thermodynamic parameters enthalpy (ΔH°), entropy (ΔS°) and Gibbs free energy (ΔG°)

indicate that at increased temperatures the sorption of metal cations on layered double

hydroxides is endothermic and spontaneous [263, 275]. Kameda et al. [275] reported the

following for increased temperatures; 1) positive enthalpy value (adsorption is endothermic),

2) negative free energy (spontaneity of the adsorptioni process increases), and 3) a positive

entropy value (implies some structural changes in sorbate and sorbent). The positive enthalpy

values also indicates that strong binding between metal cations and adsorbent exist [276].

Xu et al. [277] has studied the effect of temperature (293, 313 and 333K) on Cu(II) removal

from aqueous solution using Mg2-Al LDH and reported that as the temperature increases, ΔG°

becomes more negative (-11.24, -13.65 and -14.74 KJmol-1 at 293, 313 and 333K respectively)

and thus indicating more efficient Cu (II) removal at higher temperatures. Similar results have

been reported for sorption of Pb (II) on Mg2-Al LDH [259]. Co-Mo layererd double hydroxides

have also shown increased adsorption capacities for Mn(II) with increasing temperature

temperature at pH 5 (20.2, 26.75 and 38.1 mg/g at 298, 308 and 318 K, respectively) [278] .

50

2.7.3 Ionic strength

Effects caused by ionic strength are due to the surface potential, activity coefficient of the metal

ions in solution and the degree of aqueous complexity between the metal and electrolyte

anions [238]. The effect of activity coefficients on adsorption can be explained by the degree

of solvation of the ions, where at lower ionic strength metal cations combine with more water

molecules [259]. Due to the interaction of the metal and LDH functional groups being mainly

ionic, the formation of electrical double layer complexes is favoured at lower ionic strength

[259, 279]. It was observed that increases in ionic strength from 0.001 to 0.1M NaCl resulted

in a decrease in metal upatke capacity (98.3 to 60.9 % for Cu (II), 96.2 to 56.9 % for Zn (II)

and 91.4 to 46.8 % for Cd (II)) using hydrotalcites due to increased particle aggregation;

reduced number of surface sites and electrostatic interactions [276]. Another effect of high

ionic strength is the possibility of Na+ ions competing for cation exchange sites at the LDH

surface [276].

2.8 Conclusion

AMD associated with mining has a significant impact on the surrounding ecosystems due to

its low pH and high metal and sulphate content. Source control techniques used to prevent

AMD by eliminating water and oxygen supply to reduce pyrite oxidation are expensive and

they are ineffective to protect the environment against long and persistent pollution caused by

AMD. The drawbacks associated with source control techniques lead to the development of

migration controls to treat AMD. Active migration control techniques involve continuous

supply of alkali (such as lime, sodium hydroxide, sodium carbonate, ammonia or limestone),

and require a source of electricity and high operational and maintenance costs. Therefore, there

is need to develop AMD treatment systems with low maintenance that are effective at

neutralising acidity and removing heavy metals.

51

Bauxite refinery residues produced from the Bayer process have severe impacts on the

surrounding environmnet due to its high alkalinity. Many wet (marine discharge and lagooning)

and dry disposal methods (dry stacking and dry cake) have been employed in the management

of bauxite refinery wastes. However, these disposal methods can cause further environmental

pollution due to high risk of leakage, tailing dam instability and marine disposal had adverse

impact on ocean. Bauxite refinery residues have been used worldwide in various ecofriendly

applications such as used as building materials (cement and bricks), catalysts in some chemical

applications (such as hydrogenation, hydro-dechlorination and exhaust gas clean up), soil

amendment (improving water and phosphorous retention) and in environmental clean up

(removing heavy metals, nitrates, organics from waste water) in the hopes of reducing the

amount of land required for storage. However, the products formed from bauxite residues have

not performed as well as conventionally used ones. For example, ionising radiation level from

bricks formed from bauxite residue is 2-3 times higher compared to conventionally used

material, catalysts formed from bauxite residues had poor performance compared to

commercially used.

Neutralisation methods have been used to increase the reuse potential of neutralised bauxite

refinery waste. Seawater neutralisation of bauxite refinery residues result in the formation of

precipitates known as Bayer precipitates containing hydrotalcite, calcite and aragonite. The

formation of LDH (hydrotalcite) in the seawater neutralised bauxite residues is the primary

means of reducing alkalinity and are known for removing heavy metals from aqueous solutions.

To date, research into the use of Bayer precipitates formed from seawater neutralisation of

Bayer liquor to treat the AMD is not reported in literature. It is for this reason; Bayer

precipitates are being considered as a potential neutralisation agent for AMD water treatment.

52

The research will focus on modifying Bayer precipitates so that perform equally or better than

conventionally used alkali for AMD water treatment.

53

Chapter 3: Characterisation techniques and

Experimental method

3.1 Characterisation techniques

3.1.1 Inductively coupled plasma optical emission spectroscopy (ICP-OES)

Solutions were analysed using a VISTA-MPX CCD simultaneous ICP-OES instrument using

an integration time of 0.15 seconds with 3 replications, using the following wavelengths: Al

(308.215), Ca (317.933), Mg (285.213), Na (589.592), Co (230.786), Cu (327.393), Fe

(259.939), Mn (257.610), Ni (341.476), Be (313.07), Cd (214.440), Si (251.611), Zn (213.857),

Cr (267.716), K (766.490), As (197.197), B (249.677) and Ba (455.403). A certified standard

from Australian Chemical Reagents (ACR) containing 1000 mg/L of aluminium, calcium,

magnesium, and sodium was diluted to form a multi-level calibration curve using a Hamilton

Diluter.

3.1.2 X-ray diffraction (XRD)

X-ray diffraction patterns were collected using a Panalytical X'pert wide angle X-Ray

diffractometer, operating in step scan mode, with Co K radiation (1.7903 Å). Patterns were

collected in the range 5 to 90° 2 with a step size of 0.02° and a rate of 30 s per step. Samples

were prepared as a finely pressed powder into aluminium sample holders, which were then

placed onto aluminium sample holders.

The powder X-ray diffraction pattern for samples containing manganese (Mn) were collected

using Rigaku D/max-rA X-ray diffractometer (40 kV, 100 mA) with Mo (0.70932 Å)

54

irradiation at the scanning rate of 2 °/min in the 2θ range of 3.0-60 °. Samples were ground in

a mortar and pestle and was prepared as a finely pressed powder into Quartz holders.

The XRD patterns were matched with ICSD reference patterns using the software package

HighScore Plus. The profile fitting option of the software used a model that employed twelve

intrinsic parameters to describe the profile, the instrumental aberration and wavelength

dependent contributions to the profile.

3.1.3 Infrared Spectroscopy

Infrared spectra were obtained using a Nicolet iS50 Fourier Transform infrared spectrometer

(FTIR) with a smart endurance single bounce diamond ATR (attenuated total reflectance) cell.

Spectra over the 4000-400 cm-1 range were obtained by the co-addition of 128 scans with a

resolution of 4 cm-1 and a mirror velocity of 0.6329 m/s.

3.1.4 Thermalgravimetric analysis

Thermal decomposition of the Bayer hydrotalcite samples were carried out in a TA®

Instrument (series Q500) incorporated high-resolution thermogravimetric analyser in a flowing

nitrogen atmosphere (60cm3/min). Approximately 10 - 50 mg (dependant on sample yields) of

sample was heated in an open platinum crucible at a rate of 10 °C/min up to 750 °C. The

synthesised hydrotalcites were kept in an oven (85 °C) for 24 hrs before TG analysis. Thus, the

mass losses are calculated as a percentage on a dry basis.

3.1.5 Optical imaging

Light microscopy images were captured using a Leica M125 Light Microscope fitted with a

Leica DFC490 digital camera. Flocculation samples were inverted slowly 5 times and allowed

http://en.wikipedia.org/wiki/Attenuated_total_reflectance

55

to flocculate for one minute, before 30 µL of suspension was transferred between two glass

slides. The slides were viewed at 32x magnification. Subsequent images were analysed using

the software package ImageJ. Images were made binary (black and white) and average particle

size and area coverage of the flocculated particles was measured.

3.1.6 Freeze Drying of Mt Morgan Lime Neutralisation Sludge

Approximately 5 g of sludge produced by AMD treatment with different alkali was collected

and were freeze dried in a -80 °C freezer and then placed into the main chamber of the freeze

dryer unit until dry. The freeze dryer unit was operated at 0.013 mbar pressure with a condenser

temperature of -55 °C. Average moisture loss for the sludge samples was approximately 72 %

with a slight variation noted between sub-samples. The sludge sample was freeze dried as the

high amounts of gypsum present would convert to bassanite (CaSO4.0.5H2O) if the samples

were oven baked.

3.1.7 X-Ray Fluorescence

Samples for Wavelength Dispersive X-Ray Fluorescence (WDXRF) major element analysis

were prepared by weighing 1.15 g of the sample into a 95/5 % Pt/Au crucible followed by 8.85

g of vitreous 50:50 lithium tetraborate:lithium metaborate flux containing 0.5 % lithium iodide

as a non-wetting agent (Claisse Scientific). This sample was then mixed carefully in the

crucible before being placed into an automatic six position fusion instrument (TheOx, Claisse

Scientific). The samples were fused for 20 minutes at 1050 °C with constant agitation before

the melts were poured automatically into 40mm Pt/Au casting dishes and then cooled by a

stream of air. The resultant glass disks were then analysed using a PANalytical Axios WDXRF

equipped with a 1kW Rh tube calibrated for the analysis of 21 major elements using the

56

PANalytical WROXI protocol and associated standards. LOI was determined by igniting a

separate sample portion in a muffle furnace at 1050 °C for 30 min.

3.1.8 Process Simulation using AqMB Software

The commercially available AqMB water treatment software was used to simulate the current

lime neutralisation plant conditions used at Mt Morgan to provide insights into the theoretical

mineralogical phases that make up the lime sludge. The simulation used a doser to add lime

or sodium hydroxide (caustic) to Mt Morgan AMD water (Table 10) using sulphate corrections

for mass balance) up to pH 7.5. A thickener was then used for the solid-liquid separation

process, with design specifications as follows: minimum underflow solids 5 % w/v, feed pump

duty pressure of 100 kPa, 120 min contact zone, 9000 mm mixer impeller diameter, and 1 rpm

rake arm rotation speed.

3.1.9 Surface analysis

BET analysis was performed by a Tristar 3000 unit. Approximately 500 mg of sample was

placed inside the tube and dried at 150 ˚C overnight under flowing nitrogen to accurately

determine the dry sample weight. The sample was then analysed using a 99-data point analysis

method over a range of nitrogen gas pressures, which was used to determine the BET surface

area, the Langmuir surface area and average pore diameter.

3.1.10 X-ray Photoelectron Spectroscopy (XPS)

XPS analysis was performed using a Kratos AXIS Ultra XPS. This instrument incorporated a

165-mm hemispherical electron energy analyser, equipped with a monochromatic Al K X-

ray source (1486.6 eV) at 150 W (15 kV, 10 mA), which was incident at 45° to the sample

57

surface. Photoelectron data was collected at a take-off angle of 90°. Survey (wide) scans were

taken at analyser pass energy of 160 eV and multiplex (narrow) high resolution scans at 20 eV.

Survey scans were carried out over the 1200-0 eV binding energy range with 1.0 eV steps and

a dwell time of 100 ms. Narrow high-resolution scans were run with 0.05 eV steps and 250

ms dwell time. Base pressure in the analysis chamber was 1.0 x 10-9 mbar and during sample

analysis 1.0 x 10-8 mbar.

3.1.11 Water Quality Standards

The ANZECC guidelines for fresh and marine water which were used to determine the

effectiveness of the alkali addition strategies in this study are outlined in Table 3 [280].

Table 3: ANZECC water quality guidelines relevant to Mount Morgan AMD water

Water quality parameter Agricultural irrigation

water

Livestock drinking

water

pH 6.5 to 8.5 6.5 to 8.5

Al <5 mg/L <5 mg/L

Fe <0.2 mg/L Not sufficiently toxic

Cu <0.2 mg/L

<0.4 mg/L (sheep)

<1 mg/L (cattle)

<5 mg/L

(pigs and poultry)

Ni <0.2 mg/L <1 mg/L

Mn <0.2 mg/L Not sufficiently toxic

Zn <2 mg/L <20 mg/L

58

3.2 Experimental methods

3.2.1 Effect of Bayer liquor concentrations on the synthesis of Bayer precipitates

3.2.1.1 Bayer liquor Preparation

Bayer liquor was prepared at a range of concentrations (ranging from 1 to 10 g/L Al2O3).

Table 4 has the masses (in g) of sodium hydroxide (NaOH), sodium carbonate (Na2CO3) and

volume (in mL) of super-evaporative liquor (SEL) used to prepare these liquors. These

solutions were prepared by half filling a 2 L volumetric flask with DI water, to which the

required amount of sodium carbonate (Na2CO3) was dissolved before the addition of sodium

hydroxide (NaOH). Once Na2CO3 and NaOH dissolved, the required amount of super-

evaporative liquor (SEL) was added to the beaker and topped up to the 2 L mark and stirred for

10 minutes. Seawater used in this study was collected approximately 150 m offshore at the

Redcliffe Jetty, Moreton Bay, Queensland in March 2013. The elemental composition of Bayer

liquor solutions (1-10 g/L) and seawater has been provided in Table 5.

Table 4: Masses and volumes required to prepare different Bayer liquors

Bayer liquor

concentration (g/L

Al2O3)

Na2CO3 NaOH SEL

Mass (g) Mass (g) Volume (mL)

1 2.11 67.26 21

2 4.23 63.12 42

3 6.34 58.98 63

4 8.45 54.84 85

5 10.56 50.69 106

6 12.68 46.55 127

7 14.79 42.41 148

8 16.90 38.27 169

9 19.02 34.13 190

10 21.13 29.99 212

59

Table 5: Ions concentration (mg/L) of Bayer liquor and Seawater

Bayer liquor

concentration

(g/L Al2O3)

Concentration (mg/L)

Al Mg S Na K Ca

1 450.5 0.775 3.47 20730 30.92 <0.05

2 1190 1.40 0.79 22240 78.42 <0.05

3 1503 1.28 3.65 19570 84.29 <0.05

4 2045 1.23 5.39 20090 111.4 <0.05

5 2525 0.791 11.13 20840 123.3 <0.05

6 3125 1.27 4.95 21590 165.7 <0.05

7 3613 1.21 8.42 22080 191.8 <0.05

8 4232 1.33 8.67 22730 209.3 <0.05

9 4466 1.33 10.5 23260 237.7 <0.05

10 4893 1.10 20.14 32030 246.8 <0.05

Seawater <0.05 1256 806.9 9429 628 395

3.2.1.2 Seawater neutralisation

The Bayer precipitates were formed by the addition of seawater to Bayer liquor until the desired

pH was reached (increments of 0.25 pH units from pH 13 to 9.5). This involved the slow

addition of seawater to 5 mL of Bayer liquor until the desired pH was reached and stabilised

for more than 5 minutes. The volume of seawater added to get the desired pH for Bayer liquors

in the concentration range 1-10 g/L has been shown in Table 6. Samples were then placed in

250 mL bottles and stirred for 24 hrs before being centrifuged using a Centurion Scientific C2

Series centrifuge for 5 minutes at 2500 rpm. The supernatant was transferred into a sample

container and analysed using ICP-OES after dilution using a Hamilton dilutor, while the solid

component was washed with 150 mL of deionised H2O before being centrifuged again. The

solid component was placed in the oven overnight at temperature of 90 °C and dried.

Thereafter, the dried samples were removed from the oven and crushed to a fine powder using

an agate mortar and pestle before analysis using XRD, IR and TGA.

60

Table 6: Volume of seawater added to attain pH (13-9.25)

pH Seawater added (mL)

1g/L 2g/L 3g/L 4g/L 5g/L 6g/L 7g/L 8g/L 9g/L 10g/L

13.00 13.4 13.6 14.4 16.2 18.4 22.4 26.6 27.0 20.0 26.6

12.75 26.0 25.4 26.0 22.0 25.0 29.8 35.2 32.0 30.0 40.2

12.50 30.0 35.2 34.4 29.8 36.0 39.4 42.0 44.2 38.8 47.0

12.25 34.6 45.8 40.0 36.2 43.0 47.6 49.0 49.8 49.0 53.0

12.00 40.0 51.0 43.2 42.0 49.2 52.2 55.6 54.0 56.0 58.2

11.75 41.0 54.4 45.4 47.0 54.2 57.4 59.4 59.2 60.0 61.0

11.50 43.2 56.8 47.2 48.0 55.6 58.8 63.2 63.0 64.8 64.2

11.25 45.0 58.6 49.0 50.2 58.4 59.2 65.0 64.8 66.0 67.0

11.00 46.0 61.0 51.2 50.8 61.8 60.2 66.2 66.4 68.4 70.0

10.75 51.0 66.0 55.0 52.0 63.0 62.0 69.0 68.6 71.0 71.0

10.50 60.0 74.0 61.0 56.0 69.0 63.6 71.0 72.8 72.6 72.4

10.25 105.0 94.0 87.2 71.0 88.2 73.0 76.0 77.2 74.8 74.6

10.00 130.0 135.4 128.0 97.0 100.0 88.0 92.4 86.4 76.2 79.6

9.75 155.0 180.0 170.0 140.0 129.0 108.0 126.0 100.0 84.6 90.0

9.50

180.0 200.0 200.0 162.0 160.0 152.0 164.0 148.4 134.0 123.0

9.25 200.0 200.0 200.0 200.0 200.0 200.0 200.0 200.0 182.0 177.0

3.2.1.3 Acid neutralisation capacity of Bayer precipitates

AMD samples were collected in June 2017 from the Open Pit of Mount Morgan mine, located

in central Queensland (Table 7). Acid neutralising capacity was determined by the addition of

a known quantity (Table 8) of Bayer precipitates obtained at pH 9.25 from the seawater

neutralisation of different compositions of Bayer liquors (1-10 g/L Al2O3) to 10 mL of AMD.

The subsequent mixture was then stirred for 24 hrs before being centrifuged using an

Eppendorf Centrifuge 5702 for 5 minutes at 2500 rpm. The supernatant was then transferred

into a sample container, while the solid component was washed with 10 mL of DI water before

being centrifuged again, and then placed in the oven overnight to dry and then crushed to a fine

powder using an agate mortar and pestle. The pH of treated AMD was monitored using a

calibrated pH meter (TPS) and probes. Solutions for ICP–OES were syringe filtered using a

0.45 µm nylon filter prior to analysis.

61

Table 7: Composition of mine pit water from Mount Morgan, June 2017

pH Conductivity

(mS) SO4 (mg/L)

3.77 14.81 19342


Al Fe Mn Cu Zn Co Ni Cd Cr

1393 1.74 177 85.77 57.63 4.22 1.19 0.27 0.82


S Mg Ca Na Si K Li Sr B

6383 2580 554 632.4 26.87 7.486 0.63 0.93 0.31

Table 8: Mass of Bayer precipitates added to treat AMD

Bayer

precipitates Mass (g)

1 g/L 0.0150 0.0301 0.0399 0.0502 0.1005 0.1999 0.4002 0.5000

2 g/L 0.0151 0.0297 0.0402 0.0499 0.1001 0.2001 0.4001 0.5002

3 g/L 0.0148 0.0303 0.0397 0.0501 0.0997 0.1998 0.3999 0.4999

4 g/L 0.0152 0.0299 0.0401 0.0497 0.1005 0.2002 0.4002 0.5001

5 g/L 0.0149 0.0302 0.0403 0.0498 0.1003 0.1999 0.4003 0.4995

6 g/L 0.0150 0.0300 0.0397 0.0502 0.0998 0.2001 0.3997 0.5003

7 g/L 0.0146 0.0301 0.0403 0.0499 0.1001 0.1996 0.4002 0.4999

8 g/L 0.0151 0.0298 0.0399 0.0503 0.1003 0.2004 0.4000 0.5001

9 g/L 0.0149 0.0300 0.0402 0.0501 0.1001 0.1998 0.4001 0.5002

10 g/L 0.0151 0.0299 0.0401 0.0495 0.1000 0.1999 0.4000 0.5001

3.2.1.4 Acid digestion (ICP-OES)

Accurately weighed samples of the 1-10 g/L Bayer precipitates as shown in Table 9 were acid

digested using 2 mL HCl, 1 mL HNO3 and 2 mL DI water. The sample mixture was then heated

at 80 °C for one hour. The samples were then removed from heat and after cooling DI water

62

was added to sample solutions to make final volume 50 mL. Digested samples were then

diluted to 1:10 and 1:100 before being analysed by ICP-OES using above discussed method.

Table 9: Mass of Bayer precipitates used for acid digestion

Bayer precipitates Mass (g)

1 g/L

0.1438

2 g/L

0.1446

3 g/L

0.0557

4 g/L

0.1049

5 g/L

0.0853

6 g/L

0.1320

7 g/L

0.1534

8 g/L

0.1371

9 g/L

0.1581

10 g/L

0.1277

3.2.2 Assessing the effectiveness of Bauxite refinery residues with conventionally used

alkali

3.2.2.1 Acid mine drainage (AMD)

AMD samples were collected from the Open Pit of Mount Morgan mine, located in central

Queensland, August 2014. AMD water used in this investigation was open pit surface water.

Note, at the time of this investigation, AMD water used in the water treatment plant (lime

neutralisation) was piped from approximately 3 m below the surface. We compared

compositions of surface water and water from 3 m depth and confirmed that minimal variations

in concentrations of metals were present. Open pit AMD water sampled at the surface contained

relatively high concentrations of Mg (2265 mg/L), Al (1233 mg/L), Ca (534.3 mg/L),

Cu (77.26 mg/L), Fe (16.7 mg/L), Mn (161.5 mg/L) and Zn (48.89 mg/L), while concentrations

for other elemental species and physical properties can be found in Table 10.

63

Table 10: Composition of open pit water from Mount Morgan, August 2014

pH Conductivity

(mS)

SO4 (mg/L)

3.74 14.86 17430


Al Fe Mn Cu Zn Co Ni Cd Cr

1233 16.7 161.5 77.26 48.89 4.05 1.54 0.19 0.06


S Mg Ca Na Si K Li Sr B

5703 2265 534.3 647.8 36.94 6.24 0.39 0.77 -0.06

3.2.2.2 Alkali used to treat AMD

Lime, sodium hydroxide and sodium carbonate used in this research were of AR grade and

supplied by Labtek. Bayer liquor and Bayer precipitates were synthesised in the laboratory as

outlined below.

Synthesis of Bayer liquor

10 g/L Bayer liquor solution was prepared by the dilution of a highly concentrated real Bayer

liquor (saturated evaporative liquor - 96 g/L Al2O3) provided by an Australian alumina refinery.

This solution was prepared by half filling a 2 L volumetric flask with deionised (DI) water, to

which 21.13 g of sodium carbonate (Na2CO3) was dissolved before the addition of 29.99 g of

sodium hydroxide (NaOH). Once the Na2CO3 and NaOH were dissolved, 212 mL of saturated

evaporative liquor was added to the volumetric flask and topped up to the 2 L mark using DI

water. The mixture was then inverted several times to ensure homogeneity.

64

Synthesis of Bayer precipitates

Bayer precipitate was synthesised by the addition of seawater to 1 L of the 10 g/L Bayer liquor

until a pH of 9.25 was obtained. This latter pH value was achieved by placing the Bayer liquor

into a 10 L container equipped with an overhead stirrer (IKA, RW20) with a 4-propeller paddle

stirrer placed into solution. The stirrer was set to 400 rpm to ensure uniform mixing, while

seawater was pumped into the beaker using a Watson and Marlow 520U pump set to

1.5 mL/min using Marprene tubing (diameter 6.4 mm). Once a pH of 9.25 was reached, the

solution was allowed to stir for a further 24 hrs before being vacuum filtered. The precipitate

was then placed in an oven (90 °C) overnight before being crushed to a fine powder (< 125

µm) using a Fritsch agate ball mill.

3.2.2.3 AMD neutralisation

The treatment of AMD involved the addition of known amounts of lime, Bayer hydrotalcite,

Bayer liquor, sodium carbonate and sodium hydroxide to 25 mL of AMD water. The amount

of each alkali added to 25 mL of AMD water is shown in Table 11. The resultant mixture was

then allowed to stir for 24 hrs before being centrifuged using a Thermofischer Scientific X1

Series centrifuge for 5 minutes at 2500 rpm. The supernatant was then transferred into a sample

container, while the solid component was washed with 30 mL of DI water before being

centrifuged again. The solid component was freeze dried and then crushed to a fine powder

using an agate mortar and pestle. The pH and conductivity of treated AMD were monitored

using calibrated TPS meter and probes. Solutions for ICP–OES were syringe filtered using a

0.45 µm nylon filter prior to analysis.

65

Table 11: Amount of alkali added to 25 mL of AMD water

Alkali added

Lime (g) Sodium

hydroxide (g)

Sodium

Carbonate (g)

Bayer liquor

(mL)

Bayer

precipitates (g)

0 0.0000 0.0000 0.00 0.0000

0.0021 0.0023 0.0018 0.25 0.0018

0.0037 0.0042 0.0038 0.50 0.0045

0.0064 0.0060 0.0061 0.75 0.0062

0.0085 0.0087 0.0082 1.00 0.0081

0.0110 0.0099 0.0097 1.25 0.0099

0.0130 0.0122 0.0123 1.50 0.0125

0.0149 0.0144 0.0152 2.00 0.0148

0.0310 0.0299 0.0316 2.50 0.0298

0.0411 0.0406 0.0404 3.00 0.0408

0.0513 0.0541 0.0512 3.50 0.0501

0.0722 0.0754 0.0745 4.00 0.0760

0.1039 0.1027 0.1025 4.50 0.1010

0.1235 0.1250 0.1892 5.00 0.1253

0.2515 0.1540 0.2999 - 0.2509

0.5022 0.1896 0.4003 - 0.5025

1.0150 0.2842 0.5238 - 1.0040

- 0.4095 - - 1.2500

- 0.4989 - - 1.5000

- 1.0663 - - -


Accurately weighed samples, as shown in Table 12 were acid digested using a method

discussed in section 3.2.1.4. Digested samples were then diluted to 1:10 and 1:100 before being

analysed by ICP-OES using above discussed method.

