Transport of Nanoscale Zero Valent Iron Using ... · electrokinetic phenomena to deliver the treatment chemical (nanoscale zero valent iron) to the desired zone. Interaction of nanoscale

Transport of Nanoscale Zero

Valent Iron Using

Electrokinetic Phenomena

2006

Angus Adams

10126688

Supervisor: Dr. David Reynolds

Transport of Nanoscale Zero Valent Iron Using Electrokinetic Phenomena

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University of Western Australia ii

Abstract

DNAPL (Dense Phase NonAqueous Liquid) contamination is a large problem facing

today’s groundwater sources. One of the most prevalent classes of DNAPLs is the

halogenated aliphatics class of chemical compounds. Many techniques currently exist

for the treatment of groundwater zones contaminated with halogenated aliphatics, but

are not always feasible due to certain factors, such as cost, aquifer or hydraulic

conductivity restraints. Using electrokinetics to deliver nanoscale zero valent iron to

remediate the contamination zone is one possibility of overcoming such problem sites.

Electrokinetic phenomena are induced by applying a direct current voltage across the

target zone to induce movement of the desired species. Species can be moved via

electroosmosis, electromigration, electrophoresis or a combination of the three.

Traditional electrokinetic studies have utilised electrokinetics to induce movement of

the contaminant to the electrodes. This study however, examines the ability of

electrokinetic phenomena to deliver the treatment chemical (nanoscale zero valent

iron) to the desired zone.

Interaction of nanoscale zero valent iron slurry with varying classes of electrodes was

investigated and found to form iron cation complexes at the cathode. The ability to

transmit nanoscale zero valent iron through a porous media matrix was also

investigated, and found that transmission rates were extremely small. The attempts to

induce electrokinetically driven movement using both the cathode and the anode

indicated that both electrodes were not capable of significant movement of the

nanoscale zero valent iron through the porous media matrix, although the nanoscale

zero valent iron did exhibit a much stronger affinity for the cathode than the anode. It

was also found that the nanoscale zero valent iron was ineffective at penetrating the

porous media matrix under a hydraulic gradient, probably due to the nanoscale zero

valent iron agglomerating to form particles that were too large to effectively migrate

through the porous media matrix. It was thus determined that electrokinetic induced

movement of nanoscale zero valent iron is not feasible in cases where the nanosale

zero valent iron can not be moved due to a hydraulic gradient.


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University of Western Australia iii

Acknowledgements

Dr. David Reynolds’ mentoring and guidance throughout the study;

Cara Moreland for her support and hours of devoted editing;

Diane and Robert Adams for enabling me to get this far;

Matthew Chatley for the brainstorming and workshop skills;

Dr. Ismail Yusoff for his laboratory help.


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University of Western Australia iv

Table of Contents

Table of Contents ..................................................................................................... iv

1 Introduction......................................................................................................11

2 Literature Review..............................................................................................13

2.1 Remediation in the Saturated Zone............................................................14

2.2 Methods of Remediation in the Saturated Zone .........................................15

2.2.1 Excavation ............................................................................................15

2.2.2 Pump-and-treat......................................................................................15

2.2.3 Soil Vapour Extraction..........................................................................15

2.2.4 Thermal Treatment................................................................................16

2.2.5 In-situ Flushing.....................................................................................16

2.2.6 Passive Reactive Barriers ......................................................................16

2.2.7 Mass destruction ...................................................................................17

2.2.8 Biological Remediation.........................................................................18

2.2.9 Containment..........................................................................................18

2.3 Zero valent iron.........................................................................................18

2.3.1 Zero valent iron history .........................................................................18

2.3.2 Zero Valent Iron Chemistry...................................................................19

2.3.3 Zero Valent Iron Advantages.................................................................24

2.3.4 Nano-scale Zero Valent Iron .................................................................24

2.3.5 Zero valent iron delivery .......................................................................24

2.4 Diffusion...................................................................................................25

2.5 Electrokinetics ..........................................................................................26

2.5.1 Electroosmosis ......................................................................................27

2.5.2 Electrophoresis......................................................................................28

2.5.3 Electromigration ...................................................................................29

2.5.4 Electrolysis ...........................................................................................30

2.6 DNAPLS and chlorinated solvent contamination of groundwater ..............31

2.7 Agglomeration chemistry..........................................................................32

2.8 Site applicability .......................................................................................33

2.9 Costings....................................................................................................34


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University of Western Australia v

3 Methodology Development ...............................................................................36

3.1 Instrument Calibration ..............................................................................36

3.2 Power supply ............................................................................................37

3.3 Iron Concentration in Slurry Determination ..............................................37

3.4 Single containment vessel experiments .....................................................38

3.5 Dual containment vessel experiment with unhindered flow .......................38

3.5.1 Electrodes .............................................................................................38

3.5.2 Containment Vessel ..............................................................................40

3.5.3 Mixing method......................................................................................41

3.5.4 NaCl experiment ...................................................................................42

3.5.5 Nanoscale Zero Valent Iron Supply.......................................................43

3.5.6 Nanoscale zero valent iron experiment with no porous media................43

3.6 Dual containment vessel experiment with porous media flow and orbital

mixing method......................................................................................................45

3.6.1 Mixing method......................................................................................45

3.6.2 Orbital mixer board construction...........................................................45

3.6.3 Manufacturing of additional side ports in the connecting tube ...............46

3.6.4 Needle selection ....................................................................................47

3.6.5 Silica filling of tube/screen installation..................................................47

3.6.6 Initial containment vessel experiment with porous media flow and orbital

mixing...............................................................................................................50

3.6.7 Sampling technique development ..........................................................50

3.6.8 Second dual containment vessel experiment with porous media flow and

orbital mixing....................................................................................................50

3.6.9 Side port construction in the connecting tube ........................................51

3.6.10 Filling of connecting tube with porous media. ...................................51

3.6.11 Third dual containment vessel experiment with porous media and

orbital mixing....................................................................................................52

3.7 Dual containment vessel experiment with porous media flow and

mechanical mixing method ...................................................................................52

3.7.1 Mechanical mixing................................................................................52

3.7.2 Non-metallic mixing paddle construction ..............................................52


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University of Western Australia vi

3.8 Experiment using mechanical mixing........................................................53

3.9 Initial direct injection of nanoscale zero valent iron into porous media

experiment............................................................................................................54

3.10 Direct injection of nanoscale zero valent iron into porous media experiment

with enhanced conductivity...................................................................................55

3.11 Initial hydraulic advection experiment ......................................................56

3.12 Iron concentration sampling......................................................................56

4 Results ..............................................................................................................58

4.1 Iron concentration determination...............................................................58

4.2 Single containment vessel experiment.......................................................58

4.3 Dual containment vessel experiment with no porous media.......................60

4.3.1 Sodium Chloride (NaCl) experiments....................................................60

4.3.2 Initial zero valent iron experiment.........................................................62

4.3.3 Second zero valent iron experiment.......................................................63

4.4 Dual containment vessel experiment with porous media and orbital

mixing… ..............................................................................................................64

4.4.1 Initial experiment ..................................................................................64

4.4.2 Second experiment................................................................................65

4.4.3 Third experiment...................................................................................66

4.5 Dual containment vessel experiment with porous media and mechanical

mixing.. ................................................................................................................68

4.6 Dual containment vessel experiment with porous media and direct

injection................................................................................................................70

4.7 Dual containment vessel experiment with porous media and direct injection

with enhanced conductivity...................................................................................72

4.8 Hydraulic advection experiment................................................................74

5 Discussion.........................................................................................................76

5.1 Iron concentration determination...............................................................76

5.2 Single containment vessel .........................................................................77

5.3 Dual containment vessels with unhindered flow........................................78

5.3.1 NaCl experiment at 20 volts ..................................................................78

5.3.2 NaCl experiment at 10 volts ..................................................................79


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University of Western Australia vii

5.3.3 Initial experiment with zero valent iron and porous media.....................79

5.3.4 Second experiment with zero valent iron and no porous media..............79

5.4 Dual containment vessel experiment with porous media and orbital

mixing… ..............................................................................................................80

5.5 Experiment with mechanical mixing .........................................................81

5.6 Experiment with direct injection ...............................................................81

5.6.1 Initial direct injection experiment..........................................................81

5.6.2 Direct injection experiment with enhanced conductivity........................82

5.7 Hydraulic advection experiment................................................................83

6 Conclusion ........................................................................................................84

6.1 Electrokinetics and nanoscale zero valent iron...........................................84

6.2 Recommendations.....................................................................................84

7 Glossary ............................................................................................................86

8 References.........................................................................................................88


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University of Western Australia viii

List of Tables Table 2.1 - Suitability of zero valent metals for treatment of various compounds......19

Table 2.2 – Electroosmotic Flux Factors...................................................................28

Table 3.1 – Summary of experimentation stages.......................................................36


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University of Western Australia ix

Figure 2.1 – Diagram demonstrating three mechanisms of halogenated aliphatic

degeneration by zero valent iron...............................................................................22

Figure 2.2 – Zero valent iron reactions .....................................................................23

Figure 3.1 – Capped Pt/Ti/Cu electrodes ..................................................................40

Figure 3.2 – Aperture with silicone sealant applied to prevent leaking......................41

Figure 3.3 – Containment vessels with connecting tube ............................................41

Figure 3.4 – Top view of connecting tube with three sampling ports fitted with

flexible tubing ..........................................................................................................46

Figure 3.5 – Side view of connecting tube with three sampling ports fitted with

flexible tubing ..........................................................................................................47

Figure 3.6 – Connecting tube filled with porous media .............................................48

Figure 3.7 – Connecting tube ready for insertion between two containment vessels. .49

Figure 3.8 – Semi filled connecting tube capped with pink screens. ..........................49

Figure 3.9 – Wooden paddle used for mechanical mixing. ........................................53

Figure 3.10 – Connecting tube featuring injection of nanoscale zero valent iron

through the flexible tubing .......................................................................................55

Figure 3.11 – Containment vessel with slit in side for constant hydraulic head .........56

Figure 4.1 – Steel electrodes after operation in nanoscale zero valent iron slurry. .....59

Figure 4.2 – Slurry reaction at cathode. ....................................................................60

Figure 4.3 – NaCl experiment conducted at 20 volts. ................................................61

Figure 4.4 – NaCl experiment conducted at 10 volts. ................................................62

Figure 4.5 – Aged Zero Valent Iron Experiment Iron Concentration.........................63

Figure 4.6 – Aged Zero Valent Iron pH ....................................................................63

Figure 4.7 – Total iron concentration versus time for second experiment without

porous media............................................................................................................64

Figure 4.8 – Voiding along the top of the connecting tube ........................................65

Figure 4.9 – Connecting tube featuring voiding ........................................................66

Figure 4.10 – Voiding due to orbital motion of mixer ...............................................67

Figure 4.11 – Cathode and Anode after experimentation...........................................67

Figure 4.12 – Orbital experiment mixing experiment nanoscale zero valent iron

concentrations ..........................................................................................................68

Figure 4.13 – Nanoscale zero valent iron penetration of porous media......................69


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University of Western Australia x

Figure 4.14 – Cathode and anode after experimentation............................................69

Figure 4.15 – Mechanical mixing experiment nanoscale zero valent iron

concentrations ..........................................................................................................70

Figure 4.16 – Core sample of connecting tube featuring no visible nanoscale zero

valent iron penetration..............................................................................................71

Figure 4.17 – Direct injection experiment nanoscale zero valent iron concentration of

both anodic and cathodic containment vessels ..........................................................71

Figure 4.18 – Iron concentrations for the NaCl dosed direct injection experiment.....73

Figure 4.19 – pH and conductivity record of the NaCl dosed direct injection

experiment ...............................................................................................................73

Figure 4.20 – Amperage drawn during the NaCl dosed direct injection experiment ..74

Figure 4.21 – Core sample of connecting tube after hydraulic advection experiment.75

Figure 4.22 – Hydraulic Advection Experiment Iron Concentrations ........................75

Figure 5.1 – Powered electrodes immersed in a nanoscale zero valent iron slurry .....78

Chapter 1: Introduction

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University of Western Australia 11

1 Introduction

Dense NonAqueous Phase Liquid (DNAPL) contamination of groundwater suitable

for human consumption is prevalent throughout the world (Pankow and Cherry 1996).

DNAPLs can reside in the groundwater for years, providing a source of pollution for

decades (Pankow and Cherry 1996). One of the most prevalent DNAPLs to pollute

current groundwater supplies is trichloroethylene (TCE) (Westrick 1983).

Numerous treatment strategies exist for remediation of polluted groundwater, such as

excavation, pump and treat, passive reactive barriers and containment. These

strategies often are not feasible for many contaminated sites due to the characteristics

of the site. Many treatment strategies are not effective for sites posessing a low

hydraulic conductivity as they rely on hydraulic soil flushing.

Electrokinetics, also known as electroreclamation, electrokinetic soil processing,

electrokinetic extraction, electrodialytic remediation and electrochemical

decontamination is the application of a DC current to induce the movement of

chemical species. Electrokinetic phenomena comprise of (i) electromigration – the

movement of charged ions due to an electric potential difference, (ii) electrophoresis –

the movement of colloids or macromolecules due to an electric potential difference

and (iii) electroosmosis – the bulk movement of water due to an electric potential

difference. Electrokinetics is not affected by the hydraulic conductivity of the soil

matrix, and thus has the potential to be a treatment technique for soils possessing low

hydraulic conductivities (Van Cauwenberghe 1997). Traditional electrokinetic

remediation techniques often rely on the elecktrokinetic movement of the contaminant

to the electrode. This study focuses on the ability to move a treatment compound –

nanoscale zero valent iron – to the source of the contamination.

