Surface Complexation Modeling of CR(VI) Absorption on

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Surface Complexation Modeling of CR(VI) Absorption on Mineral Surface Complexation Modeling of CR(VI) Absorption on Mineral

Assemblages Assemblages

Ann M. Gilchrist

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SURFACE COMPLEXATION MODELING OF CR(VI) ADSORPTION ONMINERAL ASSEMBLAGES

Thesis Committee:

by

Ann M. Gilchrist

A thesis submitted to the Graduate Collegein partial fulfillment of the requirements

for the degree of Master of Science, GeosciencesWestern Michigan University

August 2013

Carla Koretsky, Ph.D., ChairR.V. Krishnamurthy, Ph.D.Alan Kehew, Ph.D.

SURFACE COMPLEXATION MODELING OF CR(VI) ADSORPTION ONMINERAL ASSEMBLAGES

Ann M. Gilchrist, M.S.

Western Michigan University, 2013

Hexavalent chromium (Cr(VI)) is a waste product of many anthropogenic

processes. Because it is highly mobile, the improper disposal of Cr(VI) has caused

widespread contamination. Because reduction and adsorption reactions may reduce the

bioavailability and mobility of Cr(VI) in environmental systems, a better understanding

of Cr(VI) adsorption behavior will improve remediation efforts.

Subsurfaces and soils are heterogeneous, however most studies focus on single

sorbate/sorbent interactions to develop surface complexation models (SCM) for

prediction of heavy metal adsorption. Theoretically, combining the SCMsdeveloped for

single sorbate/sorbent systems shouldyieldaccurate predictions of adsorption in more

complex systems (i.e., the componentadditivityapproach of Davis et al., 1998).

To assess this hypothesis, Cr(VI) adsorption on mineral assemblages of goethite,

kaolinite, montmorillonite, y-alumina, hydrous manganese oxide (HMO), and hydrous

ferric oxide (HFO) in equal surface areas, measured as a function of pH, ionicstrength

andpC02 were conducted. Double layer models (DLMs) developed for Cr(VI)

adsorption on thepure solids were used to predict Cr(VI) adsorption in the mixed solid

systems and compared with measured edges for the various mineral assemblages.

This study found that the simple additivity approach did not accurately predict

adsorption in various conditions. Additional research is required to determine if particle-

particle interactions or lack of robust single solid DLMs is more likely thecause.

Copyright byAnn M. Gilchrist

2013

ACKNOWLEDGMENTS

This work was supported by a grant from the Department of Energy, Subsurface

Biogeochemical Research (DE-SC0005362 to Koretsky). Thank you to the Department

of Geosciences, the NSF S-TEM scholarship (Mallinson Institute), Elizabeth M. Garrett

endowment and the Lauren Hughes family for additional funding during my research. I'd

like to thank my committee members, Dr. Alan Kehew and Dr. R.V. Krishnamurthy, for

their time, patience and input; my friends and family for support and encouragement. I

want to thank "team geochemistry" especially Allie Wyman for being my sounding

board, a wonderful lab companion and becoming one of my closest friends. A special

thank you to Dr. Lynne Heasley for seeing the Geoscientist in me and knowing where to

send me for cultivation. My deepest gratitude is expressed to Dr. Carla Koretsky

without whom this wouldn't have become a reality; my sincerest thanks for believing in

me, being my advisor, mentor, editor, and friend.

Ann M. Gilchrist

n

TABLE OF CONTENTS

ACKNOWLEDGMENTS ii

LIST OF TABLES vi

LIST OF FIGURES viii

CHAPTER

I. INTRODUCTION 1

1.1 Contamination and Exposure 1

1.2 Geochemistry 2

1.3 Surface Complexation Modeling 5

1.4 Hypothesis and Approach 10

II. MATERIALS AND METHODS 11

2.1 Materials 11

2.2 Experimental Methods 17

III. EXPERIMENTAL RESULTS 18

3.1 Cr(VI) Adsorption on Single Solids 18

3.2 Cr(VI) Adsorption on Binary Solid Mixtures 20

in

TABLE OF CONTENTS -CONTINUED

III. EXPERIMENTAL RESULTS -CONTINUED

3.2.1 Cr(VI) Adsorption on Mixtures of Goethite and Montmorillonite 20

3.2.2 Cr(VI) Adsorption on Mixturesof Goethite and Kaolinite 22

3.2.3 Cr(VI) Adsorptionon Mixturesof Goethite and y-Alumina 23

3.2.4Cr(VI) Adsorption on Mixtures of Goethite and HFO 26

3.2.5 Cr(VI) Adsorption on Mixtures of Goethite and HMO 29

3.3 Cr(VI) Adsorption on Multiple SolidMixtures 32

IV. DISCUSSION 38

4.1 Single Sorbate/Sorbent Surface Complexation Models 38

4.1.1 Cr(VI) Sorption on y-alumina 38

4.1.2 Cr(VI) Sorption on Hydrous Ferric Oxide (HFO) 40

4.1.3 Cr(VI) Sorption on Hydrous Manganese Oxide (HMO) 42

4.1.4 Cr(VI) Sorption on Kaolinite 44

4.1.5 Cr(VI) Sorption on Montmorillonite 47

4.1.6 Cr(VI) Sorption onGoethite 50

4.2.1 Cr(VI) Sorption onGoethite-y-Alumina Mixtures 58

4.2.2 Cr(VI) Sorption on Goethite-HFO Mixtures 60

4.2.3 Cr(VI) Sorption onGoethite-Kaolinite Mixtures 63

4.2.4 Cr(VI) Sorption on Goethite-Montmorillonite Mixtures 65

iv

TABLE OF CONTENTS - CONTINUED

IV. DISCUSSION - CONTINUED

4.2.5 Cr(VI) Sorption on Goethite-HMO Mixtures 67

4.3 Multiple Mineral Mixtures 70

4.3.1 Contribution of Individual Solid Chromate Surface Complexes in BinaryMixtures.... 70

4.3.2 Cr(VI) Sorption on Multiple Solid Mixtures 75

V. CONCLUSIONS 84

5.1 Cr(VI) Sorption Experiments 84

5.2 Surface Complexation Modeling 87

5.3 Future Work 89

APPENDICIES 91

Appendix A:_Single Solid Cr(VI) Adsorption Edge Data 92

Appendix B:_Binary Solids Cr(VI) Adsorption Edge Data 98

Appendix C:_Multiple Solids Mix Cr(VI) Adsorption Edge Data 105

BIBLIOGRAPHY HO

LIST OF TABLES

Table 4.1: Model parameters, reaction stoichiometriesand stability constants for y-alumina and Cr(VI) DLM from Reich and Koretsky (2011) used to calculateedge shown in Figure 4.1 39

Table 4.2: Model parameters, reaction stoichiometries and stability constants for HFOand Cr(VI) DLM from Dzombak and Morel (1990) for experimentalconditions in Figure 4.2. Site 1 = strong site (s) and Site 2 = weak site (w). ..41

Table 4.3: Model parameters, reaction stoichiometries and stability constants for HMOand Cr(VI) DLM from MacLeod (2013) 43

Table 4.4: Reaction stoichiometries, stability constants, and weighted some of squaresdivided by degrees of freedom for Cr(VI) adsoiption reactions on kaoliniteoptimized inFITEQL 46

Table 4.5: Reaction stoichiometries, stability constants, and WSOS/DF for Cr(VI)adsorption reactions on montmorillonite optimized in FITEQL 48

Table4.6: Model parameters, reaction stoichiometries and stability constants formontmorillonite and Cr(VI) DLM 49

Table 4.7: Reaction stoichiometries and stability constants for goethite and Cr(VI) DLMfrom Mathur and Dzombak (2006) for different model parameters 51

Table 4.8: Van Geen et al. (1994) stoichiometries and stability constants for carbonate 56

Table 4.9: Model parameters for y-alumina-goethite system 58

vi

LIST OF TABLES - CONTINUED

Table 4.10: Model parameters for goethite-HFO systems 61

Table 4.11: Model parameters for goethite-kaolinite systems 63

Table 4.12: Model parameters for goethite-montmorillonite system 65

Table 4.13: Model parameters for goethite-HMO system 68

Table 4.14: Reaction stoichiometries, stability constants, and experimental parametersfor Cr(VI) adsorption on multiple solidmixtures 77

vn

LIST OF FIGURES

Figure 1.1: Cr(VI) speciation diagram as function of pH, Eh = 1 V 3

Figure 1.2: Cr(III) speciation diagram as a function of pH, Eh=0.6 V 4

Figure 1.3: Assumed surface potential (VP) as a function of distance from the mineralsurface (x) for the CCM, modified from Goldberg (1995) 6

Figure 1.4: Assumed surface potential Q¥) as a function of distance from the mineralsurface (x) for the DLM, modified from Goldberg (1995) 7

Figure 1.5: Assumed surface potential OF) as a function of distance from the mineralsurface (x) for the TLM, modified from Goldberg (1995) 8

Figure 2.1: Powder XRD pattern (intensity on vertical axis verses 20on horizontal axis)of synthesized goethite measured 12

Figure 2.2: XRD pattern ofgoethite (intensity vs. 20) measured using Co-Ka (diagramfrom Schwertmann and Cornell, 1991. Fig. 5-6, p 75; note X-axis scale from80-20°) 12

Figure 2.3: Powder XRD pattern ofsynthesized HMO (intensity vs.20) measured usingCr Ka and converted to Cu-Ka using Jade 13

Figure 2.4: Powder XRD pattern ofHMO (intensity vs. 20) measured using Cu-Ka;diagram from Villalobos, et al, 2003. Fig. 2 p 2655 14

Vlll

LIST OF FIGURES - CONTINUED

Figure 2.5: Powder XRD pattern ofsynthesized HFO (intensity counts on vertical axisverses 20 on horizontal axis) measured using Cr Ka and converted to Co Kausing Jade (note X-axis scale 20-80°) 15

Figure 2.6: Powder XRD pattern of HFO (intensity vs. 20) measured using Co-Kadiagram from Schwertmann and Cornell, 1991. Fig. 8-1, p 106. Lower curveisHFO (2-line ferrihydrite (note X-axis scale 80-20°)) 16

Figure 3.1: Adsorption of 10"5 MCr(VI) on 2 g/L goethite in 0.01 MNaN03, measuredat 0% pC02, atmospheric, and 3-5% pC02 conditions based on colorimetricmeasurements of Cr(VI) remaining in solution after a 24 hour equilibrationperiod '"

Figure 3.2: Adsorption of10*5 3MCr(VI) on 0.045 g/L HFO in 0.1 MNaN03, measuredat atmospheric conditions based oncolorimetric measurements ofCr(VI)remaining insolution after a 4 hour equilibration period 19