66

Table 12: Mass of precipitates (obtained after AMD treatment) used for acid digestion

Alkali Mass (g)

Lime

0.0495

Sodium hydroxide 0.1008

Sodium carbonate 0.0520

Bayer liquor 0.0495

Bayer precipitates 0.0497

3.2.2.5 Leaching of Precipitates

Precipitates obtained after the treatment of AMD water with different alkalis were leached

using DI water and open pit AMD water. Approximately 0.25 g of the obtained precipitates

were added to either 20 mL of DI water or AMD water and then stirred for 24 hrs before being

centrifuged using a Thermo Fischer Scientific X1 Series centrifuge for 5 minutes at 2500 rpm.

The supernatant was then transferred into a sample container, while the solid component was

placed in an oven at 70 oC overnight to dry. The pH of treated AMD was monitored using

calibrated TPS meter and probes and the solution filtration were the same as stated under

section 3.2.2.3 Solutions for ICP–OES were syringe filtered using a 0.45 µm nylon filter prior

to analysis.

3.2.3 Heavy metal removal efficiencies of thermally activated Bayer precipitates

3.2.3.1 Preparation of Bayer precipitates and thermally activated Bayer precipitates

Bayer precipitates synthesised in previous section were consumed in these experiments.

3.2.3.2 Thermal activation of Bayer precipitates

67

Thermally activated samples were prepared by taking 10 g of Bayer precipitates into a ceramic

crucible and heating it using a SEM Muffle Furnace (100 series) with a heating rate of 20 °C

per minute up to the desired temperature (320, 380 and 440 °C) for 4 hrs. After heating to the

desired temperature for 4 hrs, samples were immediately placed in a vacuum desiccator to

ensure minimum contact with atmospheric air and water. Thermally activated samples were

then analysed using XRD, IR and BET.

3.2.3.3 Metal uptake by thermally activated and untreated Bayer precipitates

Manganese has been found in AMD up to 160 mg/L, which is considerably in excess of the

recommended discharge value (<0.2 mg/L) [281]. An aqueous solution containing 160 mg/L

Mn was prepared using AR grade manganese sulphate monohydrate (MnSO4.H2O; supplied by

PROLABO). A known amount (as shown in Table 13) of Bayer precipitate and thermally

activated Bayer precipitate (at 320 °C, 380 °C and 440 °C) was placed into 50 mL centrifuge

tubes followed by the addition of 40 mL of the metal solution at the desired pH range 3-4. The

centrifuge tubes were then placed on a Ratek rotary stirrer for 24 hrs. The tubes were removed

from the rotary stirrer and subsequently centrifuged at 2500 rpm for 5 min using a Thermo

Fischer Scientific X1 Series, pH of the solution was measured with Multi-Parameter PCSTestr

35. The obtained precipitates were placed in an oven at 70 °C to dry before being analysed by

XRD and XPS, while the solution was stored for analysis using ICP-OES.

68

Table 13: Mass of Bayer precipitates and thermally activated Bayer precipitates added

to 40 mL Mn solution

Bayer

precipitates (g)

Thermally activated Bayer precipitates (g)

320 °C 380 °C 440 °C

0.0000 0.0000 0.0000 0.00

0.0014 0.0011 0.0014 0.0012

0.0023 0.0025 0.0024 0.0024

0.0051 0.0050 0.0051 0.0050

0.0074 0.0075 0.0072 0.0074

0.0103 0.0100 0.0106 0.0101

0.0214 0.0205 0.0205 0.0203

0.0299 0.0299 0.0299 0.0300

0.0405 0.0400 0.0400 0.0405

0.0500 0.0501 0.0505 0.0501

0.0745 0.0755 0.0745 0.0747

0.1001 0.1002 0.1001 0.1000

0.2501 0.2495 0.2510 0.2501

0.5000 0.4997 0.5005 0.5000

0.7501 - - -


To determine the concentration of metals in the obtained precipitates, known amount of

precipitates (Table 14) were acid digested as discussed in section 3.2.1.4 and were then

analysed as solution using ICP-OES.

69

Table 14: Mass of precipitates used for acid digestion

Sample Mass

B.PPT

0.4085

B.PPT +Mn 0.3793

TA B.PPT 320 0.4607

TA B.PPT 320 + Mn 0.2528

TA B.PPT 380 0.1295

TA B.PPT 380 + Mn 0.1642

TA B.PPT 440 0.1649

TA B.PPT 440 + Mn 0.1713

3.2.4 AMD water treatment with Bayer precipitates and thermally activated Bayer

precipitates

3.2.4.1 AMD samples

AMD samples were collected from nine water bodies across the Mount Morgan mine, located

in central Queensland, August 2014. AMD water composition collected from different sites at

Mt. Morgan can be found in Table 15.

70

Table 15: Water compositions found at Mt Morgan mine site, August 2014

Site Containment type Concentration (mg/L)

pH SO4 Al Fe Mn Cu Zn Co Ni Mg Ca Na Si

Airfields Tailings 2.70 38000 1703 194 186 101 73 5.84 1.23 3545 431 157 52.3

Frog Hollow Slag / Waste rock

3.21 16500 1094 1045 150 87 49 3.65 0.93 1929 490 274 65

Mundic Creek East

Waste rock

2.77 19500 1532 500 134 81 45 3.59 0.88 1976 445 221 45

Mundic Creek West

Tailings / Waste rock

2.85 19780 1516 241 186 83 55 4.42 1.49 2597 495 536 62

No. 2 Mill Tailings

3.00 22500 1364 1370 161 60 39 2.88 0.9 2723 523 202 46

No. 2 South Tailings

2.86 22500 1500 169 156 61 29 3.66 0.67 2874 455 251 39

Open Pit Waste rock

3.74 17430 1233 17 162 77 49 4.06 1.54 2265 535 648 46

Shepherd's Holding

Waste rock

3.14 15000 474 153 81 16 15 1.09 0.24 2496 494 219 45

Shepherd's Spring

Waste rock

3.11 18000 1105 63 135 48 24 2.85 0.56 2526 445 260 38

71

3.2.4.2 Bayer precipitates synthesis

Bayer liquor and Bayer precipitates synthesised in the section 3.2.2 were used in these

experiments.

3.2.4.3 Thermal activation of Bayer precipitates

Seawater neutralised Bayer liquor precipitate (Bayer precipitates) was thermally activated

using a furnace with a heating rate of approximately 20 ˚C per minute up to 380 ˚C and then

held at this temperature for 4 hrs. The mass losses after thermal activation are shown in

Table 16. The thermally activated Bayer precipitates were then stored in a vacuum desiccator

to avoid being rehydrated.

Table 16: Mass loss during thermal activation

Crucible Number Mass of Bayer precipitates

before thermal activation (g)

Mass of Bayer precipitates

after thermal activation (g)

1 10.03 6.19

2 10.02 6.20

3 9.68 5.82

3.2.4.4 AMD neutralisation

The treatment of AMD water involved the addition of known amounts of Bayer precipitates

and thermally activated Bayer precipitates (as shown in Table 17) to 25 mL of AMD water.

The resultant mixture was then allowed to stir for 24 hrs before being centrifuged using a

Thermo Fischer Scientific X1 Series centrifuge for 5 minutes at 2500 rpm. The supernatant

was then transferred into a sample container, while the solid component was washed with 30

mL of DI water before being centrifuged again. The solid component was placed in the oven

overnight to dry and then crushed to a fine powder using an agate mortar and pestle. The pH

72

and conductivity of treated AMD were monitored using calibrated TPS meter and probes.

Solutions for ICP–OES were syringe filtered using a 0.45 µm nylon filter prior to analysis.

3.2.4.5 Leaching

Precipitates obtained after the treatment of AMD water with Bayer precipitates and thermally

activated Bayer precipitates were leached using DI water and respective AMD waters.

Measured amount of Bayer precipitates (as shown in Table 18) were added to 20 mL of DI

water and respective AMD water and was then stirred for 48 hrs before being centrifuged using

a Thermo Fischer Scientific X1 Series centrifuge for 5 minutes at 2500 rpm. The supernatant

was then transferred into a sample container, while the solid component was placed in the oven

overnight to dry. The pH and conductivity of treated AMD were monitored using calibrated

TPS meter and probes. Solutions for ICP–OES were syringe filtered using a 0.45 µm nylon

filter prior to analysis.

73

Table 17: Mass of Bayer precipitates and thermally activated Bayer precipitates added

to treat AMD waters

Open pit AMD Mundic West AMD Airfield dump AMD

Bayer

precipitates

(g)

Thermally

activated (g)

Bayer

precipitates

(g)

Thermally

activated (g)

Bayer

precipitates

(g)

Thermally

activated (g)

0.0000 0.0000 0.0000 0.0000 0.0000 0.0000

0.0018 0.0020 0.0021 0.0021 0.0023 0.0020

0.0045 0.0038 0.0036 0.0042 0.0045 0.0039

0.0062 0.0062 0.0060 0.0065 0.0059 0.0058

0.0081 0.0081 0.0085 0.0079 0.0085 0.0080

0.0099 0.0097 0.0098 0.0099 0.0010 0.0103

0.0125 0.0125 0.0126 0.0122 0.0129 0.0127

0.0148 0.0155 0.0147 0.0152 0.0157 0.0152

0.0298 0.0297 0.0301 0.0215 0.0301 0.0198

0.0408 0.0396 0.0405 0.0299 0.0409 0.0288

0.0501 0.0526 0.0499 0.0400 0.0515 0.0415

0.0760 0.0752 0.0789 0.0508 0.0751 0.0506

0.1010 0.1020 0.1035 0.0749 0.0993 0.0749

0.1253 0.1244 0.1250 0.1073 0.1289 0.1015

0.2509 0.2501 0.2485 0.1298 0.2520 0.1285

0.5025 0.5070 0.5085 0.2521 0.4984 0.2654

1.0040 0.7651 0.9904 0.5062 1.0041 0.5114

1.2500 0.9960 - 0.7466 - 0.7352

1.5000 1.2913 - 0.9970 - 1.0080

- - - 1.2829 - 1.2407

74

Table 18: Mass of precipitates (obtained after AMD treatment) used for leaching

experiment

AMD Treatment Mass (g)

Open pit B.PPT in DI 0.2497

TA B.PPT in DI 0.2499

Airfield B.PPT in DI 0.2507


Mundic west B.PPT in DI 0.2501


Open pit B.PPT in AMD 0.2499

TA B.PPT in AMD 0.2500

Airfield B.PPT in AMD 0.2409


Mundic west B.PPT in AMD 0.2502



Samples were prepared by method discussed in section 3.2.1.4 (acid digestion of solid samples for

ICP-OES) and Table 19 shows mass of precipitates used to prepare solutions.

Table 19: Mass of precipitates (obtained after AMD treatment) used for acid digestion

AMD Treatment Mass (g)

Open pit B.PPT 0.0497

TA B.PPT 0.2425

Airfield B.PPT 0.2546

TA B.PPT 0.2777

Mundic west B.PPT 0.2027

TA B.PPT 0.2952

75

Chapter 4: Effect of Bayer liquor composition on

the formation of Bayer precipitates

This chapter has been submitted to Science of the Total Environment for publication





76

4.1 Introduction

Bauxite refinery residue is a waste product produced by the extraction step of the Bayer process

[102, 105]. Impoundment type bauxite residue storage facilities consist of solid bauxite residue

(red mud) and an alkaline supernatant liquor (SNL); entrained liquor in bauxite residue and

leachates from the residue [282]. SNL typically has a relatively low alumina content and

alkalinity compared to super-evaporative liquor (after digestion) and underflow residue washer

liquor [24], as well as a variable composition of anions and heavy metals at trace but harmful

concentrations to the environment [282]. The composition of SNL is highly dependent on the

processing conditions (bauxite ore, digestion, liquor purification), disposal method (lagooning

and dry stacking, and whether there was prior neutralisation before disposal) and climatic

factors (rainfall and evaporation rates) [282]. Impoundment of SNL is essential to minimise

the risk of surface and groundwater contamination [24, 282]. Storage of SNL causes increased

storage capacity requirements for bauxite refinery storage facilities, and thus methods such as

seawater neutralisation have been trialled as a means of reducing SNL volumes [282]. In one

case of seawater neutralisation of SNL from impoundments, the SNL was reacted with

evaporative seawater (heated to 50 °C) and was then discharged through a labyrinth of

settlement channels to produce an inert precipitate and water suitable for estuary discharge

[282]. The precipitate that forms is a mixture of calcium carbonate and hydrotalcite (to be

referred to as Bayer precipitate) which still requires a suitable disposal technique or more

preferably a beneficial reuse option.

As the precipitate formed from seawater neutralised supernatant liquor is alkaline, it is

plausible that a possible route for reuse is the remediation of acidic waste solutions. The mining

industry has generated literally thousands of sites which are afflicted with problems associated

with AMD [13]. AMD is characterised by high sulphate and metal content and is created when

77

sulphide minerals in the waste rock and tailings are exposed to the atmosphere; thus, oxidising

to form sulphuric acid, which can leach heavy metals from surrounding rock [15, 283]. A major

environmental concern with AMD water is run-off into neighbouring waterways, and thus

needs to be treated [8, 71]. Therefore, it is proposed that the alkaline Bayer precipitate be used

in the treatment of AMD waters, and if successful, it may be commercially and socially

attractive.

However, the variability in composition of bauxite residues due to different ore compositions

and process conditions may result in variable neutralisation capacity [23, 284-286].

Consequently, there is a need to understand the reaction conditions that influence the formation

of Bayer precipitates to identify synthesis conditions that result in optimal materials for the

treatment of AMD or acid sulphate soils. The hypothesis was that relative availability of

reactive species in the Bayer liquor (aluminate, hydroxide, and carbonate) and seawater

(magnesium and calcium concentrations) can affect the formation of precipitates, particularly

Bayer hydrotalcite. Therefore, this investigation addressed the following research issues: (1)

what is the impact of Bayer liquor composition upon neutralisation efficiency of seawater? (2)

how does the Bayer liquor constitution influence the composition, structure, and stability of

the obtained precipitates upon seawater addition? and (3) how effective is the treatment of

AMD using precipitates from neutralisation of Bayer liquor? Bayer precipitates were

synthesised using a batch process that simulated the seawater neutralisation process used in

industry, and these materials were then characterised by X-ray diffraction (XRD), infrared

spectroscopy (IR), inductively coupled plasma-optical emission spectroscopy (ICP-OES) and

thermogravimetric analysis (TGA). The acid neutralising capacity (ANC) of the precipitates

provided an assessment of the ability of Bayer hydrotalcite to remove alkalinity from the

supernatant and gave an insight into effectiveness in neutralising AMD samples.

78

4.2 Impact of Bayer Liquor Composition on Seawater Neutralisation Precipitates

To illustrate the variations in the precipitates formed by seawater addition, characterisation of

materials formed at pH 9.25 with Bayer liquor at 1-10 g/L Al2O3 concentration was conducted.

A final pH of 9.25 was chosen on the basis that this pH satisfied regulations for safe disposal

of supernatant [120, 287]. Figure 8 shows XRD patterns of Bayer precipitates formed at a pH

value of 9.25. There was evidence for the formation of hydrotalcite (Mg6Al2(OH)16CO3),

mixed metal hydroxides (Mg2Al(OH)7), calcite (CaCO3), aragonite (CaCO3), and halite (NaCl)

in all obtained precipitates based on matches from respective reference patterns. Several

studies on the seawater neutralisation of bauxite refinery residues report similar mineralogical

phases [122, 288]. Greater insight regarding the precipitates formed was revealed in Table 20

which summarized the results of quantitative XRD analysis of the Bayer precipitates.

It was noted that an additional phase known as brucite (reflections observed at ca. 22 and 45°

2θ) is present in 1-3 g/L Bayer precipitates, but brucite related peaks were absent in XRD

spectra of 4-10 g/L Bayer precipitates (Figure 8). It is proposed that this observation was due

to the additional aluminium and hydroxide ions in the 4-10 g/L Bayer liquor (Table 5) that

facilitate the formation of the more thermodynamically stable hydrotalcite structure. Due to the

formation of number of hydrotalcite species (range of Mg:Al - 2:1 at pH >12, 3:1 at pH ≤10

and 4:1 at pH ≤8) during neutralisation process [33], it is no surprise that the reference pattern

does not fit perfectly with the XRD of obtained precipitates. All hydrotalcite related peaks

appeared broad and of low intensity, indicating that relatively small crystals formed [122, 287].

Taylor et al. [103] has reported that in Bayer liquors, the main alkalinity providing species are

hydroxides (OH-), aluminate ion (Al(OH)4-) and carbonates (CO3

2-). Upon the addition of

seawater (source of Mg and Ca) to Bayer liquor the following reactions occur

(Equation 16-Equation 18).

79

Equation 16: 𝑀𝑔2+(𝑎𝑞) + 2 𝑂𝐻−(𝑎𝑞) → 𝑀𝑔(𝑂𝐻)2(𝑠)

Equation 17: 2 𝐴𝑙(𝑂𝐻)4−(𝑎𝑞) → 2 𝐴𝑙(𝑂𝐻)3(𝑠) + 2 𝑂𝐻−

Equation 18: 𝐶𝑎2+(𝑎𝑞) + 𝐶𝑂32−(𝑎𝑞) → 𝐶𝑎𝐶𝑂3(𝑠)

These reactions sufficiently reduced the hydroxide concentrations, thus the hydrotalcite like

compounds start to precipitate out as shown in Equation 19.

Equation 19:

6 𝑀𝑔2+(𝑎𝑞) + 2 𝐴𝑙(𝑂𝐻)4−(𝑎𝑞) + 8 𝑂𝐻−(𝑎𝑞) + 𝐶𝑂3

2−(𝑎𝑞) + 𝑥𝐻2𝑂

→ 𝑀𝑔6𝐴𝑙2(𝑂𝐻)16(𝐶𝑂3) · 𝑥𝐻2𝑂

The detection of amorphous phases by XRD may be consistent with the presence of Al(OH)3

species (Equation 17) as aluminates were known to be present in Bayer liquor [103]

Table 20: Phase composition of Bayer precipitates formed from seawater neutralisation

of 1-10 g/L Bayer liquor

Bayer

Precipitates

Relative Percentage

Aragonite Calcite Halite Brucite Amorphous

+ LDH

1 4.6 8.4 8.4 52.9 25.7

2 5.7 5.6 9.0 30.0 49.8

3 5.8 6.4 9.7 13.4 64.7

4 11.1 8.4 5.3 - 75.1

5 2.3 9.0 7.4 - 81.4

6 0.3 10.7 6.4 - 82.6

7 0.2 12.2 1.8 - 85.8

8 0.9 13.5 0.6 - 85.0

9 0.2 13.3 2.3 - 84.2

10 0.2 13.3 0.9 - 85.6

80

(a)

(b)

Figure 8: XRD patterns for Bayer precipitates formed at pH 9.25 (a) 1-5 g/L Al2O3 b) 6-10 g/L Al2O3

81

Due to the presence of calcium in seawater, XRD patterns for all obtained Bayer precipitates

show calcite and aragonite were formed along with hydrotalcite. An interesting trend exists for

calcite and aragonite in the XRD patterns (Figure 8), whereby calcite peaks are prevalent in

the precipitates obtained from higher Bayer liquor concentrations (6-10 g/L), while both

aragonite and calcite related peaks are present in precipitates obtained at lower concentrations.

It has been reported in literature that magnesium plays an important role in the precipitation of

calcium carbonate polymorphs [289, 290]. Calcite and aragonite formation depends upon the

Mg:Ca ratio in solution [291], with relatively high magnesium in solution inhibiting the growth

of calcite relative to aragonite. This latter conclusion was in harmony with this study wherein

it was found that for higher Bayer liquor concentrations the formation of hydrotalcite

(Equation 19) resulted in a low Mg:Ca ratio. Thus, calcite formation was favoured compared

with aragonite (Table 20). Alternatively, when hydrotalcites were lesser formed at low Bayer

liquor concentrations the relatively high Mg:Ca ratio favoured the formation of aragonite.

Infrared spectroscopy was also conducted to corroborate and enhance the findings from XRD

analysis of the Bayer precipitates (Figure 9 & Figure 10). Infrared spectra for Bayer

precipitates (1-10 g/L) were comparable to peaks ascribed to synthetic hydrotalcite [292, 293].

A summary of band assignments based on the work done by Rives and Farmer is shown in

Table 21 [205, 294].

For all the obtained precipitates, a broad profile centred at 3400 cm-1 was observed which was

mainly due to a combination of the stretching modes of hydroxyl layers in the LDH structure

and water molecules (Figure 9). Peak fitting revealed that a sharp peak at 3700-3690 cm-1 was

present for the 1 and 2 g/L precipitates which was ascribed to a Mg-OH vibration in

brucite [287, 295, 296]. This latter observation was consistent with the corresponding XRD

82

patterns (Figure 10) which also indicated the presence of magnesium hydroxide under the same

conditions. In terms of the remaining sub-bands identified in the region 3690-3500 cm-1, these

are assigned to majorly be due to vibrations of Mg-Al-OH bonds in hydrotalcite or in

Mg2Al(OH)7 [297]. As the concentration of aluminium and hydroxide increased in solution,

the formation of hydrotalcite was thermodynamically more stable [121].

Alternatively, bands in the region 3500-3300 cm-1 were attributed to a number of overlapping

OH-stretching vibrations of water, presumably originating from the metal hydroxyl layers,

intercalated water and solvated anions [205]. Peaks in the lower hydroxyl region 3200-2800

cm-1 were attributed to hydrogen bonding between water molecules and interlayer water [205,

297]. For the 4 – 10 g/L Bayer precipitate samples, a general shift of the peak positions to

higher wavenumbers in the region 3500–330 cm-1 was observed. The intensity of the general

OH/water vibrational profile additionally intensified with increasing Bayer liquor

concentration, which suggested that more water was intercalated either as “free” water and/or

as solvated intercalated anions.

83

Table 21: Infrared peaks for Bayer precipitates obtained by seawater neutralisation of 1-10 g/L Al2O3 [205, 294]

Wavenumber

range (cm-1)

Sample Peaks Vibrational

mode

Assignment

1 g/L 2 g/L 3 g/L 4 g/L 5 g/L 6 g/L 7 g/L 8 g/L 9 g/L 10 g/L

3700-3690 3699 3699 - - - - - - - - Mg-OH

stretching Brucite

3690-3500

3688

3614

3681

3641

3689

3644

3683

3638

3684

3640

3685

3647

3685

3648

3682

3646

-

3652

3658

Mg-Al-OH

stretching Hydroxyl layer

3500-3300

3527

3391

3568

3417

3570

3411

3562

3412

3565

3410

3561

3423

3561

3423

3562

3429

3568

3439

3572

3445

Mg2Al-OH

MgAl2-OH

Stretching

Hydroxyl layer

3300-3100 3251

3190

3246

3234

3238 3226 3289 3270 3269 3285 3308

O-H stretching

vibrations of

H2O

H2O coordinated to

cations in brucite

layers anions

3100-2800 - 2992 3018 3074 3003 3008 3014 3064 3000 3085 H2O-CO3

2-

bridging mode H2O-CO3

2

1660-1600 1639 1639 1641 1638 1639 1635 1636 1637 1634 1634 H2O bending Interlayer water,

H2O-CO32-

1530-1460 1485 1493 1511

1477

1532

1486

1528

1483

1522

1495

1523

1486

1484

1519

1480

1527

1493

ν3 vibrational

mode of CO32-

CaCO3 (aragonite)

84

1460-1400 1428

1434

1443

1424

1443

1420

1443

1416

1458

1434

1410

1445

1407

-

1425

1423

1457

1414

ν3 vibrational

mode of CO32- CaCO3 (calcite)

1400-1350 - - 1358 1368 1379 1385

1362

1375

1389

1365 1373

Antisymmetric

stretch of

CO32-

H2O-CO32- in

hydrotalcite

interlayer

1180-1080

1173

1133

1097

1071

1175

1125

1075

1179

1129

1082

1074

1179

1128

1081

1057

1170

1126

1109

1079

1180

1145

1114

1086

1175

1108

1175

1135

1108

1083

1174

1135

1109

1085

1172

1137

1108

ν1 vibrational

mode of CO32-

CaCO3

85

(a)

(b)

Figure 9: Infrared spectra (3800 – 2800 cm-1) for (a) 1-5 g/L (b) 6-10 g/L Al2O3 samples

86

The water deformation modes observed in the region 1660-1580 cm-1 give information about

the interlayer anions of hydrotalcites (for example bands at 1655 and

1631 cm-1 indicate the presence of sulphate and carbonate as interlayer anions respectively

[33]. For all obtained precipitates, the water deformation band was observed at around 1635

cm-1, thus suggesting that the interlayer anion remains constant, and based on the position it is

proposed to be carbonate [33]. The presence of a band between 1400-1350 cm-1 also indicates

carbonate as dominant interlayer anion in hydrotalcite [205].