Zero valent iron posseses the ability to degrade halogenated aliphatics, such as TCE.

The zero valent iron oxidises halogenated aliphatics, yielding a dehalogenated

aliphatic and an iron cation (Matheson 1994). Nanoscale zero valent iron has been

shown to be even more effective than granular zero valent iron at reducing

halogenated aliphatics (Gavaskar et al. 2005b).

Chapter 1: Introduction

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Due to the inability of electrokinetics to induce movement of DNAPLs, this study’s

purpose was to investigate the ability of electrokinetic phenomena to transmit

nanoscale zero valent iron through a saturated porous media matrix. Two

containment vessels were hydraulically connected via a connecting tube filled with

porous media. The anode was positioned in a containment vessel and the cathode in

the other. The nanoscale zero valent iron was then placed in one containment vessel

and the other was monitored for an increase in iron concentration. Multiple trials

were conducted to test both the anode and the cathode for nanoscale zero valent iron

transmission. Various methods of suspending the nanoscale zero valent iron were

also tested. Samples were then analysed using an atomic absorption spectroscopy

(AAS) for total iron content. The interaction between a nanoscale zero valent iron

slurry and powered anodes and cathodes was also investigated.

Chapter 2: Literature Review

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2 Literature Review

Freeze and Cherry (1979) define groundwater contaminants as “all solutes introduced

into the hydrologic environment as a result of man’s activities” and groundwater

pollution as when “contaminant concentrations attain levels that are considered to be

objectionable”. Another definition of groundwater contamination provided by Miller

(1980) is “the degradation of the natural quality of groundwater as a result of man’s

activities”. Matthess (1982) believes polluted groundwater occurs when the

concentration of the contaminant exceeds the maximum permissible concentration for

potable water.

To put the expectations of groundwater remediation into perspective, billions of

dollars have been spent by the petroleum industry to increase yields of hydrocarbons

from identified reserves. Whilst this great sum of money has been invested in

hydrocarbon extraction, the industry considers an exceptional yield to be between 30-

40% of the total mass of petroleum products. Contrastingly, it is a normal occurrence

to expect a 99.9% removal of contaminants from a polluted groundwater source to

consider it remediated (Pankow, 1996).

Groundwater contamination and pollution has been recognized as early as the mid

nineteenth century, as evidenced by Dr. John Snow’s work connecting seepage from

privy vaults to the cholera contamination of wells in 1854 (Malman and Mac 1961).

The problem of groundwater contamination is a vast one. It was estimated that it

would take 4 to 5 years to conduct one series of test on the public water supply wells

in Illinois, which represent just below 7% of total wells in the state (Illinois EPA

1986).

In 1982, it was estimated that one percent of economically producible groundwaters

were contaminated. This contamination may be more significant than the figure

implies, due to many of the contaminated sites being in close proximity to heavily

populated areas (Gass 1982).


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Groundwater contamination can occur in four distinct ways (Barcelona et al 1990):

i) Infiltration. This is the most common mechanism for contamination of

groundwater to occur. It involves the contaminant moving from the

surface to the groundwater below it through pore spaces in the soil.

ii) Direct Migration. This occurs when a source already within the saturated

zone leaks into the surrounding groundwater, such as a pipeline.

iii) Interaquifer Exchange. The mixing of uncontaminated groundwater with

contaminated groundwater when the bodies of water are hydraulically

connected.

iv) Recharge from Surface Water. When contaminated surface water bodies

come into contact with nearby groundwater.

Sources of groundwater contamination fall into six different categories (OTA 1984):

1) Sources designed to discharge substances

2) Sources designed to store, treat, and/or dispose of substances;

discharge through unplanned release

3) Sources designed to retain substances during transport or

transmission

4) Sources discharging substances as consequence of other planned

activities

5) Sources providing conduit or inducing discharge through altered

flow patterns

6) Naturally occurring sources whose discharge is created and/or

exacerbated by human activity.

2.1 Remediation in the Saturated Zone

Various techniques exist for groundwater remediation in the saturated zone.

Irrespective of the technique utilised to clean up a contaminated site, factors such as;

i) Soil characteristics, heterogeneity and complexity

ii) Groundwater characteristics, heterogeneity and complexity

iii) Geochemical characteristics, heterogeneity and complexity


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must be analysed as they all influence remediation strategies (Henry et al 2002).

There are three classes of DNAPL remediation techniques used today. They are;

i) Containment

ii) Dissolved-Phase Destruction

iii) Saturated Zone Removal.

2.2 Methods of Remediation in the Saturated Zone

2.2.1 Excavation

The simplest method of remediating contaminated groundwater in the saturated zone

is by excavation, where the contaminated zone is excavated and removed. Suitable

excavation sites can be limited by cost, size and accessibility of contamination.

2.2.2 Pump-and-treat

Another technique is the ‘pump and treat’ method, in which a series of wells are

constructed to withdraw the contamination via pumping. Pump-and-treat is the most

used technique for remediation of chlorinated solvent sites (Henry et al 2002). The

application of this technology consists of extracting groundwater from one or more

strategically constructed wells. The contaminated material is then collected and can

then be treated externally. Pump and treat methods can be prohibitively expensive

and are also influenced by hydraulic conductivity, and in some cases, have operated

for long periods of time (sometimes over a decade) without appreciably reducing the

contamination concentration (Pankow 1996). Pump-and-treat techniques are ‘best

thought of as a management tool to prevent, by hydraulic manipulation of the aquifer,

continuation of contaminant migration’ (Mackay et al 1989), which highlights the

limited abilities of such technology.

2.2.3 Soil Vapour Extraction

Soil vapour extraction is the most accepted technique for in-situ contaminant

remediation in the vadose zone (Henry et al 2002); however it can be applied to the


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saturated zone in some cases, which is known as multiphase extraction. It involves

applying a strong vacuum (up to 660 mm of mercury) to subsurface soils and

groundwater (Henry et al 2002). Air sparging can be conducted simultaneously with

soil vapour extraction, which agitates the targeted zone with air bubbles to volatilize

the contaminants, which are subsequently extracted. Soil vapour extraction is

however limited by its inapplicability to many sites, such as deeply penetrating and

hard to get to DNAPL contamination zones, as well as sites of low hydraulic

conductivity. (Henry et al 2002).

2.2.4 Thermal Treatment

To enhance contaminant removal, certain sites can be treated thermally. Thermal

treatment involves increasing the temperature of a contaminated zone to increase

volatility and vapour pressure of the contaminant/s, which can then be removed via

soil vapour extraction. Thermal treatment is limited to sites that can use soil vapour

extraction, and can also prove to be relatively expensive in the generation of heat

(Henry et al 2002).

2.2.5 In-situ Flushing

Injection of a chemical agent into the contaminated zone to increase solubility and/or

mobility is referred to as in-situ flushing. Typical additives for flushing involve co-

solvents (often in the form of alcohol), and surfactants (Henry et al 2002). In-situ

flushing is limited in that it is not applicable to soils with low hydraulic conductivity

(Thal 2006), and is only suitable for treating the most permeable sections of the

contamination site (Henry et al 2002).

2.2.6 Passive Reactive Barriers

A widely used technique to treat contaminated groundwater in-situ is by utilising

Passive Reactive Barriers (PRB). The location of the PRB must first be ascertained,

then the existing soil must be excavated and the void space filled with the reactive

medium with relatively high hydraulic conductivity.


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Some of the initial field testing of PRBs was done using zero valent elemental iron

filings (Gillham 1993) (Gillham 1994). More exotic remediation chemicals emerged

following zero valent iron’s success, such as dissolved chemicals and genetically

engineered bacteria (Pankow, 1996), however, the material of choice for use in PRBs

is zero valent iron (Henry et al 2002). Incorrect understanding of the frequently

complex hydrogeology of various contamination sites can lead to incorrect barrier

wall placement, which can leave contaminated zones outside of the barriers untreated,

such an example is the Hill Air Force Base in Utah, which left 3000 gallons of

DNAPL untreated outside of the installed barrier wall (Henry et al 2002).

Passive Reactive Barriers have proven to be effective at treating a great number of

contaminated groundwater plumes; however, they have certain limitations (Pankow

1996):

i) They only target contaminant plume, and not the source of contamination.

They therefore have to wait for the contaminant to be leached into or

advected with the groundwater before treatment can be initiated.

ii) They are unfeasible solutions in certain situations of complex

hydrogeologic conditions, such as fractured rock.

iii) Most Passive Reactive Barriers have been installed to a depth of

approximately 15 metres, although there have been instances of depths up

to 35 metres (Henry et al 2002). They cannot penetrate deep into the soil,

rendering them wholly ineffective with deep plumes, as they cannot reach

the target zone.

iv) A comprehensive understanding of the hydrogeologic conditions at the

contamination site is required for this technology to work, as the

positioning of PRB is of utmost importance.

2.2.7 Mass destruction

Mass destruction techniques are sometimes also employed, in which a reactive

chemical is pumped to the contaminant source zone, and is flushed throughout the

zone. Chemicals such as permanganate (MnO4-), hydrogen peroxide (H2O2), sodium

(Na), potassium (K), perchlorate (ClO4), ozone (O3) and certain enzymes have been


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used in the past to oxidise organic contaminants. Reducing chemicals, such as

sodium salts of dithionite (Na2S2O4) have also been used in the past (Henry et al

2002). However, these techniques require tight controlling of chemical conditions,

such as pH and eH values, and are often costly due to the relatively expensive nature

of the chemicals involved. Hydrogeoligic structure and flow paths can also limit the

effectiveness of such techniques (Pankow 1996), thus decreasing the viability of this

technique for low hydraulic conductivity zones. Henry et al (2002) states that the

chemical additives remain largely in the most permeable zones, and thus rarely reach

lesser permeable zones.

2.2.8 Biological Remediation

The majority of bioremediation approaches rely on stimulation of biodegradation by

the addition of organic carbon. The current effectiveness of biological remediation is

limited, as demonstrated by field observations which reveal a persistence of

hydrocarbons at treated sites (Henry et al 2002).

2.2.9 Containment

Various containment strategies exist for trapping a contaminant source zone or plume

inside an impermeable barrier and preventing it from spreading further without

treatment of the contaminant. As this is not a remediation technique, and merely a

prevention of additional contamination, it shall not be considered further.

2.3 Zero valent iron

2.3.1 Zero valent iron history

Nano-scale zero valent iron is an exciting technology for treating contaminated

groundwater. Iron was first recognised and patented in 1972 as a chlorinated

pesticide degrader (Sweeny 1972). In 1981, Sweeny (1981a 1981b) utilised iron

powders to degrade various hydrocarbons, such as trichloroethylene. Additional

suggestions for using zero valent iron to degrade trichloroethylene and

trichlorotethane were made in the late 1980’s by Senzaki (1988). However, it was not


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until after this point in time that focused work was conducted on using zero valent

iron to remediate polluted groundwater. Work published in the 1990’s revealed the

power of iron at remediating contaminated groundwater (Reynolds et al 1990),

(Gillham et al 1992). In 1993 a patent was lodged by the University of Waterloo for

using zero valent iron for treating contaminated groundwater in-situ, demonstrating

the identification of zero valent iron as a remediation constituent.

2.3.2 Zero Valent Iron Chemistry

Zero valent iron has been shown to react and degrade many types of chemicals

(Gavaskar 2005a), including halogenated aliphatics, polyhalogenated aromatics and

nitrates (Zawaideh 1997) and trichloroethene (Henry et al 2002). A table listing the

various compounds zero valent iron has proven to reduce is present in Table 1.1

below.

Table 2.1 - Suitability of zero valent metals for treatment of various compounds

(Henry et al 2002).

Treatment Material Contaminants Treated Untreatable Contaminants

methanes dichloromethane

ethanes 1,2 dichloroethane

ethenes aromatic hydrocarbons

Zero valent metals propanes polychlorinated biphenyls

chlorinated pesticides chlorobenzenes

freons chlorophenols

nitrobenzenes

Cr, U, As, Tc, Pb, Cd

The standard half reaction for zero valent iron reacting to yield a ferrous cation and 2

electrons is:

Fe0 Fe2+ + 2e- This reaction has a standard reduction potential of +0.44 V (Atkins 1998).


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Alkyl halides have a typical half reaction as such, where RX indicates a halogenated

hydrocarbon, and X- represents a halogen anion:

RX + 2e- + H+ RH + X- These types of half reactions have reduction potentials ranging from +0.5 V to +1.5 V

at pH 7 (Matheson 1994), the variation is attributed to the wide range of alkyl halides

that this reaction applies to.

When combined, these two half reactions yield a thermodynamically spontaneous

reaction:

Fe0 + RX + H+ Fe2+ + RH + X-

This constitutes the most basic mechanism for halogenated hydrocarbon degradation

by zero valent iron, yielding a ferrous cation, an aliphatic hydrocarbon and a halogen

anion (Matheson 1994).

A second mechanism for degeneration of halogenated hydrocarbon by zero valent

iron is the oxidation of zero valent iron to a ferrous cation by water (Matheson 1994).