Figure 3.3: Adsorption of 10'5 MCr(IV), 0.01 MNaN03, measured at atmospheric pC02based on ICP measurements and colorimetric measurements of total Cr andCr(VI), respectively, remaining insolution after equilibration periods 21

Figure 3.4: Adsorption of10"5 MCr(IV), 0.01 MNaN03, measured at 0% pC02 basedon ICP measurements and colorimetric measurements of total Cr and Cr(VI),respectively, remaining in solution after equilibration periods 21

Figure 3.5: Adsorption of10"5 MCr(IV), 0.01 MNaN03, measured at atmospheric pC02based on colorimetric measurements of Cr(VI) remaining in solution afterequilibration periods 22

IX


Figure 3.6: Adsorption of 10"5 MCr(IV), 0.01 MNaN03, measured at 0% pC02 basedon colorimetric measurements of Cr(VI) remaining in solution afterequilibration periods 23

Figure 3.7: Adsorption of 10"5 MCr(IV), 0.01 MNaN03, measured at0% pC02 and 5%pC02 based on colorimetricmeasurements of Cr(VI) remaining in solutionafter 24 hour equilibration period 24

Figure 3.8: Adsorption of 10"5 MCr(IV), 0.01 MNaN03, measured atatmospheric pC02based on ICP and colorimetric measurements of total Cr and Cr(VI),respectively, remaining in solution after referenced equilibration periods. ...25

Figure 3.9: Adsorption of 10"5 MCr(IV), 0.01 MNaN03, measured at 0% pC02 basedon ICP and colorimetric measurements of total Cr and Cr(VI), respectively,remaining in solution after referenced equilibration periods 26

Figure 3.10: Adsorption of 10"5 MCr(IV), 0.01 MNaN03, measured at0% pC02 and5% pC02 based on colorimetric measurements of Cr(VI) remaining insolution after 24 hour equilibrationperiod 27

Figure 3.11: Adsorption of 10"5 MCr(VI), 0.01 MNaN03, measured atatmosphericpC02 based on ICP and colorimetric measurements of total Cr and Cr(VI),respectively, remaining in solution after referenced equilibration periods. .28

Figure 3.12: Adsorption of 10"5 MCr(IV), 0.01 MNaN03, measured at0% pC02 basedon ICP and colorimetric measurements of total Cr and Cr(VI), respectively,remaining insolution after referenced equilibration periods 29

x


Figure 3.13: Adsorption of 10"5 MCr(IV), 0.01 MNaN03, measured at0 and 5% pC02based on colorimetric measurements of total Cr(VI) remaining in solutionafter 24 hr equilibration period 30

Figure 3.14: Adsorption of 10"5 MCr(IV), 0.01 MNaN03, measured atatmosphericpC02 based on ICP and colorimetric measurements of total Cr and Cr(VI),respectively, remaining in solution after equilibration periods 31

Figure 3.15: Adsorption of 10"5 MCr(IV), 0.01 MNaN03, measured at 0% pC02 basedon ICP and colorimetric measurements of total Cr and Cr(VI), respectively,remaining in solution after referenced equilibration periods 32

Figure 3.16: Adsorption of 10"5 MCr(IV) on mineral mix 60 m2/g each solid in 0.01 MNaN03, measured at 0%, atmospheric and 5% pC02 based on colorimetricmeasurements of Cr(VI) remaining in solution after a 24 hr equilibrationperiod 33

Figure 3.17: Adsorption of 10'5 MCr(IV) on 150 m2/g total surface area, with equalsurface area of each solid, in 0.1 M NaN03, measured at 0% pC02 based onICP and colorimetric measurements of total Cr and Cr(VI), respectively,remaining in solution after referenced equilibrationperiods 35

Figure 3.18: Adsorption of 10"5 MCr(IV) on 150 m2/g equal surface area ofeach solid in0.1 M NaN03, measured at atmospheric pC02 based on ICP andcolorimetric measurements of total Cr and Cr(VI), respectively, remainingin solutionafter referenced equilibration periods 36

xi


Figure 3.19: Adsorption of 10"' MCr(IV) on 150 m2/g equal surface area ofeach solid in0.1 M NaN03, measured at 3-5% pC02 based on ICP and colorimetricmeasurements of total Cr and Cr(VI), respectively, remaining in solutionafter referenced equilibration periods 37

Figure 4.1: Calculated Cr(VI) adsorption edge on y-alumina using DLM with parametersin Table 4.1 as compared to experimental data from Reich & Koretsky(2011) 40

Figure 4.2: Calculated DLM adsorption edge as compared to experimental data(Dzombak and Morel, 1990, IECH3, p. 233) using parameters in Table 4.2.42

Figure 4.3: Calculated adsorption edge compared to experimental data from MacLeod(2013) using parameters in Table 4.3 44

Figure 4.4: Model fit as compared to experimental data from Wyman (personalcommunication) using parameters in Table 4.5 47

Figure 4.5: Model fit as compared to experimental data from Reich (personalcommunication) usingparameters in Table 4.7 50

Figure 4.6: Model fit as compared to experimental data (Mathur and Dzombak, 2006)using parameters in Table 4.8 52

Figure 4.7: Mathur and Dzombak (2006) model predictions as compared toexperimental data from this study using parameters shown inTable 4.8 53

xn


Figure 4.8: Mathur and Dzombak (2006) model predictions as compared to experimentaldata this study (parameters in Table 4.8) decreased solid concentration to 2g/L 54

Figure 4.9: Chromate speciation calculated using the Mathur and Dzombak (2006) DLMwith the parameters shown in Table 4.8 55

Figure 4.10: Calculated edges based on DLM this study to experimental data this studyover a range of pC02 including stability constants for carbonate on goethitefrom Van Geen et al. (1994) 57

Figure 4.11: Model predictions using y-alumina and goethite DLMs as compared toexperimental data from this study for 0% pC02 conditions 59

Figure 4.12: Model predictions using y-alumina and goethite DLMs as compared toexperimental data from this study for >5% pC02 conditions 60

Figure 4.13: Model predictions using combined goethite-HFO DLMs as compared toexperimental data from this study at 0% pC02 conditions 62

Figure 4.14: Model predictions usingcombined goethite-HFO DLMs as compared toexperimental data from this study at >5% pC02 conditions 62

Figure 4.15: Model predictions usingcombined goethite and kaolinite DLMsascompared to experimental data from this studyfor 0% pC02 conditions....64

Figure4.16: Model predictions using combined goethite and kaolinite DLMs ascompared to experimental data from this study for atmospheric pC02 64

xin


Figure 4.17: Model predictions using combined goethite-montmorillonite DLMs ascompared to experimental data from this study for 0% pC02 experiment...66

Figure 4.18: Model predictions using combined goethite-montmorillonite DLMs ascompared to experimental data from this study for atmospheric pC02conditions 67

Figure 4.19: Model predictions using combined goethite-HMO DLMs as compared toexperimental data from this study for 0% pC02 experiment 69

Figure 4.20: Model predictions using combined goethite-HMO DLMs as compared toexperimental data from this study for >5% pC02 conditions 69

Figure 4.21: Model predictions of adsorbed Cr(VI) speciation using combined goethite-y-alumina DLMs (10"5 MCr, 0.01 MNaN03) 71

Figure 4.22: Model predictions of adsorbed Cr(VI) speciation using combined goethite-HFO DLMs (10"5 MCr, 0.01 MNaN03) 72

Figure 4.23: Model predictions of adsorbed Cr(VI) speciation using combined goethite-kaolinite DLMs (10"5 MCr, 0.01 MNaN03) 73

Figure 4.24: Model predictions of adsorbed Cr(VI) speciation using combined goethite-montmorillonite DLMs (10"5 M Cr, 0.01 MNaN03) 74

Figure 4.25: Model predictions of adsorbed Cr(VI) speciation using combined goethite-HMO DLMs (10"5 M Cr, 0.01 MNaN03) 75

xiv


Figure 4.26: Prediction combined DLMs (experimental conditions Table 4.17) comparedto experimental data this study for a mixture of six solids of equal surfacearea (60 m2/g for each) 78

Figure 4.27: Prediction combined DLMs (experimental conditions Table 4.17) comparedto experimental data this study for atmospheric pC02 conditions with amixture of six solids of equal surface area(60 m /g for each) 79

Figure 4.28: Prediction using DLMs (experimental conditions Table 4.17) compared toexperimental data from this study for 5% pC02 conditions with a mixture ofsix solids ofequal surface area (60 m2/g for each) 80

Figure 4.29: Prediction using combined single solid DLMs as compared to experimentaldata for experiments with 0.1 MNaN03, 10"5 MCr(VI) and 150 m2/gsurface areaof all six solids at 0% pC02 81

Figure 4.30: Model prediction using combined single solid DLMs as compared toexperimental data for experiments with 0.1 MNaN03, 10"5 MCr(VI) and150 m2/g surface area ofall six solids at atmospheric pC02 82

Figure 4.31: Model prediction using combined single solid DLMs as compared toexperimental data for experiments with 0.1 MNaN03, 10"5 MCr(VI) and150 m2/g surface area ofall six solids at 5% pC02 83

Figure 5.1: Comparison of Cr(VI) adsorption edges at varying ionic strength and solidconcentrations OJ

Figure 5.2: Comparison ofadsorption edges atvarying ionic strength and solidconcentrations °°

xv


Figure 5.3: Comparison of adsorption edgesat varying ionic strengthand solidconcentrations 86

xvi

I. INTRODUCTION

1.1 Contamination and Exposure

Hexavalent chromium (Cr(VI)) is a byproduct of many anthropogenic activities

includingmetal plating, manufacture of pigments, corrosion inhibitors, chemical

synthesis, leather tanning, wood preservation, fuel combustion, cement production, and

sewage sludge incineration (Richard and Bourg, 1991; Grossl et al., 1997). Thedisposal

of Cr(VI) has caused widespread contamination of soils and waters. Of the 1,312

superfund sites, there are 548 known to be contaminated with chromium (U.S.

Environmental Protection Agency (EPA), 2013). Environmental Working Group

(EWG), a non-profit organization, wascommissioned to studywater from thirty-five US

cities and found that thirty-one had tap water contaminated with Cr(VI) at detectable

levels and many of those at levels above California'sproposed public health goal of 0.02

ppm (EWG, 2009; Office of Environmental Health Hazard Assessment (OEHHA),

2011).