The IR spectra of the carbonate antisymmetric stretching region (1500-1350 cm-1) displayed

multiple bands (Figure 10). The IR band assignments in Table 21 for the carbonate region

were attributed to calcite (1460-1400 cm-1), aragonite (1500-1460 cm-1), and carbonates in the

hydrotalcite interlayer (1400- 1350cm-1) [294]. Precipitates obtained for high Bayer liquor

concentrations (6-10 g/L) showed a sharp band in the region 1400-1350 cm-1 assigned to

intercalated carbonate anions in the hydrotalcite structure. The relative intensity of the 1400-

1350 cm-1 band increases as the Bayer liquor concentration increases, which suggests a possible

increase in the number of carbonate anions in the interlayer region. The most intense carbonate

band was observed in the region 1460-1400 cm-1 (characteristic of calcite – [120]) for 6-10 g/L

Bayer precipitates, while the band at around 1500-1460 cm-1 (characteristic of aragonite –

[120]) was present for 1-5 g/L Bayer precipitates in addition to calcite bands. These trends

support XRD results that calcite formation is favoured for precipitates formed at 6-10 g/L

Bayer liquor concentrations, while for 1-5 g/L Bayer liquor concentrations both aragonite and

calcite form.

87

(a)

(b)

Figure 10: Infrared spectra (1650 – 1000 cm-1) for (a) 1-5 g/L (b) 6-10 g/L Al2O3 samples

88

The precipitates formed by seawater neutralisation at pH 9.25 for the different Bayer liquor

concentrations were acid digested to determine their elemental composition (Table 22).

Magnesium quantities recorded in the lower Bayer liquor concentrations

(1-5 g/L) were greater than predicted from the stoichiometry for hydrotalcite material (i.e.

2.70). Therefore, a second magnesium mineralogical phase must be present, which based upon

XRD and IR analysis is most probably brucite (Mg(OH)2) [298]. In addition, the elemental

analysis of the precipitates revealed that the Mg:Al mass ratio approached 2.7:1 as the

concentration of Bayer liquor increased, which was typical for hydrotalcite [122]. Calcium in

the precipitates (Table 22) was identified by XRD as calcium carbonate species, calcite and

aragonite (Figure 8).

Table 22: Concentration of elements in precipitates obtained at pH 9.25 by seawater

neutralisation of Bayer Liquor

Bayer liquor concentration

(g/L Al2O3) Mg:Al


Mg Al Ca Na S

1 27.29 287.07 10.52 19.67 30.03 10.23

2 13.46 164.63 12.23 22.58 117.67 12.91

3 8.74 169.93 19.44 54.46 72.28 13.17

4 6.21 170.83 27.51 46.98 66.30 11.43

5 5.35 152.46 28.52 42.54 92.61 12.02

6 4.01 181.70 45.27 11.98 69.55 12.33

7 3.94 177.22 44.95 29.21 58.34 26.14

8 3.57 163.97 45.92 38.48 68.20 13.38

9 3.36 165.56 49.34 44.66 63.41 12.99

10 3.08 163.16 52.94 39.58 57.91 13.83

To provide a deeper insight into the seawater neutralisation of Bayer liquor with different

alumina compositions, thermal analysis of Bayer precipitates obtained after seawater

89

neutralisation of Bayer liquor (1-10 g/L Al2O3) at pH 9.25 is shown in

Figure 11. Focus was made on the mass loss between 200 and 500 °C associated with the de-

hydroxylation and de-carbonation of hydrotalcite (Equation 20) [299, 300], de-hydroxylation

of brucite (Equation 21) [301] and dehydroxylation of mixed metal oxides (Equation 22)

[299, 300].

Equation 20:

𝑀𝑔6𝐴𝑙2(𝑂𝐻)16𝐶𝑂3(𝑠)

→ 𝑀𝑔𝐴𝑙2𝑂(𝑠) + 5 𝑀𝑔𝑂(𝑠) + 𝐶𝑂2(𝑔) + 8 𝐻2𝑂(𝑔) + 31

2 𝑂2(𝑔)

Equation 21: 𝑀𝑔(𝑂𝐻)2 → 𝑀𝑔𝑂 + 𝐻2𝑂

Equation 22: 𝑀𝑔2𝐴𝑙(𝑂𝐻)7 → 𝑀𝑔2𝐴𝑙𝑂(𝑂𝐻)2 + 2𝐻2𝑂 + 31

2𝑂2 +

1

2𝐻2

The DTG curves of all the Bayer precipitates obtained after seawater neutralisation of Bayer

liquor (1-10 g/L Al2O3) at pH 9.25 have been peak fitted and shown in

Figure 11. The assignments of the peaks were determined from work done by

Palmer et al. [300], the bands at around 300-330 °C are assigned to the removal of weakly

bonded interlayer water, whereas the band around 335-350 °C are assigned to the initial

dehydroxylation of the brucite like layers of hydrotalcite structure in the obtained precipitates,

bands at 360-370 °C shows slight decarbonation of aragonite and the final bands at

370-385 °C are assigned to the simultaneous dehydroxylation and decarbonation of

hydrotalcite structure in the Bayer precipitates.

90

It was noted that the decomposition of the hydroxyl layers and interlayer anions undergoes

slight changes based on the Bayer liquor concentration (Figure 11). The decomposition

temperature increased from 330 to 365 °C as the Bayer liquor concentration increased. Based

on the observation of increase water and carbonate anions in the infrared spectra of these

precipitates it is thought that a more structured interlayer is formed and is the reason behind

the increased thermal stability. Bayer precipitates obtained between 1-5 g/L Bayer liquor

observed decomposition temperatures between 303 and 352 °C with mass loss of 29.29 %

(1 g/L), 24.56 % (2 g/L), 24.39 % (3 g/L) and 22.45 % (4 g/L). A mass loss between 17.6 and

16.8 % was observed for precipitates obtained from higher concentrations of Bayer liquor

(6-10 g/L), with the 10 g/L Bayer precipitates having the lowest mass loss. The increased mass

losses and lower decomposition temperatures were result of the formation of brucite (as shown

in XRD and IR spectra), which has a lower thermal stability than hydrotalcite, formation of

hydrotalcite with a more simplistic interlayer region (not as many anions intercalated) or

different composition of the brucite-like layers of hydrotalcite (Mg:Al ratio) [300].

91

(a)

(b)

Figure 11: DTG curves of the Bayer precipitates in the dehydroxylation/decarbonation region (a) 1-5 g/L (b) 6-10 g/L Bayer liquor

92

4.3 Impact of Bayer liquor composition on neutralisation efficiency

The neutralisation curves obtained by seawater addition to different Bayer liquor compositions

(1-10 g/L Al2O3) are shown in Figure 12. As a general observation, three distinct reaction

zones were evident: (1) an initial decrease from pH 13 to 12.5; (2) an inflection point between

pH 12.5 to 10; and (3) the plateauing of the pH at values less than pH 10. Also noted was the

fact that as the Bayer liquor alumina concentration increased, not only did the amount of

seawater required to reduce the pH increase but also the final solution pH was lower. For

example, for solutions with up to 3 g/L alumina the final pH value was approximately 10

whereas for Bayer liquor with 8 g/L alumina or greater the final pH was ca. 9.25. This

observation is believed to be related to the reduced concentration of carbonate for low Bayer

liquor concentrations, which in turn results in a smaller amount of calcium carbonate being

formed (buffering agent).

93

Figure 12: pH variation as a function of seawater addition to Bayer liquor

with different alumina concentrations

To gain an insight into the chemistry occurring, which may explain the pH behaviour in

Figure 12, examination of the concentrations of major ions in solution such as magnesium,

aluminium and calcium was made (Figure 13-Figure 15). The initial concentration of

magnesium in seawater was 1256 mg/L, however, upon its addition to Bayer liquor the

concentration of Mg in the supernatant remained below 100 mg/L until a pH of ca. 11, before

rapidly rising as the pH falls below 11 (Figure 13). The initial increase in magnesium

concentration corresponds well with the inflection points (approximately 11.5 for 1-5 g/L

Bayer liquor and 10.5 for 6-10 g/L Bayer liquor) of the neutralisation curves (Figure 12).

94

Figure 13: ICP-OES of remaining Mg2+ in seawater neutralised

Bayer liquor supernatants

Consideration of the behaviour of aluminium ions in solution provided an insight as to the

process occurring. Although the initial concentration of aluminium differs for each Bayer

liquor sample, a general trend was observed whereby it rapidly decreased with the addition of

seawater until depleted at around pH 12.25 (Figure 14). These trends for pH, magnesium and

aluminium indicate hydrotalcite forms between pH 12.5-9.5. XRD of the precipitates confirms

hydrotalcite was present in the neutralisation precipitates.

95

Figure 14: ICP-OES of remaining Al3+ in seawater neutralised

Bayer liquor supernatants

The calcium concentration in the supernatant appears to depend on the Bayer liquor

concentration, with deviations from the norm being observed for Bayer liquors

1-4 g/L. At lower Bayer liquor concentrations, a steady rise in solution concentration was

observed which indicated that calcium was not readily consumed in the formation of

precipitates. One rationale for this observation was that there was insufficient carbonate in the

Bayer liquor to form calcium carbonate. However, at higher Bayer liquor concentrations (5-10

g/L), calcium was being consumed as the pH declined to 10.75 before steadily rising. Based on

the observation of increased intensity of peaks related to calcium carbonate species by infrared

spectroscopy (Figure 12), it is proposed that calcium was involved in the formation of

calcite/aragonite.

96

Therefore, based on previous studies, the driving force behind the reduction of pH of Bayer

liquors was due to the formation of hydrotalcite and calcium carbonate species [121, 122, 296,

302]. It was proposed that at higher Bayer liquor concentrations the increased amount of

aluminium (Al(OH)4-), carbonate (CO3

2-) and hydroxide (OH-) present enabled a greater

amount of hydrotalcite to form; hydrotalcite contains between 12 and 20 hydroxyl units (16

units in the standard 3:1 structure - Mg6Al2(OH)16(CO3)·xH2O); thus, resulting in significant

decrease in pH when it formed. For the lower Bayer liquor concentrations (1-5 g/L) tested, the

inflection points of the neutralisation curve appeared at lesser seawater addition; requiring in

some cases half the volume of seawater before the start of the inflection point. This latter

observation was believed to be related to the reduced concentration of carbonate for low Bayer

liquor concentrations, which in turn resulted in a smaller amount of calcium carbonate being

formed. Excess calcium was indeed (Figure 17) for the lower Bayer liquor concentration

solutions (1-3 g/L), confirming that there was insufficient carbonate in the Bayer liquor to form

calcium carbonate.

97

Figure 15: ICP-OES of remaining Ca2+ in seawater neutralised Bayer liquor

supernatants

4.4 AMD Treatment with Bayer Precipitates

The various Bayer precipitates formed by addition of seawater until a final pH of 9.25 was

obtained, were added to AMD to raise solution pH and remove dissolved species (Figure 16).

Data is in Appendix 1 (Table SI 4.1-Table SI 4.10)

98

Figure 16: Neutralisation curve obtained by the addition of

Bayer precipitates to AMD

The ideal pH range for the discharge of treated AMD solutions into local water bodies should

be between 6.0 and 8.5 [281] and it was recorded that all precipitates successfully met this

latter condition. Neutralisation of AMD water was assumed to be predominately through the

release of hydroxyl ions and carbonate species due to the dissolution of brucite (present in 1-3

g/L Bayer precipitates), mixed metal hydroxide species (hydrotalcite and Mg2Al(OH)7), and

CaCO3 species present in all Bayer precipitates (Equation 23-Equation 26).

Equation 23: 𝑀𝑔(𝑂𝐻)2 + 2 𝐻+ → 𝑀𝑔2+(𝑎𝑞) + 2 𝐻2𝑂(𝑙)

99

Equation 24:

𝑀𝑔6𝐴𝑙2(𝑂𝐻)16𝐶𝑂3. 𝑥𝐻2𝑂(𝑠) + 𝐻+(𝑎𝑞)

→ 6 𝑀𝑔2+(𝑎𝑞) + 2 𝐴𝑙(𝑂𝐻)3(𝑠) + 𝐻𝐶𝑂3−(𝑎𝑞) + 𝑥𝐻2𝑂(𝑙)

Equation 25: 𝑀𝑔𝐴𝑙(𝑂𝐻)7 + 𝐻+(𝑎𝑞) → 𝑀𝑔2+(𝑎𝑞) + 𝐴𝑙(𝑂𝐻)3 + 𝑥𝐻2𝑂(𝑙)

Equation 26: 𝐶𝑎𝐶𝑂3(𝑠) + 𝐻+(𝑎𝑞) → 𝐶𝑎2+(𝑎𝑞) + 𝐻𝐶𝑂3−(𝑎𝑞)

Gibbsite formed due to the dissolution of hydrotalcite and mixed metal oxides (Equation 24

& Equation 25) can further react with acid and increase the pH as shown in Equation 27.

Equation 27: 2 𝐴𝑙(𝑂𝐻)3 + 𝐻+ → 2 𝐴𝑙3+ + 3𝐻2𝑂

Highest final pH values (9.17-9.11) were achieved by addition of precipitates obtained from

lower Bayer liquor concentrations (1-3 g/L); while the maximum pH values for (4-5 g/L Bayer

precipitates) were 8.67, and 8.10-8.03 for 6-10 g/L Bayer precipitates. The difference in final

solution pH can be described in terms of buffering phenomena.

Various authors have reported that the presence of calcium carbonate species (such as calcite)

buffer solution pH, making it difficult to increase the pH >8 as calcium carbonate dissolution

decreased with increased pH, Ca2+ and HCO3- content [303-307]. In the case of precipitates

obtained from lower (1-3 g/L) Bayer liquor concentrations, high pH is attained due to a lack of

carbonate buffering effect (lower Bayer liquor concentrations has less carbonate content). The

other reason behind the high pH attained by lower Bayer liquor concentration precipitate was

the presence of brucite (Mg(OH)2), which has a higher solubility than hydrotalcite [308]. In

100

contrast, precipitates obtained from higher Bayer liquor concentrations (6-10 g/L) had

relatively high carbonate content, and thus achieved a lower maximum pH.

To facilitate interpretation of the neutralization curves displayed in Figure 16, the change in

concentration of major dissolved species of interest (Al, Mn, Cu, Zn and Fe) as a function of

Bayer precipitate addition was also monitored (Figure 17-Figure 21). In general, the addition

of Bayer precipitates reduced the concentrations all metals to satisfy discharge limits set by

ANZECC guidelines as shown in Table 3 [280]. Increasing the pH of AMD water showed the

precipitation of metals of interest occurred in the following order: Fe, Al, Cu, Zn, and Mn. Iron

appeared to be the easiest metal being removed from solution, with the precipitation of all Fe

(below instrument detection limits <0.05 mg/L) at pH 5.75 for 1-2 g/L Bayer precipitates, pH

4.5 for 3-4 g/L Bayer precipitates and pH 4.10 for 5-10 g/L Bayer precipitates as shown in

Figure 17. In AMD waters, iron is normally present in the ferric (Fe3+) state due to oxygenation

by turbulence. The precipitation of iron hydroxides compound at pH>3.5 has been reported

[309] and as shown in Equation 23-Equation 26, dissolution of different alkaline species

present in Bayer precipitates released hydroxide ions and thus facilitates the removal of iron

by hydroxide formation. Rahman et al. [308] additionally reported the formation of Fe-Al LDH

after the reaction between Fe and Mg-Al LDH.

101

Figure 17: Variation of iron concentration in AMD as a function of solution pH when

1-10 g/L Bayer precipitates were added

The removal of aluminium from AMD water after treatment with 1-10 g/L Bayer precipitates

is shown in Figure 18. For all Bayer precipitates, the aluminium precipitation starts at pH >4

and its concentration fell below water quality discharge limits (Table 3) at pH ≥6.5. Above pH

5.5, aluminium begins to precipitate out as Al(OH)3 [80]. The aluminium concentration

remained constant below pH 4, thus indicating aluminium did not simultaneously precipitate

out with iron. A slight increase in aluminium concentrations was observed above pH 8, due to

the formation of aluminate ions [80], however, the aluminium concentraton remained below

the ANZECC guidelines [280].

102

Figure 18: Variation of aluminium concentration in AMD as a function of solution pH

when 1-10 g/L Bayer precipitates were added

The removal data for Cu and Zn with the addition of obtained Bayer precipitates

(1-10 g/L) is shown in Figure 19. It has been observed that Cu and Zn concentrations begin to

decreaese at pH ≥4 and thus indicating that their simultaneous precipitation occur with

aluminium. However, complete Cu and Zn removal occurs at pH >8 for 1-4 g/L Bayer

precipitates, pH >7.5 for 6-7 g/L Bayer precipitates c and at pH >5.9 for 8-10 g/L Bayer

precipitates. The treatment of acidic Cu and Zn solution with Mg-Al LDH has reported the

removal of Cu from solution by hydroxide precipitation [246, 308], and another study reported

the removal of Cu and Zn by isomorphic substitution with magnesium in LDH structures [266].

103

(a)

(b)

Figure 19: Variation of concentration of dissolved components in AMD as a function of

solution pH when Bayer precipitates were added

(a) Copper (b) Zinc

104

The removal of manganese from AMD water can be problematic due to the high pH (greater

than 9) required to produce manganese precipitates [80]. Mn begins to precipitate out as

hydroxides at pH between 9 and 9.5 [80] with complete removal of Mn requiring a pH of 10.5

[310]. Issues with raising the pH this high relate to concerns about failure to comply with

discharge water pH requirements and the dissolution of aluminium hydroxide. The removal of

manganese from AMD water with the addition of 1-10 g/L Bayer precipiates is shown in

Figure 20. The removal of manganese begins at pH >7.5 for 1-3 g/L Bayer precipitates, at pH

>6.5 for 4-7 g/L Bayer precipitates and at pH >5.8 for 8-10 g/L Bayer precipitates. Bayer

precipitates (1-3 g/L) containing brucite were able to achieve a pH >9, and thus were able to

achieve complete removal of Mn. Due to the buffering effect of calcium carbonate in 4-10 g/L

Bayer precipitates a maximum pH of 8.5 was achieved, which was insuffecient for the complete

removal of Mn.

Figure 20: Variation of manganese concentration in AMD as a function of solution pH

when 1-10 g/L Bayer precipitates were added

105

It was noted that treatment of AMD with 1-10 g/L Bayer precipitates caused an increase in

magnesium and calcium concentrations in the treated water (Figure 21). Increase in calcium

concentrations was believed to be due to the dissolution of calcium carbonate species (calcite

and aragonite) as shown in Equation 26, present in precipitates obtained after seawater

neutralisation of Bayer liquor [311]. The observed rise in Mg concentration in treated water

was ascribed to the dissociation of mixed metal hydroxide species (hydrotalcite and

Mg2Al(OH)7) and brucite ((Mg(OH)2) present in Bayer precipitates obtained from 1-3 g/L

Bayer liquor ) [298]. Calcium and magnesium are considered as essential elements for animal

nutrition. According to ANZECC guidelines, livestock can tolerate 1000 mg/L of calcium in

their drinking water; however, there is insufficient information available to set acceptable limits

for magnesium in livestock drinking water [280]. However, relatively high concentrations of

magnesium can cause water hardness and thus result in problems associated with scaling of

equipment. For AMD waters treated with 1-10 g/L Bayer precipitates the concentration of

calcium after treatment was below the concentration limit set by ANZECC guidelines (i.e. 1000

mg/L) [280].

106

(a)

(b)

Figure 21: Variation of concentration of dissolved components in AMD as a function of

solution pH when Bayer precipitates were added

(a) Magnesium (b) Calcium

107

4.5 Conclusion

The potential for application of seawater neutralised supernatant liquor waste from the alumina

refining industry to neutralize AMD has been demonstrated in this chapter. Furthermore, the

hypothesis that the abundance of reactive species in the Bayer liquor and seawater can impact

the quality of the precipitates formed has been evaluated.

Bayer liquor constitution was found to influence the composition of the precipitates formed.

The major materials formed were hydrotalcite (Mg6Al2(OH)16CO3.xH2O), calcite (CaCO3),

aragonite (CaCO3), mixed metal hydroxides (Mg2Al(OH)7) and halite (NaCl). Brucite

(Mg(OH)2) was present for lower Bayer liquor concentrations (1-3 g/L Al2O3) under conditions

wherein the concentration of aluminium and hydroxyl species was unable to produce solely

hydrotalcite. The ratio of calcite to aragonite formed was related to the Mg:Ca ratio with calcite

dominating at high alumina levels and aragonite relatively more prevalent at low alumina

values. Amorphous aluminium hydroxide may also be present in the precipitate.

In turn, the neutralizing capacity of the Bayer precipitates also varied with Bayer liquor

composition. More seawater was required for neutralization of the Bayer liquor as the alumina

concentration increased. Concomitantly, the final solution pH obtained was reduced as the

alumina concentration increased. The degree of buffering capacity of the Bayer liquor was

proposed to be a key aspect controlling this latter behavior. Examination of the concentration

of individual ions as a function of solution pH supported the idea that hydrotalcite was formed

in addition to lesser materials such as calcium carbonate and brucite.

The presence of hydroxide and carbonate species in the hydrotalcite material and the existence

of calcium carbonate species in the Bayer precipitates, was demonstrated to be amenable for

108

the treatment of AMD. All Bayer precipitates evaluated, successfully neutralised AMD. In

addition to neutralising the acidity of AMD, all Bayer precipitates investigated, decreased the

concentration of Fe, Al, Cu and Zn to within acceptable discharge limits (ANZECC). The

removal of Mn to within acceptable limits could only be obtained using the 1-3 g/L Bayer

precipitates due to their ability to attain a higher pH. However, an increase in Al was observed

once a pH of 8.5 was obtained; ANZECC limits for pH were no longer met.

The results of this study showed that Bayer precipitates had potential as new materials in the

treatment of AMD samples. Further investigations are required to explore the influence of

AMD composition upon the effectiveness of Bayer precipitates and also to determine if the

precipitates can be modified to further enhance performance. Particular emphasis should be

placed upon a means for removing dissolved manganese species without solubilizing

aluminium.

109

Chapter 5: Assessing the effectiveness of Bauxite

refinery residues with conventionally used alkali






110

5.1 Introduction

AMD is a challenging environmental problem created by numerous mining operations due to

its low pH and high metal and sulphate content [13, 35]. The toxic characteristics of AMD can

permanently damage surrounding ecosystems, thus suitable management and treatment

methods to remediate affected water bodies are required [14]. For example, in January 2013,

AMD from the open pit at Mount Morgan, overflowed into the Dee river and thus decreased

the pH and increased the heavy metal content; ultimately causing the death of aquatic animals

and birds [10]. Active treatment methods of AMD water typically involve alkali addition in

order to raise the pH to between 6 and 9 [298]. In this latter pH range the concentrations of

dissolved metals generally decreases due to the formation of insoluble metal hydroxides and

oxyhydroxides [78]. Various alkalis like lime (CaO), limestone (CaCO3), sodium hydroxide

(NaOH), and sodium carbonate (Na2CO3) have been used to modify pH and remove heavy

metals from AMD water as precipitates [90].

Lime is arguably the most widely used alkali applied to remediate AMD solutions [84], mainly

due to its relatively low cost, availability and simplicity of treatment plant [312]. Lime

neutralisation is currently being used at Mount Morgan to control the volume of AMD in the

open pit to avoid overflow events [93]. A disadvantage of the process is the voluminous sludge

that is produced; sludge typically settles slowly to 10 % of the volume of water treated albeit

as much as 50 % sludge volume has been observed [312]. The combination of slow settling

rate, low sludge density, and excessive volumes of sludge formed from the neutralisation

process can result in a costly process.

111

It has been reported that AMD solutions are inherently composed of many dissolved

components [12] and thus the optimal alkali addition strategy may not be the same for all AMD

types. Additionally, the question arises as to the potential benefits of employing alternate

alkaline solutions or materials. An innovative means of treating AMD solutions may involve

the application of alkaline solutions or materials produced as waste products from other major

industries. As discussed previously, Bayer precipitates formed by the seawater neutralisation

of Bayer liquor have the potential to treat AMD water. Due to the causticity of these waste

materials, they may prove an interesting alternative to the traditional application of lime for

treating AMD waters [298]. The fact that a region such as Queensland contains not only a

substantial bauxite refining industry [313] but also numerous AMD problems generated by the

mining industry [314], makes this outlined approach attractive.

Therefore, the aim of this study was to compare the performance of Bayer liquor and Bayer

precipitates, with respect to material requirements and discharge water quality, with

commercially available alkali commonly used in the treatment of AMD water. The

fundamental hypothesis was that waste alkaline materials may provide both performance and

economic benefits in relation to AMD treatment. The research questions addressed were: (1)

can the waste alkali materials raise the pH to the required levels to meet water discharge limits;

(2) is it possible to reduce dissolved metal concentrations to satisfy regulations; and (3) what

the scientific explanation for differences in performance for the various alkali’s is. ANZECC

guidelines were used to determine the required discharge water quality values for this study

[280]. Bench scale tests were conducted using AMD water collected from the open pit at the

abandoned Mount Morgan mine in Queensland, Australia.

112

5.2 Characterisation of Bayer Precipitate

The XRD pattern of Bayer precipitate employed in this study (Figure 22) revealed that the

material was composed of a number of mineralogical phases including: 1) hydrotalcite

(Mg6Al2(OH)16(CO3)∙4H2O); 2) calcite (CaCO3 – rhombohedral); 3) aragonite (CaCO3 –

orthorhombic); and 4) magnesium hydroxide (Mg(OH)2). These detected materials were

consistent with phases previously observed in precipitates formed from the seawater

neutralisation of Bayer liquor [27, 302, 315]. However, it was noted that brucite (Mg(OH)2) is

normally only observed when magnesium is more prevalent than aluminium at pH values

greater than 10 [298]. The broadness of the d003 and d006 peaks (approximately 12 and 25° 2θ)

was characteristic of hydrotalcite and indicated that this material had a poorly crystalline

structure probably due to hydroxide layers being partially askew [316]. The Bayer hydrotalcite

formed had a d-spacing of 7.8 Å, typical of a carbonate hydrotalcite material [258]. Based on

the mineralogical composition of Bayer precipitate, species involved in the neutralisation of

AMD would be majorly hydrotalcite and calcium carbonate.