The ferrous ion then further oxidises to a ferric cation by the following half equation:

Fe2+ Fe3+ + e- This oxidation reaction can be coupled with the reduction half equation to reduce the

alkyl halide shown above to yield:

Fe2+ + RX + H+ Fe3+ + RH + X-

This is a second mechanism for the degradation of an alkyl halide (Matheson 1994)

by zero valent iron and is portrayed in Figure 2.1 as reaction (B).

Matheson (1994) describes a third mechanism, portrayed by reaction (C) in Figure

2.1, which involves the zero valent iron reacting with water to yield the ferrous cation,

the hydroxyl anion and hydrogen gas (H2). It is a combination of the two following

half reactions:

Fe0 Fe2+ + 2e-

H2O + e- H2 + OH-


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To produce:

Fe0 + 2H2O Fe2+ + 2H2 + 2OH-

The H2 gas generated can then continue on to react with an alkyl halide in a reaction

known as an addition reaction to yield a dehalogenated aliphatic, a halogen anion and

a proton in the following manner:

RX + H2 RH + X- + H+ It is important to note that the H2 can only react with the alkyl halide if a suitable

catalyst is present. Matheson (1994) states that the iron surface, defects or additional

solid constituents may provide such catalysis.


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Figure 2.1 – Diagram demonstrating three mechanisms of halogenated aliphatic

degeneration by zero valent iron (Matheson 1994).


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It is important to note that the protons generated from the above reactions are capable

of further reducing chlorinated hydrocarbons. Subsequently, it has been shown that

hydrogenation is insignificant, and an oxide layer encapsulates most iron particles.

Alkyl halides are now thought to react with zero valent iron in corrosion pits in which

Fe0 is exposed, as shown in the top most diagram in Figure 2.2, for the oxide layer

acting as a semi-conductor to facilitate the reduction-oxidation reaction, as shown in

the middle diagram in Figure 2.2, or for the oxide layer to coordinate Fe2+ to reduce

the alkyl halide (Center for Groundwater Research 2002).

Figure 2.2 – Zero valent iron reactions


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(Center for Groundwater Research, 2002)

2.3.3 Zero Valent Iron Advantages

Using zero valent iron has the following advantages as a remediation technology

(Zawaideh 1997):

i) It is relatively inexpensive

ii) It is non-toxic

iii) It degrades certain chemical faster than other techniques of remediation,

such as biotic remediation

iv) It has a high energy effectiveness.

2.3.4 Nano-scale Zero Valent Iron

Nano-scale zero valent iron is more effective at reaching deep zones of

contamination, and is more effective at contaminant degradation than iron of larger

sizes (Geiger et al 2003). Nano-scale zero valent iron can induce greater rates of

reaction because of its greater specific surface area, which allows a greater exposure

of the iron particle to the contaminant per unit weight of iron than other larger

particles (Tratnyek 2003). Additionally, as particle size decreases and tends towards

10nm, thermodynamic properties, such as work-function and free energy begin to

alter and can increase reactivity (Campbell et al 2002). Gavaskar et al (2005b) has

found that nanoscale zero valent iron is significantly more reactive than granular iron,

and states that it can remediate a plume in a much shorter time scale. Additionally,

injection of nanoscale zero valent iron has proved to be less arduous (Gavaskar

2005b). Henry et al (2002) states that nanoscale zero valent iron has a superior pore

penetration ability when compared to larger particulate zero valent iron.

2.3.5 Zero valent iron delivery

Delivery mechanisms for nanoscale zero valent iron to DNAPL source zones include

pneumatic fracturing and injection, direct push injection and closed-loop recirculation

wells (Gavaskar 2005a), (Quinn et al 2005). Difficulty in administering the zero

valent iron to the target area has been expressed using these methods. Electrokinetics


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has the possibility of providing a solution to the problem of administering nanoscale

zero valent iron to the intended zone.

2.4 Diffusion

Diffusion in a liquid medium is the net flux of a certain constituent from a zone of

higher concentration to a zone of lower concentration (Quickenden, 2003). Diffusion

can also be explained by the Second Law of thermodynamics, which states that ‘The

entropy of an isolated system increases in the course of a spontaneous change’

(Atkins 1998). Diffusion occurs irrespective of bulk fluid motion or electrical

potential gradients.

Diffusion can be described by Fick’s laws, the first being:

dx

dCDF !=

where

F = mass flux of species per unit area per unit time.

D = diffusion coefficient.

C = solute concentration.

dx

dC = the rate of change of concentration with respect to distance.

The negative term is used to specify that bulk motion is from higher concentration to

a lower concentration, and no vice-versa (Fetter 1994).

Fick’s second law stats that the rate of change of concentration with respect to time is

equal to the product of the diffusion coefficient and the rate of change of the rate of

change of the concentration with respect to distance (Fetter 1994).

2

2

dx

CdD

dt

dC=

Where dC/dt = rate of concentration change with respect to time.


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The diffusion process when flowing through a porous media is slightly different, due

to two reasons. One being that the distance a particle must travel is increased due to

the fact that the particle must flow around the media. The second reason is that a

large percentage of the cross sectional area of the length the particle is flowing

through is blocked by the presence of the porous media. To account for this, the

effective diffusion (D*) is calculated in the following manner;

D* = wD where w is a determined empirical coefficient, typically ranging between 0.5 to 0.01.

A nonempirical relationship was determined in 1971, such that:

D* = D (√τ) Where τ = tortuosity (the actual length of flow path divided by the straight-line

distance between the start and end point of the flow) (Fetter 1994).

2.5 Electrokinetics

Electrochemical remediation has a number of terms, such as electrokinetic

remediation, electroreclamation, electrokinetic soil processing, electrokinetic

extraction, electrodialytic remediation and electrochemical decontamination. All these

terms refer to the application of a low-intensity direct electrical current (DC) between

an anode and cathode situated at the site of contamination to induce or increase one or

more transport processes (Lageman et al 2003). In a soil matrix, electric current tends

to be conveyed through micropores, which is the location of many contaminants, such

as DNAPLs (Lageman et al 2003). Several phenomena arise from the application of

such an electric field, such as:

1) Electroosmosis

2) Electrophoresis

3) Electromigration

4) Electrolysis

The first pioneering effort of using an electric field to improve the chemical quality of

soils was in the 1930s. Puri and Anand (1936) used an electric potential difference to


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determine if it could influence the extraction of sodium ions in soil. Utilisation of

electroosmosis was used by Casagrande (1948) to stabilise soil formations by

dewatering. Electrokinetic research boomed in the late 1980s and early 1990s

(Lageman et al 2003), with more than 400 papers written in this time. It is an

indication of the significant level of interest in this field.

2.5.1 Electroosmosis

Electroosmosis is the bulk movement of fluid due to the imposition of an electric

field. Most soil surfaces possess, to some degree, a charge, predominantly a negative

polarity. The negative charge present on the surface of the particles attract positive

ions to them, forming what is know as a ‘double-layer’, or ‘zeta-potential’ (Zeng

2001). When an electric field is applied, the positive ions accumulated at the surface

of the particles begin to move towards the cathode. This movement also draws the

surrounding fluid with it via friction (Electroosmosis 2006), thus initiating water flux.

The water flow rate is determined by the forcing due to a potential difference, and the

frictional forces experienced at the solid-liquid interface. Total flow rate (qA) is

determined by

AL

Vkq eA

=

Where

ke = electroosmotic permeability

L

V = electrical potential gradient

A = cross sectional area.

Electroosmotic flow can be determined by:

Aikq ee=

where q is electroosmotic flow rate, ke is electroosmotic permeability, ie is electrical

potential gradient and A is the area normal to the flow. Electroosmotic flow can be

affected by a number of factors, and are summarised in the table below:


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Table 2.2 – Electroosmotic Flux Factors

Electroosmotic Flux Affecting Factor Effect

Electric field Changes in proportion to potential applied Buffer pH Increases as pH increases Ionic Strength Decreases as ionic strength increases Temperature Decreases as temperature increases

Organic Modifier Generally decreases as concentration increases

Negative Surfactant Increases as concentration increases Positive Surfactant Decreases as concentration increases Neutral hydrophilic polymer Decreases as concentration increases

Electroosmosis induced flows are not affected by pore size (Zeng 2001). Therefore

electroosmosis has the potential to be an effective mechanism for treatment of soils

sites which feature poor hydraulic conductivity due to pore sizing and therefore

difficult to treat using methods reliant on hydraulic conductivity.

2.5.2 Electrophoresis

Electrophoresis is the movement of colloids or macromolecules induced by an electric

field. Due to the varied nature of colloidal particles and macromolecules, it is

extremely difficult to characterise electrophoresis. Probably the largest sector that

deals with electrophoresis is the biotechnology sector. Gel phoresis is used

extensively in this field to spearate nucleic acids and proteins base on their ability to

move through a gel under an electric (DC) potential.

The force (Fe) experienced by a charged particle under an electric gradient is

EqFe !=

where

q = charge

E = electric field

This force is countered by the frictional force (Ff), which acts against the movement

of the particle.

fvFf !=


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where

v = velocity of the particle

f = friction coefficient

These two equations can be used to determine the effective electrophoretic mobility

factor (µ), where;

E

v

f

q==µ

This derived effective electrophoretic mobility is not necessarily a good

approximation for nanoscale zero valent iron because of its physical and chemical

properties. Factors such as particle size, surface charge density, pH and solution ionic

strength all have an influence on the effective electrophoretic mobility (Taylor et al

2004).

The Smoluchowski equation derives a relationship between the zeta potential and

effective electrophortic mobility as such (Taylor et al 2004):

!

"#µ =

where

ξ = zeta-potential

ε = electric permitivity

η = viscosity

Electrophoretic induced movement is difficult to characterise for nanoscale zero

valent iron, due to the varying nature of the nanoscale zero valent iron particle size

distribution and effective surface charge.

2.5.3 Electromigration

Electromigration is the movement of charged species, such as Fe2+ or OH- to the

electrode of opposite charge. Migrational flux (Jjm) is dependant on effective ionic


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mobility, electrical potential, valence and temperature (Acar, 1993). The relationship

of migrational flux and its variable is as follows:

)(*EcuJ jj

m

j !"##=

where *

ju = effective ionic mobility

cj = molar concentration

E = electrical potential

Although no completely correct method to determine effective ionic mobility has

been devised, extending the Nerst-Townsend-Einstein relation yields:

RT

FzDu

jj

j

*

*=

where *

jD = effective diffusion coefficient

zj = valence

F = Faraday’s constant

R = universal gas constant

T = absolute temperature

The effective ionic mobility of a species is typically an order of magnitude larger than

the effective diffusion coefficient and, assuming a unit electrical gradient, is

approximately 40 times the valence (Acar 1993). This highlights the importance of

electromigration, and it’s much larger influence on mobility of charged species than

the diffusion mechanism.

2.5.4 Electrolysis

Electrolysis is the application of an electric current to induce a non-spontaneous

chemical reaction, such as the splitting of H2O to H+ and OH- (Atkins 1996). When

electrodes are inserted into an aqueous medium, two important reactions may take


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place, generation of oxygen gas and protons at the anode, and generation of hydrogen

gas and hydroxyl ions at the cathode, as shown below:

2H2O O2 + 4H+ + 4e- anode

4H2O + 4e- 2H2 + 4OH- cathode

It is important to note that the pH of the bulk solution remains constant as the number

of protons produced equals the number of hydroxyl ions produced.

2.6 DNAPLS and chlorinated solvent contamination of groundwater

DNAPLs are defined as Dense NonAqueous Phase Liquids. The term is used in

hydrogeological circles to describe a liquid that is immiscible with water and has a

specific gravity greater than water. When situated with water, DNAPLs form a

separate phase and do not mix to any significant degree. A great number of DNAPLs

are chlorinated hydrocarbons, such as trichloroethylene (U.S. Geological Society

2006).

Dense nonaqueous phase liquids accumulate in groundwater as pools that can slowly

release contaminants into the surrounding groundwater over multiple decades

(Pankow and Cherry 1996). DNAPLS are a real threat to groundwater quality

because of their ability to migrate below the water line in aquifers and their persistent

presence once there (Groundwater Protection and Restoration Group 2006), that they

remain the largest cleanup problems (Anonymous 1995) and are amongst the most

prolific groundwater contaminant (Pankow and Cherry 1996).

Remediation of DNAPLs subsurface pools have been shown to rapidly collapse the

pollutant plume originating at the DNAPL pool. It is therefore important to remediate

the pool of DNAPL, and not concentrate solely on the emanating plume (U.S.

Geological Society 2006).

The Ground Water Supply Survey (Westrick 1983) found that one of the two most

prevalent volatile organic chemicals in groundwater was trichloroethylene. This


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highlights the significance of trichloroethylene as a contaminant of groundwater

supplies, and thus the importance of developing strategies to reduce such

contamination.

Chlorinated solvents are problematic for a number of reasons:

1) Their high volatilities lead people to believe that it is safe to

disposal of solvents by pouring them on the ground, and that it

would all volatialise into the atmosphere. Although a large amount

does, a significant component of the solvent can penetrate the soil

and enter the groundwater.

2) The high densities enable the solvent to easily penetrate through

the vadose zone and the groundwater zone.

3) The low absolute solubilities result in the contamination having a

long life span, because it can not be effectively dissolved away by

the groundwater

4) The high relative solubilities result in the saturated level of

chlorianted solvents to be much higher than the safe concentrations

for human consumption.