In September 2010, the U.S. EPA released a toxicological review of Cr(VI)

describing the health effects of oral route exposure (U.S. EPA 2010). Studies compiled

in this document indicate statistically significant increases in oral tumors, stomach cancer

and evidenceof mutation to DNA upon exposure to Cr(VI) levels of 0.5 mg/kg-day or

higher. Therefore, human oral exposure to Cr(VI) is considered likely to be carcinogenic

(U.S. EPA, 2010).

1.2 Geochemistry

As a transition metal, chromium has many oxidation states. However, in the Eh-

pH range of natural systems, the primary forms are Cr(VI) and Cr(III) (Richard and

Bourg, 1991). Hexavalent chromium compounds are strong oxidizing agents, very

soluble and their speciation is heavily pH dependent. Within the pH range of natural

waters, under oxidizing conditions at pH <6.3 the hydrochromate anion (HCrOO is

dominant in solution. The chromate anion (CriV2) dominates in oxidizing conditions at

pH >6.3 (Figure 1.1). At concentrations greater than 0.01 M significant quantities of

other species ofCr(VI), such asCr20y"2 and HCr207", form. However, this level of

contamination is much higher than the concentrations of Cr used in this study, and thus

these species do not appear on Figure 1.1. Cr(III) species dominate in moderately

oxidizing to reducing conditions and react with various ligands, including hydroxyl, to

form a variety of aqueous complexes (Rai et al., 1989; Richard and Bourg, 1991). Under

reducing conditions in the pH range of 3 to 10, the following Cr(III) species are present

in order from low pH to high pH: Cr3+, Cr(OH)2+ and Cr(OH)3 (Figure 1.2).

Cr(Vi) SpeciationActivity: 10s M, Eh = Iv

PH

Figure 1.1: Cr(VI) speciation diagram as function of pH, Eh = 1 V.

The oxidation state of chromium has a large effect on its mobility and toxicity,

with Cr(VI) being more mobile and toxic than Cr(III). However, Cr(VI) is reduced to

Cr(III) by reacting with organic matter and other reducing agents. Cr(III) may also

readily adsorb to many solids, thereby reducing its mobility. Highly organic soils have

high chromium retention in the insoluble form. Conversely, organic-poor soilsretain less

chromium and tend to have higher levels of soluble chromium (EPA, 2011).

Cr(lll) SpeciationActivity: 10-5 M, Eh = 0.6 V

Figure 1.2: Cr(III) speciation diagram as a function of pH, Eh=0.6 V.

Adsorption, the binding of a chemical species on a solid surface, may also impede

Cr(VI) movement in the environment (Richard and Bourg, 1991; Mesuere and Fish,

1992). The adsorption behavior of Cr(VI) is dependent upon pH andthe presence of

competing ions, including carbon dioxide, and ionic strength (Richard and Bourg, 1991).

Many studies have been conducted to observe the adsorption behavior of Cr(VI) on

single minerals (e.g., Davis and Leckie, 1980; Zachara et al., 1987; Ainsworth et al.,

1989; Dzombak and Morel, 1990; Grossl et al., 1997; Villalobos et al., 2001) anda few

oncomposite materials or soils (e.g., Zachara et al., 1987; Khaodhiar et al., 2000; Weng

et al., 2001; Hellerich and Nikolaidis, 2005; Smith and Ghiassi, 2006). However, most

studies have not explored the potential effects of mineral-mineral interactions on Cr(VI)

adsorption.

1.3 Surface Complexation Modeling

Thermodynamically-based surface complexation models (SCM) are used to

describe adsorption reactions at equilibrium. SCMs are based on the assumptions that

adsorption occurs at specific coordination sites, that the reactions themselves result in the

creation of surface charge and that the adsorption reactions can be quantitatively

described using mass law equations. Surface complexes are typically assumed to have

one of two structural configurations for modeling purposes: innersphere complexes,

which have no water molecules between adsorbing ion and surface, creating strong bonds

and outersphere complexes, which contain at least one water molecule between the

absorbing ion and the surface, with predominantly electrostatic interactions binding the

ions to the surface.

Different types of SCM have been developed, using various assumptions

regarding surface complexes and the development of charge at the solid surface. The

constant capacitance model (CCM) assumes all surface complexes are innersphere, that

there is no formation of surface complexes with the background electrolyte ions, and that

the surface is comprised of a single charged plane (Figure 1.3).

Decay of charge

Figure 1.3: Assumed surface potential (4') as a function ofdistance from the mineralsurface (x) for the CCM, modified from Goldberg (1995).

The double layer model (DLM) assumes all surface complexes are innersphere,

that there are no surface complexes that form withthe background electrolyte and that

there are two planes ofcharge at the surface. The first plane is located at the mineral

surface with the electrostatic chargeof the surface balanced by a secondplane, an

adjacent layerof counter-ions in solution (Figure 1.4).

H^

¥

°0

1st Plane

of charge

C<j

Diffuse Layer 2nd

pianeof charge

A"

Even charge

distribution

Figure 1.4: Assumed surface potential (¥) as a function of distance from themineral surface (x) for the DLM, modified from Goldberg (1995).

The triple layer model (TLM) allows explicit formation of both inner and

outersphere surface complexes. Protons andhydroxyl ions, together with strongly bound

ions,are described as innersphere complexes, while complexes with weakly bound ions,

including thebackground electrolyte, are described as outersphere complexes. The TLM

includes three planes of charge representing the adsorption potential with capacitance of

the planes differing in each adjacent layer as they become more distant from the solid

surface (Figure 1.5).

1st Charge 2nd Charge 3rd Charge

Plane Plane Plane

Figure 1.5: Assumed surface potential (VF) as a function of distance from the mineralsurface (x) for the TLM, modified from Goldberg (1995).

SCMs are typically calibrated for single sorbate/sorbent systems. Theoretically,

combining single sorbate/sorbent models (the component additivity approach of Davis et

al., 1998)to predict adsorption behavior should yield accurate results, however very few

studies have been conducted to verify this assumption. Some studies on metal cations

indicate that the uncertainties associated with the single sorbate/sorbent SCMs are larger

8

than the effects of mineral-mineral interactions (Lund et al., 2008; Landry et al., 2009).

However, a study on Cr(VI) usingbinary oxide mixtures of titanium and amorphous iron

oxyhydroxide, and alumina and amorphous ironoxyhydroxide found that the predictions

of the SCMs exhibited reduced adsorptioncompared to the experimental results for

systems containing titanium and either aluminum or iron oxides (Honeyman, 1984).

Particles suspended in fluid are in constant, random motion (Brownian motion).

Coagulation occurs when particles with similar properties overcome energy barriers to

collide and/or react with one another in fluid. Heterocoagulation is the term for the

collisions or interactions of particles that have dissimilarproperties. In theoryparticle-

particle interactions could reduce thenumber of surface sites available to the sorbate by

adhering to one another or increase them by coating particles thereby providing a larger

surface area for binding.

The underpredictions of the SCMs in Honeyman (1984) were attributed to

heterocoagulation reactions between the solids in solution butthis was not due to coating,

asa prior study found that amorphous iron and alumina do not form coatings ontitanium

oxides (Honeyman, 1984). Honeyman presents various hypotheses to explain the

enhanced observed adsorption in the binary mixtures containing titanium oxides,

ultimately concluding that the most probably explanation is formation ofa "hybrid site"

inthe region between the two types ofparticles which is not accounted for using the

simple component additivity approach. This indicates that the application ofmodels

calibrated for the single sorbate/sorbent using component additivity could potentially be

inaccurate in natural soils unless such reactions are accounted for in the models

(Honeyman, 1984).

1.4 Hypothesis and Approach

The central hypothesis explored in this study is that surface complexation models

developed for single sorbate/sorbent systems cannot correctly predict sorption behavior in

the presence of multiple minerals, unless mineral-mineral interactions are explicitly

included. To test this hypothesis, Cr(VI) adsorption on binary mixtures of goethite with

hydrous manganese oxide (HMO), hydrous ferric oxide (HFO), kaolinite,

montmorillonite or y-alumina is measured as a function of pH, ionic strength, and pC02.

Further experiments are conducted with mineral assemblages of goethite, kaolinite,

montmorillonite, y-alumina, hydrous manganese oxide (HMO), and hydrous ferric oxide

(HFO) in equal surface areas again measured as a function of pH, ionic strength and

PCO2. Double layer models (DLM) originally developed for Cr(VI) adsorption on pure

solids are used to predict Cr(VI) adsorption in the mixed solid systems, and the model

predictions are compared with measured edges for the mineral assemblages. This allows

mineral-mineral interactions and the applicability of SCMs derived for simple systems to

more complex systems to be assessed. Quantitative knowledge gained regarding these

interactions should result in more accurate predictions of Cr(VI) for mixtures of minerals

using models derived for single sorbate/sorbent systems, and should permit those

involved in remediation of contaminated sites to more accurately predict Cr(VI) behavior

in heterogeneous subsurfaces.

10

II. MATERIALS AND METHODS

2.1 Materials

Goethite was synthesized using the alkaline method by ageing a solution of 1 M

Fe(N03)3 • 9 H20 and 5 M KOH, under constant agitation, at 70°C for 60 hrs

(Schwertmann and Cornell, 1991). The aged mixture was poured into several 50-mL

centrifuge tubes, and then centrifuged for ~8 minutes at 3600 rpm to separate the

precipitate from supernatant. The supernatant was decanted, ultrapure water (DDI; >18

MH) was added and the precipitate re-suspended. This procedure was repeated -5-6

times, after which the remaining precipitate was frozen overnight, and then placed on a

freeze-drier tor 3 days. Approximately 0.1-0.2 g of the freeze-dried solid was degassed

at 130°C for 10-18 hrs (Mazeina and Navrotsky, 2007) and surface area was confirmed

by 11 pt N2-BET analysis (Quantachrome Nova 2200 SA Analyzer). Production of

goethite was also confirmed by powderX-raydiffraction with a Cr anode (Rigaku

Miniflex) and Jade 6.0 was used to convert the wavelengths from Cr to Co for

comparison withpublished patterns (Figure 2.1 XRD pattern of synthesized goethite;

Figure 2.2 Schwertmann and Cornell, 1991 goethite XRD).

11

Figure 2.1: Powder XRD pattern (intensity on vertical axis verses 20 on horizontal axis)of synthesized goethite measured.

o

I— .2

CO

70 0

Uu

80 SO 50 40 30 "20 Co K a

Figure 2.2: XRD pattern of goethite (intensity vs. 20) measured using Co-Ka(diagram from Schwertmann and Cornell, 1991. Fig. 5-6, p 75; note X-axis scalefrom 80-20°).