5.3 AMD Characteristics

Open pit AMD water contained relatively high concentrations of dissolved metals and

displayed a characteristic low pH value of 3.74 (Table 10). Compared to the values observed

by Bosman [312], Mt Morgan AMD water had a high sulphate concentration

(17,430 mg/L), low iron concentration (16.7 mg/L), and significantly higher concentrations of

aluminium (1233 mg/L) and magnesium (2265 mg/L). Based on the geological study

performed by Taube [317] at Mt Morgan, the primary sources of aluminium and magnesium

were probably from feldspar and dolomite. A previous study of Mt Morgan AMD water from

the open pit in 2002, also showed major differences to the water composition in this study; an

113

increase in pH (2.7 to 3.74), SO4 (13600 to 17430 mg/L), Al (780 to 1233 mg/L), Mg (1280 to

2265 mg/L), Cu (44.54 to 77.26 mg/L), Mn (71.28 to 161.5 mg/L) and Zn (21.97 to 48.89

mg/L), and reductions in Fe (253 to 16.7 mg/L) and Na (830 to 648 mg/L). The reduction in

iron concentration from 2002 to 2014, was believed to be due to the precipitation of Fe(OH)3,

which occurs at pH values above 3.5 in oxygenated waters caused by turbulence [318]. A

sediment sample taken from the open pit also showed the presence of jarosite

(KFe+33(OH)6(SO4)2).

114

Figure 22: XRD pattern of Bayer precipitate formed by the seawater neutralisation of

Bayer liquor

The rise in metal concentrations (Cu, Mn and Zn) in open pit water was probably a by-product

of the lime dosing plant built in 2006 to reduce water volumes in the open pit [319]. This plant

treats the AMD water by raising the pH to 7.5 thus precipitating the heavy metals present before

discharging the metal free water into the Dee River. The heavy metal rich sludge is transferred

to an adjacent tailings beach [320]. It is therefore proposed, the decreasing water volumes and

leaching of metals from the metal rich sludge has caused an increase in metals in the AMD

115

water over time. The possibility for release of metals from the sludge when exposed to the

open pit AMD water has been confirmed in this study (Section 5.6).

5.4 AMD neutralisation using various alkalis

Treatment efficiency of AMD by different alkaline materials depends upon various factors such

as concentration and oxidation state of metals, pH of AMD water and hydrolysis reactions that

occur upon addition of alkali to AMD [85]. Figure 23 shows the pH behaviour as a function

of the different alkali species of interest to the AMD water. Data is in Appendix 2 (Table SI

5.1-Table SI 5.5). Neutralisation was assumed to be predominately through the buffering

capacity of hydroxyl and carbonate groups in alkali [298]. It was evident that increasing the

pH of AMD water consumed a substantial amount of alkalinity agent, for example, to achieve

a pH between 6 and 7 for 25 mL of AMD water, 1.6 mmol lime, 4.7 mmol sodium hydroxide,

1.7 mmol of sodium carbonate, 2.5 mL of 10 g/L Bayer liquor and 0.5025 g of Bayer precipitate

were required. The dissolution of alkali increased the pH and promoted the formation of

insoluble metal hydroxide and carbonate precipitates that could potentially be removed by a

solid-liquid separation process [37].

Table 23 provides a snapshot of the concentrations of Al, Mg, Mn, Cu, Zn, Si, Fe and Ni at 3

stages during treatment as a function of solution pH (metal concentrations at approximately pH

3.5, 6.5 and 9). The full range of pH values tested (Figure 23), corresponding AMD water

compositions, and curves related to the removal of metals for each alkali at a particular pH is

shown in Figure 24. In order to evaluate the effectiveness of various alkalis for treatment in

removing metals from AMD, the metal ion removal results (Table 23) were compared with

Australian and New Zealand guidelines (Table 8) [280]. Increasing the pH of AMD water

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showed the precipitation of metal hydroxide and carbonate precipitates occurred in the

following order: Fe, Al, Cu, Zn, Ni and then Mn. It was possible that iron and manganese

could be selectively precipitated out of solution, however for the remaining metals, overlap in

the precipitation pH range existed.

Figure 23: Neutralisation curves for different alkaline materials as indicated

117

Table 23: Mount Morgan mine pit water treatment using various alkaline materials and

their metal removal capacity in mg/L

Alkali pH Amount

added


Al Mn Cu Zn Fe Ni

Lime

3.72 0.0 mmol 1233 161.5 77.26 48.89 16.70 1.54

6.75 1.6 mmol 0.53 85.40 0.11 0.75 0.05 <0.05

9.19 3.3 mmol 0.60 0.45 <0.05 <0.05 <0.05 <0.05

Percentage removed (%) 99.9 99.7 99.9 99.9 99.7 96.8

Sodium

hydroxide

3.71 0.0 mmol 1233 161.5 77.26 48.89 16.70 1.54

6.80 3.12 mmol 0.11 136.8 0.757 10.67 <0.05 <0.05

9.46 7.1 mmol 3.46 <0.05 2.59 <0.05 <0.05 <0.05


Sodium

carbonate

3.72 0.0 mmol 1233 161.5 77.26 48.89 16.70 1.54

6.26 1.7 mmol 2.97 131.60 3.53 21.34 <0.05 1.37

9.15 4.9 mmol 1.95 6.80 3.24 <0.05 <0.05 <0.05


Bayer liquor

3.69 0.0 mL 1233 161.5 77.26 48.89 16.70 1.54

6.49 2.5 mL 2.28 111.70 2.99 23.80 <0.05 1.07

8.95 5.0 mL 4.77 3.78 0.55 <0.05 <0.05 <0.05


Bayer

precipitates

3.75 0.0000 g 1233 161.5 77.26 48.89 16.70 1.54

6.05 0.5025 g 2.65 67.20 <0.05 <0.05 <0.05 <0.05

8.00 1.5000 g 0.91 27.74 <0.05 <0.05 <0.05 <0.05

Percentage Removed (%) 99.9 82.82 99.9 99.9 99.7 96.8

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5.4.1 Iron removal

Iron species in AMD waters are normally in the ferric (Fe3+) state due to oxygenation by

turbulence. However, deeper water that has not been disturbed can have iron in the ferrous

(Fe2+) state [318]. The water collected in this study was from a depth of only 3 m (with the

deepest section of the open pit being 40 m) in the Mt Morgan open pit and it is therefore

reasonable to assume that the majority of the 16.7 mg/L iron existed as Fe3+. As the pH of the

AMD water was gradually increased to in excess of 6, iron precipitated to the point it was

below detection limits.

Lime

Sodium hydroxide

119

Figure 24: Trends for Al, Mg, Mn, Cu, Zn, Si, Fe and Ni removal from AMD solution

when using different alkaline materials

The removal of iron was relatively rapid with a sharp decline in iron concentration in the pH

range 3 and 4, observed for all alkalis investigated (Figure 24). For all alkali tested, 99.7 % of

Sodium carbonate Bayer liquor

Bayer precipitates

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iron was removed from the open pit AMD waters; thus meeting water quality guidelines

outlined by ANZECC (Table 3) [280]. Iron in the ferric state should readily precipitate as

oxyhydroxide compounds (FeO(OH)) as shown in Equation 28 at pH values greater

than 3.5 [309].

Equation 28: 𝐹𝑒(𝑎𝑞)3+ + 2 𝑂𝐻(𝑎𝑞)

− → 𝐹𝑒𝑂𝑂𝐻(𝑠) + 𝐻(𝑎𝑞)+

Formation of oxyhydroxides of Fe tends to effect the mobility of other metals like Mn, Ni, As

and Mo through sorption or co-precipitation [78, 321]. At pH 8, most of the Mn can be

removed if the Fe concentration is more than four times the concentration of Mn in water [84].

However, Fe concentration in open pit water was too low (16.7 mg/L) and thus there was not

enough iron present to promote the co-precipitation of Mn, Ni, As and Mo (Table 10).

5.4.2 Aluminium removal

Aluminium was the second metal found to precipitate out of solution within a pH range 4 to 7

(Figure 24). The average initial concentration of aluminium in the open pit AMD water was

1212 mg/L. Treatment results for all alkali materials showed maximum removal of aluminium

between pH 7 and 8, with a slight increase in aluminium concentration at pH values above 8.5.

At pH 5.5, Al begins to precipitate as Al(OH)3 (Equation 29), however this species can

redissolve when the pH is increased to 8.5 as aluminate ions (Al(OH)4-) as shown in

Equation 30 [80].

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Equation 29: 𝐴𝑙(𝑎𝑞)3+ + 3 𝑂𝐻(𝑎𝑞)

− → 𝐴𝑙(𝑂𝐻)3 (𝑠)

Equation 30: 𝐴𝑙(𝑂𝐻)3 (𝑠) + 𝑂𝐻(𝑎𝑞)− → [𝐴𝑙(𝑂𝐻)4](𝑎𝑞)

−

Current acceptable release limits of aluminium into waterways from industry have been

reported to be 5 mg/L [280]. All alkalis achieved a water discharge quality (aluminium removal

percentage of greater than 99.5 %) with acceptable aluminium concentrations in the pH range

6.5 to 8.5 based on ANZECC guidelines (Table 3) [280], with lime showing a significantly

greater retention of aluminium in the solid phase at pH values above 9. This is proposed to be

due to the formation of calcium aluminium hydroxide co-precipitates based on the work by

Packter and Khaw [322], who showed precipitation of this latter mineral with increasing

calcium concentrations in mixed cation solutions.

5.4.3 Copper removal

Based on the Eh-pH curves of a Cu-S-H2O system at pH 3.5 to 4, it is believed Cu2+ ions are

present as the dominant species [323]. The average initial concentration of Cu in the open pit

AMD water was 79.32 mg/L, which was substantially above the ANZECC water quality

guidelines for Cu (<5 mg/L) [280]. However, the results in Table 23 showed that by increasing

the pH above 4 (Figure 24), greater than 95 % removal of Cu for all alkali materials was

achieved; which resulted in concentrations less than those stipulated by ANZECC guidelines

(Table 3).

In the case of lime, Bayer liquor, and Bayer precipitates, the concentration of Cu fell below

instrumental detection limits of 0.05 mg/L (Table 23). Baltpurvins et al. [324] conducted a

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study on the solubility domains of copper hydroxide precipitate using lime, and found that

multiple phases formed depending on the Cu2+:SO42-:OH- ratio, temperature, and reaction time.

The precipitation of copper hydroxides from sulphate rich solutions typically results in the

formation of brochantite (Cu4SO4(OH)6) as shown in Equation 31 and tenorite (CuO)

(Equation 32), as other phases such as posnjakite [Cu4SO4(OH)6.H2O] and spertiniite

(Cu(OH)2) are precursors to these more thermodynamically preferred minerals, respectively

[325]. Baltpurvins et al. [324] further found that gypsum (CaSO4.2H2O - most dominant

species precipitated during lime neutralisation) had little influence on the formation of

brochantite and tenorite.

Equation 31: 𝐶𝑢(𝑎𝑞)2+ + 𝑆𝑂4 (𝑎𝑞)

2− + 6 𝑂𝐻(𝑎𝑞)− ↔ 𝐶𝑢(𝑆𝑂4)(𝑂𝐻)6 (𝑠)

Equation 32: 𝐶𝑢𝑆𝑂4 (𝑎𝑞) + 2 𝑂𝐻(𝑎𝑞)− ↔ 𝐶𝑢𝑂(𝑠) + 𝑆𝑂4 (𝑎𝑞)

2− + 𝐻2𝑂

5.4.4 Zinc removal

The average initial concentration of Zn in the AMD water was 48.62 mg/L, which was in excess

of ANZECC guidelines (Table 3) for discharge [280]. In aerobic natural waters, Zn can form

various complexes with numerous ions (carbonate and hydroxyls) present in water [326].

Below pH 7, Zn exists as Zn2+ and in the pH range 7 to 8.2 ionic Zn gets converted to a

hydroxycarbonate precipitate termed hydrozincite (Zn5(CO3)2(OH)6) (Equation 33). In

sulphate rich environments, Zn forms ZnSO4 (Equation 34) at pH <7 [79]. Therefore, the

>95 % removal of Zn in this study is proposed to be due to the formation of ZnSO4 which gets

converted to hydrozincite at higher pH. The treatment of Mt Morgan AMD water with lime,

sodium carbonate, Bayer liquor and Bayer precipitates reduced the concentration of Zn below

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the instrumental detection limit of 0.05 mg/L (Table 23) and ANZECC guidelines (Table 3)

[280].

Equation 33: 5𝑍𝑛(𝑎𝑞)2+ + 2 𝐶𝑂3 (𝑎𝑞)

2− + 6 𝑂𝐻(𝑎𝑞)− → 𝑍𝑛5(𝐶𝑂3)2(𝑂𝐻)6 (𝑠)

Equation 34: 𝑍𝑛(𝑎𝑞)2+ + 𝑆𝑂4 (𝑎𝑞)

2− → 𝑍𝑛𝑆𝑂4 (𝑠)

Lime and Bayer precipitate were discovered to be particularly effective at removing Zn from

AMD solution at a lower pH range (pH 6 to 7) than the other alkaline materials (pH 8.5 to 9.5)

(Table 23). The calcium content in lime and Bayer precipitates (as CaCO3) resulted in an

increase in the amount of gypsum that formed during the neutralisation of AMD water (XRD

in section 3.4.2). A study by Huang et al. [327] found that the formation of gypsum flocs from

a sulphate wastewater and lime had a Zn uptake capacity of 0.06 mg/g at pH 6 and 0.10 mg/g

at pH 7. Therefore, a similar adsorption and encapsulation process is proposed to be occurring

in the neutralisation of Mt Morgan AMD water for lime and Bayer precipitates.

5.4.5 Nickel removal

The average initial concentration of Ni in the open pit AMD water was only 1.49 mg/L, which

was considerably less than other metal species present; nevertheless, according to ANZECC

guidelines discharge requires concentration levels <0.2 mg/L (Table 3) [280]. In aqueous

systems, Ni is present as ionic nickel below pH 6.6, while between 6.6 and 8.2 ionic Ni changes

to species such as NiCO3 (Equation 35) or NiSO4 (Equation 36) depending upon the sulphate

concentration in solution [79]. Treatment of AMD water with all alkali materials showed the

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removal of Ni to below instrumental detection limits <0.05 mg/L at pH values around 8. In the

case of lime, sodium hydroxide and Bayer precipitate, acceptable discharge limits were

achieved at pH values greater than 6. Based on the work by Olds et al. [328], increased removal

of Ni at lower pH values for lime, sodium hydroxide and Bayer precipitate may be due to the

greater surface area of precipitates that form when using these alkali materials (See section

3.4.3 for more details).

Equation 35: 𝑁𝑖(𝑎𝑞)2+ + 𝐶𝑂3 (𝑎𝑞)

2− → 𝑁𝑖𝐶𝑂3 (𝑠)

Equation 36: 𝑁𝑖(𝑎𝑞)2+ + 𝑆𝑂4 (𝑎𝑞)

2− → 𝑁𝑖𝑆𝑂4 (𝑠)

5.4.6 Manganese removal

The removal of manganese from complex water matrices such as AMD water using chemical

precipitation can be problematic due to the highly alkaline conditions (greater than 9) required

to produce manganese precipitates [80]. Issues with raising the pH this high relate to concerns

about excessive chemical usage, failure to comply with discharge water pH requirements and

the dissolution of aluminium hydroxide. Theoretically, Mn can exist in numerous oxidation

states, but in natural waters only Mn2+ and Mn4+ are relatively stable [329]. Depending upon

pH conditions manganese can form oxides, hydroxides or carbonate compounds [330],

however between pH 9 and 9.5 Mn is precipitated as hydroxides as shown in Equation 37 [80]

with complete removal of Mn requiring a pH of 10.5 [310].

Equation 37: 𝑀𝑛(𝑎𝑞)2+ + 2𝑂𝐻(𝑎𝑞)

− → 𝑀𝑛(𝑂𝐻)2 (𝑠)

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The average initial concentration of manganese in the AMD water was 162 mg/L, which was

above acceptable release limits. Sodium hydroxide decreased the concentration of Mn to

acceptable levels (<0.2 mg/L) at pH 9.46, however slight increases in aluminium concentration

were observed (Table 23 shows aluminium concentration increased to 3.46 mg/L at pH 9.46

from 0.82 mg/L at pH 8.10). Bayer precipitates were able to remove 57 % of manganese

(67.2 mg/L remaining in solution) at pH 6.05 compared to 45.6 % removal at pH 6.75 for lime

(next best performing alkali). This result clearly showed that Bayer precipitates perform better

at the removal of manganese at lower pH values than the more conventionally used lime. It is

proposed that the incorporation of manganese in reformed hydrotalcite (substitution of Mg with

Mn in the structure) enabled the removal of manganese at lower pH than the other alkalis. A

more in-depth study (outside the scope of the current work) will be required to confirm this

proposed mechanism, in particular, the reformation of hydrotalcite at pH 6-7 after dissolution

in acid and the degree of Mn inclusion in the structure.

5.5 Precipitate Analysis

5.5.1 Elemental composition

The precipitates obtained after the treatment of AMD water with different alkalis to a pH

between 6.5 and 7.5 were analysed using ICP–OES to determine their elemental composition

(Table 24). The Bayer precipitate had the highest concentration of magnesium (105.13 mg/g)

compared to the other alkaline materials wherein Mg ranged from 16.68 to 34.70 mg/g. This

was probably due to magnesium content present in Bayer precipitates in the form of

hydrotalcite (Mg6Al2(OH)16(CO3)·4H2O). In acidic conditions, hydrotalcite in Bayer

precipitate dissociates [272, 298] and thus releases hydroxyl units required for the

neutralisation of AMD waters. Magnesium in the precipitates formed after neutralisation, was

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most likely some form of magnesium hydroxide species or even could be due to the reformation

of the hydrotalcite structure [140]. All precipitates had very high concentrations of Al, Fe, Cu,

Zn, Si and Mn as shown in Table 24. The concentration of aluminium in lime (59.75 mg/g)

and Bayer precipitates (96.87 mg/g) was significantly lower than sodium hydroxide (122.57

mg/g), sodium carbonate (157.02 mg/g) and Bayer liquor (133.94 mg/g). In the absence of

excess Ca, gibbsite (Al(OH)3) was the primary phase found for sodium hydroxide, sodium

carbonate and Bayer liquor precipitates (Section 5.5.2).

Table 24: Metal concentrations (mg/g) in precipitates between pH 6.5 and 7.5

Alkali pH Mg Ca Al Mn Cu Zn Si Fe

Lime 6.75 16.68 141.2 59.75 4.05 3.89 2.46 2.21 0.87

Sodium hydroxide 6.70 24.57 5.78 122.6 3.24 7.95 3.86 3.30 0.81

Sodium carbonate 7.37 27.11 31.94 157.0 6.18 10.11 6.16 4.53 1.95

Bayer liquor 6.49 34.70 10.35 133.9 4.46 6.42 3.90 3.90 1.27

Bayer precipitates 7.57 105.1 44.41 96.87 3.33 2.76 1.80 2.56 0.86

5.5.2 X-ray diffraction

The addition of lime to Mt Morgan AMD water resulted in gypsum (CaSO4.2H2O – reference

98-002-7221) being the primary phase formed (Figure 25). Gypsum was also a dominant phase

formed with the addition of Bayer precipitate to the Mt Morgan AMD water, however calcite

(CaCO3) and hydrotalcite were also detected in the precipitates formed. The presence of

hydrotalcite in the precipitate showed that hydrotalcites can reform after they undergo

dissolution in AMD water. These assignments were supported by the infrared spectra of the

precipitates (dominated by sulphate bands between 1200 and 900 cm-1 and 700 and 500 cm-1 –

Appendix 2, Figure SI 5.1), as well as by high concentrations of Mg, Ca and Al found in the

acid digested precipitates (Table 24).

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The remaining alkaline materials used to neutralise the AMD water (Bayer liquor, sodium

hydroxide and sodium carbonate), produced precipitates with similar XRD patterns that

primarily consisted of amorphous material and gypsum. Gypsum forms for AMD water treated

with sodium carbonate and sodium hydroxide due to the presence of 460 mg/L of calcium in

the untreated AMD water (Table 15). Based on the location of the broad band in Figure 25,

with respect to the gibbsite reference pattern 98-041-3987, water chemistry and simulations

performed using caustic additions using AqMB software (Table 25) it is proposed that the

amorphous phase was primarily gibbsite (Al(OH)3).

Table 25: Possible phases precipitated at pH 7.5 using lime and sodium hydroxide based

on AqMB simulations

Mineral phase Chemical Formula Lime

(mol/hr)

Sodium

hydroxide

(mol/hr)

Birnessite Mn7O13 1.00 0.49

Copper hydroxide Cu(OH)2 16.96 24.47

Copper carbonate CuCO3 12.66 5.67

Ferrihydrite Fe2O3.0.5H2O 3.71 3.76

Gibbsite Al(OH)3 812.1 822.1

Gypsum CaSO4.2H2O 2052 77.26

Hydrotalcite-CO3 Mg6Al2(OH)16(CO3)·4H2O 93.6 94.36

Muscovite KAl3Si3O10(OH)1.8 F0.2 3.87 0

Paragonite NaAl3Si3O10(OH)2 0 4.34

Zinc carbonate ZnCO3.H2O 15.29 15.81

AqMB simulations were also able to provide more details regarding the mineralogical phases

which were difficult to characterise by XRD (due to the presence of highly crystalline gypsum

which exhibited intense reflections which obscured other peaks) (Table 25). Simulations were

128

run for lime and sodium hydroxide, with AMD water dosed to pH 7.5 and allowed to react for

2 h prior to solid-liquid separation. Gypsum was the primary phase formed by the addition of

lime, at a rate of 2052 mol/hr, followed by gibbsite (812 mol/hr) and hydrotalcite (93.6 mol/hr).

This was an interesting finding as the XRD patterns of the lime precipitates were unable to

identify the formation of gibbsite. The addition of caustic in the simulation found gibbsite to

be the primary phase formed, at a rate of 812 mol/hr, followed by hydrotalcite (194 mol/hr)

and gypsum (77.26 mol/hr). These results support the details in the XRD pattern shown in

Figure 25. Based on the concentrations of sulphate in the feed (18630 mg/L) and resultant

thickener overflow in AqMB, the addition of lime results in a 42 % reduction of sulphate in the

discharge water (10780 mg/L) compared to a 1.6 % reduction for caustic (18310 mg/L).

Interestingly, different aluminosilicates were predicted by AqMB simulations to precipitate

based on the alkaline material used; in the case of lime, muscovite formed (KAl3Si3O10(OH)1.8

F0.2), while for caustic solutions paragonite formed (NaAl3Si3O10(OH)2). This latter situation

was consistent with the dependency of the formation of paragonite and muscovite on the ratio

of sodium (Na) and Potassium (K) [331].

129

Figure 25: XRD pattern of precipitates obtained from treatment of AMD with different

alkaline materials

Metal hydroxide phases could not be detected by XRD; however, heavy metals were present

in the precipitates based on ICP-OES for the acid digested precipitate samples. The AqMB

simulations predicted that the main metal phases forming were copper hydroxide and

carbonate, zinc carbonate, and birnessite (Table 25). In the presence of lime, Cu was

precipitated out in relatively similar amounts of hydroxide (16.96 mol/hr) and carbonate

(12.66 mol/hr), while the use of caustic in the neutralisation process favoured the formation of

130

copper hydroxide (24.47 mol/hr). Both lime and caustic removed similar quantities of Zn

(15.29 and 15.81 mol/hr, respectively), while lime precipitated out approximately double the

amount of manganese than caustic at the same pH (1.00 compared to 0.49 mol/hr). These

simulation results appeared to reflect the trends observed in the actual treatment of the AMD

water at pH 7.5 using lime and sodium hydroxide (Figure 24).

The most effective alkali materials for the removal of metals were found to be lime and Bayer

precipitates; high gypsum generating precipitates after the neutralisation of AMD water.

Huang et al. [327] studied the removal of Ni, Cu and Zn from a sulphate wastewater using

gypsum and determined the removal mechanism to be a combination of adsorption on gypsum

colloids followed by encapsulation during the subsequent precipitation processes. An increase

in metal removal was found when there was a high Ca2+:SO42- ratio, solution pH, ionic strength

and surface area [327]. It was noted, that the addition of gypsum had a negligible adsorption

of heavy metals, but rather it was the formation of high surface area gypsum flocs from metal

rich sulphate wastewaters that allowed for their removal [327]. Therefore, the increased

removal efficiency of heavy metals for lime and Bayer precipitates was postulated as due to

gypsum being the major mineralogical phase formed.

5.5.3 Particle size analysis

Sorption of metals on aluminium/iron hydroxide precipitates has been reported to be related to

their surface area [332], while the uptake of Ni, Cu and Zn from sulphate wastewaters has been

linked to the surface area of gypsum flocs [327]. Magnesium hydroxides have also been found

to show metal uptake capacity through adsorption onto the surface of Mg(OH)2 particles [333].