5) The low interfacial tension between chlorinated solvent and water

allow the solvent to penetrate into small pore spaces.

6) The low degree of retardation by soil material results in the

solvent not being significantly retarded and thus allowing the

chlorinated solvent to move with the groundwater

7) The low degradability of chlorinated solvent result in the

substance remaining in the groundwater for a long period of time.

2.7 Agglomeration chemistry

Zero valent iron is not a polar substance, and carries no overall charge (hence the

zero-valent term). However, zero valent iron has been known to agglomerate and

form colloidal particles (Thomas, D., 2005, pers. comm., 16 September), which

requires an attractive driving force to draw the particles together. Since they do not

have an overall charge, an explanation for the formation and maintenance of these


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colloids is that they are held together by Van Der Waals forces. It is the effect of

these Van Der Waals forces that result in the polarity of zero-valent iron, which thus

leads to agglomeration. It was attempted to exploit this polarity by using

electrokinetics to induce the zero valent iron to move from the position of application

to the desired position.

2.8 Site applicability

Electrokinetic phenomena have been used with success in many distinctly different

soil types (Acar 1997). Electromigration rates are not particularly dependant on fluid

permeability, rather pore water electrical conductivity and tortuosity. Electrokinetic

remediation is a viable technique in both saturated and unsaturated zones (Van

Cauwenberghe 1997).

Electrokinetics is suitable for zones of low hydraulic conductivity (Van

Cauwenberghe 1997). In such soils the low hydraulic conductivity makes traditional

soil flushing techniques such as pump-and-treat unfeasible. This fact is immensely

important, as electrokinetic inducement of nanoscale zero valent iron may prove to be

a solution to halogenated aliphatic groundwater contamination in zones which are not

suitable for techniques amenable for sites with high hydraulic conductivity.

Before a site can be deemed suitable to be electrokinetically remediated, certain

parameters need to be ascertained (Van Cauwenberghe 1997). Spatial electrical

conductivity variability must be examined at the site to determine if it will interfere

with the voltage gradient. Pore water pH must be determined to gain an

understanding of how it may affect the nanoscale zero valent iron. Pore water

electrical conductivity must also be taken into account, to establish the anticipated

efficiency of the technique. The chemical make-up of the soil and pore water must

also be considered as it has the potential to interact and react with the nanoscale zero

valent iron (Van Cauwenberghe 1997).

Electrokinetics does have limitations however, and is not suitable for use irrespective

of the site. Electrokinetic remediation is not suitable when the pH conditions are such


_____________________________________________________________________


that the anode is susceptible to unacceptable levels of corrosion. Sites which contain

chemical species that may influence the pH when exposed to an electrical gradient

must be examined for suitability. Foreign anomalies such as rubble or metallic

building bodies may affect the effectiveness of the electrokinetic phenomena (Van

Cauwenberghe 1997).

2.9 Costings

Nanoscale zero valent iron varies in price depending on supplier and current market

prices. Gavaskar (2005a) quotes prices varing from US$ 20/lb to US$ 70/lb. Factors

such as raw material cost, licencing fees and manufacturing costs all impact on the

price of nanosccale zero valent iron.

PRBs using zero valent iron have lower costs than pump-and-treat and have higher

initial outlay, but maintenance and long-term operation costs are lower (Henry 2002).

Commercially, the longest running PRB costs US$ 50 000 per year, as opposed to the

US$ 300 000 spent before on the same site using pump-and-treat.

Factors which influence costings include (Van Cauwenberghe 1997):

1) Electricity price

2) Labour cost

3) Initial contaminant concentrations

4) Target contaminant concentrations

5) Conductivity of pore water

6) Concentration of other ions.

7) Soil characteristics

8) Moisture content

9) Extent of contaminantion

10) Zone preparation

Acar (1997) found the energy expenditure to electrokinetically remediate a site to be

between 325 kWhm-3 to 700kWhm-3. Assuming an energy cost of 10 cents/kWh, this

translates to $33 m-3 to $70 m-3. Van Cauwenberghe (1997) quotes energy


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consumption rates of 500 kWhm-3, which corresponds to a cost of $50 m-3. Further

prices quoted from various vendors range from $25 m-3 to $300 m-3 (Van

Cauwenberghe 1997).

Gavaskar (2005a) conducted field trials on three separate sites using zero valent iron

and found that it cost US$ 289 000 to treat a contamination site in Hunters Point

(USA) of 1287 m3 in size containing 6.4 kg of TCE. Another site in Jacksonville

(USA) of size 1265 m3 containing an estimated 27.7 kg of TCE cost US$ 412 000 to

remediate. A third 12426 m3 site in Lakehurst (USA) incurred a cost of US$ 255 500

to remediate. The three sites give costs of $US 224 m-3, US$ 326 m-3 and US$ 21 m-3

respectively. The breakdown of the costs of remediating the three sites is not

consistent, as is the type of zero valent iron used, and therefore the cost per cubic

metre is not entirely consistent. This discrepancy in cost between the sites may also

be due to differences in TCE source zone location, TCE contamination extent, extent

of remediation and aquifer variability.

Chapter 3: Methodology Development

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3 Methodology Development

The experimental procedure was developed throughout the experimental period. The

experimentation began with investigation of effects of both the cathode and anode in

contact with a nanoscale zero valent iron slurry. The mass transport of chemical

species known to be susceptible to electrokinetic effects (sodium and chloride ions)

between two hydraulically connected containment vessels was then conducted.

Following this, electrokinetic mass transport of nanoscale zero valent iron was

attempted between a congruent pair of hydraulically connected containment vessels.

The experimental set-up was then transmogrified to simulate a groundwater

environment more closely by forcing the nanoscale zero valent iron to flow through

porous media. Mixing methods to maintain the nanoscale zero valent iron suspended

in solution was also experimented with. Induced movement of nanoscale zero valent

iron that had been directly injected into the porous media were also attempted.

Finally, movement of the nanoscale zero valent iron by application of a hydraulic

gradient was trialled. These major steps are summarised in the table below, and

documented in more detail later in this chapter.

Table 3.1 – Summary of experimentation stages Stage Experiment 1 Nanoscale zero valent iron interaction with electrodes in a single containment vessel

2 Electrokinetic movement of nanoscale zero valent iron between two hydraulically connected containment vessels

3 Electrokinetic movement of nanoscale zero valent iron through porous media featuring orbital mixing

4 Electrokinetic movement of nanoscale zero valent iron through porous media featuring mechanical mixing

5 Electrokinetic movement of nanoscale zero valent iron directly injected into porous media

6 Electrokinetic movement of nanoscale zero valent iron directly injected into porous media with enhanced conductivity

7 Hydraulic advection of nanoscale zero valent iron through porous media

3.1 Instrument Calibration

Various parameters were measured in the experiments to characterise the effect of

electrokinetic phenomena on nanoscale zero valent iron. All pH, temperature and

conductivity measurements were conducted using a TPS Conductivity-Salinity-pH-

Temp. Meter, Model WP-81. The pH probe was calibrated using a 2-point calibration


_____________________________________________________________________


technique. It was first rinsed with de-ionised water, dried, then immersed in a pH

7.00 calibration standard solution Biolab pH 7 potassium dihydrogen orthophosphate

buffer solution, batch number AF405379 and allowed to equilibrate. After

equilibrium was reached, the probe was removed and rinsed with de-ionised water

again, dried, and placed in Rowe Scientific pH 4.00 calibration standard solution –

potassium hydrogen phthalate, code CB 2660, batch AK051017 and allowed to reach

equilibrium again. The slopes given from the calibrations ranged between 98.9% and

98.2%.

The conductivity probe was calibrated using a 1-point calibration technique. The

probe was rinsed with de-ionised water, and then immersed in 58Scm-1 calibration

solution, allowed to equilibrate and then calibrated, the probe was then considered fit

for use.

3.2 Power supply

A Powertech dual tracking DC power supply, model MP – 3092 was used to supply

power to the electrodes throughout the research. It was capable of supplying a

maximum voltage of 40 volts, and a maximum current of 3 amps. The power supply

had two outputs, capable of being used independently or in a master/slave

configuration. Voltage used in the experiments never exceeded 20 volts, and typical

currents were approximately 0.01 amps.

3.3 Iron Concentration in Slurry Determination

The container of iron was thoroughly shaken for 1 minute before sampling. A 120

mL sample was poured into a measuring cylinder on a tared electronic scale. The

sample was then weighed, and it was then attempted to calculate the percentage iron

content.

Another 100g sample was poured into a drying container. After 16 hours of heating at

80 degrees Celsius, the drying container was reweighed. A percentage iron

calculation was then conducted.


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3.4 Single containment vessel experiments

A test was devised to first initiate experimentation with zero valent iron and

electrokinetics to observe any forthcoming effects. A 2L single containment vessel

was initially filled with water. 20 mL of zero valent iron was then added to the water

and mixed metal oxide electrodes were inserted into the slurry. The electrodes were

positioned at opposite corners of the containment vessel, and the voltage set to 20

volts. The voltage was applied for 8.5 hours. The containment vessel was left

uncovered for the duration of the experiment to prevent the possible build-up of

noxious gases.

Following this test, two stainless steel nails 100mm in length and 38mm in diameter

were used as electrodes. These were placed in opposite corners of a 2L containment

vessel. The vessel was filled with tap water and 10 mL of zero valent iron slurry was

added. A voltage of 20 volts was applied between the electrodes in the uncovered

bucket for a period of 41 hours.

3.5 Dual containment vessel experiment with unhindered flow

The dual containment vessel experiment with unhindered flow was designed to begin

experimentation of movement of species from one vessel to another using

electrokinetics. The nanoscale zero valent iron was to be placed in one containment

vessel with the aim to enhance its movement into another containment vessel through

a connecting tube using electrokinetics.

3.5.1 Electrodes

Care was taken during electrode selection as not all materials were deemed suitable

for usage. Iron electrodes were avoided due to the inability of the analytical

technique used for analysis of iron content to distinguish between zero valent iron and

iron released from the electrode. Copper electrodes are susceptible to corrosion (Lee,

2005) and were therefore not suitable. Similarly, any other common metal that could


_____________________________________________________________________


corrode was not accepted because of the possibility of influencing the iron in solution.

Mixed metal oxide electrodes were also considered due to their notable performance

and relatively low cost. However, they were also eventually rejected as suitable

electrodes due to the inability of to be completely certain that there were no iron

compounds present that could influence results.

The electrodes that were eventually selected consisted of three layers. The inner core

comprised of copper for its ability to carry charge as copper has the second highest

electrical conductivity of all known elements (5.88 x 107/ S m-1) (Kittel, 1986). The

outer layer of the electrode was a fine plating of platinum due to platinum’s ability to

resist chemical attack. This was the most vital section of the electrode, as it would be

in contact with the aqueous solution, and therefore must not contaminate it with

foreign iron atoms. Between the platinum and copper layers was a layer of titanium,

which provided a buffer between the solution it was to be placed in and the copper

core, in case the fine platinum plating was penetrated due to chemical corrosion or,

the more likely event of mechanical scratching.

The electrodes were purchased from McCoy Engineering in lengths averaging 23 cm.

They were all cut from a single strand of electrode and therefore had an exposed

copper core at each end. This was undesirable, as the copper at the tip of the end of

the electrode that was immersed in the aqueous solution would be exposed to

chemical attack and could rapidly corrode, thus contaminating the containment

vessels. To prevent this from occurring, it was necessary to cap the end with a non-

permeable material. Both ends of the electrodes were ground on a bench grinder.

This achieved two goals. Firstly, it coarsened one end and removed any unwanted

compounds so that a good connection could be made to the power source. Secondly,

on the other end, it resulted in a stronger bonding of the capping substance. An epoxy

resin was mixed, comprising of 2 parts by weigh Araldite BY 157 TS LC from

Vantico (>60% Bisphenol A, >10% Butandiol diglycidyl ether, 10-30% Bisphenol F )

and 1 part by weight hardener Aradur 2764 – CH from Vantico. PVC electrical

cowling of 20 mm outside diameter was then cut in lengths of approximately 30 mm,

into which the mixed epoxy was injected. The electrodes were then inserted along

their longest axis into the cowling to a depth of approximately 2 cm. They were then


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fixed in place and the epoxy allowed to harden over 24 hours. As mentioned above,

both ends had been ground with a bench grinder, which had removed the platinum

layer, leaving the titanium middle layer exposed. Although one end was encased in

epoxy resin, an approximately 8mm section immediately above the epoxy was not

sealed (due to it being outside the encapsulating epoxy), and had had the platinum

ground off, thus exposing the titanium beneath it. This was not considered a

significant problem, because the titanium would oxidise, forming a TiO2 layer that

electrically insulates and stops the copper from corroding. As this was not a large

section of the electrode, the reduction in capacity to deliver current to the bulk

solution was not considered significant. Following the hardening period, the

electrodes were then deemed suitable for use, and are shown in Figure 3.1.

Figure 3.1 – Capped Pt/Ti/Cu electrodes

3.5.2 Containment Vessel

The testing apparatus consisted of two 15 litre plastic vessels, each featuring an

aperture in one side. These apertures were fitted with a circular plastic fitting, with an

o-ring on the inside diameter. This left an aperture in the side of each vessel of 50

mm radius. The two vessels were joined by a 100 mm length of clear Perspex tube.