12

Synthesis of hydrous manganese oxide (HMO) was conducted following the

alkametric titration method by drop-wise introduction of a 100 mL solution of 0.29 M

KOH and 0.14 M KMn04 into 900 mL of 0.02 M Mn(N03)2 • 4 H20 (Stroes-Gascoyne

et al., 1987). This mixture was then allowed to equilibrate for 1 hr, centrifuged and

rinsed, as described for goethite, until the conductivity of the supernatant was less than

that of 0.001 M NaN03. The remaining precipitate was frozen overnight, and then

freeze-dried for 3 days. The surface area was measured by 11 pt N2-BET analysis after

degasing -0.1 g at 90°C for 24 hrs. XRD analysis with a Cr anode was used to obtain the

powder diffraction pattern, and then Jade 6.0 was used to convert from Cr to Cu Ka for

comparison with published patterns (Figure 2.3 XRD pattern of synthesized HMO;

Figure 2.4 HMO XRD pattern from Villalobos, et al., 2003).

Figure 2.3: Powder XRD pattern ofsynthesized HMO (intensity vs.20) measured usinjCr Ka and converted to Cu-Ka using Jade.

13

1 5-Mn02

1.4A

55 65 75

Figure 2.4: Powder XRD pattern of HMO (intensity vs. 20) measured using Cu-Ka;diagram from Villalobos, et al, 2003. Fig. 2 p 2655.

Hydrous ferric oxide (HFO) synthesis entailed slow titration of a 500 mL solution

of 0.2 M Fe(N03)3. 9 H20 with 1 M NaOH during constant stirring to a pH of-7.4,

which was maintained for 72 hrs (Schwertmann and Cornell, 1991). The aforesaid

centrifugingand rinsing process was repeated-6 times, and then the precipitate was

frozen overnight and freeze-dried. The surface area was determined by 11 pt N2-BET

analysis afterdegasing ~0.3-.04 g at 80°C for 24 hrs (Lund et al., 2008). A powder XRD

diffraction patternmeasured with Cr Ka confirmed the product to be HFO. The 2D

values were converted from Cr to Co Ka using Jade 6.0, for comparison with published

patterns (Figure 2.5 XRD pattern of synthesized HFO; Figure 2.6Schwertmann and

Cornell, 1991 HFO XRD).

14

Figure 2.5: Powder XRD pattern of synthesized HFO (intensity counts on vertical axisverses 20 on horizontal axis) measured using Cr Ka and converted to Co Ka using Jade(note X-axis scale 20-80°).

15

•<0 5 0 4 0 SO "20 Co A- a

Figure 2.6: Powder XRD pattern of HFO (intensity vs. 20) measured using Co-Kadiagram from Schwertmann and Cornell, 1991. Fig. 8-1, p 106. Lower curve is HFO (2-line ferrihydrite (note X-axis scale 80-20°)).

The remaining minerals required for the edge experiments were purchased from

the following sources: montmorillonite (SWy-2) and kaolinite (KGa-lb) were obtained

from the Clay Minerals Society, and y-alumina (y-Al203) was purchased from Inframat

Advanced Materials. Surface areas of these minerals were confirmed by measurement

and comparison to published prior works. Montmorillonite (SWy-2) as purchased from

the Clay Minerals Society was determined to have a surface area of 32 m /g (Akatia et

al., 2011). The surface area of kaolinite (KGa-lb) from the Clay Minerals Society was

determined to be 13.6 m2/g (Lund et al., 2008). Inframat Advanced Materials y-alumina

(y-Al203) showed a surface area of233 m2/g (Reich and Koretsky, 2011).

16

2.2 Experimental Methods

Chromate adsorption edges were measured over a pH range of -3.5 to 10 with

0.001, 0.01 or 0.1 M NaN03 used as the background electrolyte, 10"5 MCr(VI) and total

solid concentrations for mixed solid experiments were set by equal mass or equal surface

area for a total solids between 3.75 to 6.15 g/L. To evaluate the potential competition

between chromate anions and carbonate for the sorption sites, some of the edges were

measured under atmospheric conditions, N2 atmosphere in a Coy anaerobic glovebox (0%

pC02) or in a Coy glovebox with N2 and 3-5% pC02.

Each individual edge is measured using the following procedure. Ultrapure water

(>18 MQ\ the prescribed dose of background electrolyte and Cr(VI) are mixed well in a

1000 mL volumetric flask and 60 mL is removed as a control. The appropriate amount of

solid(s) is then added to the remaining solution. The batch slurry is allowed to equilibrate

at circumneutralpH, under constant mixing, for 1 hr. The pH is then lowered using 1 M

and 0.1 M HN03 to -3.5 and 60 mL of slurry removed. The batch slurry is then titrated

upward using 0.1 and 1 M NaOH. At each -0.5 pH increment, a 60 mL aliquot is

removed and placed on a LabQuake rotating shaker. After a set period of mixingtime

(typically 24 hrs, 48 hrs, 1 weekand 2 weeks) -15 mL of slurry is removed from each

aliquot and the pH rechecked. Eachslurry sample is centrifuged, the supernatant syringe-

filtered (0.45 jam) and tested for Cr(VI)using UV/VIS spectrophotometry according to

the diphenylcarbazide method (Greenberg et al., 1992). Using a PerkinElmer Optima

2100DV, total Cr is analyzed by ICP-OES using standards rangingfrom 0 to 1000 ppb

total Cr with matrix-matching using the specified background electrolyte and spiked with

1000 ppb yttrium for an internal standard.

17

III. EXPERIMENTAL RESULTS

3.1 Cr(VI) Adsorption on Single Solids

Single solid adsorption edges for Cr(VI) on goethite were conducted for

comparison to edges in mixed solid systems (Figure 3.1) These were conducted in

atmospheric, 0% pC02 and 3-5%pC02 environments, and allowed to equilibrate for a 24

hr period, with the remaining Cr(VI) in solution measured colorimetrically. Nearly 100%

adsorption of Cr(VI) is maintained from pH -3 to 6 in the lower pC02 conditions and

from pH -3 to 5 in the 3-5% pC02 environment (lO^5 MCr(VI), 0.01 MNaN03, 2 g/L

solid). In the lower pC02 conditions, between pH -6.5 and 8.9, there is a rapid decrease

in Cr(VI) adsorbed with increasing pH (adsorption edge), with less than 10% of the

Cr(VI) adsorbed at pH < 8.5 and a pH50 (the pH with 50%of the Cr(VI) adsorbed) of

-7.5. However, in the 3-5% pC02 environment, the adsorption edge spans from pH -5.2

to 8 with pHso -6.2.

A Cr(VI)adsorption edge on pure HFO was measured over a pH range of-3.5 to

10 with 0.1 MNaN03 as the background electrolyte, 10"53 MCr(VI) and a solidconcentration of 0.045 g/L to replicate experimental conditions reported in Dzombak and

Morel (1990); a 4 hr equilibration timewas used with Cr(VI) remaining in solution

measured colorimetrically (Figure 3.2). Cr(VI) adsorption of-90 to 100% is maintained

below pH 5.5 followed by a rapid decrease in the percent Cr(VI) adsorbed with

increasing pH. The pH at which 50% Cr(VI) adsorption occurs ( pH50) is -6.5.

18

100

90

80

70

60

50

40

30

20

10

0

2 g/L goethite, 0.01 NaN03,105 M Cr042"

t»raj? i ~ *_ _ •* *......Ah4*.

*■♦

—&4

• ♦ "- • "

7

pH

♦ J g + ^A

11

♦ 0% pC02

4 atm pC02

• 5% pC02

Figure 3.1: Adsorption of 10"5 MCr(VI) on2 g/L goethite in 0.01 MNaN03, measuredat 0% pC02, atmospheric, and 3-5% pC02 conditions based on colorimetricmeasurements of Cr(VI) remaining in solution after a 24 hour equilibration period.

-Q

T5<

100

80

60 k

40

20

.045 g/L HFO, 0.1 NaN03/10"5 3M Cr(VI)

♦ ♦

4- ♦

7

PH

10 11

Figure 3.2: Adsorption of 10"5 3MCr(VI) on 0.045 g/L HFO in0.1 MNaN03, measuredat atmospheric conditions based on colorimetric measurements of Cr(VI) remaining insolution after a 4 hour equilibration period.

19

3.2 Cr(VI) Adsorption on Binary Solid Mixtures

Adsorption edges were measured under atmospheric and 0% pC02 conditions for

a variety of solids physically mixed with goethite. These experiments were completed

using equal surface areas of the two solids (-2.1 - 6.15 g/L total solid) with 10"5 M

Cr(VI) in 0.01 M NaN03 (see Chapter 2 for details).

3.2.1 Cr(VI) Adsorption on Mixtures of Goethite and Montmorillonite

The target surface area for each mineral was 60 m2/g for a total surface area of

120 m /g. For these experiments, solid concentrations were set at 2 g/L goethite and 1.9

g/L montmorillonite providing a total solid concentration of 3.9 g/L. Adsorption edges

were measured at atmospheric conditions over a pH range of-3.5 to 10 with 0.01 M

NaN03 as the background electrolyte and 10"5 MCr(VI) (Figure 3.3). All experimental

parameters were maintained and the experiment was repeated under 0% pC02 to assess

carbonate competition for the adsorption sites (Figure 3.4). The resulting edges show

adsorption of-90-100% of the Cr(VI) below pH 6.2 and a rapid decrease in adsorption

between -6.5 and -7.2, with pHso occurring at -6.8. Cr(VI) adsorption continues to

decrease with increasing pH, with -3% adsorbed at the highest pH measured (-9.8). The

lack of variation between edges measured at 0% and atmospheric pC02 suggests that

carbonate competition for adsorption sites is negligible under atmospheric conditions.

Adsorption occurs rapidly with no significant differences in edges measured after 24 hr

or 2 week equilibration times. The similarity in the results calculated from UV/VIS

spectrophotometric measurements of Cr(VI) remaining in solution and from ICP-OES

measurements of total Cr in solution indicates that no Cr(III) is released to solution.

20

100

80

60

40

20

0

Equal Surface Area 60 m2/g Atmosphere'2 g/L Goethite - ~1.9 g/L Montmorillonite

~G&

V

-

7

PH

...^..SEL*. ^K

10 11

♦ 24 hr ICP

< 24 hr UV/VIS

4 48 hr ICP

48 hr UV/VIS

• 1 wk ICP

01 wk UV/VIS

»2wklCP

D 2 wk UV/VIS

Figure 3.3: Adsorption of 10"5 MCr(IV), 0.01 MNaN03, measured at atmospheric pC02based on ICP measurements and colorimetric measurements of total Cr and Cr(VI),respectively, remaining in solution after equilibration periods.