As the concentration of iron in Mt Morgan AMD water was only 16.7 mg/L, while the

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magnesium, aluminium and calcium concentrations were 2265, 1233 and 534 mg/L,

respectively, it is proposed that metal uptake was through adsorption on different phases of

magnesium and aluminium hydroxide (more prevalent in Bayer precipitates compared to lime),

and gypsum (to a much greater extent for lime). The dissolution of hydrotalcite in Bayer

precipitates, increases the amount of aluminium and magnesium available to form Mg(OH)2

and Al(OH)3 precipitates as the pH becomes alkaline [272, 298].

Optical images and Image J values for the average size (μm) of the precipitates that formed

during the neutralisation of AMD water with lime, sodium hydroxide, sodium carbonate, Bayer

liquor and Bayer precipitates are shown in Figure 26. Average particle size of the precipitates

formed was in the order lime (730 µm) > Bayer precipitates (184 µm) > sodium carbonate (62

µm) > sodium hydroxide (59 µm) > Bayer liquor (20 µm). Based on the size of the flocs that

formed for the different alkali, it appeared that surface area alone was not driving the metal

uptake capacity. Therefore, based on the work by Huang et al. [327], it was believed that the

encapsulation of metals in larger flocs (precipitates) was a key factor in metal removal.

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(a) Average size 730μm

(b) Average size 59μm

(c) Average size 62μm

(d) Average size 20μm

(e) Average size 184μm

Figure 26: Image J particle size analysis of optical images of precipitates formed during

AMD treatment a) with lime at pH 6.75 b) sodium hydroxide at pH 6.80 c) sodium

carbonate at pH 6.26 d) Bayer liquor at pH 6.49 and e) Bayer precipitates at pH 6.50

5.6 Performance versus Operational Considerations

The hydrochemical and geochemical behaviour of metals in solids must be considered during

the treatment of AMD to avoid the release of metal contaminants once disposed of [90, 334].

Depending upon the site conditions, in some cases, sludges are further treated for metal

recovery or can be converted into an inert material via stabilisation or solidification prior to

disposal to avoid further contamination [334, 335]. Currently at Mt Morgan, lime neutralised

precipitates (sludge) are disposed of on a tailings beach that has periods of immersion with

open pit water. This form of disposal was chosen to utilise any residual caustic remaining in

the sludge to slowly raise the pH of the AMD water in the open pit. However, XRF analysis

133

of samples collected at the Mt Morgan tailings beach (Table 26) shows that the metals have

been leached back into the AMD water (reductions in Cu – 92 %, Mn – 87 %, and Zn – 96 %),

and thus are being recirculated through the neutralisation tanks. Only slight reductions in

calcium were observed (22.2 compared to 20.9 %).

Precipitates obtained after treatment of AMD water with different alkalis were leached using

deionised water and open pit AMD water to determine the extent of metal leaching, and thus

precipitate stability. Concentration of metals (mg/L) in DI water and AMD waters after 24 hr

contact are provided in Table 27 & Table 28, respectively. In DI water, the Bayer precipitates

appeared to be the most stable with leached Al, Cu, Zn and Fe concentrations being less than

0.05 mg/L (detection limit), while only 1.30 mg/L of Mn leached. Minimal changes in pH

were also observed (remained neutral at around 7.25). Bayer liquor and sodium hydroxide also

showed minimal leached Al, Cu, Zn and Fe, however at pH values of 6.04 and 5.67,

respectively, an increased amount of Mn was leached (18.3 and 25.3 mg/L) compared to the

Bayer precipitates (1.30 mg/L). Interestingly, the Mn concentration for lime, which had the

lowest pH value of 5.46, only observed 7.99 mg/L. The main mineralogical difference between

Bayer precipitates and lime with the other alkaline materials was the content of gypsum,

therefore, the increased stability of Mn in these precipitates was thought to be related to

adsorption mechanisms previously explained [327]. Overall, lime and sodium carbonate

released the greatest amount of Cu (2.10 and 2.19 mg/L) and Zn (3.83 and 2.29 mg/L) back

into solution.

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Table 26: XRF data of freeze dried tailings beach and lime neutralisation plant sludge

from Mt Morgan

Measurement Lime neutralisation plant sludge Tailings beach sludge

Initial (g) 1.15 1.16

Final (g) 10.0 10.0

Loss On Ignition (%) 27.0 23.8

Sum 101.6 100.6

SiO2 (%) 0.617 8.58

Al2O3 (%) 13.4 7.07

Fe2O3 (%) 0.137 3.61

Na2O (%) 0.305 0.183

MgO (%) 1.47 0.577

K2O (%) 0.003 0.032

CaO (%) 22.2 20.9

TiO2 (%) 0.004 0.023

Mn3O4 (%) 0.391 0.052

P2O5 (%) 0.01 0.028

SO3 (%) 35.2 35.6

ZnO (%) 0.303 0.012

CuO (%) 0.521 0.040

NiO (%) 0.008 0.001

The exposure of the precipitates to AMD water for 24 hrs showed greater amounts of leached

species. As observed for AMD water, precipitates obtained after AMD treatment with Bayer

precipitates had the greatest pH buffering capacity (5.73 after 24 hrs), due to the formation of

hydrotalcite and calcite as shown in Figure 25; compared to all other alkaline materials whose

pH was similar to the original 3.75 of the AMD water (between 3.80 and 3.90). This pH

buffering capacity of Bayer precipitates, meant that Bayer precipitates when added to AMD

135

water reduced the concentrations of Cu from 77.3 to 16.2 mg/L, Zn from 48.9 to 34.7 mg/L

and Al from 1233 to 42.9 mg/L from AMD water compared to other alkaline materials which

showed an increase in the metal concentrations due to leaching of these metals to AMD water

as shown in Table 28.

Table 27: Metals leached in DI water from precipitates obtained after treatment of

AMD water with different alkali

Alkali pH Concentrations (mg/L)

Mg Ca Al Mn Cu Zn Fe

Lime 5.46 108 569 <0.05 7.99 2.10 3.83 <0.05

Sodium hydroxide 5.67 244 73.1 <0.05 25.3 <0.05 <0.05 <0.05

Sodium carbonate 5.78 177 86.9 <0.05 13.7 2.19 2.29 <0.05

Bayer liquor 6.04 298 94.0 <0.05 18.3 <0.05 <0.05 <0.05

Bayer precipitates 7.25 147 209 <0.05 1.30 <0.05 <0.05 <0.05

Table 28: Metals concentration in AMD water from precipitates obtained after

treatment of AMD water with different alkali

Alkali pH Concentrations (mg/L)

3.75 Mg Ca Al Mn Cu Zn Fe

Mt Morgan AMD 3.75 2265 535 1233 162 77.3 48.9 16.7

Lime 3.80 2547 483 1284 158 121 63.1 1.24

Sodium hydroxide 3.82 2542 562 609 194 192 145 <0.05

Sodium carbonate 3.87 2386 579 634 163 209 92.8 <0.05

Bayer liquor 3.90 2616 567 583 187 154 115 <0.05

Bayer precipitates 5.73 3332 517 42.9 168 16.2 34.7 <0.05

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However, for all the tested alkaline materials including Bayer precipitates this was not the case

for Mn when leaching the precipitates with AMD water. Table 28 showed leaching for all

tested alkaline materials (due to low pH) which resulted in enhanced Mn concentrations

ranging from 158 to 194 mg/L. In acidic conditions hydrotalcite present in Bayer precipitates

dissociates, hence why Bayer precipitates showed the highest concentration of leached Mg

(resulted in increase in concentration to 3332 from 2265 mg/L compared to concentrations

ranging from 2386 to 2616 mg/L for other alkaline materials). It should be noted, that the

concentrations of Mg, Cu, Mn and Zn were significantly higher than the original AMD water,

which supported the theory that the increased concentrations observed in this study compared

to Edraki, et al. [77] were due to the dissolution of precipitates in the disposed lime sludge.

Overall, Bayer precipitates were the most stable in the presence of AMD water.

This study has found Bayer precipitates to perform as well as lime, with the added benefits of

increased sludge stability and the removal of manganese at lower pH. However, the mass

required for attaining the desired pH is higher for Bayer precipitates (40 g/L to attain pH 7.5),

compared to lime (6.16 g/L to attain pH 7.7), and thus associated plant neutralising capacities

would be negatively affected. An added incentive for using Bayer precipitates is that they are

a waste by-product of the alumina industry; prepared from seawater neutralised residual Bayer

liquor. Bayer precipitates are also highly soluble in acidic conditions, whereas the hydrophobic

properties of lime means extensive mixing is required [84, 90]. By taking into account the cost

and behaviour of lime, it is suggested that using Bayer precipitates for AMD neutralisation and

removal of metals is an attractive alternative both in terms of effectiveness and that one industry

waste can be used to treat another’s waste.

137

5.7 Conclusions

This study evaluated the hypothesis that waste Bayer liquor and Bayer precipitates from the

alumina refining industry for the neutralisation of AMD. The first question was whether the

waste alkali materials could enhance the pH to achieve water discharge limits. All investigated

alkaline materials successfully raised the pH of treated AMD waters to meet discharge limits

i.e. pH 6.5-8.5. The second research question was in relation to the effectiveness of removal

of dissolved metal concentrations to satisfy regulations. This study found that lime and Bayer

precipitates were more effective in removing the metals present in AMD from the Mount

Morgan site than either sodium hydroxide, sodium carbonate or Bayer liquor. The removal

capacity of Bayer precipitates and lime was approximately the same for Al, Cu, Fe, Zn and Ni

and shown to achieve acceptable discharge limits, whereas in the case of Mn, at lower pH (6.5

to 7.5) Bayer precipitates were more efficient than lime to decrease its concentration. For

complete Mn removal, pH > 9 was required. However, raising the treated AMD water to this

latter pH was not viewed as acceptable; since not only was the caustic nature of the water

elevated but also aluminium content increased due to dissolution events. Therefore, satisfying

discharge limits for Mn remains challenging.

The final research question concerned the search for a scientific explanation to explain

differences in performance for the various alkali’s. The ability of the precipitates to encapsulate

heavy metals was determined to be more important than surface area. Sludge produced after

treatment with Bayer precipitates was more stable and showed minimum metal leaching as

compared to sludge produced after treatment with other alkali. The mass of material required

for attaining the desired pH was higher for Bayer precipitates compared to lime, but the capital

cost for a system using lime was considered high due to its hydrophobic nature and the resultant

extensive mixing required.

138

By considering the cost and behaviour of lime, it was suggested that the use of tailings for the

treatment of another mining waste was an interesting prospect that has the potential to reduce

the footprint of both industries, and therefore making them more sustainable.

139

Chapter 6: Enhanced removal of Mn (II) by

Bayer precipitates and thermally activated Bayer

precipitates

This chapter has been submitted to Minerals Engineering for publication





140

6.1 Introduction

The seawater neutralisation of Bayer liquor (liquid waste from alumina refinery) has been

shown in Chapter 4 to form Bayer precipitates comprising of hydrotalcite (Mg/Al LDH) and

calcium carbonate species (calcite and aragonite). In addition it was noted in Chapter 5 that

Bayer precipitates were effective in not only neutralising AMD to desired pH range but also

removed heavy metals, except manganese, to acceptable discharge limits

[281]. Theoretically, Mn can exist in numerous oxidation states, but in natural waters only

Mn2+ and Mn4+ are relatively stable [329]. Depending upon pH conditions, manganese can

form oxides, hydroxides or carbonate compounds [330]. However, between pH 9 and 9.5 Mn

is precipitated as hydroxides [80], with complete removal of Mn requiring a pH of 10.5 [310].

Issues with the aforementioned approach include: raising the pH requires potentially excessive

chemical usage; elevated pH values may not satisfy discharge water pH requirements; and

aluminium species can become soluble which leads again to problems with water quality.

Hence, the discovery of more effective sorbents for Mn is warranted.

Literature study reveals that calcination of layered double hydroxides generates mixed metal

oxides due to the removal of interlayer water, interlayer anions and hydroxyl groups during the

heating process [203, 336] . However, when exposed to water and anions, calcined LDH is able

to regenerate the layered structure [206, 207]. This conversion of metal oxides into layered

double hydroxides has been referred to as regeneration, restoration, structural memory effect

or simply memory effect. This property of regenerating the layered structure has been used

previously for removal of cations and anions from aqueous systems [190, 337].

At present, there does not appear to be sufficient information in relation feasibility of using

Bayer precipitates obtained from the seawater neutralisation of Bayer liquor to remove heavy

141

metals from aqueous solution. Palmer et al. [140] reported that seawater neutralisation of red

mud resulted in the formation of precipitates containing 40 – 60 % Bayer hydrotalcite and that

the thermal activation of these resultant precipitates increased their uptake capacity for oxy-

anions such as vanadate, arsenate and molybdate as compared to thermally activated red mud

alone. The possibility of using thermally activated Bayer precipitates for manganese

remediation from solution is therefore raised. Consequently, this study aimed to develop

optimal thermally activated Bayer precipitates for the removal of Mn from acidic solutions.

The hypothesis was that selection of the correct activation procedures may produce higher

capacity materials for manganese uptake. The research questions addressed included: (1) what

is the impact of thermal activation on Bayer precipitates? (2) to what extent is manganese

removed from solution when using thermally activated Bayer precipitates? and (3) by what

mechanism do thermally activated Bayer precipitates capture manganese? Both Bayer and

thermally activated Bayer precipitates were made and then tested by a series of bench top trials

to determine the capacity for manganese removal from solution. Several characterization

methods were then employed to provide insight into the sorption process involved.

6.2 Effect of Thermal Activation of Bayer Precipitates

The XRD patterns of Bayer precipitates (B.PPT) and thermally activated Bayer precipitates

(TA B.PPT) are shown in Figure 27. The XRD pattern of Bayer precipitate was composed of

several mineralogical phases including: 1) hydrotalcite (Mg6Al2(OH)16(CO3)∙4H2O), 2) calcite

(CaCO3 – rhombohedral), 3) aragonite (CaCO3 – orthorhombic) and 4) halite (NaCl).

142

Figure 27: XRD pattern of Bayer precipitates and thermally activated Bayer

precipitates

The broadness of the d003 and d006 peaks (approximately 12 and 25° 2θ) was characteristic of

poorly crystalline hydrotalcite [316], and the d-spacing of 7.8 Aᵒ indicated that this material

was a carbonate hydrotalcite [258]. Palmer et al. [300] reported that hydrotalcites formed at

high pH have 2:1 Mg:Al ratio, while those formed at pH 8-10 have Mg:Al ratio 3:1 [300]. The

Bayer precipitates containing hydrotalcite structure in this study were prepared at pH 9.25 and

thus are proposed to have predominantly a Mg, Al ratio of 3:1.

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Thermal decomposition has been reported to increase the reactivity of hydrotalcite due to de

hydroxylation and decarbonation of the layered structure [300, 338]. The thermal

decomposition of Bayer precipitates in the region of 300-330 °C are assigned to the removal

of weakly bonded interlayer water. Thermal decomposition events occurring from 335–350 °C

have been recorded to be the initial dehydroxylation of the brucite like layers of hydrotalcite

structure, while mass losses between bands at 350-385 °C were assigned to the simultaneous

dehydroxylation and decarbonation of hydrotalcite to form mixed metal oxides [300, 338]. The

degree of decarbonation/dihydroxylation of hydrotalcite in Bayer precipitates is dependent on

the temperature and duration of time used for thermal activation, however care must be taken

to avoid permanent decomposition of the layers at high temperature for extended period.

Implementation of the thermal activation process which involved heating the Bayer precipitate

samples for 4 h at 320, 380 and 440 °C resulted in various changes to the XRD patterns.

Thermal decomposition has been reported to increase the reactivity of hydrotalcite due to de

hydroxylation and decarbonation of the layered structure [300, 338]. Increasing the thermal

activation temperature appeared to progressively decrease the crystallinity of hydrotalcite as

indicated by the decrease in intensity and broadening of the d003 peak (ca. 12o 2θ). This

observation suggested that the interlayer spaces had collapsed due to decarbonation and

dehydroxylation [339, 340] (Equation 20).

The identification of peaks at 44 and 78o 2θ which were assigned to magnesium aluminate also

supported this latter conclusion. Also, apparent when the sample was heated to 320 oC was the

dominance of the XRD pattern by reflections ascribed to calcium carbonate. This latter

behaviour was consistent with the diminishment of the hydrotalcite crystallinity.

144

Heating to 380 and 440 oC resulted in the appearance of relatively small peaks assigned to the

presence of calcium oxide (55o 2θ) which indicated slight decarbonation of calcium carbonate

(Equation 38)

Equation 38: CaCO3(s) → CaO(s) + CO2(g)

The absence of MgO (periclase) in the XRD patterns inferred that Bayer precipitates had not

been entirely decomposed to simple oxides as illustrated in Equation 20.

Further information was gathered from the FT-IR spectra of Bayer precipitate and thermally

activated Bayer precipitates (Figure 28). The infrared spectrum of Bayer precipitate showed a

broad band centred at around 3400 cm-1, which was assigned to a number of overlapping OH-

stretching vibrations and stretching vibrations of metal hydroxyl layers, intercalated water and

solvated anions [205]. The shoulder at ca. 3000 cm-1 was believed to be due to the interaction

between hydroxyls and interlayer carbonate anions, this band was attributed to the bridging

mode of H2O-CO32- [205]. Corresponding water bending modes were observed at 1621 cm-1,

which suggested there were a number of hydroxyls units in the structure [341]. The intense

band at 1372 cm-1 in Bayer precipitates was attributed to the anti-symmetric stretching mode

of carbonate anions hydrogen bonded with interlayer water [341], while the ν3 vibrational

modes of carbonate were observed at 1100 cm-1 [294].

The IR spectra of all thermally activated Bayer precipitates (320,380 and 440 °C), observed a

decrease in intensity of the band centred at 3400 cm-1 (intercalated water and solvated anions)

and the interlayer water band at 1660-1600 cm-1, which nearly disappeared. This latter

145

behaviour was consistent with partial decarbonisation and dehydroxylation of the hydrotalcite

structure in Bayer precipitates.

A number of shifts in the IR band positions were also observed Figure 28. The broad band

centred at 3496 cm-1 for Bayer precipitates also shifted to lower wavenumber for all thermally

treated Bayer precipitates (3422 cm-1) due to a reduction in the extent of hydrogen bonding

upon heating the material. The carbonate ν3 antisymmetric stretching vibration also shifted

from 1372 to 1406 cm-1, and this behaviour was accompanied by the appearance of a new

vibration at 1497 cm-1. These latter changes in the IR spectra suggested that the carbonate

anions acting as an interlayer anion in hydrotalcite present in Bayer precipitates moved into the

metal ion layer upon heating due to removal of water from hydrotalcite which changed the

symmetry of carbonate anions from C2v to B2h [342, 343].

146

(a)

(b)

Figure 28: IR spectra of Bayer precipitates and thermally activated Bayer precipitates

(a) high wavenumber region (b) low wavenumber region

The N2 adsorption-desorption isotherms of Bayer precipitates and thermally activated Bayer

precipitates are displayed in Figure 29. The adsorption-desorption isotherms exhibited the

characteristics of type IV isotherms with a H4 type hysteresis loop according to the IUPAC

classification [344]. The H4 type hysteresis loop is similar to H3 type and is ascribed to the

presence of slit-shaped micropores of packing plate-like particles [344]. The specific area of

Bayer precipitates and thermally activated Bayer precipitates calculated by the BET method

was 26.44, 59.54 (320 °C), 103.22 (380 °C) and 114.09 m2/g (440 °C), respectively. The pore

size distribution and total volume were determined by the BJH method based on the desorption

branch. The results showed that the average pore diameter was ca. 32.686, 61.842, 37.858 and

34.373 A° and total pore volume was 0.014, 0.103 (320 °C), 0.097 (380 °C) and 0.091 cm3/g

(440 °C) for Bayer precipitates and thermally activated Bayer precipitates, respectively. The

147

increase in surface area with thermal treatment indicated a decrease in crystal size which was

supported by the broad reflections in the XRD patterns (Figure 27). Previous investigations of

thermally activated layered double hydroxides suggested the increase in surface area and pore

volume was due to the removal of water and carbon dioxide which resulted in the formation of

an additional mesoporous region [339, 345].

(a)

(b)

(c)

(d)

Figure 29: Adsorption-desorption isotherm: (a) Bayer precipitates (b) Thermally

activated Bayer precipitates 320 °C (c) Thermally activated Bayer precipitates 380 °C (d)

Thermally activated Bayer precipitates 440 °C

6.3 Impact of Bayer and Thermally Activated Bayer Precipitates upon Manganese

Concentration in Solution

148

The addition of Bayer precipitates and thermally activated Bayer precipitates (320, 380 and

440 °C) to Mn solution (pH 3.25) generally caused an increase in the pH of solution,

magnesium and calcium concentration; and decrease in manganese concentration (Figure 30-

Figure 33). Data is in Appendix 3 (Table SI 6.1-Table SI 6.4).

Figure 30: Concentration of Mn,Mg and Ca after addition of Bayer precipitates

149

Figure 31: Concentration of Mn, Mg and Ca after addition of TA B.PPT at 320 °C

Figure 32: Concentration of Mn,Mg and Ca after addition of TA B.PPT at 380 °C

150

Figure 33: Concentration of Mn, Mg and Ca after addition of TA B.PPT at 440 °C

The pH obtained after the addition of 0.5 g of Bayer precipitates and thermally activated Bayer

precipitate prepared at 320, 380 and 440 °C is 7.97, 8.45, 10.25 and 10.67, respectively. The

neutralisation mechanism was proposed to be an acid-base type reaction, whereby the

numerous OH- units in the hydrotalcite and mixed metal oxide structures and CO32- species (in

HT structure, calcite or aragonite) behaved as the base and reacted with H+ ions present in the

acidic metal solution [298]. The gibbsite (Al(OH)3) formed from the dissolution of Bayer

hydrotalcite (Equation 24) further consumes acid (pH 3.0 to 4.7) as shown in Equation 27,

and thus also contributes to the rise in pH of solution [298].

For thermally activated Bayer precipitate heated at 320 °C the solution pH rise was slightly

greater than that measured for Bayer precipitates. In contrast, thermally activated Bayer

precipitates heated at 380 and 440 °C exhibited a significantly higher final solution pH of >10.

151

The presence of CaO formed by partial decarbonation of CaCO3 (Equation 38) may explain

this latter observation. The presence of water was expected to result in the formation of slaked

lime (Equation 39) which in turn partially dissociated releasing OH- ions and calcium ions

(Equation 40).

Equation 39: 𝐶𝑎𝑂 + 𝐻2𝑂 → 𝐶𝑎(𝑂𝐻)2

Equation 40: 𝐶𝑎(𝑂𝐻)2 → 𝐶𝑎2+ + 2 𝑂𝐻−

Evidence for the former chemical reactions was gained from inspection of the calcium ion

concentration as a function of precipitate addition (Figure 32-Figure 35). Notably, when Bayer

precipitate was added to the manganese solution the solution calcium concentration was always

less than 50 mg/L. This outcome may have been due to partial dissociation of calcium

carbonate species as shown in Equation 26. Similar behaviour was seen with the 320 oC

thermally activated material. In contrast, a relatively rapid increase in Ca2+ ions were recorded

for samples heated at 380 °C (Figure 32) and 440 °C (Figure 33). Heating of the Bayer

hydrotalcite to 380 oC enhanced the concentration of calcium ions to 121 mg/L. Further raising

of the heating temperature to 440 oC induced a substantial promotion in calcium concentration

to 388 mg/L. This latter behaviour was in harmony with the XRD patterns in Figure 27 which

showed that calcium oxide was formed at these higher temperature values.

In all four tests, magnesium ions were determined to generally increase in concentration as the

addition of Bayer or Thermally Activated Bayer precipitates was increased. Dissolution of

hydrotalcite in Bayer precipitates and mixed metal oxides in thermally activated Bayer

152

precipitates in the acidic conditions can release magnesium ions as shown in Equation 41 and

Equation 42.

Equation 41:

𝑀𝑔6𝐴𝑙2(𝑂𝐻)16(𝐶𝑂32−). 𝑥𝐻2𝑂 (𝑠) + 12 𝐻+(𝑎𝑞)

→ 6 𝑀𝑔2+(𝑎𝑞) + 2𝐴𝑙(𝑂𝐻)3(𝑠) + 𝐶𝑂32−(𝑎𝑞) + 𝑥𝐻2𝑂

Equation 42:

𝑀𝑔𝐴𝑙2𝑂4 (𝑠) + 5 𝑀𝑔𝑂 (𝑠) + 12𝐻+ → 6 𝑀𝑔2+(𝑎𝑞) + 2 𝐴𝑙(𝑂𝐻)3(𝑠) + 3 𝐻2𝑂(𝑙)

However, with the thermally activated materials (380 and 440 °C) there appeared a distinct

maximum in the magnesium concentration after which point further addition of precipitate

resulted in a notable decrease or even an absence of magnesium ions in solution

(Figure 32-Figure 33). It has been reported that dissolution of layered double hydroxides

occurs in acidic medium and at pH above 8 reformation of hydrotalcite structure is initiated

[298]. The regeneration of layered structure by thermally activated layered double hydroxide

in water containing carbonate has also been reported [206]. As in the case of Bayer precipitates,

maximum attained pH was 8.03, therefore reduction in magnesium concentration was not

observed. While for thermally activated Bayer precipitates high pH was attained and thus

reduction in magnesium concentration was observed due to reformation of hydrotalcite

(hydrotalcite reformation for thermally activated Bayer precipitates is observed in Figure 34).