After evidence of severe leaking around the aperture and screws fixing the circular

plastic fitting, a silicon-based sealant was applied liberally to any outside surfaces

suspected of leakage, as shown in Figure 3.2. After 24 hours of setting, the

containment vessels were again connected via the connecting tube and filled with


_____________________________________________________________________


water and it was tested for leaks. This procedure was repeated until there was no

leakage and the finished set-up is shown in Figure 3.3.

Figure 3.2 – Aperture with silicone sealant applied to prevent leaking

Figure 3.3 – Containment vessels with connecting tube

3.5.3 Mixing method

Mixing of the water in the containment vessels would advect the nanoscale zero

valent iron, thus masking any movement of nanoscale zero valent iron due to

electrokinetic effects. It was therefore decided to not stir the containment vessel

solutions in this experiment.


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3.5.4 NaCl experiment

Tests were conducted regarding using electrokinetics to induce movement of charged

ionic species. NaCl was chosen as the ionic species to conduct the experiment due to

their non-hazardous nature, ease of acquisition and low cost. These tests were

designed to observe electrokinetic phenomena in action, and to ensure that the

methodology and equipment were correct. As such, the containment vessels were the

ones used in the zero valent iron experiments, the electrodes used were fabricated

platinum/titanium/copper electrodes (the same as the zero valent iron experiment

electrodes) and the connecting tube between the containment vessels had the same

dimensions as the tubes used in the zero valent iron experiments.

3.5.4.1 NaCl test at 20 volts

24 litres of 23.4°C tap water, with a maximum total iron concentration of 0.16 mgL-1

(Water Corporation, 2006) was added to two containment vessels connected by a 100

mm connecting tube between the apertures. The aperture in the containment vessel

containing NaCl was blocked so that there was no advection from one containment

vessel to the other. 100 g of sodium chloride was added to one of the two

containment vessels. The fabricated Pt/Ti/Cu electrodes were positioned such that

they were suspended above the aperture fitted in each containment vessel, and

projected downwards across the diameter of the aperture, the cathode (negative

electrode) being positioned in the containment vessel dosed with NaCl. The pH and

conductivity probes were positioned in the corner of the containment vessel that held

the anode (positive electrode), closest to the aperture and electrode. The containment

vessel dosed with NaCl was then stirred for two minutes to ensure thorough mixing.

The blockage between the two containment vessels was then removed after motion in

the stirred containment vessel had ceased, and conductivity was monitored every

minute until it stabilised. Although the water in the containment vessel that was not

dosed with additional NaCl was saline to a small degree, the experiment was designed

to measure and characterise the change in salinity and therefore, this small amount of

additional salt was not a problem.


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3.5.4.2 NaCl test at 10 volts

A second test was run with slightly different operating parameters to determine the

effect of voltage on electromigration. First the containment vessels were filled with

24 litres of 22.5°C tap water with maximum Fe concentration 0.16 mgL-1, (Water

Corporation, 2006). The connecting tube was blocked at the aperture of the

containment vessel containing the negative electrode, to ensure no advection occurred

between buckets. 101 g of NaCl was added to the containment vessel holding the

negative electrodes, and stirred for two minutes to ensure dissolution. The cathode

was then placed such that it was suspended into the NaCl doped containment vessel,

projecting downwards across the aperture, and the anode positioned similarly in the

other containment vessel. The blockage was then removed and conductivity was then

measured every minute until the conductivity levels stabilised. The conductivity

probe was agitated before readings were taken to give a more representative sample.

3.5.5 Nanoscale Zero Valent Iron Supply

Each sample of nanoscale zero valent iron used in the various experiments in this

document was obtained in the following manner. The container of the nanoscale zero

valent iron was shaken vigorously for 1 minute to ensure homogeneity. The required

volume of nanoscale zero valent iron was poured into a measuring cylinder. It was

then transferred into the required containment vessel.

3.5.6 Nanoscale zero valent iron experiment with no porous media

Following the NaCl experiments, it was decided to continue with the no porous media

experiments and conduct a similar experiment, but this time using zero valent iron in

place of NaCl.

3.5.6.1 Initial nanoscale zero valent iron experiment with no porous

media

Two containment vessels were connected via 100 mm length of 49.8 mm diameter

clear Perspex tube and filled with normal tap water. The buckets were placed on a


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purpose made wooden platform sitting on top of a Ratek EOM5 Orbital Mixer. A

mixed metal oxide electrode was connected via dual strand copper wiring to both the

positive and negative port of a Powertech dual tracking DC power supply model MP –

3092, and a potential difference of 20 volts was applied. This voltage was used

because a large voltage was desired to induce the electrokinetic effects, without

overloading the power supply. A mixed metal oxide electrode was positioned at the

aperture of each containment vessel. A water sample was taken from the containment

vessel containing the positive electrode, and the solution from the same vessel was

monitored for pH and conductivity. 72 grams of zero valent iron that was received in

August 2005 was introduced to the containment vessel containing the negative

electrode. The zero valent iron varied in size from a powdery substance to one

roughly spherical piece with a radius of approximately 2 cm. The orbital mixer was

not operated, due to the possibility of the mixing conveying some of the iron to the

positively charged containment vessel because the connecting Perspex tube did not

hold any inhibiting material, i.e. filled solely with water. After 300 minutes, the

orbital mixer was powered and mixing of the water contained in the containment

vessels began. The experiment continued to run for a further 90 minutes with the

orbital mixer running. Water was sampled from the surface, just outside the aperture

of the positively charged containment vessel for further analysis.

3.5.6.2 Second nanoscale zero valent iron experiment with no porous

media

The two containment vessels were connected by a 100mm, 49.8mm inside diameter

piece of tubing. 24 litres of water were added to the containment vessels, which were

then isolated from each other by application of a plug, thus blocking flow from one

containment vessel to the other. Electrodes were connected 20 volts via the power

source and positioned at both ends of the connecting tube in the containment vessels.

31 mL of the freshly prepared iron slurry was then introduced into the negative

containment vessel. The pH probe was placed in the corner of the negative

containment vessel, closest to the side aperture. The pH in the anodic containment

vessel was monitored, and water samples were taken by submergence of a 21 mL

sample vial at the aperture of the connecting tube, on the anodic side.


_____________________________________________________________________



orbital mixing method

After the experiments with a connecting tube free of porous media, an experimental

set-up that simulated a groundwater environment was required. As such, it was

decided to fill the connecting tube with inert 250 micron silica beads to imitate porous

media in the subsurface.

3.6.1 Mixing method

The zero valent iron was significantly denser than water, and was observed to descend

to the bottom of the containment vessel upon introduction to the water body. In order

to have the iron more heterogeneously distributed throughout the containment vessel

it was introduced into, it was deemed necessary to agitate the water body. Various

methods of mixing were posed as suitable means to suspend the iron in the aqueous

solution. Magnetic stirrer plates and stirrer bars were quickly disregarded, due to the

interactions the imposed magnetic fields would have on the zero valent iron. Electric

mixers were considered, however, concern was raised over the ability of the electric

motor to endure constant operation for hours at a time. Two electric mixers were

purchased from a department store for usage, and the accompanying documents did

not recommend stirring for more than one and five minutes respectively, so they were

deemed unsuitable and not used. All stirrer fittings available for both Sunbeam™ and

Breville™ electric mixers also contained high levels of iron, which could influence

results. It was then decided upon to use a Ratek EOM5 orbital mixer, which uses

elliptical motion of a base-plate to induce agitation in the containment vessels.

3.6.2 Orbital mixer board construction

A board was constructed for the containment vessels to be placed upon. A 750 mm x

450 mm wooden board was measured to fit the lip of the Ratek EOM5 orbital mixer

plate. Four rectangular wooden stoppers were attached to the underside of the

wooden board, positioned such that they were situated along the outside of each side


_____________________________________________________________________


of the Ratek EOM5 orbital mixer plate; this was to prevent any slippage whilst the

mixer plate was in motion.

3.6.3 Manufacturing of additional side ports in the connecting tube

Samples from within the porous media in the connecting tube were to be taken, using

a needle and syringe. The connecting tube was 100mm long, had an inside diameter

of 49.8 mm and an outside diameter of 51.8 mm. Once fitting inside the containment

vessels was complete, the tube length between the two containment vessels was

69mm. Manufacturing of three sampling ports was conducted using three 7 mm

diameter tubes. These were fitted along the longitudinal axis of the tubing at

equidistant intervals. These tubes were inserted into the connecting tube, and

PARFiX silicone sealant was applied to completely seal the join. The completed

connecting tube apparatus can be seen in Figure 3.4 and Figure 3.5.

Figure 3.4 – Top view of connecting tube with three sampling ports fitted with flexible tubing


_____________________________________________________________________


Figure 3.5 – Side view of connecting tube with three sampling ports fitted with flexible tubing

3.6.4 Needle selection

A needle was required to sample in the connecting tube porous media. An envisaged

problem would be the aperture of the needle becoming clogged by the porous media.

To combat this possible problem, a 10 mL needle with side port injection capabilities,

part # 008962SGE was acquired from Alltech Associates Australia. This needle

featured a side-port aperture, rather than the more conventional location at the tip of

the needle, which would decrease the chance of blockage from porous media

particles.

3.6.5 Silica filling of tube/screen installation

In order for the 250 micron silica porous media to remain inside the connecting tube

after installing it into the apertures of the containment vessels, PARFiX silicone

sealant was applied to the circular edge of the connecting tube. A single sheet of

Chux® Regular Superwipes was then placed on the edge and pressure was applied to

fix the sheet to the connecting tube edge. After 24 hours of drying, an additional


_____________________________________________________________________


sheet was affixed on top of the first sheet, rotated by 90 degrees, so that the small

apertures in the sheet were perpendicular to the first sheet. After the PARFiX silicone

sealant had set, 250 micron silica bead porous media were poured into the connecting

tube and packed by application of pressure. Another sheet of Chux® Regular

Superwipes was then fixed in place with silicone sealant on the top end of the

connecting tube, thus sealing the silica inside. A fourth sheet of Chux® Regular

Superwipes was rotated 90 degrees, then affixed on top of the first sealing sheet on

the top end of the tube as seen in Figure 3.6. The connecting tube was then fitted into

the containment vessel apertures ready for usage.

Figure 3.6 – Connecting tube filled with porous media

The filling of the connecting tube with porous media method was improved following

the initial effort, by standing the connecting tube upright in a container holding water

before addition of porous media. This was done to compact the porous media to

reduce the chance of a large void forming in the silica after insertion into the

containment vessel apertures. After filling the tube with silica, the end was capped

with Chux® Regular Superwipes in the same manner as outlined previously, and is

shown in Figure 3.7 and Figure 3.8.


_____________________________________________________________________


Figure 3.7 – Connecting tube ready for insertion between two containment vessels.

Figure 3.8 – Semi filled connecting tube capped with pink screens.


_____________________________________________________________________


3.6.6 Initial containment vessel experiment with porous media flow and

orbital mixing

Two containment vessels were connected by insertion of a 49.8 mm inside diameter

connecting tube. The tube was filled with porous media beads and an electrode was

positioned over the aperture of each containment vessel. The containment vessels had

24 L of tap water added to them; 30 mL of zero valent iron slurry was then added to

the cathodic containment vessel. Preliminary testing of pH, temperature and

conductivity was conducted. The entire set-up was positioned on the orbital mixer on

a setting 2.5, and an initial sample of the anodic containment vessel water was done.

Sampling was to be conducted periodically after.

3.6.7 Sampling technique development

The proposed sampling technique consisted of inserting a needle connected to a

syringe through the sample port on the side of the connecting tube, into the porous

media inside of it. The plunger on the syringe was then pulled, allowing water into

the syringe. However, after extracting approximately 1.5 mL, the needle ceased to

function. The suspected problem was that the hole was plugged by a silica bead, thus

preventing flow into or out of the needle. It was thus deemed an unsuitable technique

for sampling. It was decided upon to sample in the same manner as in previous

experiments, which consisted of manually stirring the solution in the containment

vessel to ensure homogeneity, and immersing a 21 mL sampling vial in the solution in

the containment vessel at the aperture to obtain the water sample.

3.6.8 Second dual containment vessel experiment with porous media flow

and orbital mixing

The experiment was set-up in the same manner as the previous experiment, with two

containment vessels connected by a connecting tube filled with porous media. 24 L

of water was added, with a cathode positioned at the aperture of one containment

vessel and an anode positioned at the aperture of the other containment vessel. The

containment vessels were placed on a board on top of the orbital mixer. The power


_____________________________________________________________________


source delivered 20 volts to the electrodes, and the orbital mixer was set on setting of

2.5. 30 mL of nanoscale zero valent iron was added to the cathodic containment

vessel. Water was sampled in the anodic containment vessel just outside the aperture

of the connecting tube in 21 Ml sample vials.

3.6.9 Side port construction in the connecting tube

To improve the filling of the connecting tube with porous media, it was required to

drill an aperture into the side of the connecting tube to allow the porous media to be

poured into it. A 7mm aperture was drilled into the side of the tube, equidistant from

both ends. The aperture was sealed after filling with porous media by plugging with 7

mm diameter flexible tubing, followed by application of silicone PARFiX sealant

around the join.

Following filling of the connecting tube, it was thought that easier filling could be

achieved by positioning the filing aperture closer to one end. This allowed the

connecting tube to be tipped on one side when the tube was semi-filled, which moved

all the porous media to the end furthest from the filling aperture, thus allowing easier

addition of further porous media.