Equal Surface Area 60 m2/g 0% pC02~2 g/L Goethite - ~1.9 g/L Montmorillonite

♦ 24hrlCP

O 24 hr UV/VIS

A 48 hr ICP

4 48 hr UV/VIS

* 1 wk ICP

O 1 wk UV/VIS

• 2 wk ICP

Figure 3.4: Adsorption of 10"5 MCr(IV), 0.01 MNaN03, measured at 0% pC02 basedon ICP measurements and colorimetric measurements of total Cr and Cr(VI),respectively, remaining in solution after equilibration periods.

21

3.2.2 Cr(VI) Adsorption on Mixtures of Goethite and Kaolinite

The target surface area for each mineral was -60 m2/g for a total surface area of

-120 m2/g. For these experiments, 2 g/L goethite and 4.15 g/L kaolinite were used,

resulting in a total solid concentration of 6.15 g/L. Adsorption edges were measured at

atmospheric and 0% pC02 conditions over a pH range of-3.5 to 10 with 0.01 M NaN03

as the background electrolyte and 10"5 MCr(VI) (Figure 3.5 and 3.6). Approximately 90-

100%of Cr(VI) is adsorbed at pH < 6.4 with pH50 occurring at -7.1. With increasing pH,

Cr(VI) sorption decreases continually to the highest measured pH (-9.8), at which the

quantity of Cr(VI) adsorbed is below detection limits. As with goethite and

montmorillonite, the adsorption edges vary little between 0% and atmospheric pC02.

Again, rapid adsorption is evident in both sets of data with insignificant changes in the

quantity of Cr(VI) adsorbed between the 24 hr and 2 week equilibration times.

100

80

60

40

20

0

Equal Surface Area "60 m2/g Atmosphere~2 g/L Goethite "4.15 g/L Kaolinite

7

PH

O

8 9 10 11

0 24 hr UV/VIS

A 48 hr UV/VIS

01 wk UV/VIS

D 2 wk UV/VIS

Figure 3.5: Adsorption of 10"5 MCr(IV), 0.01 MNaN03, measured atatmospheric pCO;basedon colorimetric measurements of Cr(VI) remaining in solution after equilibrationperiods.

22

Si

<

U

Equal Surface Area ~60 m2/g 0%pCO2~2 g/L Goethite ~4.15 g/L Kaolinite

100 & igA aaa-8 S°'

80

60

40

20

0

O

o:0

**3

TJ2TO

nOr" A

7

PH

10 11

O 24 hr UV/VIS

,a 48 hr UV/VIS

• 2 wk UV/VIS

Figure 3.6: Adsorption of 10"5 MCr(IV), 0.01 MNaN03, measured at 0% pC02 basedon colorimetric measurements of Cr(VI) remaining in solution after equilibration periods.

3.2.3 Cr(VI) Adsorption on Mixtures of Goethite and y-Alumina

A total surface area of-120 m2/g was reached byconducting experiments with

equal surface areas ofeach mineral of60 m2/g. Solid concentrations of2 g/L goethite

and 0.26 g/L y-alumina were used, resulting in a total solidconcentration of 2.26 g/L.

Adsorption edges were measured colorimetrically aftera 24 hr equilibration period, at

0% pC02 and 5%pC02conditions over a pH range of-3.5 to 10 with 0.01 MNaN03 as

the background electrolyte and 10"5 MCr(VI) (Figure 3.7). In the 0% pC02 environment

from -3.3 to 6.8, nearly 100% of the Cr(VI) is adsorbed; with increasing pH, adsorption

decreases swiftly with Cr(VI) adsorbed less than detection limitsat pH 9.5; the pH50

23

occurs at -7. The adsorption edge shifts to the left from 0% to 5% pC02 with -100%

adsorption maintained below pH 5, the pH50 at 6 and <10% adsorbed at -8 pH.

100

90

80

70

60

50

40

30

20

10

0

Equal Surface Area 60 m2/g Each Solid2 g/L goethite, 0.2598 g/L y-alumina

♦♦ :■■♦■-%♦ -♦ +

«••

* 0% pC02

• 5% pC02

7

PH

10 11

Figure 3.7: Adsorption of 10"5 MCr(IV), 0.01 MNaN03, measured at 0% pC02 and 5%pC02 based on colorimetric measurements ofCr(VI) remaining insolution after 24 hourequilibration period.

Adsorption ofCr(VI) was shown to be rapid, exhibiting little change inedges

measured after 24hr to 2 week equilibration times, in experiments using a total surface

area of-120 m2/g (3.25 g/L goethite and 0.5 g/L y-alumina. Adsorbed Cr calculated

from Cr(VI) insolution using UV/VIS spectrophotometry was also similar to that

calculated from total Cr measured using ICP-OES indicating that little or no Cr(III) was

released into solution (Figure 3.8). Also, little variation inthe sorption edge span from

24

0% pC02 to atmospheric pC02 in these experiments indicated that low pC02

concentrations do not significantly affect the Cr(VI) adsorption (Figures 3.8 and 3.9).

100

80

60

40

20

0

Equal Surface Area 122.4 m2/g Atmosphere~3.25 g/L Goethite ~.5 g/L g-Alumina

fy^wfc|jt

«S

. •* -m

7

PH

11

♦ 24 hr ICP

k 48 hr ICP

A 48 hr UV/VIS

#1 wklCP

Olwk UV/VIS

• 2wklCP

• 2 wk UV/VIS

Figure 3.8: Adsorption of 10"5 MCr(IV), 0.01 MNaN03, measured at atmospheric pC02based on ICP and colorimetric measurements of total Cr and Cr(VI), respectively,remaining in solution after referenced equilibration periods.

25

100

80

| 60•o<w 40

<5^

20

Equal Surface Area 122.4 m2/g 0%pCO2~3.25 g/L Goethite ~.5 g/L g-Alumina

D.

7

PH

♦

-cr

♦ 24 hr ICP

0 24 hr UV/VIS

a 48 hr ICP

•h 48 hr UV/VIS

#lwklCP

01 wk UV/VIS

D 2 wk UV/VIS11

Figure 3.9: Adsorption of 10"5 MCr(IV), 0.01 MNaN03, measured at 0% pC02 basedon ICP and colorimetric measurements of total Cr and Cr(VI), respectively, remaining insolution after referenced equilibration periods.

3.2.4 Cr(VI) Adsorption on Mixtures of Goethite and HFO

Atotal surface area of 120 m2/g was achieved using 2 g/L goethite and 0.2 g/L

HFO (i.e. 60 m2/g for each solid), with atotal solid concentration of2.2 g/L. Adsorptionedges were measured at 0% pC02 and 5% pC02 conditions after a 24 hr equilibrationtime, and over a pH range of-3.5 to 10 with 0.01 MNaN03 as the background

electrolyte, and 10*5 MCr(VI). In 0% pC02 nearly all ofthe Cr(VI) is adsorbed betweenpH -3.3 to 7; with increasing pH, adsorption declines to negligible levels at pH 9; thepH50 occurs at -7.5. Within the 5% pC02 environment the edge shifts left with -100%

26

adsorption maintained below pH 5.4, pHso at -6.7 and <10% Cr(VI) adsorbed at -8.7 pH

(Figure 3.10).

Equal Surface Area 60 m2/g Each Solid2 g/L goethite, 0.22 g/L HFO

* 0% pC02

5% pC02

Figure 3.10: Adsorption of 10"5 MCr(IV), 0.01 MNaN03, measured at0% pC02 and5% pC02 based on colorimetric measurements of Cr(VI) remaining in solution after 24hour equilibration period.

In experiments with 150 m2/g ofeach solid (5 g/L goethite and 0.5 g/L HFO), no

significant difference was observed in edges measured under 0% pC02compared to

atmospheric pC02 indicating with low pC02 conditions competition for sorption sites is

negligible (compare Figure 3.11 and Figure 3.12). Adsorption of Cr(VI) was shown tobe

rapid with insignificant differences observed forequilibration times of 24 hr up to 2

weeks, except for the 0% pC02 dataat pH>8; at pH >8, adsorption increases somewhat

27

with increasing equilibration time (Figure 3.12). The quantity of Cr adsorbed calculated

from UV/VIS spectrophotometry and ICP-OES are similar suggesting that Cr(III) does

not accumulate in the aqueous phase.

100

90

80

70

60

50

40

30

20

10

0

Equal Surface Area 150 m2/g Atmosphere~5 g/L Goethite and ~ 5 g/L HFO

~0~

__JL

7

pH

"€&10 11

♦ 24 hr ICP

O 24 hr UV/VIS

A 48 hr ICP

A 48 hr UV/VIS

• 1 wk ICP

Olwk UV/VIS

• 2wklCP

• 2wk UV/VIS

Figure 3.11: Adsorption of 10"5 M Cr(VI), 0.01 M NaN03, measured at atmosphericpC02 based on ICP and colorimetric measurements of total Cr and Cr(VI), respectively,remaining in solution after referenced equilibration periods.

28

100

90

80

70

60

50

40

30

20

10

0

Equal Surface Area 150 m2/g 0%pCO2~5 g/L Goethite and ~.5 g/L HFO

....mtmmmmmi

7

pH

1-

o

o -

"-••<> <P# *

9 10 11

♦ 24hrlCP

0 24 hr UV/VIS

A 48 hr ICP

A,48hrUV/VIS

• 1 wk hr ICP

01 wk UV/VIS

• 2wklCP

D2wkUV/VIS

Figure 3.12: Adsorption of 10"5 MCr(IV), 0.01 MNaN03, measured at 0% pC02 basedon ICP and colorimetric measurements of total Cr and Cr(VI), respectively, remaining insolution after referenced equilibration periods.

3.2.5 Cr(VI) Adsorption on Mixtures of Goethite and HMO

Atotal surface area of 120 m2/g was achieved by combining equal surface areas

of60 m2/g of goethite (2 g/L) and HMO (0.09 g/L). Adsorption edges were measured

under 0 and 5%pC02, with a pHrange of-3.5 to 10, 0.01 MNaN03 as the background

electrolyte, and 10"5 MCr(VI) (Figure 3.13). In the 0% pC02 atmosphere adsorption is

>90% below pH-6.5 and the pH50 is -7.5. The percentage of Cr(VI) adsorbed decreases

with increasing pH from 7.5, reaching -14% adsorbed at pH-10. The adsorption edge

29

shifts to the left in the 5% pC02 environment, with >90% below pH 5, pH5o ~6.3 and

-14% adsorbed at pH -8. This suggests carbonate competition for adsorption sites at

elevated pC02 (Figure 3.13).