Comparison of Mn removal performance by the addition of Bayer precipitates and thermally

activated Bayer precipitates was shown in Figure 30-Figure 33. In all cases it was evident that

manganese anions could be completely removed from solution by the various Bayer precipitate

materials. Nevertheless, it was also apparent that the mass of precipitate required to reduce the

153

manganese concertation to practically zero was significantly less when thermally activated

material was employed (0.75 g of Bayer precipitates and 0.25 g (320 °C), 0.05

(380 °C) and 0.04g (440 °C) of thermally activated Bayer precipitate). Notably, the ability of

the precipitates heated to 380 and 440 oC was very similar in terms of manganese removal

performance. This latter behaviour indicated that the underlying change in the material

composition responsible for the removal of manganese had already been completed by 380 oC,

but not completed by 320 oC (as more material was required in this case compared to the higher

temperature treated samples).

6.4 Examination of Precipitates after Removal of Manganese

The three main mechanisms responsible for heavy metal removal by layered double hydroxides

have been reported to be (1) precipitation (2) surface complexation and (3) isomorphic

substitution [238]. Table 29 confirms the presence of Mn in precipitates formed after treatment

using the four Bayer precipitates (unmodified, and thermally activated at 320, 380 and 440 °C).

To help understand the phenomena responsible for the solution behaviour described in

Figure 30-Figure 33, XRD patterns of precipitates obtained after treatment at pH 8.03 for

Bayer precipitate and pH >8 for thermally activated Bayer precipitates have been recorded

(Figure 34). These latter pH values were chosen because manganese removal is usually

preferred in this range.

154

Table 29: Concentration (mg/g) of Mg, Al and Mn in precipitates before and after

treatment with Mn solution.

Sample Mg: Al Concentration (mg/g)

Mg Al Mn

B.PPT

3.31 208.81 63.00 -

B.PPT +Mn 3.21 196.02 61.07 6.91

TA B.PPT 320 3.40 254.29 74.71 -

TaB.PPT 320

+ Mn 3.29 202.73 61.63 15.24

TA B.PPT 380 3.38 274.17 80.93 -

TA B.PPT 380

+ Mn 3.33 221.29 66.35 23.27

TA B.PPT 440 3.53 309.58 87.66 -

TA B.PPT 440

+ Mn 3.52 249.10 70.81 19.54

The precipitates produced after the treatment of the Mn solution predominately consist of

gypsum (CaSO4.2H2O) formed by the reaction of calcium (as CaCO3 and/or CaO) in all four

Bayer precipitates and sulphate in the Mn solutions (Figure 34). Gypsum formation during the

treatment of AMD has been reported when treated with lime and limestone [346, 347]. The

absence of bands associated with CaCO3 and CaO, observed in the original precipitates

(Figure 27), confirm their dissolution during the treatment process.

155

Figure 34: XRD pattern of precipitates obtained after treatment

with Mn solution

Overlapping hydrotalcite peaks with gypsum were also observed. There appears to be a

significant reduction in hydrotalcite peak intensity, however it is unclear how much

hydrotalcite remained after the treatment process. A decrease in intensities of LDH peaks

during the removal of metals from aqueous solution has been reported previously [243, 246].

It should be noted that the XRD patterns were unable to provide any information on how and

if Mn was associated with hydrotalcite in the samples. Zhang et al. [242] has also reported the

156

reconstruction of calcined Mg-Al hydrotalcite after dispersing into an aqueous solution of Pb2+,

resulted in the broadening and reduction of peak intensities.

Minor other phases found in the XRD pattern were rhodochrosite (MnCO3) and hausmannite

(Mn2+Mn3+O4); precipitation of these Mn phases at pH >7 in aerated aqueous solutions has

been previously reported [348, 349]. More specifically, rhodochrosite formation has been

reported in the presence of carbonate ions during the treatment of Mn solution with limestone

and sodium carbonate between pH 7 and 11 [350]. The dissolution of calcite and hydrotalcite

in the Bayer precipitates would provide a source of carbonate ions. The presence of MnCO3

(low intensity peak at 14.48° 2θ) was most noticeable in the precipitates formed after treatment

with B.PPT. and 320 °C TA B.PPT. Decarbonation of the Bayer hydrotalcite and partial

conversion of calcite to CaO for 380 and 440 °C Bayer precipitates would reduce the amount

of available carbonate to form MnCO3. The precipitation of lead carbonates from a Pb2+

solution using Mg-Al LDH was reported to be due to the availability of carbonates released by

decomposition of LDH structure [246].

The residual concentrations of Mn, Mg and Al in solution after treatment were determined by

ICP-OES (Table SI 6.1-Table SI 6.4). A continuous increase in Mg concentration, along with

a ca. 50 % reduction in Mn by pH 7.76, and complete removal of Mn at pH 8.03 was observed

for Mn solutions treated with B.PPT. A similar trend was noted for TA B.PPTs, however

ca. 50 % Mn removal was achieved at pH 6.53, with complete Mn removal being achieved

between pH 7.5 and 8. Stanimirova et al. [249] has reported that layered double hydroxides

remove metals as hydroxides at high pH (pH 11-13), while at low pH (6.5-8) metal cations in

the solution can substitute for Mg2+ cations in layered structures and mixed metal oxides. As

the Mn removal for Bayer precipitates and thermally activated Bayer precipitates begins at a

157

pH less than 8, and that the Mg concentration rises, it is proposed that isomorphic substitution

of Mg2+ by Mn2+ may be taking place.

To gain a better insight into the bonding environments in hydrotalcite structures, before and

after the treatment of Mn solutions, surface analysis was conducted using X-ray Photoelectron

Spectroscopy (XPS) for B.PPT and 380 °C TA B.PPT. The wide scan XPS spectra of Bayer

precipitates and thermally activated Bayer precipitates showed Mg2p, Al2p, C1s, O1s, Ca2p

and Na1s (Figure 35). More detailed information was gathered using narrow scan XPS. The

Bayer precipitates and thermally activated Bayer precipitates exhibited a Mg2p peak

(Figure 36) at 50.3 and 50 eV, respectively, which were characteristic of Mg2+ in a layered

double hydroxide structure [351], while the Al2p peak (Figure 37) at 74.3 and 74.2 eV,

respectively, were characteristic of an octahedral arrangement of Al3+ in layered double

hydroxides [352]. This data agreed with XRD data (Figure 27) that hydrotalcite was a major

phase in all four Bayer precipitates. The observed shift in binding energy for Mg2p (0.3 eV)

and Al2p (0.1 eV) after thermal activation is proposed to be due to the dehydroxylation and

decarbonation of hydrotalcite present in Bayer precipitates. Zhang et al. [242] has observed

similar shifts in the binding energy for Mg2p and Al2p after thermal activation of synthetic

Mg-Al layered double hydroxides.

158

Figure 35: Wide scan XPS spectra

The Mg2p peaks observed for precipitates obtained after the treatment of Mn solution with

Bayer precipitates and thermally activated Bayer precipitates were deconvoluted into three

components as shown in Figure 36. The peak occurring at 50.3 eV corresponds to Mg2+ atoms

in the layered double hydroxide structure [351], while the peaks occurring at 48.8 and 51.3 for

B.PPT-Mn and at 48.5 and 51.2 for TA B.PPT-Mn corresponds to Mg2+ and is assigned to

Mg-O bond [352].The Al2p XPS signal obtained after treatment of Mn solution with B.PPT

and TA B.PPT were fitted with a single peak at 74.38 and 74.52 eV, respectively, and occurred

nearly at the same value for octahedral arrangement of Al3+ in layered double hydroxide

structures [352].

159

(a)

(b)

(c)

(d)

Figure 36: Deconvolution of Mg 2p peak (a) B.PPT (b) B.PPT-Mn (c) TA B.PPT and (d)

TA B.PPT-Mn

160

(a)

(b)

(c)

(d)

Figure 37: Al 2p (a) B.PPT (b) B.PPT-Mn (c) TA B.PPT and (d) TA B.PPT-M

The intensity of O1s peak in all spectra were similar (Figure 38); generally consisting of a M-

O bond (527-530 eV), M-OH/M-CO3 bond (531-533 eV), and a peak at 535 eV due to adsorbed

water [353, 354]. For the 380 °C thermally activated sample, the peak occurring at 532.48 ±

0.1 showed an increase in intensity, while the 531.32 ± 0.1 peak showed a decrease in intensity.

Jamie et al. [352] observed similar results after the calcination of a Mg-Al layered double

hydroxide, and reported that a decrease in crystalline size was responsible for this observation.

The removal of adsorbed water with heat treatment, also resulted in a reduction in the 535 eV

peak intensity for 380 °C TA B.PPT (Figure 38(c))

161

(a)

(b)

(c)

(d)

Figure 38: Deconvolution of O1s peak (a) B.PPT (b) B.PPT-Mn (c) TA B.PPT and

(d) TA B.PPT-Mn

162

a)

(b)

Figure 39: Mn 2p peak after treatment with (a) Bayer precipitate (b) Thermally

activated Bayer precipitate (380 oC)

The existence of Mn in the precipitates formed after treating the Mn solution was identified by

the appearance of characteristic peaks of Mn at approximately 642 eV (assigned to Mn2p 3/2)

and 653 eV (assigned to Mn2p 1/2). To understand the removal mechanism of Mn by Bayer

precipitates and thermally activated Bayer precipitates deconvolution of Mn2p 3/2 were

performed (shown in Figure 39). It has been observed that Mn2p 3/2 has two contributions for

the peak at 641.8 (B.PPT treated sample) and 641.6 eV (TA B.PPT treated sample), indicative

of Mn2+ and Mn3+; and assigned to Mn-O bond formed during surface complexation reactions

[355]. A peak at 644 and 643.6 eV, respectively, has been assigned to Mn4+ most likely as a

Mn-CO3 bond, which is consistent with the formation of rhodochrosite (Figure 34) by surface

induced precipitation [355].

The shift in the binding energy (EB) values of Mg2p (eV) and Al2p (eV) were observed for

precipitates obtained after Mn removal with Bayer precipitates. Those shifting values are in the

range of instrumental error (±0.1 eV), signifying that the local bonding environments and the

163

chemical states of Mg2p and Al2p are not altered. However, the EB values of Mg2p and Al2p

of precipitates obtained after removal with thermally activated Bayer precipitates were shifted

by 0.3 eV units, suggesting that the bonding environment has changed. It has been reported

that layered double hydroxides remove metal ions by forming inner sphere complexes at pH

values greater than 7, while at pH less than 7 metal removal can occurr via outer sphere

complexes [239, 241, 274]. In this study, it has been noted that Mn removal from solution

begins at around pH 6, and thus it is proposed that Mn removal may occur by the formation of

both inner sphere and outer sphere complexes.

Table 30: XPS results of Bayer precipitates and thermally activated Bayer precipitates

before and after treatment with Mn solution

Sample Binding energy (eV)

Mg2p Al2p Mn2p 3/2 Mn2p 1/2

B.PPT 50.32 74.27 - -

B.PPT-Mn

48.80

50.25

51.26

74.38 641.85

644.00

653.73

655.55

TA B.PPT 50.01 74.24 - -

TA B.PPT-Mn

48.54

50.28

51.16

74.52 641.66

643.60

653.40

655.34

Based on the above analysis, the removal of Mn2+ by Bayer precipitates and thermally activated

Bayer precipitates was a complex process, controlled by isomorphic substitution between Mn2+

and Mg2+, coupled with the precipitation of Mn as oxides and carbonates and adsorption by

chemical binding with surface hydroxyl groups or by electrostatic interactions.

164

6.5 Conclusion

This chapter assessed the hypothesis that thermal activation of Bayer precipitates will increase their

metal uptake capacity; manganese removal was the focus of this research. The thermal activation

of Bayer precipitates prepared by heating for 4 hrs at 320, 380 and 440 °C were characterised using

XRD, IR and BET. A reduction in Bayer hydrotalcite crystallinity and increase in surface area

were observed for the thermally activated Bayer precipitates. These changes in physical properties

was due to the decarbonation and dehydroxylation (loss of carbonate and hydroxyl units) of the

mixed metal hydroxide layers in Bayer hydrotalcite. At higher thermal activation temperatures

(380-440 °C) partial decarbonation of calcium carbonate species also formed calcium oxide. An

optimum temperature of 380 oC was determined on the basis that not only was the manganese

removal performance acceptable but also the amount of material required was minimized.

All materials tested completely removed manganese ions from solution. The removal capacities for

manganese were found to increase with thermal activation temperature; increasing the

activation temperature increased the maximum pH that could be obtained. The 380 and 440 °C

thermally activated Bayer precipitates were able to achieve a pH >10, while the 320 °C Bayer

precipitate was only able to reach similar maximum pH values as Bayer precipitates (between

8 and 8.5). The increased neutralisation capacity of the 380 and 440 °C thermally activated

Bayer precipitates was proposed to be due to CaO in the samples (decomposition of calcite at

high temperatures); hydrolysis of CaO forms slaked lime. All precipitates successfully

removed Mn from simulated solutions, however increases in Mg and Ca concentrations

(dissolution of hydrotalcite, calcite and calcium oxide) were observed in the treated samples.

The concentrations of Na, Mg, and Ca were within ANZECC guidelines.

165

The final phase of this research was to determine the mechanism for Mn removal using Bayer

precipitates and thermally activated Bayer precipitates at different temperatures. XRD, XPS

and ICP_OES confirmed the reformation of hydrotalcite; when added to acidic solutions it

dissociates, however as the pH rises hydrotalcite regenerates and incorporates Mn into its

structure (isomorphic substitution). Results also indicate that surface precipitation and surface

complexation reactions are also responsible for the removal of Mn, along with the formation

of gypsum, hausmannite and rhodochrosite.

166

Chapter 7: Performance of Bayer materials for

treating different mine wastewater This chapter has been submitted to Journal of Water Process Engineering for publication





167

7.1 Introduction

In previous chapters, it has been confirmed that Bayer precipitates were indeed effective in

neutralising AMD to a desired pH range and capable of removing heavy metals, except

manganese, to meet acceptable discharge limits. In addition, the performance of Bayer

precipitates was shown to be comparable to lime and thermally activated Bayer precipitates

were more effective in removing manganese from solution as compared to Bayer precipitates.

However, in chapter 5 the treatment of AMD with Bayer precipitates involved AMD water of

only one composition i.e. open pit AMD water from Mount Morgan mine. Bosman reviewed

the variability of AMD water compositions from six different South African mines, which

showed large variations in pH (1.8 to 5.0), calcium (30 to 800 mg/L), magnesium

(10 to 660 mg/l), ferrous (5 to 1250 mg/L), ferric (5 to 3350 mg/L), aluminium

(2 to 200 mg/L) and sulphate (600 to 13390 mg/L); concentrations of heavy metals were not

provided [312]. This variability in AMD water quality illustrates the difficulties in developing

a single treatment process for all AMD waters. The problem is further escalated when

variations in mine water compositions at the same mine site are considered, as reported by

Edraki et al. [77] for ten water holding areas at Mount Morgan mine. The following variations

were observed across the ten sites; pH (2.6 to 3.8), sulphate (8390 to 56240 mg/L), Al

(209 to 3074 mg/L), Ca (426 to 514 mg/L), Mg (1051 to 6101 mg/L), Fe (13 to 1487 mg/L),

Na (106 to 830 mg/L), Cu (3.27 to 138 mg/L), Mn (51.1 to 355 mg/L), and Zn (7.11 to 81.4

mg/L).

From the previous discussion, it can be seen that AMD solutions are inherently composed of

many dissolved components [12] and thus the optimal alkali addition strategy may not be the

same for all AMD types. Therefore, this chapter evaluated a range of AMD water compositions

to establish what impact variations in water quality had upon treatment methods using Bayer

168

precipitates and thermally activated Bayer precipitates. It was hypothesised that Bayer

precipitates may be capable of remediating AMD water of different types, thus expanding the

applicability of this latter approach. Specific research questions addressed included: (1) how

does the performance of Bayer precipitates change with variable AMD water composition? (2)

what impact does mine water composition have on the quality of treated water and sludge

produced, and (3) what impact does mine water composition have on sludge stability.

Consequently, AMD samples were collected and analysed from Mount Morgan mine,

Queensland, and bench scale tests conducted to determine the influence of AMD composition

upon Bayer precipitate treatment.

7.2 Variations in Mine Water Composition at Mt Morgan Mine Site

Table 15 shows the analysis of the AMD solutions collected from Mt Morgan. All the samples

were acidic with pH values in the range 2.70 (Airfield) to 3.74 (Open Pit). Sulphate

concentrations in AMD waters varied from 15,000 mg/L (Shephard’s holdings) to 38,000 mg/L

(Airfield dumps), while dissolved metal concentrations (Mn, Cu, Zn, Co, and Ni) ranged from

113 to 367 mg/L). It was also noted that significant differences in iron concentrations existed,

with the highest values observed at No. 2 Mill (1370 mg/L) and Frog Hollow (1045 mg/L),

while the lowest concentration observed was at Open Pit (16.7 mg/L). The relatively large

surface area (approximately 650 m in length and up to 500 m wide) of the Open Pit water was

conducive to wind turbulence impacts which generate oxygenated waters that precipitate

Fe(OH)3 at pH values above 3.5, hence the low concentrations of dissolved iron recorded [318].

In comparison, the other water bodies were considerably smaller and sheltered from the wind.

A general trend was noted concerning the containment type holding the water and water

quality. Waters located in dams constructed/surrounded by tailings exhibited lower pH and

169

higher concentrations of sulphate, magnesium, transition metals (Al, Fe) and dissolved metals

(Mn, Cu, Zn, Co and Ni). This observation was consistent with the fact that tailings were

remnants of ore bodies that have been milled and processed to recover the desired mineral

commodity; therefore, they are typically characterised as being fine material with increased

reactivity to oxidation processes. In contrast, waste rocks are larger and as such less reactive.

Overall, it was evident that the concentrations of metals in the AMD samples were substantially

in excess of the discharge limits set by Australian and New Zealand guidelines for fresh and

marine water quality [280].

Previous studies by Kim et al. [356] regarding AMD in the Donghae mine area in Korea

disclosed that pH was inversely proportional to the sulphate and dissolved metal

concentrations. This relationship also existed for the Mt Morgan water compositions

(Table 31), with Airfield dump showing the highest sulphate concentration (38,000 mg/L),

lowest pH (2.70) and highest dissolved metal concentrations (367 mg/L; Mn, Cu, Zn, Co, and

Ni). The reverse trend was also observed for water samples with pH values ranging between

3.11 and 3.74 (Shepard’s Holding, Shepherds Spring, Frog Hollow and Open Pit), which had

relatively low sulphate concentrations (15,000, 18,000, 16,500, and 17,430 mg/L, respectively)

and dissolved metal concentrations (113, 210, 291, and 294 mg/L respectively) compared to

Airfield.

7.3 Performance of Bayer Precipitates with Variable AMD Water Composition

To assess the impact of variable AMD water composition had on a treatment process, Bayer

precipitates were used to treat Open Pit, Mundic West, and Airfields samples. These particular

water samples were chosen on the following basis. Open Pit water was the main AMD water

source at Mt Morgan and this solution was periodically treated by lime neutralisation to

170

maintain dam water levels. Airfields was chosen to evaluate the impact of a lower pH (2.70

compared to 3.74), higher sulphate (38000 compared to 17430 mg/L), aluminium (1703

compared to 1233 mg/L), magnesium (3545 compared to 2265 mg/L) and total dissolved metal

concentrations (367 compared to 294 mg/L) would have an impact on the performance of Bayer

precipitates, while Mundic West was chosen as it had mid-range values between Airfields and

Open Pit. Open Pit water had the highest concentrations of Ca (535 compared to 431 mg/L)

and Na (648 compared to 157 mg/L).

Table 31: Concentrations of heavy metals in AMD water from three sites at Mt. Morgan

AMD

Site

pH Conductivity Concentrations (mg/L)

Ca Na Mg Al Mn Cu Zn Fe Ni

Open pit 3.74 14.86 535 648 2265 1233 161.5 77.26 48.89 16.7 1.54

Airfield

dump

2.70 16.37 431 157 3545 1703 186.3 101.2 72.8 194.4 1.23

Mundic

west

2.85 17.35 495 536 2597 1516 188.5 83 54.96 241 1.49

The treatment of AMD waters need to: (1) neutralise the acid, and (2) remove metals.

Therefore, increasing masses of Bayer precipitates (B.PPT) and thermally activated Bayer

precipitates (TA B.PPT) were added to three Mt Morgan AMD waters (Open Pit, Airfields,

and Mundic West) to assess their acid neutralising capacity. Initially the Bayer precipitates

and thermally activated Bayer precipitates were characterized using XRD (Figure 40). Bayer

precipitates have been previously characterised [281], and found to consist of primarily

hydrotalcite, calcite and aragonite (Chapter 4). The same Bayer precipitates were thermally

activated for 4 h at 380 °C, which resulted in the formation of an amorphous mixed Mg, Al

oxide (MgAl2O4 and MgO) and calcite. Thermally activated Bayer precipitates has shown an

increase in their uptake capacity for manganese in chapter 6. Remnants of the hydrotalcite

171

structure were visible in the XRD pattern (Figure 40); however, the relatively small intensity

of the 003 peak indicated that the interlayer spaces had collapsed [339, 340]. Based on the

study by Palmer et al. [300] Bayer precipitate showed the loss of water and carbonate when

heated to 380 °C and these losses were attributed to the dehydroxylation and decarbonation of

the hydrotalcite structure, resulting in the formation of an amorphous Mg,Al oxide.

Figure 40: XRD pattern of Bayer precipitates and thermally activated Bayer

precipitates

172

The ideal pH range for the discharge of wastewater in surface water systems should be between

6.0 and 8.5 [281]. Addition of both B.PPT and TA B.PPT caused the pH of the AMD waters

to increase to a maximum pH of around 8 (Figure 41). To obtain this latter desired pH range;

1.0 g (pH 7.5), 1.0 g (pH 7.48) and 0.99 g (pH 7.47) of Bayer precipitates and 0.48 g (pH 7.3),

0.46 g (pH 7.21) and 0.46 g (pH 7.77) of thermally activated Bayer precipitate were required

for 25 mL of Open Pit, Airfield and Mundic West AMD, respectively. Data is provided in

Appendix 4 (Table SI 7.1-Table SI 7.6). The neutralisation mechanism is proposed to involve

interaction of OH- species in the hydrotalcite structure with sulphuric acid in the AMD waters

as illustrated in Equation 43 & Equation 44 [298].

Equation 43:

𝑀𝑔6𝐴𝑙2(𝑂𝐻)16(𝐶𝑂32−). 𝑥𝐻2𝑂 (𝑠) + 12 𝐻+

→ 6 𝑀𝑔2+(𝑎𝑞) + 2 𝐴𝑙(𝑂𝐻)3(𝑠) + 𝐶𝑂2 (𝑔) 𝑥𝐻2𝑂 (𝑙)

Equation 44:

𝑀𝑔𝐴𝑙2𝑂4 (𝑠) + 5 𝑀𝑔𝑂 (𝑠) + 12𝐻+ → 6 𝑀𝑔2+(𝑎𝑞) + 2 𝐴𝑙(𝑂𝐻)3(𝑠) + 3 𝐻2𝑂(𝑙)

173

(a)

(b)

(c)

Figure 41: Neutralisation curves for Open Pit, Airfield, and Mundic West AMD water

treated with (a) Bayer precipitates and (b) thermally activated Bayer precipitates:

Comparison curve representing addition of Bayer precipitates and thermally activated

Bayer precipitates to acidified water shown in (c)

174

The gibbsite (Al(OH)3) created from the dissolution of B.PPT and TA B.PPT further consumed

acid (pH 3.0 to 4.7) according to Equation 45; thus, also contributing to the rise in pH.

Equation 45: 𝐴𝑙(𝑂𝐻)3 + 3 𝐻+ ↔ 𝐴𝑙3+ + 3 𝐻2𝑂

To aid understanding, the pH curves obtained by the addition of Bayer precipitates and

thermally activated Bayer precipitates to acidified DI water (H2SO4) is shown in Figure 41(c).

It was noted that even with relatively minor addition of Bayer precipitates a significant increase

in pH resulted. This behaviour indicated that hydroxyl units of hydrotalcite present in Bayer

precipitates reacted with H+ ions present in acidified DI water. The calcium carbonate species

present in the Bayer precipitates and calcium oxide in thermally activated Bayer precipitates

(formed by slight decarbonation of CaCO3) can also contribute to rise in pH by reacting with

acid (previously described in Chapter 6).

It was observed that for all AMD waters relatively small increase in pH was apparent for the

initial addition of Bayer precipitates and thermally activated Bayer precipitates. In addition,

the analysis results revealed that iron (Fe) and then aluminium (Al) was precipitating at this

stage. A notable difference in Figure 41 was the higher initial pH of the Open Pit water (3.74)

compared to Airfields (2.70) and Mundic West (2.85), and the appearance of an additional

neutralisation reaction taking place for Airfields and Mundic West between pH 3 and 3.5

(inflection starting at 0.008 g of B.PPT and 0.0036 g TA B.PPT and finishing at 0.03 g B.PPT

and 0.02 g TA B.PPT) which is proposed to be due to iron precipitation (as discussed in Section

3.3). TA B.PPT also had an increased acid neutralising capacity, requiring approximately half

the amount of material as B.PPT. This latter behaviour is postulated to be due to reduction in

carbonate content in thermally activated Bayer precipitates (caused by decarbonation of CaCO3

175

and hydrotalcite structure) as compared to Bayer precipitates. Reduction in carbonate content

reduced the buffering effect and thus increasing solution pH was easier in this case.