3.6.10 Filling of connecting tube with porous media.

A third method was then employed to further the efforts to mitigate voiding occurring

in the connecting tube porous media. Both ends were sealed using PARFiX silicone

sealant and Chux® Regular Superwipes in the same manner as outlined previously.

Once the silica gel had set after 24 hours, the porous media was inserted into the

connecting tube through the previously manufactured 7 mm sample port. The porous

media was dry to allow easy insertion. Following dry packing, the tube was

immersed in water, which resulted in the porous media compacting further.

Additional dry porous media were then poured through the small aperture and

compacted using a thin pine skewer. This process was repeated until no more porous

media could be compressed into the connecting tube. At this stage, the screening

cloths on both sides were convex, bulging outwards due to the pressure exerted by the


_____________________________________________________________________


porous media. A flexible polymer tube piece with outside diameter 7 mm was

inserted into the small aperture to seal it, and PARFiX silicone sealant was applied to

the joint and allowed to set to complete the join.

3.6.11 Third dual containment vessel experiment with porous media and

orbital mixing

The identical experimental set-up was used for this experiment as used in the previous

experiment, and sampling was conducted periodically in the same manner. The

hydraulically connected tube packed with silica beads was properly packed with no

voiding across the top. pH was periodically sampled in the anodic containment

vessel, and water samples were also taken to be analysed for iron content by

submergence of a 21 mL sample vial at the aperture of the connecting tube in the

anodic containment vessel. The experiment was run until the porous media in the

connecting tube eroded to form a void space across the length of the connecting tube

containing the porous media.


mechanical mixing method

3.7.1 Mechanical mixing

The orbital mixers were used on a number of experiment runs. However, it was

suspected that they could cause erosion of the porous media in the connecting tube.

To alleviate this problem, a XUI 13 mm Hammer drill, Model XHD-200 variable

speed drill was fitted with a manufactured wooden paddle to agitate the water

contained in the containment vessel.

3.7.2 Non-metallic mixing paddle construction

A 9.6 mm diameter, 390mm long piece of wooden dowel was fitted and glued into the

side of an 80 mm x 39mm x 13 mm rectangular piece of wood. A 10 mm hole was

drilled approximately 20 mm into a rectangular piece of wood. A piece of dowel was

inserted and glued into the hole to form a non-ferrous mixing paddle. This dowel


_____________________________________________________________________


could be inserted into the chuck of an electric power drill to rotate and mix the fluid in

the containment vessel. The apparatus is shown in Figure 3.9

Figure 3.9 – Wooden paddle used for mechanical mixing.

3.8 Experiment using mechanical mixing

This experiment was conducted to examine the effects of placing the anode in the

containment vessel that the nanoscale zero valent iron was in and placing the cathode

in the other containment vessel that had no zero valent iron in it. 24 L of tap water

was added to two containment vessels connected by a tube packed with porous media.

The electrodes were suspended over a wooden pole placed across the containment

vessels, and were placed at the ends of the connecting tube. A voltage of 20 volts was

applied to the electrodes. The fabricated wooden paddle was fitted into the XUI 13

Hammer drill and then suspended into the anodic containment vessel. The drill was

operated at regular intervals to re-suspend any nanoscale zero valent iron that had

settled out of the suspension. 30 mL of nanoscale zero valent iron slurry was added to

the anodic containment vessel. The experiment was run for one week. Sampling was

conducted periodically and consisted of agitating the fluid in the containment vessel

to ensure the sample was homogenous and taking the sample from just below the

surface near the electrode. pH was also monitored in the cathodic containment vessel.


_____________________________________________________________________


The water that was removed lowered the depth of the water in the containment

vessels. To combat the drop in water level, tap water was added each day to maintain

the volume of water used in the experiment. This added water was not considered

enough to alter any physical or chemical process due to slight changes in

concentration. The experiment was run for 7.5 days, as per the previous experiment,

because for the electrokinetic effect on moving the nanoscale zero valent iron

particles to be considered useful, it must be able to move the particles the 100mm in

this time interval.

3.9 Initial direct injection of nanoscale zero valent iron into porous

media experiment

Two containment vessels connected by a porous media filled connecting tube were

filled with 24 L of tap water. Electrodes were suspended just outside the aperture of

each containment vessel and a voltage of 20 volts was applied. The connecting tube

had a 7 mm diameter flexible tube (termed injection ports) inserted into each of the

three 7 mm apertures along the top of the connecting tube, and PARFiX silicone

sealant sealed the joints. Approximately 1.5 mL of nanoscale zero valent iron was

injected into the middle flexible tube as seen in Figure 3.10. It was the intention that

electrokinetic processes induce movement of the nanoscale zero valent iron to an

electrode. Water in both containment vessels was agitated with a non-metallic stirring

pole and sampled periodically just underneath the surface near the electrodes. The

experiment was run for 7.8 days as it was deemed that if the nanoscale zero valent

iron could not be moved in this time period, it would not be effective as a remediation

technique.


_____________________________________________________________________


Figure 3.10 – Connecting tube featuring injection of nanoscale zero valent iron through the flexible tubing

3.10 Direct injection of nanoscale zero valent iron into porous media

experiment with enhanced conductivity

A subsequent experiment was conducted again featuring direct injection of nanoscale

zero valent iron into the connecting tube. In this experiment, the injection site was

220 mm from the aperture in the anodic containment vessel and 670 mm from the

aperture in the cathodic containment vessel. This was done as it was suspected that

the nanoscale zero valent iron would have a greater affinity for the cathode than the

anode. In the event of the nanoscale zero valent iron moving to the cathodic

containment vessel, the longer pathway through the porous media would impart more

confidence in the validity of the result. 50 g of sodium chloride (NaCl) was added to

each containment vessel to determine if the increased conductivity would enhance the

electrokinetic effects on the nanoscale zero valent iron. pH and conductivity were

monitored in the cathodic containment vessel, as was the amperage drawn by the

power supply. Cathodic containment vessel water samples were taken by

submergence of a 21 mL sample vial at the connecting tube aperture. The experiment

was run for 10.2 days, because for the electrokinetic transport of nanoscale zero


_____________________________________________________________________


valent iron to be considered in the field, it would have to be able to move 100mm in

this time period.

3.11 Initial hydraulic advection experiment

A connecting tube filled with porous media was inserted between two containment

vessels. A slit was cut in the side of a containment vessel, to ensure a constant

hydraulic head. The containment vessels were filled so that the water level was at the

slit that was cut into the side of the containment vessel and 30 mL of nanoscale zero

valent iron was added to the other containment vessel. 10 pore space volumes (780

mL) of water were added to the containment vessel containing the iron, to provide a

hydraulic gradient. The zero valent iron containment vessel was stirred frequently to

mitigate settling. The final set-up is shown in Figure 3.11

Figure 3.11 – Containment vessel with slit in side for constant hydraulic head

3.12 Iron concentration sampling

The samples were analysed for total iron content using a SpectraAA-100 atomic

Absorption Spectroscopy (AAS) Machine. The machine was calibrated using a two

point calibration technique, using standards of 10 ppm total iron and 3.9 ppm total


_____________________________________________________________________


iron. The readings were reliable to a minimum concentration of 2 ppm. The machine

did exhibit a slight amount of creep in the measurements, and to mitigate this, the

machine was re-calibrated every 20 samples. Samples were prepared by addition of a

70% nitric acid (HNO3) solution, to ensure the dissolution of all the iron.

Chapter 4: Results

_____________________________________________________________________


4 Results

The results for the numerous experiments are presented below. The implications of

the results are discussed in the discussion chapter.

4.1 Iron concentration determination

A 120 mL sample of nanoscale zero valent iron slurry weighed 99.9g. This gave a

specific gravity of less than unity. 8325.0120

9.99= .

A sample of nanoscale zero valent iron was dried to determine the slurry’s water

content. The results showed that the drying vessel that had been dried over 16 hours

contained solids weighing 68.2 g. This resulted in a water content to be 100 - 68.2 =

31.8 %.

4.2 Single containment vessel experiment

After 15 minutes, the test involving mixed metal oxide electrodes was stirred, which

produced a fizzing noise, probably due to gas generation at the electrodes. An hour

later, a significant amount of effervescence was observed at the cathode. The surface

around the cathode also had a brittle film form. Two hours after the electrodes were

supplied power, a brown sludge had formed around the cathode (shown in Figure 4.2),

and the bubbling continued, which was accompanied by an audible fizzing noise.

There was little change to the anode, however approximately 75 % of the mixed metal

oxide coating had been removed from the cathode.

The test using steel electrodes began in a similar fashion to the mixed metal oxide

electrode test, with bubbling occurring at the cathode. A brown sludge formed at the

cathode approximately 3 hours after the electrodes were powered. This brown sludge

continued to grow and propagate over the surface of the fluid until the cessation of the

experiment. The electrodes were examined after the conclusion of the experiment.

The cathode appeared unchanged, whereas the anode was coated in a thick brown

Chapter 4: Results

_____________________________________________________________________


coating. This brown coating was easily wiped away by a cloth, exposing a black

coloured surface. While the thickness of the cathode did not appear to be changed,

the anode was significantly thinner. The thickness of both electrodes before the

experiment was 3.9 mm, and after the experiment the cathode was still 3.9 mm in

diameter, but the anode was 3.6 mm in diameter. Both electrodes are shown in Figure

4.1

Figure 4.1 – Steel electrodes after operation in nanoscale zero valent iron slurry.

Chapter 4: Results

_____________________________________________________________________


Figure 4.2 – Slurry reaction at cathode.

4.3 Dual containment vessel experiment with no porous media

4.3.1 Sodium Chloride (NaCl) experiments

The results for both the NaCl experiment run at 20 volts, and the experiment operated

at 10 volts revealed the ions in solution did indeed migrate from one containment

vessel to the other. The increase in concentration of ions was indicated by the

increase in conductivity, as they are approximately proportional (Zimmt, 1993).

4.3.1.1 NaCl experiment at 20 volts

The conductivity results from the NaCl experiment conducted at 20 volts are shown

below in Figure 4.3.

Chapter 4: Results

_____________________________________________________________________


NaCl expt

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

0.00 200.00 400.00 600.00 800.00 1000.00 1200.00

Time (mins)

Co

nd

ucti

vit

y (

mS

)

Figure 4.3 – NaCl experiment conducted at 20 volts.

It can be seen from Figure 4.3 that during the first 22 minutes of the experiment, the

conductivity fluctuated noticeably. The conductivity of the solution in the anodic

vessel then increased over time, thus indicating the migration of ions from the dosed

cathodic containment vessel to the anodic containment vessel. The conductivity

plateaued after approximately 4 hours.

4.3.1.2 NaCl experiment at 10 volts

The conductivity results from the NaCl experiment conducted at 10 volts are shown

below in Figure 4.4.

Chapter 4: Results

_____________________________________________________________________


NaCl 10V Run

0.00

1.00

2.00

3.00

4.00

5.00

6.00

7.00

8.00

0 200 400 600 800 1000 1200

Time (mins)

Co

nd

ucti

vit

y (

mS

)

Figure 4.4 – NaCl experiment conducted at 10 volts. It can be seen from the above graph that the conductivity rapidly increased from

below 1 mS/cm to approximately 5.3 mS/cm within 12 minutes. The conductivity

then fluctuated for 5 hours and then exhibited an upward trend.

4.3.2 Initial zero valent iron experiment

Upon powering the electrodes in the initial zero valent iron experiment, effervescence

was observed at both electrodes, being more pronounced at the negative electrode.

As time progressed, the pH was observed to drop from 7.45 to 7.05 in 190 minutes.

Accurate measurement of the pH was not possible following initiation of the orbital

mixer, due to the pH probe fluctuations. After 390 minutes, no migration of the iron

was visually observed.

Figure 4.5 and Figure 4.6 below show the iron concentration and pH level

respectively for the first zero valent iron experiment.

Chapter 4: Results

_____________________________________________________________________


Aged ZVI Experiment

-0.005

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0 50 100 150 200 250 300 350 400

Time (mins)

Fe

co

nc

ne

tra

tio

n (

mg

/L)

`

Figure 4.5 – Aged Zero Valent Iron Experiment Iron Concentration

Aged ZVI Experiment

6.2

6.4

6.6

6.8

7

7.2

7.4

7.6

0 50 100 150 200 250 300 350 400

Time (mins)

pH

Figure 4.6 – Aged Zero Valent Iron pH

4.3.3 Second zero valent iron experiment

Although efforts were made to mitigate nanoscale zero valent iron advection from the

cathodic containment vessel to the anodic containment vessel, the removal of the plug

created a great deal of water movement that advected the iron from one containment

vessel to the other. Due to the absence of porous media between the two containment

vessels, eddies were induced that advected a significant amount of the zero valent iron

slurry from the negative containment vessel to the positive containment vessel when

the plug was removed. There was also a slight hydraulic head difference between the

Chapter 4: Results

_____________________________________________________________________


containment vessels due to the addition of the nanoscale zero valent iron slurry, which

also conveyed nanoscale zero valent iron through the connecting tube. This

compromised the experiment, as nanoscale zero valent iron had been moved by non-

electrokinetic phenomena. Figure 4.7 shows the results of the analysis of the samples

taken during this experiment.

No Porous Media Experiment

0

1

2

3

4

5

6

7

8

-200 0 200 400 600 800 1000 1200 1400

Time (mins)

Fe

Co

nc

en

tra

tio

n (

mg

/L)

Figure 4.7 – Total iron concentration versus time for second experiment without porous media.