100

90

80

70

60

50

40

30

20

10

0

Equal Surface Area 60 m2/g Each Solid2 g/L goethite, 0.09 g/L HMO

" B .♦_

7

pH

m* m ♦

10 11

♦ 0% pC02

• 5% pC02

Figure 3.13: Adsorption of 10"5 MCr(IV), 0.01 MNaN03, measured at0and 5% pC02based on colorimetric measurements of total Cr(VI) remaining in solution after 24 hrequilibration period.

Experiments conducted with a total surface area of 326.8 m /g achieved by

combining equal surface areas of 163.4 m2/g each ofgoethite (4 g/L) and HMO (0.4 g/L)

indicated Cr(VI) sorption was rapid with little variation from 24 hr to 2 week

equilibration times (Figure 3.14). Adsorption is >90% below pH -7, and the pH50 is

-7.8. The percentage of Cr(VI) adsorbed decreases with increasing pH from 7.8,

30

reaching quantities below detection limits at pH-9.5. There is negligible dependence of

the edges on pC02 at lower pC02 conditions (Figures 3.14 and 3.15). Thecalculated

quantity of Cr adsorbed does not vary significantly based on total Cr ICP-OES

measurements verses Cr(VI) UV/VIS spectrophotometric data.

100

90

80

70

60

50

40

30

20

10

0

Equal Surface Area 163.4 m2/g Atmosphere~4 g/L goethite ~0.4 g/L HMO

ITiic • " - "

D

7

PH

•A*.

•*o~

9 10 11

♦ 24hrlCP

<> 24 hr UV/VIS

A48hrlCP

A 48 hr UV/VIS

• 1 wk hr ICP

01 wk UV/VIS

• 2wkhrlCP

• 2 wk UV/VIS

Figure 3.14: Adsorption of 10"5 MCr(IV), 0.01 MNaN03, measured at atmosphericpC02 based on ICP and colorimetric measurements oftotal Cr and Cr(VI), respectively,remaining in solutionafter equilibration periods.

31

100

80

1 60<

U40

20

Equal Surface Area 163.4 m2/g 0%pCO2~4 g/L goethite ~0.4 g/L HMO

7

PH

10 11

♦ 24 Goe HMO ICP

<0 24GoeHMOUV

A48 Goe HMO ICP

£48 Geo HMO UV

• lwk Goe HMO ICP

OlwkGoeHMOUV

• 2wk Goe HMO ICP

D2wkGoeHMOUV

Figure 3.15: Adsorption of 10"5 M Cr(IV), 0.01 MNaN03, measured at 0% pC02 basedon ICP and colorimetric measurements of total Cr and Cr(VI), respectively, remaining insolution after referenced equilibration periods.

3.3 Cr(VI) Adsorption on Multiple Solid Mixtures

Adsorption edges were measured over a pH range of-3.5 to 10 under 0% pC02,

atmospheric and 3-5% pC02 for physical mixtures of solids of equal surface area (60

m2/g each) consisting of 2.0 g/L goethite , 0.222 g/L HFO , 0.09 g/L HMO , 0.258 g/L y-

alumina ,4.412 g/L kaolinite, and 1.875 g/L montmorillonite , providinga total solid

concentration of8.857 g/L. Concentrations of 10"5 MCr(VI) in0.01 Mbackground

electrolyte of NaN03 were used for these experiments, with equilibration times of 24 hrs

(Figure 3.16).

32

Under 0% pC02 conditions, nearly 100% Cr(VI) adsorption is maintained at pH

values below -6.5 with the pHso at -7.5. The Cr(VI) adsorption percentage decreases

with increasing pH, with -5% Cr(VI) adsorbed at pH 9.5 (Figure 3.16). Atmospheric

pC02 exhibits a slight reduction in adsorption from the 0% pC02 condition, with -100%

maintained below pH 5.7, pHso at 6.7 and -7% sorption at pH 10.35. The 5% pC02

experiment shows a further reduction in Cr(VI) sorption, with 100% adsorbed at pH

<5.35 and pHso at 6.3. This suggests competition between carbonate and chromate for

sorption sites.

All Solids Equal Surface Area 60 m2/g Each100

90

80

70

60

50

40

30

20

10

0

1 ♦

rm •^•"

>-*X*"

7

PH

~ ♦--■

10 11

♦ 0% pC02

«5%pC02

A Atm pC02

Figure 3.16: Adsorption of 10"5 MCr(IV) on mineral mix 60 m2/g each solid in 0.01 MNaN03, measured at 0%, atmospheric and 5% pC02 based on colorimetric measurementsof Cr(VI) remaining in solution after a 24 hr equilibration period.

33

Adsorption edges were also measured over a pH range of-3.5 to 10under 0%

pC02, atmospheric and 3-5% pC02 for physical mixtures of solids of equal surface area

(150 m2/g each) consisting ofgoethite (5.3 g/L), montmorillonite (4.7 g/L), kaolinite (11

g/L), HFO (0.5 g/L), HMO (0.4 g/L) and y-alumina (0.6 g/L). Concentrations of 10*5 MCr(VI) with 0.1 Mbackground electrolyte of NaN03 used for these experiments, with

equilibration times of 24 hours to 2 weeks (Figure 3.17).

Under 0%pC02 conditions, nearly 100% Cr(VI) adsorption is maintained at pH

values below-6.9 with the pH5o at -7.5. The Cr(VI)adsorption percentage decreases

with increasing pH, with -8% Cr(VI) adsorbed at pH 9.6. Rapid Cr(VI) adsorption

occurs, as indicated by the lack of dependence of adsorption on equilibration time. The

negligible difference between the Cradsorption edges calculated using ICP-OES

compared to UV/VIS spectrophotometry indicates a lack of Cr(III) in solution (Figure

3.17).

34

100

90

80

70

60

50

40

30

20

10

0

All Solids ~150 m2/g; 0.1 M NaN03 0%pC02

7

PH

...Ijp.

.'X

: ><b

9 10 11

♦ 24 hr UV-VIS

O 24 hr ICP

H48hr UV-VIS

D48hrlCP

A1 wk UV-VIS

A1 wk ICP

© 2 wk UV-VIS

2wklCP

Figure 3.17: Adsorption of 10'5 MCr(IV) on 150 m2/g total surface area, with equalsurface area of each solid, in 0.1 M NaN03, measured at 0% pC02 based on ICP andcolorimetric measurements of total Cr and Cr(VI), respectively, remaining in solutionafter referenced equilibration periods.

Under ambient pC02 and at pH < -7.1, Cr(VI) adsorption is above 90% (Figure

3.18). Adsorption decreases with increasing pH, with -7% Cr(VI) adsorbed at pH -9.5;

the pHso occurs -8.1. Analogous with the binary mixtures, there is no significant effect

of pC02 in 0% compared to atmospheric conditions or for equilibrationtime from 24 hrs

to 2 weeks, and no indication of Cr(III) in solution based on the similarity of UV/VIS and

ICP-OES data.

35

All Solids ~150 m2/g; 0.1 M NaN03 Atmosphere

♦ 24 hr UV-VIS

O 24hr ICP

• 48 hr UV-VIS

u48hrlCP

A, 1 wk UV-VIS

a 1 Wk ICP

# 2 wk UV-VIS

2wklCP

Figure 3.18: Adsorption of 10"5 MCr(IV) on 150 m2/g equal surface area ofeach solid in0.1 M NaN03, measured at atmospheric pC02 based on ICP and colorimetricmeasurements of total Cr and Cr(VI), respectively, remaining in solution after referencedequilibration periods.

Under >5% pC02, Cr(VI) adsorption on the mixed solidassemblages again

decreases with increasing pH with nearly 100% sorptionat -4.2, decreasing to 90%at pH

-5.9, followed by a rapid decline in adsorbed Cr with increasing pH (pHso occurs at

-6.6); Cr(VI) adsorption is no longer at a detectible level at pH > 8.5. There is little

change in adsorption with equilibration time and no significant difference between edges

calculated from ICP-OES compared to UV/VIS spectrophotometric measurements

(Figure 3.19). Also, as with the binary mixes, the elevated level of pC02 (3-5%) appears

to promote carbonate competition with theCr(VI) anion for adsorption sites, and

therefore decreases overall Cr adsorbed.

36

All Solids ~150 m2/g: 0.1 M NaN03 3-5% pC02

100

90

80

70

60

50

40

30

20

10

0

♦ 24 hr UV-VIS

O 24 hr ICP

B48hr UV-VIS

D48hrlCP

Alwk UV-VIS

AlwklCP

© 2 wk UV-VIS

2wklCP

Figure 3.19: Adsorption of 10"5 MCr(IV) on 150 m2/g equal surface area ofeach solid in0.1 MNaN03, measured at 3-5% pC02 based on ICP andcolorimetric measurements oftotal Cr and Cr(VI), respectively, remaining in solution afterreferenced equilibrationperiods.

37

CHAPTER

IV. DISCUSSION

4.1 Single Sorbate/Sorbent Surface Complexation Models

4.1.1 Cr(VI) Sorption on y-alumina

The adsorption behavior of Cr(VI) on y-alumina as a function of pH, ionic

strength, and pC02 was previously studied to develop and compare three types of surface

complexation model: the CCM. DLM, and TLM (Reich and Koretsky, 2011). The

experimental parameters were varied to ensure the models were calibrated over a range of

conditions (0.001-0.1 MNaN03, 10"4 to 10"5 MCr(VI) with 5 g/L solid, 0-2.5% pC02).

Cr(VI) adsorption under these conditions is -100% below pH 6.5, with thepH^o occurring

between 6.5 and 8. Ionic strength does not have a large effect on the edges, except at the

combination of high ionic strength, high pC02 and low pH, where Cr(VI) adsorption is

slightly repressed. Reich and Koretsky (2011) found little difference in the fits that could

be achieved using the three surface complexation models. The CCM produced the best

fit to the experimental data, but the stability constants are ionic-strength dependent and

therefore only valid for the study conditions. The TLM, which requires more parameters

than either the CCM or DLM was not able to better describe the Cr(VI) adsorption

behavior than either of the simpler models.

Because widely used thermodynamic databases such as JCHESS, MINTEQ, and

Geochemist's Workbench include DLM stability constants for many sorbates and

sorbents, it is convenient to include the DLM stability constants for Cr(VI) adsorption on

y-alumina (Reichand Koretsky. 2011). DLM parameters, reaction stoichiometries and

38

stability constants from Reich and Koretsky (2011; Table 4.1) were entered into the

thermodynamic database MINTEQ. Adsorption edges calculated usingthese parameters

withthe default thermodynamic database for all aqueous species agreewell with

experimental data from Reich and Koretsky (2011; Figure 4.1).