To evaluate the effectiveness of B.PPT and TA B.PPT for the removal of metals from AMD

waters, the removal of Al, Mn, Cu, Zn, Fe, and Ni was examined (Figure 42 & Figure 43;

Table 32 & Table 33). In general, both B.PPT and TA B.PPT effectively removed heavy

metals from AMD water and reduced the concentrations of Al, Cu, Zn, Fe and Ni to acceptable

limits for all three AMD samples (below the detection limits of the instrument). Data is

provided in Appendix D (Table SI 7.1-Table SI 7.6). It was noted that as the AMD pH

increased with addition of Bayer precipitates and thermally activated Bayer precipitates, iron

was the first metal to be precipitated and when the pH was raised to 4 the presence of iron could

not be detected. However, at pH ≤4, the aluminium concentration remained constant and thus

indicated no simultaneous precipitation of aluminium occurred in this pH range. For all AMD

solutions, the aluminium precipitation started at pH ≥4 and the Al concentration was not

measurable at pH ≥6.5. Remaining metals such as Cu, Zn and Ni also started to precipitate at

pH ≥4 and thus indicated that simultaneous precipitation occurred with aluminium.

Open Pit

176

Airfield Sump

Mundic West

Figure 42: Heavy metal removal curves for a) open pit b) airfield sump and c) mundic

west treated with Bayer precipitates

177

Open Pit

Airfields Sump

Mundic West

Figure 43: Heavy metal removal curves for a) open pit b) airfield sump and c) mundic

west treated with thermally activated Bayer precipitates

178

Thermally activated Bayer precipitate was found to be more effective in the removal of Mn

(lower concentration in discharge water) from all three AMD waters compared to Bayer

precipitate (Table 32 & Table 33). After treatment with Bayer precipitates, the percentage

removal for Mn in Open Pit, Mundic West and Airfield was 82.8, 74.0 and 76.4 %, respectively;

whereas the removal percentage of Mn for TA B.PPT was 91.6, 83.7 and 85.6 %, respectively.

179

Table 32: Heavy metal concentration (mg/L) in AMD waters after treatment with Bayer precipitates

AMD water pH Amount

added (g)


Al Mg Ca Na Mn Cu Zn Fe Ni

Open Pit

3.75 0.0000 1233 2265 535 648 161.5 77.26 48.89 16.70 1.54

6.05 0.5025 2.65 3594 472.9 672.4 67.20 <0.05 <0.05 <0.05 <0.05

7.57 1.0040 0.91 3710 735.3 645.6 27.74 <0.05 <0.05 <0.05 <0.05

Percentage removed (%) 99.9 - - - 82.82 * * * *

Airfield

2.70 0.0000 1703 3545 431 157 186.3 101.2 72.8 194.4 1.23

6.07 0.4984 <0.05 5997 508.6 201 108.8 <0.05 <0.05 <0.05 <0.05

7.48 1.0041 <0.05 5784 514.2 198 48.42 <0.05 <0.05 <0.05 <0.05


Mundic

west

2.85 0.0000 1516 2597 495 536 188.5 83 54.96 241 1.49

6.08 0.2485 <0.05 4279 489.8 602.6 175.2 <0.05 16.72 <0.05 <0.05

7.47 0.9904 <0.05 4702 573.7 568.1 44.4 <0.05 <0.05 <0.05 <0.05


* Percentages unable to be calculated due to concentrations being less than detection limits

180

Table 33: Heavy metal concentration in AMD waters after treatment with thermally activated Bayer precipitates

AMD water pH Amount

added (g)


Al Mg Ca Na Mn Cu Zn Fe Ni

Open pit

3.75 0.0000 1233 2265 535 648 161.5 77.26 48.89 16.70 1.54

6.90 0.3169 1.46 3794 762.9 821.4 40.2 <0.05 <0.05 <0.05 <0.05

7.88 0.8070 1.38 3680 794.4 1059 13.6 <0.05 <0.05 <0.05 <0.05


Airfield

2.70 0.0000 1703 3545 431 157 186.3 101.2 72.8 194.4 1.23

6.70 0.3196 <0.05 5433 767.8 368.2 60.3 <0.05 <0.05 <0.05 <0.05

7.84 0.7750 <0.05 5143 831.9 584.6 30.41 <0.05 <0.05 <0.05 <0.05


Mundic

west

2.85 0.0000 1516 2597 495 536 188.5 83 54.96 241 1.49

7.20 0.4666 1.41 4694 761.6 646.3 39.8 <0.05 16.72 <0.05 <0.05

8.01 0.8018 1.59 4929 907 657.4 27.1 <0.05 <0.05 <0.05 <0.05


* Percentages unable to be calculated due to concentrations being less than detection limits

181

A variety of researchers reported that thermal activation of layered double hydroxide increased

the surface area, which in turn enhanced the heavy metal removal capacity [242, 357]. It is

proposed that similar chemistry occurred in this study which explained the increase in Mn

removal by thermally activated Bayer precipitates as compared to Bayer precipitates. Another

insight was the fact that maximum Mn removal was recorded for open pit water followed by

airfield and mundic west water, which may be due to the formation of gypsum rich precipitates.

An increase in metal removal was found when there was a high Ca2+:SO42- ratio, solution pH,

ionic strength and surface area [327]. It was noted that the addition of gypsum had a negligible

adsorption of heavy metals, but rather it was the formation of high surface area gypsum flocs

from metal rich sulphate wastewaters that allowed for their removal [327]. Therefore, the

increased Mn removal efficiency for open pit was postulated as due to gypsum being the major

mineralogical phase formed as detected in XRD (Figure 45). It was apparent that treatment of

AMD solutions with Bayer precipitates and thermally activated Bayer precipitates caused an

increase in magnesium, calcium, and sodium concentrations in the remediated water. Increase

in sodium and calcium concentrations was believed to be due to the dissolution of salt (halite

as shown in Figure 40) and calcium carbonate species (calcite and aragonite) present in

precipitates obtained after seawater neutralisation of Bayer liquor [311]. In order to reduce the

concentration of sodium in treated water, proper washing of Bayer precipitates was

recommended [298]. The observed increase in Mg concentration in treated water was ascribed

to the dissociation of hydrotalcite (Mg6Al2(OH)16(CO3)·4H2O) and brucite (Mg(OH)2) present

in Bayer precipitates and mixed metal oxides (MgAl2O4 (s), MgO) present in thermally

activated Bayer precipitates. Calcium and magnesium are considered as important elements

for animal nutrition. According to ANZECC guidelines, livestock can tolerate 1000 mg/L of

calcium in their drinking water; however, there is insufficient information available to set

acceptable limits for magnesium in livestock drinking water. Nevertheless, relatively high

182

concentrations of magnesium promote water hardness and thus may cause problems associated

with scaling of equipment. For all three AMD solutions evaluated the concentration of calcium

after treatment with Bayer precipitates was below the concentration limit set by ANZECC

guidelines.

Liang et al. [238] stated that removal of heavy metal ions by synthetic layered double

hydroxides was by one of four mechanisms: (1) surface precipitation of metal hydroxides onto

LDH surfaces; (2) adsorption to the surface hydroxyl groups; (3) isomorphic substitution; and

4) chelation with a functional ligand in the interlayer region. In this study no chelating agent

was used, so metal removal by hydrotalcite in Bayer precipitates and thermally activated Bayer

precipitates was proposed to be by the remaining three mechanisms as shown in Figure 44.

Stanimirova et al. [249] reported that calcined layered double hydroxides remove metals as

hydroxides at high pH (pH 11-13), whereas at low pH (6.5-8) metal cations in the solution can

substitute the Mg2+ cations of the initial metal oxides. From the above discussion, it was

proposed that removal of heavy metal from AMD by hydrotalcite structure in the Bayer

precipitates or thermally activated Bayer precipitates (hydrotalcite reforms from metal oxides)

was a complex process, mainly controlled by adsorption, precipitation or isomorphic

substitution of divalent metal ions in the layered structure of hydrotalcite.

183

Figure 44: Metal removal mechanism using LDH

184

7.4 Produced Sludge Composition and Stability

XRD analysis indicated that precipitates formed by the addition of Bayer precipitates and

thermally activated Bayer precipitates to AMD water were relatively amorphous (Figure 45).

The crystalline materials in these precipitates were composed of primarily gypsum

(CaSO4.2H2O), hydrotalcite, and calcite (CaCO3). Gypsum and calcite assignments were

supported by infrared spectra of the precipitates (dominated by sulphate bands between 1200

and 900 cm-1, and carbonate bands between 1500-1300 cm-1 and 1100-1000 cm-1 (Appendix 4,

Figure SI 7.1-Figure SI 7.2) [358]. The XRD pattern of precipitates obtained after treatment

of AMD with Bayer precipitates showed more intense hydrotalcite related peaks compared to

thermally activated Bayer precipitates. This observation was proposed to be due to the

incomplete/partial dissolution of hydrotalcite in Bayer precipitates. Matias et al. [273] reported

that at pH >4, the dissolution of Mg-Al hydrotalcite was incongruent due to the formation of a

passive layer of an amorphous Al(OH)3 that prevents further dissolution of hydrotalcite.

Relatively high concentrations of Mg, Ca and Al found in the acid digested precipitates

(Table 34 & Table 35), alongwith the observation of the reformation of hydrotalcites as the

pH becomes alkaline in Chapter 6, supports the assignment of hydrotalcite in the XRD pattern

for precipitates obtained after treatment with B.PPT and TA B.PPT.

Even though metal hydroxide phases could not be detected by XRD, due to relatively low

concentrations in the CaSO4 and CaCO3 matrix, it was proposed that metal hydroxides existed

in the precipitates based on the concentration of metals in the acid digested samples

(Table 34 & Table 35).

185

(a)

(b)

Figure 45: XRD patterns of precipitates obtained after AMD water treated with

(a) Bayer precipitates (b) thermally activated Bayer precipitates

186

The chemical composition of sludge/precipitate obtained after AMD treatment was highly

variable and was mainly influenced by the initial composition of AMD water and the

precipitate used to neutralise it [80]. As shown in Table 31, Open Pit AMD water had high

Mn (161.5 mg/L) content as compared to Fe (16.7 mg/L), therefore sludge produced after

treatment of Open Pit AMD water had a high concentration of Mn (3.33 mg/g for Bayer

precipitates, 2.35 mg/g for thermally activated Bayer precipitates) compared to Fe (0.86 mg/g

for Bayer precipitates, 0.18 mg/g for thermally activated Bayer precipitates). All precipitates

showed higher concentration of Cu than Zn (Table 34 and 35), this was in harmony with higher

Cu concentrations in all AMD waters (Table 31). The increased levels of magnesium relative

to other metals (Cu, Zn, Al, Fe, Ca, S and Si) in the precipitate were proposed to be due to the

presence of hydrotalcite in precipitates obtained from AMD treatment with Bayer precipitates.

Seawater neutralisation of Bayer liquor introduced a significant amount of magnesium to Bayer

precipitates [298]. However, high Mg concentrations in precipitates obtained after treatment of

AMD with thermally activated Bayer precipitates was probably due to the reformation of

hydrotalcite (Mg6Al2(OH)16(CO3)·4H2O) or brucite (Mg(OH)2) as pH became

alkaline [140, 272, 298]. It has been noted that all obtained precipitates have a significant

amount of calcium and sulphur and this observation was probably due to the formation of

gypsum (detected by XRD in Figure 45).

187

Table 34: Metal concentrations (mg/g) in precipitates obtained by AMD treatment with

Bayer precipitates between pH 7.0 to 8.0

AMD site pH Concentrations (mg/g)

Mg Ca Al Mn Cu Zn Si S Fe

Open pit 7.57 105.13 54.41 96.87 3.33 2.76 1.80 2.56 - 0.86

Airfield sump 7.48 157.60 49.65 81.28 3.15 1.46 1.08 1.05 20.23 1.17

Mundic west 7.47 140.60 51.04 92.72 3.43 1.49 1.00 1.47 20.32 4.37

Table 35: Metal concentrations (mg/g) in precipitates obtained by AMD treatment with

thermally activated Bayer precipitates between pH 7.0 to 8.0

AMD site pH Concentrations (mg/g)

Mg Ca Al Mn Cu Zn Si S Fe

Open pit 7.88 205.65 55.50 85.05 2.35 1.17 0.74 1.02 22.84 0.18

Airfield sump 7.84 188.57 51.62 94.18 2.66 1.50 1.02 1.07 25.33 2.60

Mundic west 8.02 184.28 50.42 95.92 2.83 1.44 1.00 1.33 22.06 3.72

An ideal sludge from the neutralisation of AMD waters should exhibit minimal heavy metal

leaching, otherwise additional measures need to be put in place to restrict the movement of

leachates in storage facilities [281]. Therefore, precipitates obtained after treatment of AMD

solutions with Bayer precipitates and thermally activated Bayer precipitates were leached for

24 hrs using DI water and respective AMD samples to determine the extent of metal leaching

which occurred (Table 36). Leaching with AMD solutions was evaluated as mines may store

the sludge in existing tailings exposed to the AMD solution [334]; whereas the study of DI

water was performed to gain an understanding of leaching due to exposure to rain water.

The addition of precipitates obtained after AMD treatment into DI water resulted in minimal

changes in pH as shown in Table 36, while calcium and magnesium were the major ions

188

released in all instances regardless of precipitate tested. The release of Mg and Ca ions into DI

water was proposed to be due to the dissolution of hydrotalcite and gypsum present in

precipitates (detected by XRD). Lebedev et al. [359] reported the incomplete dissolution of

gypsum in water at 25 °C, while the partial dissolution of Mg-Al hydrotalcite in near neutral

solution has also been reported [273]. Notably, there was more hydrotalcite detected in Bayer

precipitates compared to thermally activated Bayer precipitates (Figure 45). Therefore, the

concentration of Mg leached into DI water was higher for precipitates obtained from AMD

treatment with Bayer precipitate (Table 36). In contrast, sludge exposed to DI water showed

no discernible leaching of Cu, Zn, and Fe. This observation was consistent with the proposal

that these latter species were present in the form hydroxides/carbonates as these species exhibit

minimal solubility under the test conditions employed [360]. In contrast, it was observed that

Al and Mn showed limited leaching (Table 36). It has been reported that Mn begins to

precipitate out as hydroxides at pH >9 and is present as Mn2+/Mn4+ at lower pH [310]. The re-

dissolution of Al(OH)3 as aluminate ions (Al(OH)4-) were noted as the pH was raised above 7

[361]. Hence, Mn and Al materials leached to a relatively small extent into DI water after

24 hrs.

189

Table 36: Metals leached from precipitates obtained after treatment of AMD waters with Bayer precipitates and thermally activated

Bayer precipitates

AMD pH Treatment Concentrations (mg/L)

Mg Ca Al Mn Cu Zn Fe

Open pit 7.25 B.PPT in DI 147 209 <0.05 1.30 <0.05 <0.05 <0.05

7.28 TA B.PPT in DI 33.48 69.96 1.4 <0.05 <0.05 <0.05 <0.05

Airfield 7.25 B.PPT in DI 78.4 88.28 0.783 1.057 <0.05 <0.05 <0.05

7.31 TA B.PPT in DI 59.35 81.3 2.09 0.607 <0.05 <0.05 <0.05

Mundic west 7.29 B.PPT in DI 45.75 116.4 0.74 1.20 <0.05 <0.05 <0.05

7.30 TA B.PPT in DI 49.26 89.01 1.52 0.856 <0.05 <0.05 <0.05

Open pit

3.75 AMD 2265 535 1233 161.5 77.26 48.89 16.7

6.05 B.PPT in AMD 3332 517 42.9 168 16.2 34.7 <0.05

6.55 TA B.PPT in AMD 3724 460 1.76 84.82 <0.05 <0.05 <0.05

Airfield

2.70 AMD 3545 431 1703 186.3 101.2 72.8 194.4

6.14 B.PPT in AMD 3403 428 6.49 168.3 1.91 14.47 <0.05

6.51 TA B.PPT in AMD 5251 509.2 0.548 123.6 <0.05 2.727 <0.05

Mundic west

2.85 AMD 2597 495 1516 188.5 83 54.96 241

6.10 B.PPT in AMD 4189 483.4 2.37 201 1.05 11.83 <0.05

6.59 TA B.PPT in AMD 4565 456 <0.05 138 <0.05 0.649 <0.05

190

Exposure of the various sludge materials to AMD solutions resulted in substantially different

behaviour compared to the situation with pure water (Table 36). The addition of sludge

produced after the treatment of AMD waters, resulted in an increase in pH of AMD waters

(raised pH between 6.05 - 6.15 for Bayer precipitates and between 6.51-6.55 for TA B.PPT)

when submersed in their respective AMD waters for 24 hrs. This increase in pH was believed

to be due to the re-dissolution of hydrotalcite (as detected by XRD) in the sludge. It has been

noted that the iron concentration fell below the instrumental detection limit after the addition

of sludge to their respective AMD waters. This latter observation was ascribed to the rise in

pH which promoted iron precipitation as hydroxides at pH > 4. There was also a considerable

decrease in the Al, Cu and Zn concentration due to the rise in pH. An increase in Mg and Ca

concentrations was observed, which was consistent with the dissolution of hydrotalcite,

gypsum, and calcite present in precipitates in acidic conditions. From the aforementioned

result, it can be said that sludge produced after treatment of AMD with Bayer precipitates and

thermally activated Bayer precipitates was beneficial in treating AMD solutions, with respect

to pH as well as complete removal of Fe.

7.5 Conclusions

This chapter evaluated the hypothesis that Bayer precipitates and thermally activated precipitates

can effectively treat different AMD solutions. Nine AMD samples were analysed with all being

acidic with a pH between 2.7 and 3.75, as well comprising of high metal and sulphate content.

Sulphate and metal concentrations were found to be dependent on its source; AMD water from

tailings had higher metal and sulphate concentrations compared to AMD solutions from waste

rocks.

191

Bayer precipitates and thermally activated Bayer precipitates (mixture of amorphous oxides

and calcite) were found to raise the solution pH to meet regulatory values. However, thermally

activated Bayer precipitate (380 °C) had a higher neutralising capacity (required approximately

half the material as B.PPT). The composition of the AMD water did not appear to exert a

significant influence on the acid neutralising capacities of Bayer precipitates or thermally

activated Bayer precipitate; as it was discovered that all AMD samples could be treated to meet

ANZECC pH requirements for discharge.

In terms of the ability of the precipitates to remove dissolved ions from AMD, this study found

that both Bayer precipitates and thermally activated Bayer precipitates were able to satisfy

discharge requirements, irrespective of the initial concentrations of Al, Cu, Fe, Zn and Ni in

the AMD sample. Albeit, the complete removal of Mn was not achieved, therefore, satisfying

discharge limits for Mn remains a challenge. Nevertheless, thermally activated Bayer

precipitates were found to be more efficient overall than Bayer precipitates.

The final research question concerned the stability of produced sludge after the treatment of

AMD waters with Bayer precipitates and thermally activated Bayer precipitates. All produced

sludges were rich in metals and showed minimum metal leaching when added to DI water. This

would be beneficial for sludges stored in separate dams to original tailings; leachate

compositions produced from runoff would not pose significant threats. However, lime

neutralised sludges are disposed of in pre-existing tailing dams and exposed to AMD water

(common practice). Therefore, the sludges produced by B.PPT and TA B.PPT were exposed

with their respective AMD waters for 24 hrs to determine their stability. All sludges caused an

increase in pH due to the re-dissolution of hydrotalcite, which triggered a reduction in Fe, Al,

Zn and Cu concentrations.

192

Chapter 8: Conclusions

This thesis has revealed that the innovative addition of alkaline Bayer precipitate waste from

the alumina industry to AMD can potentially be a viable technical solution. Bayer precipitates

formed from the seawater neutralisation of Bayer liquors are composed of hydrotalcite and

calcium carbonate. The use of Bayer precipitates would benefit society and industry in two

ways: (1) it is capable of treating AMD waters, and (2) lessens waste storage requirements for

the alumina refinery.

Bayer precipitates were found to both neutralise AMD and remove heavy metal ions. The

identity of the precipitates depended upon Bayer liquor composition (1-10 g/L Al2O3) with the

main mineralogical phases being hydrotalcite and calcium carbonate. An additional brucite

phase was determined for the 1-3 g/L Bayer precipitates, which caused higher neutralisation

pH values to be achieved. All Bayer precipitates (1-10 g/L) successfully treated AMD by

neutralising the pH and removing heavy metals, with manganese being the only heavy metal

not to meet ANZECC guidelines.

The performance of Bayer liquor and Bayer precipitate was compared with conventionally used

alkalis for AMD neutralisation such as lime, sodium hydroxide, and sodium carbonate. Target

ions such as Al, Cu, Fe, and Zn were successfully removed by both Bayer precipitates and

alkali’s to meet discharge limits. However, Mn was problematic due to its inherent solubility

at elevated pH. Nevertheless, Bayer precipitate was shown to have an enhanced ability to

remove manganese at lower pH (6.5 to 7.5) relative to lime, with residual Mn concentrations

of 32.30 and 85.40 mg/L, respectively. The encapsulation of heavy metals in gypsum was

found to be a key mechanism in the removal of heavy metals. Overall, Bayer precipitate were

found to be a potential alternative for the treatment of AMD water.

193

The ability of Bayer precipitates to remove heavy metals from aqueous solution was enhanced

by thermal activation; smaller amounts of material were required, increased heavy metal uptake

capacity achieved, and higher pH values attained. An optimal treatment temperature of 380 oC

was discovered and this related to formation of calcium oxide and mixed metal oxide

(magnesium aluminate). Surface area also increased due to decarbonation and dehydroxylation

of hydrotalcite.

The robustness and versatility of Bayer precipitates and thermally activated Bayer precipitates

for the treatment of AMD waters was demonstrated; treatment of AMD of varying water

composition was successful in meeting ANZECC discharge requirements using both Bayer

precipitate and thermally activated Bayer precipitate. The resultant sludge was found not only

stable to leaching by pure water but also beneficial if added to AMD.

Future research should address issues such as economic feasibility of the outlined AMD

treatment process. Issues such as transport costs, process design and sludge handling should

be considered. In addition, real Bayer liquor solutions should be sourced from alumina

refineries and evaluated.

194

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222

Appendix

Appendix 1: Supplementary information for Chapter 4

Table SI 4.1: ICP-OES analysis for AMD water treated with 1 g/L Bayer precipitates

Mass (g) pH Concentration (mg/L)