4.4 Dual containment vessel experiment with porous media and

orbital mixing

4.4.1 Initial experiment

The first attempt at filling the connecting tube with porous media resulted in the

formation of a significant cavity space located along the top of the long axis of the

connecting tube. This resulted in the rapid advection of the aqueous solution

containing nanoscale zero valent iron through the connecting tube, along the top of

the tube through the space with no porous media, into the other containment vessel.

The cavity was suspected to be caused by compaction of the porous media once wet,

thus reducing the volume occupied by porous media and leaving a void space above,

seen in Figure 4.8. The advection of the nanoscale zero valent iron through the cavity

Chapter 4: Results

_____________________________________________________________________


from one containment vessel to the other would mask any electrokinetic transport, and

hence the experiment was stopped with inconclusive results.

Figure 4.8 – Voiding along the top of the connecting tube

4.4.2 Second experiment

When the connecting tube filled by the second method was installed into the

containment vessel apertures, a cavity formed in the same position as before, i.e.

along the long axis above the porous media. The reason for this cavity space

formation could not be explained by the compaction of silica after it was wet, since it

was installed into the connecting tube in an aqueous matrix. The Chux® Regular

Superwipes screening cloth affixed over the ends of the connecting tube were flush

with the tube ends during filling with porous media because the tube was standing

upright. However, when turning the connecting tube on its side to fit it into the

containment vessel apertures, the screening cloth bulged outwards, increasing the

available volume for containment of the porous media, and thus resulting in the cavity

space across the long axis of the tube. Upon addition of the nanoscale zero valent

iron, it was visibly seen to immediately flow into the connecting tube through the

cavity along the top of the tube, as seen in Figure 4.9. The experiment was then

Chapter 4: Results

_____________________________________________________________________


stopped, again because of the advection of the nanoscale zero valent iron masked any

electrokinetic induced movement.

Figure 4.9 – Connecting tube featuring voiding

4.4.3 Third experiment

The third experiment yielded much better results than the previous two experiments

due to the porous media in the connecting tube not containing a large void. The

experiment proceeded to run in a satisfactory manner until 92 hours after

commencement, when the waves induced by the elliptical motion of the orbital mixer

caused the media in the connecting tube to erode away. The erosion of the porous

media resulted in a void forming across the top of the connecting tube, similar to the

previous two experiments, seen in Figure 4.10. This allowed water to be advected

from one containment vessel to the other solely without passing through the porous

media. When removed from the containment vessel, the cathode was coated in a

black coating that could not be easily removed, seen in Figure 4.11.

Chapter 4: Results

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Figure 4.10 – Voiding due to orbital motion of mixer

Figure 4.11 – Cathode and Anode after experimentation

Figure 4.12 shows the total iron concentration for samples taken over the duration of

the experiment.

Chapter 4: Results

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Orbital mixing Expermient with no Voiding

-0.2

0

0.2

0.4

0.6

0.8

1

0 1000 2000 3000 4000 5000 6000 7000

Time (mins)

Fe

Co

nc

en

tra

tio

n (

mg

/L)

Figure 4.12 – Orbital experiment mixing experiment nanoscale zero valent iron concentrations

A slight increase in total iron concentration was initially observed, followed by a

decrease after 50 hours. The slightly increasing trend is then observed once more up

until 92 hours, when a large spike in iron concentration is observed, coinciding with

the time the cavity was formed from the eroded porous media. The concentration of

iron in all the samples was very small (less than 0.2 mg/L), with the exception of the

last sample.


mechanical mixing

For the duration of the experiment, water slowly leaked from the join between the

connecting tube and containment vessel. The leakage was very slow, less than a drop

every 10 minutes. However, when this water loss was combined with additional

water loss from evaporation and removal for sampling, it had the potential to induce

iron migration by the formation of a hydraulic head. To prevent the hydraulic head

forming, both containment vessels were periodically topped up to exactly 24 L with

additional tap water. Nanoscale zero valent iron penetration can be seen in Figure

4.13 and the electrodes after the experiment are shown in Figure 4.14.

Chapter 4: Results

_____________________________________________________________________


Figure 4.13 – Nanoscale zero valent iron penetration of porous media

Figure 4.14 – Cathode and anode after experimentation

Figure 4.15 shows the total iron concentration results for the duration of the

experiment.

Chapter 4: Results

_____________________________________________________________________


Mechanical Mixing Experiment

0

0.2

0.4

0.6

0.8

1

0 2000 4000 6000 8000 10000 12000

Time (mins)

Fe

Co

nc

en

rati

on

(m

g/L

)

Figure 4.15 – Mechanical mixing experiment nanoscale zero valent iron concentrations

It can be seen that the iron concentrations for this experiment remained fairly constant

over the entire period. The concentration reached a peak level of 0.118 mg/L, fell to a

low of 0.017 mg/L, and had a range of 0.101 mg/L.

A visual inspection of the core of the porous media in the connecting tube revealed

the nanoscale zero valent iron to have penetrated into the porous media on the anodic

side. The porous media was starkly white from the cathodic end to 18 mm from the

anodic end, when it was contrastingly a dark black. The iron penetrated 18 mm in

over 7.5 days, resulting in a transmission rate of 2.39mm/day.


injection

After a visual inspection of the porous media core following completion of this

experiment, the nanoscale zero valent iron directly injected into the side injection port

did not seem to move significantly. In fact, it had not even entered the porous media

in the main connecting tube. The nanoscale zero valent iron could be easily observed

visually, as it was a black colour, and the porous media was starkly white. After the

187 hours (7.8 days) had passed, the porous media 1 mm below the injection port had

not changed colour, and was still a very clear white colour. After inspection, the

Chapter 4: Results

_____________________________________________________________________


entire core sample did not appear to have any trace of black nanoscale zero valent

iron, as seen in Figure 4.16. Total iron concentrations can be seen in Figure 4.17.

Figure 4.16 – Core sample of connecting tube featuring no visible nanoscale zero valent iron penetration

Direct Injection Experiment

0

0.2

0.4

0.6

0.8

1

0 5000 10000 15000

Time (mins)

Fe

Co

nc

en

tra

tio

n (

mg

/L)

Anodic Fe

Concentration

(mg/L)

Cathodic Fe

Concentration

(mg/L)

Figure 4.17 – Direct injection experiment nanoscale zero valent iron concentration of both anodic and cathodic containment vessels

Chapter 4: Results

_____________________________________________________________________


The iron levels in the anodic containment vessel were significantly higher than the

iron levels monitored in the cathodic containment vessel. This was assumed to be due

to residual iron from a previous experiment being present in the anodic containment

vessel. This was not deemed to be problematic because the concentrations were

analysed for a change in iron concentration, and not absolute concentration. Thus, the

general discrepancy between the anodic and cathodic containment vessels iron

concentrations is not an indication of electrokinetic transport phenomena, and merely

a difference in baseline iron concentrations. The maximum and minimum iron

concentrations in the anodic containment vessel were 0.323 mg/L and 0.064 mg/L

respectively, and had a range of 0.259 mg/L. The maximum and minimum iron

concentrations in the anodic containment vessel were 0.113 mg/L and 0.014 mg/L

respectively, and had a range of 0.099 mg/L.


injection with enhanced conductivity.

The nanoscale zero valent iron that was injected into the injection port did not visibly

move after 9 days in this experiment. Visual inspection of the core revealed the

porous media to be completely white with no black sections, thus indicating the

nanoscale zero valent iron had not moved through the connecting tube. Figure 4.18

shows the total iron concentration levels for the duration of the experiment.

NaCl Dosed Direct Injection Experiment

0

0.2

0.4

0.6

0.8

1

0 2000 4000 6000 8000 10000 12000 14000 16000

Time (mins)

Fe

Co

nc

en

tra

tio

n (

mg

/L)

Chapter 4: Results

_____________________________________________________________________


Figure 4.18 – Iron concentrations for the NaCl dosed direct injection experiment

It can be seen from Figure 4.18 that the total iron concentration remained relatively

constant over the entire duration of the experiment. Peak concentration was 0.25

mgL-1 and the lowest concentration was 0.16 mgL-1. The concentration range was

0.09 mgL-1. Figure 4.19 shows the conductivity to immediately increase from 4.31 mS/cm to 9.70

mS/cm in a time span of 8 hours. The conductivity then remains fairly constant,

fluctuating by only 0.58 mS/cm for the rest of the experiment’s duration. The pH also

climbed from an initial value of 8.27, and exceeded a pH of 10 after 33 hours. It then

further increased to a peak value of 11.42, and then fluctuated between 10.7 and 11.25

for the remainder of the experiment.

NaCl Dosed Direct Injection Experiment

0.00

2.00

4.00

6.00

8.00

10.00

12.00

0 5000 10000 15000 20000

Time (mins)

pH

Conductivity (mS/cm)

Figure 4.19 – pH and conductivity record of the NaCl dosed direct injection experiment

As seen in Figure 4.20, the current drawn at the beginning of the experiment was

similar to other experiments, at 0.02 amps. The enhanced salinity did have a marked

effect on the amperage drawn, peaking at double the original reading.

Chapter 4: Results

_____________________________________________________________________


NaCl Dosed Direct Injection Experiment Current

Levels

0

10

20

30

40

50

0 2000 4000 6000 8000 10000 12000 14000 16000

Time (mins)

Am

pe

rag

e (

mA

)

Figure 4.20 – Amperage drawn during the NaCl dosed direct injection experiment

4.8 Hydraulic advection experiment

The volume required to provide 10 pore space volumes was calculated in the

following manner.

Volume of connecting tube = 105.22!!"

= 196.35 mL

Void volume = 35.1964.0 !

= 78.5 mL

A core sample was taken following the completion of the experiment, seen in Figure

4.21, to ascertain the degree of nanoscale zero valent iron penetration. After a period

of 380 minutes, the iron had penetrated a length of 15 mm. This correlated to a

transmission rate of 2.37 mm/hr.

Chapter 4: Results

_____________________________________________________________________


Figure 4.21 – Core sample of connecting tube after hydraulic advection experiment

Figure 4.22 shows the total iron concentration for the duration of the experiment

Hydraulic Advection Experiment

0.000

0.200

0.400

0.600

0.800

1.000

0 100 200 300 400

Time (mins)

Fe

co

nc

en

tra

tio

n (

mg

/L)

Figure 4.22 – Hydraulic Advection Experiment Iron Concentrations

The iron concentration fluctuated from a peak value of 0.23 mgL-1 to a minimum

value of 0.11 mgL-1. There did not seem to be any clear upward trend in the total iron

concentrations for this experiment.

Chapter 5: Discussion

_____________________________________________________________________


5 Discussion

5.1 Iron concentration determination

Since iron is more than seven times the density of water, a specific gravity of less than

unity was not expected. Upon visual inspection of the slurry, the same gas that was

suspected for the pressure build up in the packaging was present in the slurry as an

emulsified froth. It was thought that this gas had a specific gravity less than unity,

and thus was the explanation for the very low density of the slurry. As the details of

the gas were not divulged to the author, the iron concentration was therefore not able

to be ascertained by weighing a known volume. This is because when as the density

of a constituent was not known, an additional variable exists, making the system of

equations used an unsolvable system.

The sample that was dried in the drying oven may have gained weight depending on

the degree of enhanced oxidation of the nanoscale zero valent iron. If the iron

corroded very rapidly due to the elevated temperatures, each iron atom is capable of

bonding with 3 oxygen atoms. Although not every iron atom would react in this way,

a large degree of oxidation in the elevated temperatures in the drying oven would

result in the oxygen atoms contributing significantly to the weight of the sample. The

molecular mass of oxygen and iron is 16.0 g/mol, and 55.8 g/mol respectively. Three

additional oxygen molecules would contribute 48 g, or 2.461008.5548

48=!

+% of

the weight.

When first opened, there was a significant spillage of the iron slurry. This was due to

the encapsulating plastic withholding a build up of pressure from the sample. When

the packaging was opened, an expulsion of the build up of gas was combined with a

large leakage of the nanoscale zero valent iron slurry container. This leakage may

have removed a significant amount of water from the slurry, and would hence

increase the slurry’s iron concentration.


_____________________________________________________________________


The slurry had been made at an unknown time in the past and so it was known that the

slurry had been existent long enough for an amount of evaporation to occur. This

would also have increased the iron concentration.

This method for determining the iron concentration in the slurry was deemed to be the

optimum method, and gave an iron concentration of 682 g/L.

5.2 Single containment vessel

The major change witnessed with the steel anode was explained by corrosion. The

observed change agreed with the concept of the zero valent iron undergoing an

oxidation reaction at the anode, converting from Fe0 to Fe2+ and/or Fe3+. Once

formed, the ferrous and/or ferric ions could then be solvated by the surrounding water

molecules. This would result in the reduction in mass and diameter observed with the

steel anode. The anode surface was black, suggesting the formation of either FeO or

Fe3O4. This did not occur with the mixed metal oxide anode because its external

surface did not contain significant amounts of Fe0 to be oxidised.