Table 4.1: Model parameters, reaction stoichiometries and stability constants for y-alumina and Cr(VI) DLM from Reich and Koretsky (2011) used to calculate edge shownin Figure 4.1.

Model Parameters

Reich and

Koretsky y-alumina DLM

Surface Area (m /g) 233

Solid Concentration (g/L) 5

Site Density (sites/nm2) site 1 1.49

Cr(VI) Concentration (M) 10-5

NaN03 Concentration (M) 0.01

Surface Complexation Reactions

Log StabilityConstant (K)

SOH + H+ = SOH2+ 7.3

SO_ + H+ = SOH+ -8.6

(SOH2)2C03 = 2SOH + C03"2 +2H+ 24.3

SOH2Cr04*= SOH + CKV2 +H+ 10.3

39

Reich & Koretsky, 2011

y-Alumina DLM

♦ R&K 2011 Data

— DLM R&K 2011

Figure 4.1: Calculated Cr(VI) adsorption edgeon y-alumina usingDLM withparametersin Table 4.1 as compared to experimental data from Reich & Koretsky (2011).

4.1.2 Cr(VI) Sorption on Hydrous Ferric Oxide (HFO)

A DLM for hydrous ferric oxide was developed by Dzombak and Morel (1990)

using a vast array of experimental data from published work. The model incorporates a

wide variety of anion and cation adsorption reaction constants (including Cr(VI)), solid

concentrations, ionic strengths and pC02 conditions (Dzombak and Morel, 1990). This is

thedefault DLM in the chemical equilibrium database MINTEQ. A calculated Cr(VI)

adsorption edge on HFO using the DLM inMINTEQ with parameters shown in Table 4.2

agrees well with experimental data reported inDzombak and Morel (1990, Figure 4.2).

40

Table 4.2: Model parameters, reaction stoichiometries and stability constants for HFOand Cr(VI) DLM from Dzombak and Morel (1990) for experimental conditions in Figure4.2. Site 1 = strong site (s) and Site 2 = weak site (w).

Model Parameters

Dzombak and

Morel HFO

DLM

Surface Area (m2/g) 600

Solid Concentration (g/L) 0.05

Site Density (sites/nm2) site 1(s) 2.26

Site Density (sites/nm2) site 2 (w) 0.056

Cr(VI) Concentration (M) 10-7.55


Surface Complexation ReactionsLog Stability

Constant

>FesOH + H+ = >FesOH2+ 7.29

>FewOH + H+ = >FewOH2+ 7.29

>FesO" + H+ - >FesOH -8.93

>FewO" + H+ = >FewOH -8.93

>FesC03" + H20 = >FesOH + C03"2 +H+ 12.78

>FewC03" + H20 = >FewOH + C03"2 +H+ 12.78

>FesC03H + H20 = >FesOH + C03"2 + 2H+ 20.37

>FewC03H + H20 = >FewOH + C03-2 + 2H+ 20.37

>FesCr04"+H20 = >FesOH + Cr04"2 + H+ 10.85

>FewCr04"+H20 = >FewOH + Cr04"2 + H+ 10.85

>FesOHCr04"2 = >FesOH + Cr04'2 3.9

>FewOHCr04"2 = >FewOH + Cr04*2 3.9

41

"O

-Q

T5TO

100

80

60

40

20

u

<5^

Dzombak and Morel, 1990

HFO DLM

7

PH

10 11

D&M 1990 Data

DLM D&M 1990

Figure 4.2: Calculated DLM adsorption edge as compared to experimental data(Dzombak and Morel, 1990, IECH3, p. 233) using parameters in Table 4.2.

4.1.3 Cr(VI) Sorption on Hydrous Manganese Oxide (HMO)

The HMO DLMof Tonkin et al. (2004) was expanded to include chromium

adsorption ina recent study (MacLeod, 2013). Stability constants and reaction

stoichiometries for Cr(VI) adsorption onHMO were derived for experimental conditions

ranging from pH -3-10, ionic strengths of0.001 to 0.1 MNaN03, 10 or 20 g/L solid,

pC02 conditions of 0-5%, and Cr(VI) concentration of 10"5 or 10"4 7M. MacLeod (2013)showed that equilibration isreached in24 hrs for experiments with 20 g/L but that lower

HMO concentrations (10 g/L) require longer equilibration times. For experiments with

10"5 MCr(VI), 100% adsorption does not occur unless >20 g/L solid is present. Cr(VI)adsorption on HMO is strongly pH and ionic strength dependent. MacLeod (2013) notes

42

little change in the adsorption edges with varying pC02. However, carbonate competition

for soiption sites was anticipated, and MacLeod (2013) suggests that the lack ofpC02

dependence is due to experimental error.

MacLeod (2013) tested a number of DLM reaction stoichiometries andstability

constants for Cr(VI) adsorption using the HMO DLM developed by Tonkin et al. (2004).

He adopted a DLM with a single chromate surface complex (Table 4.3). The model fits

experimental data over a broad range ofconditions reasonably well (e.g., Figure 4.3).

Table 4.3: Model parameters, reaction stoichiometries and stability constants for HMOand Cr(VI) DLM from MacLeod (2013).

Model Parameters

MacLeod, 2013HMO DLM

Surface Area (m2/g) 746


XOH Site Density (sites/nm2) 1.0848

YOH Site Density (sites/nm2) 0.6102

Cr(VI) Concentration (M) 10"5




XO" + H+ = SOH+ -2.35

YO" + H+ - SOH+ -6.06

XOHCr04"2 = XOH + Cr04-2 8.57

YOHC03"2 = YOH + C03-2 17.17

43

100

MacLeod Thesis, 2013

HMO DLM

7

pH

10 11

MacLeod

Data 2013

•MacLeod

(2013) DLM

Figure 4.3: Calculated adsorption edge compared to experimental data from MacLeod(2013) using parameters in Table 4.3.

4.1.4 Cr(VI) Sorption on Kaolinite

DLM stability constants for kaolinite protonation and deprotonation were adopted

from Sverjensky and Sahai (1996), and the exchange capacity for the ion exchange site of

30 meq/100 g was adopted from Bordon and Giese (2001), with Lund et al. (2008)

determining the surface area to be 13.6 m2/g. Due to different activity coefficient

conventions, the ion exchange stability constant of-2.5 (Landry, et al., 2009) used in

44

FITEQL required conversion for use in MINTEQ, accordingto

log Kminteq = log ((10AKFiTEQL*site density/solid concentration))

Chromate surface complex stoichiometries and stability constants were derived

using the nonlinear least squares optimization program FITEQL with experimental data

from Wyman for untreated kaolinite (personal communication; Figure 4.4). FITEQL was

used to optimize possible chromate adsorption stability constants for a variety ofpotential

reactions (Table 4.4) and to provide an overall variance indicating the goodness offit to

the data. This variance is reported as theweighted sum of squares divided bythedegrees

of freedom (WSOS/DF) with the lowest value generally being most desirable, but values

between 0.1 and 20 indicating an acceptable fit to the data (Mathur and Dzombak, 2006),

and values >0.1 suggesting overdetermination of the data fitting exercise. It was assumed

that Cr04"2 would adsorbto the variable charge (>SOH) site, and not to the fixed charge

site.

Stability constants for several chromate surface complexes (>SOHCr04* ,

>SHCr04, and>SCr04") were optimized individually, and then pairs of complexes were

optimized together, using the individually optimized stability constants as starting values.

The resulting reaction stoichiometries, best fit stability constants, and corresponding

WSOS/DF values are shown in Table 4.4. The lowest WSOS/DF was obtained with

formation ofboth >SCr04" and SOHCr04"2 (highlighted line inTable 4.4). The optimized

stability constants for formation ofthese complexes were entered into MINTEQ, together

with the experimental parameters shown inTable 4.5, resulting ina good fit to the

experimental data (Figure 4.4).

45

Table 4.4: Reaction stoichiometries, stability constants, and weighted some of squaresdivided by degrees of freedom for Cr(VI) adsorption reactions on kaolinite optimized inFITEQL.

Model Parameters

Modified from

Landry, et al.Kaolinite DLM

Surface Area (m2/g) 13.6a


Variable Charge Site (SOH) Density (sites/nm2) 4.8184b

Fixed Charge Site (XH) Density (sites/nm2) 0.099c

Cr(VI) Concentration (M) io-5


Surface Complexation ReactionsLog StabilityConstant (K)

SOH + H+ = SOH2+ 2.1b

SO" + H+ = SOH+ -8.1b

XH + Na+ = XNa + H+ -8.2a

SCr04"+H20 = SOH + Cr04"2 +H+ 9.7

SOHCKV2 = SOH + Cr04"2 4.2

aLandry et al. (2009) b Sverjensky and Sahai (1996), c. Exchangecapacity 30 meq/lOOg Bordonand Giese (2001)

46

100

Modified Landry, 2009

Kaolinite DLM

♦ Wyman

Data 2012

— DLM This

Study

Figure 4.4: Model fit as compared to experimental data from Wyman (personalcommunication) using parameters in Table 4.5.

4.1.5 Cr(VI) Sorption on Montmorillonite

The montmorillonite surface area of 32 m2/g reported bythe Clay Minerals

Society was used for all calculations. DLM stability constants for montmorillonite

protonation and deprotonation were adopted from the kaolinite model of Sverjensky and

Sahai (1996) and Akafia et al. (2011), and the exchange capacity for the ion exchange

site of 85 meq/100 g was taken from Bordon and Giese (2001). The stability constant for

sodium/hydrogen exchange on the fixed charge site used for the kaolinite DLM was used

for the montmorillonite model as well. Furthermore, it was assumed that Cr04" would

adsorb to the variable charge (>SOH) site, and not to the fixed charge site. As with the

47

kaolinite DLM, FITEQL was used to optimize chromate surface complex stability

constants for a variety of potential reactions (Table 4.6). Experimental data from Reich

for Cr(VI) adsorption on montmorillonite (personal communication; Figure 4.5) was

entered in FITEQL and stability constants for several chromate surface complexes

(SOHCr04"2, SHCr04, and SCr04") were optimized individually, and then pairs of

complexes were optimized together, using the individually optimized stability constants

as starting values. The resulting reaction stoichiometries, best fit stability constants, and

corresponding WSOS/DF values are shown in Table 4.6. The WSOS/DF closest to 1 was

obtained with formation of both SCr04"and SOHCr04"2 (highlighted line inTable 4.6).

The optimized stability constants for formation of these complexes were entered into

MINTEQ, together with the experimental parameters shown in Table 4.7, resulting in a

slight under estimation of Cr(VI) adsorption as compared to the experimental data

(Figure 4.5), however the model was only optimized to 8 data points.