Al Ca Cu Fe Mg Mn Zn

0 3.75 1393 554 85.77 1.74 2580 177 57.63

0.0150 3.9 1202 598.2 80.32 1.714 2702 176.4 55.55

0.0301 5.85 588.6 635 74.57 <0.05 3055 172.9 55.54

0.0399 6.25 237.8 659.5 65.01 <0.05 3290 169.2 53.29

0.0502 6.52 36.1 678.2 46.93 <0.05 3629 168.3 51.1

0.1005 8.21 1.374 682 0.086 <0.05 3910 59.48 1.139

0.1999 8.97 0.182 691.4 0.025 <0.05 3837 2.604 0.594

0.4002 9.1 0.313 703.4 0.046 <0.05 3807 0.05 0.623

0.5000 9.17 0.314 725 0.077 <0.05 3755 0.038 0.935




0 3.75 1393 554 85.77 1.74 2580 177 57.63

0.0151 3.91 1229 589.7 78.38 1.53 2667 172.3 54.59

0.0297 4.38 966.7 654 71.15 0.634 3077 166.6 52.07

0.0402 5.8 546.7 672 62.03 <0.05 3143 159.7 49.16

0.0499 6.21 235.2 700.1 55.95 <0.05 3426 157.6 49.71

0.1001 7.98 0.773 713.2 0.108 <0.05 3949 77.43 1.169

0.2001 8.9 1.253 715.9 0.027 <0.05 3929 8.659 0.651

0.4001 9.08 0.372 725.9 0.02 <0.05 3847 0.086 0.737

0.5002 9.15 0.203 730.3 0.021 <0.05 3845 0.029 0.712

223




0 3.75 1393 554 85.77 1.74 2580 177 57.63

0.0148 3.92 1285 602 80.77 1.587 2670 173.8 56.39

0.0303 3.98 1130 616.3 65.27 1.421 2770 165.3 52.56

0.0397 4.51 725.8 678.8 63.56 <0.05 3059 162.3 50.63

0.0501 6.03 514 685 55.26 <0.05 3182 160.7 49.91

0.0997 7.62 0.466 695 0.157 <0.05 3814 101.7 1.729

0.1998 8.83 0.412 707 0.038 <0.05 4051 15.65 0.554

0.3999 8.95 0.225 712 0.05 <0.05 4043 1.75 0.574

0.4999 9.11 0.212 761 0.029 <0.05 3951 0.096 0.672




0 3.75 1393 554 85.77 1.74 2580 177 57.63

0.0152 3.91 1245 614.6 76.5 1.261 2582 168.7 53.83

0.0303 3.97 1080 696.5 68.05 1.171 2844 164.3 50.57

0.0401 4.12 861.7 714.5 63.3 <0.05 2914 161 49.34

0.0497 5.72 668.8 752 57.89 <0.05 3043 155.7 46.7

0.1001 6.91 2.664 763 1.453 <0.05 3923 116.2 3.957

0.2002 8.35 0 767.9 1.406 <0.05 4094 22.95 0.812

0.4002 8.65 0 759 0.203 <0.05 3981 13.37 1.403

0.5001 8.67 0 792.5 0.435 <0.05 3974 10.39 1.525




0 3.75 1393 554 85.77 1.74 2580 177 57.63

0.0149 3.92 1234 585.6 77.15 1.673 2570 170 53.81

0.0302 3.96 1088 639.5 66.64 1.072 2800 162.8 49.44

0.0403 4.08 969.6 661.1 61.79 <0.05 2883 159.7 47.9

0.0498 5.52 856.4 676.8 55.89 <0.05 2947 153.2 44.12

0.1003 6.2 10.61 683 8.696 <0.05 3640 135.1 22.49

0.1999 7.98 0.714 723 0.061 <0.05 4033 40.61 0.147

0.4003 8.61 1.368 776.8 0.072 <0.05 3944 8.535 0.141

0.4995 8.67 1.18 780 0.032 <0.05 3932 7.432 0.123

224




0 3.75 1393 554 85.77 1.74 2580 177 57.63

0.0150 3.9 1232 613.2 73.05 1.387 2753 170.6 50.69

0.0300 3.98 997.8 640.1 59.89 0.766 2803 167.4 46

0.0397 4.02 973.2 635.6 58.11 <0.05 2807 168.8 44.66

0.0502 5.54 780.9 694.8 47.25 <0.05 3033 164.7 39.69

0.0998 6.15 19.09 732.7 6.927 <0.05 3494 157.4 17.5

0.2001 7.57 0.643 742 0.162 <0.05 3929 95.78 0.292

0.3997 7.82 0.725 756.5 0.196 <0.05 3959 47.36 0.11

0.5003 8.1 0.654 740.6 0.194 <0.05 3946 13.46 0.118




0 3.75 1393 554 85.77 1.74 2580 177 57.63

0.0146 3.91 1301 586.5 75.02 1.637 2622 172.2 53.39

0.0301 3.98 1127 637.8 62.96 0.752 2832 163.7 47.76

0.0403 4.02 840.8 668.4 53.13 <0.05 2950 157 43.37

0.0499 5.52 335.3 679.8 48.01 <0.05 3405 154.9 42.22

0.1001 6.17 0.396 739 0.231 <0.05 4008 107.9 1.733

0.1996 7.38 0.927 748.9 0.172 <0.05 4062 21.59 0.116

0.4002 7.84 1.131 750.6 0.108 <0.05 4033 13.05 0.071

0.4999 8.06 1.24 795.3 0.078 <0.05 4028 4.684 0.083




0 3.75 1393 554 85.77 1.74 2580 177 57.63

0.0151 3.91 1250 600.8 70.38 1.547 2673 167.7 50.63

0.0298 3.99 807.9 657.6 59.29 0.741 2898 159.6 46.62

0.0399 4.03 745.7 661 51.45 <0.05 3008 151 41.79

0.0503 5.49 519.1 675.2 42.79 <0.05 3339 144.1 35.97

0.1003 6.15 0.298 712 0.277 <0.05 4078 104 2.662

0.2004 7.35 0.565 764.7 0.194 <0.05 4045 43.14 0.098

0.4000 7.81 0.721 765 0.1 <0.05 4034 23.65 0.098

0.5001 8.03 1.38 771.9 0.083 <0.05 4025 6.892 0.091

225




0 3.75 1393 554 85.77 1.74 2580 177 57.63

0.0149 3.91 1275 579.9 76.11 1.563 2585 171.3 53.34

0.0300 4.01 1099 625.6 66.35 0.752 2774 166.5 49.77

0.0402 4.03 869.6 670.3 59.05 <0.05 2950 164 47.72

0.0501 5.49 650.6 709.2 49.47 <0.05 3305 157.8 43.01

0.1001 6.01 1.391 728 0.318 <0.05 3877 121.7 3.854

0.1998 7.39 0.725 733 0.196 <0.05 3959 47.36 0.11

0.4001 7.79 1.312 755 0.151 <0.05 3890 14.9 0.093

0.5002 8.09 1.318 758 0.108 <0.05 3892 10.94 0.078




0 3.75 1393 554 85.77 1.74 2580 177 57.63

0.0151 3.93 1230 610.6 76.59 1.547 2602 167.1 50.92

0.0299 3.99 1174 644.6 66.4 0.527 2626 161.2 46.4

0.0401 4.01 1055 714.8 54.18 <0.05 2754 155.2 40.75

0.0495 5.47 868.1 749.9 45.25 <0.05 3202 150.8 36.67

0.1000 5.85 1.569 752 0.546 <0.05 3615 104.2 5.767

0.1999 7.18 0.928 758.5 0.062 <0.05 4046 30.22 0.112

0.4000 7.8 1.38 771.9 0.083 <0.05 4034 6.892 0.091

0.5001 8.05 1.24 795.3 0.078 <0.05 4033 4.684 0.083

226


Table SI 5.1: ICP-OES analysis for AMD water treated with lime


Al Mn Cu Zn Si Fe Ni

0 3.72 1233 161.5 77.26 48.89 36.94 16.7 1.54

0.0021 3.73 1216 157 71.87 47.81 36.11 15.24 1.34

0.0037 3.79 1178 153.6 71.59 48.52 35.7 14.13 1.33

0.0064 3.80 1205 156.3 70.98 47.27 35.67 7.75 1.32

0.0085 3.80 1186 157.7 73.01 48.69 35.52 4.67 1.34

0.0110 3.84 1219 158.6 71.46 47.9 36.79 3.54 1.33

0.0130 3.86 1169 154.3 71.33 47.89 36.56 2.98 1.32

0.0149 3.86 1222 158.6 73.57 49.75 36.67 2.78 1.34

0.0310 3.89 953.4 150.2 71.79 48.65 24.91 1.28 1.32

0.0411 3.93 776.7 154.9 72.19 48.5 17.9 0.58 1.34

0.0513 4.02 578.9 151.1 69.78 47.19 14.86 0.33 1.31

0.0722 4.09 277.8 150 62.7 47.59 6.31 0.17 1.26

0.1039 4.39 2.88 134.2 4.44 18.24 2.86 0.06 0.82

0.1235 6.75 0.53 85.4 0.11 0.75 0.68 0.05 -0.01

0.2515 9.19 0.6 0.45 0.05 0.14 1.85 0.04 0

0.5022 13.31 0.61 0 0.04 0.01 0.89 0.13 -0.02

1.0150 13.46 1.39 0.07 0.1 0.12 0.02 0.05 0.02

227

Table SI 5.2: ICP-OES analysis for AMD water treated with sodium hydroxide



0.0000 3.71 1233 161.5 77.26 48.89 36.94 16.7 1.54

0.0023 3.73 1456 186.7 92.12 59.01 42.73 16.67 1.73

0.0042 3.73 1444 184.1 90.22 56.75 42.7 14.49 1.64

0.0060 3.74 1496 192.8 90.26 57.34 40.42 6.12 1.69

0.0087 3.75 1518 195.5 96.09 60.72 44.32 3.33 1.76

0.0099 3.77 1394 183 89.58 57.25 39.23 2.56 1.69

0.0122 3.79 1390 184.7 92.66 57.93 34.22 1.42 1.7

0.0144 3.81 1409 183.7 90.83 57.4 34.36 1.86 1.69

0.0299 3.84 1228 180.8 89.93 57.16 31.41 0.78 1.66

0.0406 3.87 1045 182.1 89.36 57.16 23.41 0.34 1.7

0.0541 3.89 883.9 179.3 87.57 56.5 16.48 0.17 1.63

0.0754 3.93 307.6 173.1 81.68 57.05 9.51 0.08 1.62

0.1027 4.08 125.6 177.1 70.89 57.13 11 -0.06 1.59

0.1250 6.80 0.107 136.8 0.757 10.67 4.579 0.04 0.497

0.1540 7.70 0.285 52.48 0.004 0.112 1.554 0.009 0.016

0.1896 8.10 0.82 23.06 0.1 0.05 0.43 -0.1 0.08

0.2842 9.46 3.46 0.73 0.06 -0.01 -0.65 -0.13 0.03

0.4095 12.64 87.3 -0.02 0.06 -0.01 -0.85 -0.11 0.03

0.4989 13.13 239 18.2 6.69 5.53 8 -0.12 0.18

1.0663 13.12 100.5 1.5 2.59 2.86 27.33 -0.1 0.07

228

Table SI 5.3: ICP-OES analysis for AMD water treated with sodium carbonate



0.0000 3.72 1233 161.5 77.26 48.89 36.94 16.7 1.54

0.0018 3.72 1175 152.1 77.04 50.1 37.15 14.38 1.43

0.0038 3.73 1184 151.6 76.88 50.53 35.83 14.42 1.44

0.0061 3.75 1210 155.1 76.71 50.87 35.96 14.4 1.49

0.0082 3.77 1190 154.2 77.09 51.34 36.02 12.53 1.47

0.0097 3.78 1165 151.7 76.67 51.58 38.14 9.2 1.44

0.0123 3.79 1160 151 76.17 50.3 37.15 6.66 1.44

0.0152 3.79 1199 155.3 75.83 50.44 37.17 4.14 1.43

0.0316 3.80 1183 154.9 77.58 50.88 36.48 1.8 1.46

0.0404 3.82 1093 152.8 76.3 50.52 31.13 1.05 1.41

0.0512 3.84 940.9 153 75.57 49.67 26.85 0.53 1.45

0.0745 3.89 685 152.2 75.79 50.23 18.31 0.2 1.47

0.1025 3.91 395.7 152.5 73.2 49.55 10.61 0 1.46

0.1892 6.26 2.97 131.6 3.53 21.34 4.64 0 1.37

0.2999 7.35 0.32 135 1.94 2.15 2.83 0 0.61

0.4003 8.04 0.55 18.2 2.02 0.36 2.08 0 0.21

0.5238 9.15 1.95 6.8 3.24 0.12 1.4 0 0.12

229

Table SI 5.4: ICP-OES analysis for AMD water treated with Bayer liquor

Volume (mL) pH Concentration (mg/L)


0.00 3.73 1233 161.5 77.26 48.89 36.94 16.7 1.54

0.25 3.75 1119 144.50 75.92 46.62 35.66 1.70 1.06

0.50 3.77 1051 144.50 75.51 46.65 31.04 0.92 1.05

0.75 3.82 807.9 143.30 75.20 45.90 21.61 0.35 1.04

1.00 3.97 639.3 142.5 72.68 46.74 16.25 0.16 1.06

1.25 4.21 332.6 136.20 70.98 43.35 11.10 0.16 1.00

1.50 4.97 133 144.9 54.4 44.91 4.87 0.19 1.36

2.00 6.49 2.28 139.1 2.99 23.8 6.79 -0.06 1.07

2.50 7.85 1.31 111.7 0.45 0.44 1.87 -0.06 0.09

3.00 8.29 0.31 57.9 0.38 0.09 1.2 -0.1 0.08

3.50 8.56 0.41 23.3 0.3 0.02 0.08 0.02 0.06

4.00 8.69 0.27 11 0.3 0.01 0.96 -0.12 0.01

4.50 8.95 1.04 7.3 0.35 0.03 0.68 -0.11 0.08

5.00 8.95 4.77 3.78 0.55 0.16 0.77 0.31 0.03

230

Table SI 5.5: ICP-OES analysis for AMD water treated with Bayer precipitates



0.0000 3.75 1233 161.5 77.26 48.89 36.94 16.7 1.54

0.0018 3.75 1227 157.2 74.2 47.75 37.48 15 1.36

0.0045 3.81 1242 158.3 73.75 47.44 38.49 8.22 1.34

0.0062 3.81 1256 160.3 73.25 46.02 36.26 6.5 1.37

0.0081 3.81 1312 166 75.72 47.3 36.62 4.53 1.44

0.0099 3.82 1288 164.1 72.59 46.22 36.27 3.56 1.32

0.0125 3.82 1282 161.7 73.17 46.93 37.1 2.84 1.36

0.0148 3.85 1254 158 71.43 45.86 35.09 3.31 1.33

0.0298 3.88 1225 158.9 68.27 44.99 33.9 1.74 1.29

0.0408 3.92 1208 162.3 63.07 43.43 31.37 1.7 1.29

0.0501 3.93 1120 159.6 57.76 42 28.14 1.2 1.25

0.0760 3.94 697.2 155.5 51.34 40.01 7.92 0.36 1.19

0.1010 3.95 548.8 158.4 44.47 36.7 5.81 0.16 1.08

0.1253 4.01 363.1 151.7 32.42 31.79 6.09 0.1 1.01

0.2509 4.02 421.1 152.9 34.63 31.74 6.38 0.14 0.96

0.5025 6.05 2.65 67.2 0.34 0.48 0.98 0.46 -0.01

1.0040 7.57 0.91 32.3 0.11 0.16 1.28 0.05 -0.02

1.2500 7.80 0.55 32.64 0.55 0.26 1.56 0.01 0.03

1.5000 8.00 0.19 27.74 0.19 0.03 1.47 0.01 0.01

231

Figure SI 5.1: IR spectrum of precipitates obtained from treatment of AMD with

different alkali

232


Table SI 6.1: Mn, Mg, Al, Ca and S concentration (mg/L) in solution after treatment

with Bayer precipitates

pH


Mg Mn Ca Al S

3.25 0 163.1 0 0 117.7

3.41 6.15 155.3 12.31 1.99 106.9

3.99 12.47 152.3 11.25 2.6 102.5

5.42 16.31 151.5 10.45 <0.05 108

5.79 17.62 150.1 10.82 <0.05 117.7

6.28 15.59 143.6 12.04 <0.05 102.2

6.53 19.76 141.9 11.38 <0.05 107.9

6.75 21.39 136.7 12.21 <0.05 91.01

6.91 24.08 134 11.4 <0.05 103.2

7.1 27.46 132 10.2 <0.05 106.2

7.25 35.62 129.1 11.74 <0.05 108.5

7.46 41.02 122.8 12.78 <0.05 108.5

7.76 76.08 79.49 16.5 <0.05 118.7

7.97 124.8 5.43 27.89 <0.05 107

8.03 147 0.68 43.96 <0.05 104.2

233


with thermally activated Bayer precipitates at 320 °C

pH Concentration (mg/L)

Mg Mn Ca Al S

3.25 0 163.1 0 0 117.7

3.65 3.07 155.3 13.08 1.04 98.33

3.99 11.24 152.3 7.17 1.37 106.8

5.25 16.3 149.6 11.25 <0.05 101.9

6.05 17.4 148 11.47 <0.05 103

6.31 18.15 143.6 8.4 <0.05 100.6

6.54 20.15 134.4 23.53 <0.05 99.34

6.62 29.79 115.4 12.48 <0.05 101.3

6.71 37.07 92.45 12.79 <0.05 98.16

6.95 50.27 65.28 14.06 <0.05 94.47

7.21 68.35 21.65 13.83 <0.05 93.03

7.65 79.88 1.83 42.92 <0.05 91.29

8.15 77.55 0.03 41.09 <0.05 89.24

8.45 66.85 0.0209 50.61 <0.05 87.23

234




Mg Mn Ca Al S

3.25 0 163.1 0 0 117.7

3.67 6.88 154.5 5.37 1.88 100.5

4.13 9.19 151.5 8.85 1.53 100

5.67 14.13 149.6 8.57 <0.05 100.2

6.1 15.8 148 9.51 <0.05 99.16

6.35 18.93 143.5 11.48 <0.05 109.6

6.53 34.53 92.57 12.85 <0.05 89.29

6.7 54.71 50.02 16.84 <0.05 102.8

6.89 75.71 13.23 25.14 <0.05 93.92

7.45 71.45 0.76 29.59 <0.05 103.1

7.8 60.91 0.7 36.21 <0.05 96.81

8.36 21.28 0.55 72.71 <0.05 88.35

9.42 0.68 0.1 104.7 <0.05 64.19

10.25 0.2327 0.24 120.5 <0.05 56.5

235




Mg Mn Ca Al S

3.25 0 163.1 0 0 117.7

3.7 4.01 155.3 6.14 1.42 104.7

4.25 14.16 151.5 12.6 <0.05 96.27

5.72 13.1 146.6 26.98 <0.05 100.8

6.56 14.29 135.7 16.32 <0.05 98.46

6.68 15.91 124.6 15.89 <0.05 88.65

6.83 29.5 77.12 27.51 <0.05 96.07

6.99 42.54 47.33 44.67 <0.05 87.9

7.81 53.61 0.75 60.89 <0.05 92.06

7.95 42.37 0.03 72.93 <0.05 75.78

8.5 32.49 0.01 108.6 <0.05 81.16

9.45 16.69 0 112.7 1.16 76.68

10.55 1.18 0.08 229.7 1.35 78.73

10.67 0.0429 0.0004 387.5 1.24 77.6

236


Table SI 7.1: ICP-OES analysis for Open pit AMD water treated with Bayer

precipitates


Al Mn Cu Zn Fe Ni

0.0001 3.75 1233 161.5 77.26 48.89 16.7 1.54

0.0024 3.75 1227 157.2 74.2 47.75 15 1.36

0.0045 3.81 1242 158.3 73.75 47.44 8.22 1.34

0.0062 3.81 1256 160.3 73.25 46.02 6.5 1.37

0.0081 3.81 1312 166 75.72 47.3 4.53 1.44

0.0099 3.82 1288 164.1 72.59 46.22 3.56 1.32

0.0125 3.82 1282 161.7 73.17 46.93 3.31 1.36

0.0148 3.85 1254 158 71.43 45.86 2.84 1.33

0.0298 3.88 1225 158.9 68.27 44.99 1.74 1.29

0.0408 3.92 1208 162.3 63.07 43.43 1.7 1.29

0.0501 3.93 1120 159.6 57.76 42 1.2 1.25

0.076 3.94 697.2 155.5 51.34 40.01 0.36 1.19

0.101 3.95 548.8 158.4 44.47 36.7 0.01 1.08

0.1253 4.01 421.1 152.9 34.63 31.74 0.01 0.96

0.2509 4.28 2.65 67.2 0.34 0.48 0.01 0.03

0.5025 6.05 0.91 32.64 0.15 0.26 0.01 0.03

1.004 7.57 0.55 32.3 0.11 0.16 0.01 0.03

1.25 7.61 0.19 27.74 0.1 0.03 0.01 0.03

237

Table SI 7.2: ICP-OES analysis for Mundic West AMD water treated with Bayer

precipitates


Al Mn Cu Zn Fe Ni

0.0001 2.85 1516 188.5 83 54.96 241 1.49

0.0023 3.01 1516 191.7 83.1 55.58 150.9 1.49

0.0045 3.03 1495 188.8 81.9 54.96 130.9 1.47

0.0059 3.05 1504 187.2 82.1 54.65 67.71 1.52

0.0085 3.09 1534 189.7 83.4 54.72 34.35 1.52

0.0101 3.27 1522 189.6 81.9 55.9 19.79 1.53

0.0129 3.44 1561 190.7 81.8 56.48 19.72 1.52

0.0157 3.59 1542 190.3 82.5 54.92 13.69 1.5

0.0301 3.75 1550 190.3 83.3 55.7 12.86 1.51

0.0409 3.78 1521 187.1 78.9 55.04 9.44 1.47

0.0515 3.76 1489 185.5 76.5 53.86 9.27 1.5

0.0751 3.76 1149 183.8 63.3 49.53 3.43 1.4

0.0993 3.75 982.4 183.6 54.5 46.29 3.37 1.34

0.1289 3.8 761.7 185.6 49.2 43.06 2.19 1.23

0.252 4.25 5.51 175.2 1.46 16.72 0.01 0.7

0.4984 6.07 1.06 83.1 0.13 0.28 0.01 0.02

1.0041 7.48 1.26 44.4 0.13 0.19 0.01 0.01

238

Table SI 7.3: ICP-OES analysis for Airfield AMD water treated with Bayer

precipitates


Al Mn Cu Zn Fe Ni

0.0001 2.7 1702 186 101.2 72.8 194.4 1.23

0.0024 2.88 1722 184.4 101.8 72.4 127.9 1.23

0.0036 2.89 1854 198.3 108.9 76.9 98.6 1.24

0.006 2.91 1779 191.6 104.9 74.7 88.7 1.2

0.0085 2.98 1788 189 102.1 73.4 48.4 1.24

0.0098 3.15 1810 190.2 100.2 73.5 46.7 1.19

0.0126 3.35 1715 181.2 97.4 70.1 39 1.23

0.0147 3.45 1772 190.6 103.8 74.5 38.7 1.24

0.0301 3.73 1742 186.9 103.3 72.8 28.4 1.19

0.0405 3.79 1734 184.5 101.2 72.9 24.7 1.22

0.0499 3.8 1775 188.5 103.4 74 26.8 1.23

0.0789 3.8 1469 183.8 86.4 67.6 4.6 1.15

0.1035 3.81 1227 187.4 77.4 67.3 3.9 1.16

0.125 3.85 927.2 162.4 66 61.4 2.34 1.09

0.2485 4.32 371.8 154.3 39.2 48.6 0.01 0.79

0.5085 6.5 0.004 108.8 2.1 0.52 0.01 0.04

0.9904 7.47 0.003 48.42 1.5 0.12 0.01 0.003

239

Table SI 7.4: ICP-OES analysis for Open pit AMD water treated with Thermally

activated Bayer precipitates


Al Mn Cu Zn Fe Ni

0.0001 3.75 1233 161.5 77.26 48.89 16.7 1.54

0.0013 3.76 1105 148.6 77.9 49.03 5.748 1.12

0.0024 3.76 1113 148 76.4 49.28 5.264 1.13

0.0038 3.77 1115 149.3 77.3 47.94 3.446 1.1

0.0050 3.78 1165 154.1 81 49.93 2.599 1.13

0.0060 3.79 1118 147.6 76.7 48.47 2.359 1.09

0.0078 3.8 1067 139.7 72.5 54.69 2.088 1.26

0.0096 3.8 1113 144.3 74.7 47.76 1.848 1.09

0.0185 3.81 1140 152.5 74.8 45.58 0.958 1.06

0.0247 3.87 992.3 146.9 69.7 45.56 0.578 1.04

0.0328 3.91 945.1 169.4 75.6 46.71 0.285 1.1

0.0470 3.95 569.2 149.8 57.6 40.63 0.101 0.97

0.0637 4.04 248.3 151.2 47.9 38.9 0.014 0.93

0.0777 4.24 17.6 145.8 20.9 31.39 0.005 0.85

0.1563 4.78 0.96 77.1 1.1 0.86 0.004 0.06

0.3168 6.2 1.46 40.2 0.6 0.1 0.004 0.02

0.4781 7.3 1.38 29.5 0.6 0.05 0.004 0.02

0.6225 7.55 1.37 20.3 0.6 0.04 0.004 0.01

0.8070 7.88 1.21 13.6 0.6 0.04 0.004 0.01

240

Table SI 7.5: ICP-OES analysis for Mundic West AMD water treated with

Thermally activated Bayer precipitates


Al Mn Cu Zn Fe Ni

0.0001 2.85 1516 188.5 83 54.96 241 1.49

0.0013 2.91 1425 173.4 81.4 56.4 148.4 1.16

0.0026 2.95 1385 168.4 80.6 54.6 128.6 1.14

0.0041 3.02 1574 184 87.3 59.7 62 1.17

0.0049 3.1 1401 170.6 79.7 55.1 37.1 1.19

0.0062 3.2 1368 169.5 79.8 55.1 18.8 1.18

0.0076 3.45 1402 171.8 81.3 55.8 15.8 1.17

0.0095 3.54 1489 182.6 86.4 60 19.1 1.18

0.0134 3.65 1482 181.8 85 59.2 30.3 1.17

0.0187 3.76 1444 173.5 81 56.5 31.9 1.17

0.0250 3.82 1467 174.9 79.7 56.2 29.2 1.14

0.0318 3.86 1459 177.5 78.4 56.2 27 1.15

0.0468 3.97 1177 178.3 72.1 54 8.3 1.07

0.0671 4.01 628.5 162.7 55.2 46.9 1.65 1

0.0811 4.18 279.4 160.6 46 43 0.38 0.95

0.1576 4.78 1.97 110.1 0.74 1.58 0.05 0.15

0.3164 5.92 1.73 47.7 0.15 0.12 0.05 0.02

0.4666 7.2 1.59 39.8 0.09 0.02 0.05 0.01

0.6231 7.84 1.47 37.2 0.07 0.02 0.05 0.01

0.8018 8.01 1.41 27.1 0.07 0.02 0.05 0.01

241

Table SI 7.6: ICP-OES analysis for Airfield AMD water treated with Thermally

activated Bayer precipitates


Al Mn Cu Zn Fe Ni

0.0001 2.7 1702 186 101.2 72.8 194.4 1.23

0.0013 2.83 1539 169.8 92.7 65.7 149.3 0.95

0.0024 2.88 1556 170.3 94.1 66 91.6 0.99

0.0036 2.92 1552 170.6 93.6 66.3 62.8 0.99

0.0050 3.01 1595 175.1 96.3 68.2 39.6 0.97

0.0064 3.18 1681 191.9 100 71.3 18.9 0.97

0.0095 3.45 1637 177.6 98.7 69.6 19.9 0.98

0.0124 3.6 1610 172.4 93.3 66.8 31.2 0.97

0.0180 3.74 1634 170.1 92.3 66.1 44.8 0.93

0.0259 3.77 1684 173.5 94.5 67.4 44.6 0.94

0.0316 3.78 1599 167.2 90.1 64.9 41.9 0.92

0.0468 3.81 1582 178.7 86.4 66.2 24 0.88

0.0634 3.85 1178 171.5 73.1 60.4 8.1 0.86

0.0803 3.91 1125 207.5 81.3 70.6 4.9 0.79

0.1659 5.2 1.34 150.5 1.76 12.6 0.01 0.57

0.3196 6.77 0.6 60.3 0.16 0.13 0.01 0.03

0.4595 7.77 0.65 46.7 0.13 0.05 0.01 0.01

0.6300 7.83 0.43 37.22 0.11 0.04 0.01 0.01

0.7754 7.84 0.38 30.41 0.09 0.03 0.01 0.01

242

Figure SI 7.1: IR spectrum of precipitates obtained from treatment of different AMD

waters with Bayer precipitates

243

Figure SI 7.2: IR spectrum of precipitates obtained from treatment of different AMD

waters with thermally activated Bayer precipitates

Documents

CID MINE DRAINAGE TREATMENT OPTIONS USING ... Kaur_Thesis...Acid mine drainage water, characterised as having a low pH, and high metal and sulphate concentrations, has a detrimental