The brown sludge that formed in both experiments (Figure 5.1), could be explained

by formation of ferric oxide (Fe2O3). Commonly known as rust, it has a characteristic

brown appearance that can be seen in Figure 5.1. Both experiments featured the

brown sludge forming at the cathode. The theory that the iron was reacting with the

oxygen generated by the electrodes was quickly discounted because the generation of

oxygen occurs at the anode, and not at the cathode, which was where the brown

sludge appeared. The appearance of the brown sludge can be explained by

combination of hydroxyl radicals and positively charged solvated iron particles. Once

solvated, the positively charged iron particles migrate by the process of

electromigration to the cathode. (OH)- radicals are generated at the cathode due to the

electrolysis of water. The (OH)- ions combine with the positive Fe ions to form a

brown iron hydroxide solid. It was this iron-hydroxide solid that was observed at the

cathode.


_____________________________________________________________________


Figure 5.1 – Powered electrodes immersed in a nanoscale zero valent iron slurry

5.3 Dual containment vessels with unhindered flow

5.3.1 NaCl experiment at 20 volts

The fluctuation at the beginning of the experiment (up until approximately 22

minutes) can be explained by the anode’s influence on the dissolved salts in solution.

As the water contained a small amount of charged ions, the electrode induced these

ions to movement close to the conductivity sensor. As the ions passed the probe, the

probe would record the increase in conductivity. These reading would give a false

reading of the actual conductivity, as it measured the higher conductivity of the

immediate surroundings, and not the overall conductivity of the anodic containment

vessel. It can be seen that following the initial fluctuations, the readings did indeed

stabilise, and give more credible results. The steady increase in conductivity

following the initial 22 minutes show that the electrodes did indeed function in the

desired manner, and induced electrokinetic phenomena to move the charged ions in

solution.


_____________________________________________________________________


5.3.2 NaCl experiment at 10 volts

The 10 volt experiment did not exhibit the same initial fluctuations as the 20 volt

experiment, indicating that the lower voltage did not induce such a large force on the

existing ions in solution as the 20 volt experiment, thereby the number of ions that

were moved into close proximity of the conductivity probe was not as great. The

probe was moved at certain times to refresh the water surrounding it. This had a

pronounced effect on the reading, which accounts for the fluctuations observed for the

first 5 hours. Again, the results demonstrate the electrokinetic phenomena to be

powerful enough to induce a significant ion flux at this lower voltage.

5.3.3 Initial experiment with zero valent iron and porous media

The initial test used nanoscale zero valent iron that had been procured the previous

calendar year. The slurry had almost completely no water content and had

agglomerated to a significant degree. As a result of this, the iron did not disperse into

the containment vessel, rather it settled quickly to the bottom. The observed pH

fluctuations that made it difficult to measure the exact pH was explained by the

movement of the water body continually exposing the probe to a different section of

water, however, the pH did drop approximately more than half a pH point after

mixing. This pH change was thought to be due to hydrolysis of water occurring at the

electrodes.

5.3.4 Second experiment with zero valent iron and no porous media

The total iron concentration results verify that the sampling technique used is capable

of detecting iron influxes. The concentration of iron peaked at a level of 6.96 mg/L,

which was more than any other experiment. The fluctuating nature of the total iron

concentration is due to agitation variance. The higher occurrences of iron

concentration coincide with times of agitation. The induced eddies that conveyed

nanoscale zero valent iron between the containment vessels upon removal of the plug

was the reason why the levels of total iron concentration are so high. Therefore, the

merits of this experiment are that it demonstrated the analysis technique was capable


_____________________________________________________________________


of detecting nanoscale zero valent iron when it was known to be definitely present,

and that agitation does re-suspend previously settled iron particles in the solution.


orbital mixing

The packing of the connecting tube with porous media proved to be quite difficult. In

an effort to mitigate the effects of voiding across the top of the connecting tube, the

porous media was packed very compactly. This compaction may have made it more

difficult for transmission of particles than if it had been packed more loosely. As the

third experiment did not immediately feature voiding across the top of the connecting

tube, the total iron concentration analysis of this experiment yielded interesting

results. Due to the erosion caused by the elliptical motion of the orbital mixers in this

experiment, a void formed across the connecting tube.

This void allowed the advection of iron particles into the anodic containment vessel

before the final sample was taken (taken at t = 5872 minutes, total iron concentration

= 0.919 mg/L. This influx of iron due to non-electrokinetic effects explains the

extremely high iron level recorded at this time. Once this sample is disregarded, the

iron concentration is seen to be quite stable, and very low. The apparent slight

increase in iron concentrations is not due to electrokinetic phenomena, but rather to

instrumentation creep. This is evidenced by the drop in apparent total iron

concentration at t = 3000 minutes, which coincided with the re-calibration of the AAS

machine. All remaining total iron concentration levels were very low, indicating the

nanoscale zero valent iron had not successfully been moved through the connecting

tube to the anodic containment vessel in the time period.

When removed from the containment vessel, the cathodic electrode was coated in a

black layer. This was presumed to be a coating of nanoscale zero valent iron. This

layer had significantly adhered to the cathode, and did not easily rub-off by hand or

scaping and rubbing with paper. This validated the notion that the nanoscale zero

valent iron is attracted to cathodic sources.


_____________________________________________________________________


5.5 Experiment with mechanical mixing

The mechanical mixing experiment results show no significant influx of nanoscale

zero valent iron into the cathodic containment vessel. The slight variances in the

concentrations are attributed to the random fluctuations of the AAS machine, and also

the re-calibrations that were conducted at t = 1090 minutes and t = 6300 minutes.

Overall, the total iron concentrations are insignificant and show no influx of iron

particles over the duration of the experiment.

It is thought that the mechanical agitation of the nanoscale zero valent iron slurry in

the anodic containment vessel resulted in a forcing of the nanoscale zero valent iron

through the porous media in the connecting tube. The penetration of the dark

coloured iron into the porous media reveals the ability of the nanoscale zero valent

iron to move through media, thus demonstrating the iron particle transmission ability

of the porous media.

After more than a week of being submerged in a nanoscale zero valent iron slurry, the

anode did not accumulate a solid black coating, rather a thin film of water and

nanoscale zero valent iron that could easily be removed by wiping with either a finger

or a piece of paper. It is this fact, coupled with the observed coating on the cathode,

which suggests the nanoscale zero valent iron has an affinity for the cathode and not

the anode.

5.6 Experiment with direct injection

5.6.1 Initial direct injection experiment

The total iron concentrations varied considerably between the anodic and cathodic

containment vessels. This is due to the presence of residual iron from a previous

experiment in the anodic containment vessel. This was not considered to be

problematic, as the water samples from each containment vessel were analysed for

changes in total iron concentration, and not absolute iron concentrations. The

relatively large decrease observed after 5.5 days in both anodic and cathodic samples

is explained by the re-calibration of the AAS machine. The total iron concentration

fluctuations for both containment vessels were very small, indicating no significant

transmission of nanoscale zero valent iron through the porous media.


_____________________________________________________________________


Mixing prior to each sampling run may have moved iron through connecting tube,

however, since mixing was done equally on both sides, and only just before sampling,

it was not thought to have a significant effect. The flexible injection port tubing was

removed and examined after the experiment. The tubing was sliced longitudinally

and inspected. White porous media was observed at the base of the tubing where it

had been inserted into the connecting tube, to a thickness of 3 mm. This white porous

media had been forced into the injection port tubing when the tubing had been

installed. On top of the white porous media was the nanoscale zero valent iron slurry

that was a distinct black fluid. The iron slurry did not appear to have moved into the

connecting tube and had remained in the injection port for the duration of the

experiment. This suggests that the electrokinetic effects had no significant influence

on the nanoscale zero valent iron, because it did not seem to have been moved.

Ferric (Fe3+) and ferrous (Fe2+) ions could move towards the negatively charged

cathode due to electromigration, but not to the anode due to electro-repulsion. As

both containment vessels experienced similarly insignificant total iron concentration

increases, the transmission of Fe2+ and Fe3+ ions was determined to be insignificant.

5.6.2 Direct injection experiment with enhanced conductivity

The total iron concentration did not significantly increase over time. The fluctuations

observed were due to the small fluctuations of the AAS machine, and not due to an

influx of nanoscale zero valent iron. This coupled with the evidence that the

nanoscale zero valent iron did not leave the injection port flexible tubing suggests that

electrokinetic phenomena were unsuccessful in mobilising the nanoscale zero valent

iron.

The solutions in the two containment vessels were dosed with additional NaCl,

however, the porous media in the connecting tube was wet with non-dosed tap water.

This resulted in the connecting tube acting as a resistive barrier between the two

containment vessels because of its lower relative salinity. The amperage increased to

30 mA after 3.5 days, which indicated that the ions had penetrated through the porous


_____________________________________________________________________


media. The amperage further increased to 40 mA after 10 days, further enforcing the

notion that the added ions had gone through the connecting tube. The enhanced ion

concentrations and thus higher currents involved in this experiment show that

elevated conductivity does not have an appreciable effect on the ability to move

nanoscale zero valent iron using electrokinetic phenomena.

5.7 Hydraulic advection experiment

Despite 10 pore space volumes of fluid passing through the containment vessel, the

nanoscale zero valent iron had only moved a small fraction the length of the

connecting tube, (15 mm (or 15 %) of the length of the tube). This demonstrated that

the nanoscale particles had difficulty travelling through the porous media network of

voids, as they had moved such a small distance after such a comparatively large

discharge.

This experiment was conducted after all previous experiments, resulting in using the

most aged nanoscale zero valent iron slurry. Since the slurry had had the most time

agglomerate, it would have had the largest mean particle size. It is therefore

inconclusive as to if the nanoscale zero valent iron had better penetration capabilities

at a younger age.

However, this experiment does add credence to the theory that the nanoscale zero

valent iron is difficult to move due to its difficulty in fitting through the void

networks. It has therefore not been established if electrokinetic phenomena can be

used to induce movement of nanoscale zero valent iron. It has however been shown

that electrokinetic phenomena are not able to induce a meaningful transmission of

nanoscale zero valent iron in cases where the nanoscale zero valent iron cannot be

moved effectively by a hydraulic gradient.

Chapter 6: Conclusion

_____________________________________________________________________


6 Conclusion

6.1 Electrokinetics and nanoscale zero valent iron

Electrokinetic effects were shown to possess the ability to move charged aqueous

ions, such as sodium and chloride ions. The experiments conducted showed that the

nanoscale zero valent iron was not able to be moved through the porous media

effectively by electrokinetic effects. The nanoscale zero valent iron particles did not

move significantly towards either the anode or the cathode in a number of

experiments. It was also shown that the nanoscale zero valent iron was also not able

to be moved through the porous media under a hydraulic gradient. It was therefore

concluded that electrokinetic phenomena could not move nanoscale zero valent iron

in situations where the hydraulic inducement of nanoscale zero valent iron is not

possible.

The nanoscale zero valent iron did have an affinity for the cathode and was not

attracted to the anode significantly. This was evidenced in the altered appearances of

the cathode in the orbital mixing experiment and the anode in the mechanical mixing

experiment.

6.2 Recommendations

An aspect of the experiment that had a large degree of variability was the packing of

the connecting tube with porous media. The packing process used required a great

deal of time to be spent packing the porous media. It was not determined whether the

packing process impeded nanoscale zero valent iron flow because it was so closely

packed. It is therefore recommended that it be ascertained whether the packing of the

porous media has an impact on the ability of the nanoscale zero valent iron to pass

through it.

The difference in the interaction of the nanoscale zero valent iron slurry with the

cathode and with the anode provides an interesting area for further research. The

Chapter 6: Conclusion

_____________________________________________________________________


apparent affinity of the nanoscale zero valent iron particles to the cathode and the lack

of reaction between slurry and anode require more investigation.

It is also recommended to investigate the interaction between the nanoscale zero

valent iron and silica porous media particles at various pH levels. This will give a

greater understanding of the processes involved in the various experiments, and

possible retardation mechanisms.

A further recommendation is to investigate the usage of additional chemical species

such as surfactants or polymers containing both hydrophobic and hydrophilic

constituents to interact with the nanoscale zero valent iron. This has the possibility to

prevent or mitigate agglomeration. Preventing agglomeration has the potential to

increase the ability of the nanoscale zero valent iron to penetrate the porous matrix

and also decrease the force required to move the average sized particle.

Chapter 7: Glossary

_____________________________________________________________________


7 Glossary

AAS – Atomic Absorption Spectroscopy.

Agglomeration – adherence of particles into single particle.

Aliphatic – An organic carbon compound in which the atoms are joined in open

chains.

Alkane – A hydrocarbon that contains only single bonds.

Alkyl Halide – An alkane that has had at least one hydrogen replaced with a halogen.

Anode – The positive electrode, where oxidation occurs.

Cathode – The negative electrode, where reduction occurs.

DNAPL – Dense Non-Aqueous Phase Liquid.

Electrode – An electrically conductive structure that transfers electrons.

Ferric – An iron cation that has a charge of 3+.

Ferrous – An iron anion that has a charge of 2+.

Halogenated – A compound containing at least one element that is a halogen.

Halogen – An element coming from group 17 of the periodic table.

Hydrocarbon – An organic chemical consisting of hydrogen and carbon.

Halogenated Hydrocarbon – A hydrocarbon that has had at least one hydrogen

replaced with a halogen atom.

Chapter 7: Glossary

_____________________________________________________________________


Pore Space Volume – The volume of a designated zone that is comprised of voids.

Saturated Zone – The zone in which all pore spaces are completely filled with water.

Vadose zone – The zone between land surface and water table that contains water

content less than saturation.

ZVI – Zero Valent Iron.

Chapter 8: References

_____________________________________________________________________


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