Table 4.5: Reaction stoichiometries, stability constants, and WSOS/DF for Cr(VI)adsorption reactions on montmorillonite optimized in FITEQL.


Log StabilityConstant (K) WSOS/DF

SCr04' + H20 = SOH + Cr04"2 + H+ 8.1 1.00

SHCr04 + H20 = SOH + Cr04'2 + 2H+ 12.14 0.81

SOHCrCV2 = SOH + CrCV2 3.9 2.43

SCr04*+ H20 = SOH + Cr04"2 + H+SHCr04+ H20 = SOH + Cr04"2 + 2H+

-

No

Convergence

SHCr04+H20 = SOH + Cr04"2 +2H+SOHCr04"2 = SOH + Cr04'2

12.04

3.270.56

SCr04"+ H20 = SOH + Cr04"2 + H+SOHCKV2 - SOH + CKV2

8.04

3.250.85

SCr04"+H20 = SOH + Cr04'2 +H+SHCr04+H20 = SOH + Cr04'2 + 2H+

SOHCr04"2 = SOH + Cr04"2

™ No

Convergence

48

Table4.6: Model parameters, reaction stoichiometries and stability constants formontmorillonite and Cr(VI) DLM.

Model Parameters

Montmorillonite

DLM

Surface Area (m /g) 32d


Variable Charge Site (SOH) Density (sites/nm2) 10b

Fixed Charge Site (XH) Density (sites/nm2) l.llc


NaNC>3 Concentration (M) 0.01

Surface Complexation ReactionsLog StabilityConstant (K)

SOH + H+ = SOH2+ 2.1b

SO" + H+ = SOH+ -8.1b

XH + Na+ = XNa + H+ -8.2a

SHCr04'+H20 = SOH + Cr04*2 + H+ 12.04

SOHCr04'2 = SOH + Cr04"2 3.27

a. Landry et al. (2009), b. Sverjensky and Sahai (1996), c. Exchangecapacity 85 meq/lOOg Bordon and Giese (2001), d. Surface areaClay Minerals Society

49

100

Montmorillonite DLM

6

PH10

♦ Reich Data

2012

•DLM This

Study

Figure 4.5: Model fit as compared to experimental data from Reich (personalcommunication) using parameters in Table 4.7.

4.1.6 Cr(VI) Sorption on Goethite

The DLM for chromate adsorption on goethite was developed by Mathur and

Dzombak (2006) to fit data from Ainsworth et al. (1989). Mathur and Dzombak selected

a surface area of60m2/g based onanarithmetic mean of literature values reported up to

1995. These parameters and stability constants (Table 4.8) were entered into MINTEQ to

ensure the model fit would match to the Ainsworth et al. (1989) data for which they were

developed (Figure 4.6).

50

Table 4.7: Reaction stoichiometries andstability constants for goethite and Cr(VI) DLMfrom Mathur and Dzombak (2006) for different model parameters.

Model Parameters

Mathur and

Dzombak

Goethite DLM

This StudyGoethite

DLM

Surface Area (m2/g) 60 35

Solid Concentration (g/L) 0.84 3.25

Site Density (sites/nm2) site 1 2 2

Cr(VI) Concentration (M)10-6.3 10"5

NaN03 Concentration (M) 0.1 0.01



LogStabilityConstant

(K)

SOH + H+ = SOH2+ 6.93 +/- 0.07 6.93

SO" + H+ = SOH+ -9.65 +/- 0.05 -9.65

SCr04"+H20 = SOH + Cr04"2 + H+ 11.17+/-0.19 10.98

SHCr04+H20 = SOH +Cr(V2 + 2H+ 17.11 +/-0.47 16.64

SOHCrCV2 = SOH + Cr04"2 4.05 +/- 0.21 2.05a

Log Kvaluesa shifted in MINTEQ to match experimental data from this study.

51

Mathur and Dzombak (2006) Goethite DLMNo pC02/

0.84g/L goethite, 0.1 NaN03,1063 M Cr042"♦ Ainsworth

et al Data

DLM This

Study

— — M&D

(2006) DLM

10 11

Figure 4.6: Model fit as compared to experimental data (Mathur and Dzombak, 2006)using parameters in Table 4.8.

With the assumption that this DLM was calibrated over a wide range ofsolid

concentrations, ionic strengths and sorbate concentrations, it was used to predict Cr(VI)

adsorption for the experimental conditions used in this study (Table 4.8). However, thisresulted in an overprediction ofCr(VI) adsorption, which could reflect differences in the

surface area used by Mathur and Dzombak (2006) compared to the surface area ofthe

goethite used in this study. The surface area of 60 m2/g used by Mathur and Dzombak(2006) was almost double that found through 11 point N2 BET analysis on severalsynthesized batches ofgoethite for this study, which repeatedly fell between 30 and 40

52

m/g. Interestingly, Ainsworthet al. (1989) reported a surface area of 33.7 m/g for the

goethiteused to calibrate the Mathur and Dzombakmodel. Reducingthe input surface

area from 60 m2/g to 35 m2/g still results inan overprediction ofCr(VI) sorption for the

experimental conditions used this study(Figures 4.7 and 4.8). Because the goal of this

study is to test the end memberDLMs in mixed solid systems, the Mathur and Dzombak

DLM was recalibrated using experimental data from this study.

"D

Si

<

100

80

60

h 40

20

Mathur and Dzombak (2006)Goethite DLM

Atm pC02, 3.25 g/L solid, 10 5M, 0.01 M NaN03 Cr(VI)

». \ ♦ Experimenta\ I Data This

Study

— — M&DDLM

DLM This

Study

Figure 4.7: Mathur andDzombak (2006) model predictions as compared toexperimental data from this study using parameters shown in Table 4.8.

53

100

80

Mathur and Dzombak (2006) DLM0% pC02, 2 g/L solid, 10"5 M Cr(VI), 0.01 M NaN03

♦ \

\

\

\

' " ' 1 V

♦ Experimenta

I Data This

Study

— — M&DDLM

DLM This

Study

Figure4.8: Mathur and Dzombak (2006) model predictions as compared to experimentaldata this study (parameters in Table 4.8) decreased solid concentration to 2 g/L.

Due to a lack of convergence in FITEQL, MINTEQ was used to fit one stability

constant by eye to better matchthe experimental data from this study. These experiments

were conducted at atmospheric pCC>2, however, Mathur and Dzombak (2006) state that

carbonate is not expected to compete for sorption sites unless it is at high levels, so it was

left out of the initial model. The fitting procedure used was as follows: the goethite DLM

developed by Mathur and Dzombak (Table 4.8) was used, with all stability constants kept

constant except one. Stability constants for each of the three chromate surface complexes

(>SOHCr04"2, >SHCr04, and >SCr04") were first entered at the low limit ofMathur and

54

Dzombak (2006). Then the Cr(VI) surface speciation was assessed with MINTEQ to

determine the contribution and distribution of each chromate complex and the total

sorption of Cr (Figure 4.9). This data suggested that adjusting the stability constantof

>SOHCr04"2 while maintaining the stability constants for >SHCr04, and >SCr04' from

Mathur and Dzombak (2006) would shift the sorption edge to lower pH, thereby better

fitting the experimental data from this study (Figures 4.7 and 4.8). However, the adjusted

DLM under predicts sorption compared to the Ainsworth et al. (1989) data (Figure 4.6).

M&D Chromate Speciation Distribution

-—total Crsorbed

- SCr04-

— SHCr04

— SOHCi-04-2

Figure 9: Chromate speciation calculated using the Mathur and Dzombak (2006) DLMwith the parameters shown in Table 4.8.

Based on the experimental results for Cr(VI) adsorption on goethite, carbonate

species are likely compete for sorption at pC02 levels above 4.99% as indicated by the

55

reduced Cr(VI) adsorption in the >5% pC02 environment (Figure 3.1). Therefore, the

carbonate surface species >SC03H, and >SC03" were added to the DLM (with stability

constants for formation of these surface complexes from Van Geen et al., 1994 (Table

4.9). This model was then used to predict Cr(VI) adsorption edges for experiments

conducted with 10"5 MCr(VI), 2 g/L goethite, an ionic strength of 0.01 MNaN03, and

pC02 conditions of0%, atmospheric and 5%, resulting in an acceptable fit to the

experimental data (Figure 4.10).

Table 4.8: Van Geen et al. (1994) stoichiometries and stability constants for carbonateadsorption on goethite.

Van Geen, et al., 1994Carbonate Surface Complexation Reactions

>FeC03" + H20 = >FeOH + C03 + H+

>FeC03H + H20 = >FeOH + C03"2 + 2H+

56


12.71

20.78

Goethite DLM This Study with Carbonate2 g/L goethite, 0.01 NaN03,105 M Cr(VI)

♦ Atm pC02

Data

Atm pC02

DLM

• 0% pC02

Data

0%pCO2

DLM

* 5% pC02

Data

5%pC02

DLM

Figure 4.10: Calculated edges based on DLM this study to experimental data this studyover arange ofpC02 including stability constants for carbonate on goethite from VanGeen et al. (1994).

4.2 Binary Mineral Mixtures

The DLMs developed for Cr(VI) adsorption on pure solids (above) were used topredict Cr(VI) adsorption in binary systems. The combined model predictions werecompared with measured edges reported in Chapter 3, Section 3.2. MINTEQ can be usedto calculate adsorption on amaximum of five surfaces, with aseparate DLM for eachsolid. Therefore, MINTEQ was used with models developed for the individual solids topredict Cr(VI) adsorption in the presence of multiple solids. This is afirst step forassessing the applicability of SCMs derived for simple systems to more complex systems.

57

4.2.1 Cr(VI) Sorption on Goethite-y-Alumina Mixtures

The adsorption behavior of Cr(VI) on y-alumina-goethite as a function of pH,

ionic strength, and pC02 isdetailed in Section 3.2.3. The surface complexation reactions

and stability constants for y-alumina (Table 4.1) and goethite (protonation, deprotonation

andCr(VI) adsorption reactions and stability constants in Table 4.8 andcarbonate

adsorption reactions and stability constants inTable 4.9) DLMs were used to calculate

Cr(VI) and carbonate adsorption for the mixed solid system. Experimental conditions for

the y-alumina-goethite experiments with 0 or 5% pC02 (Table 4.10) were entered into

MINTEQ, and the resulting adsorption edges were compared to the experimental data

(Figures 4.11 and 4.12). At 0% pC02, the predicted edge from the y-alumina and goethite

DLMs follows the data closely, however, at elevated pC02 (>5%) the model

underpredicts adsorption at pH >7.5.

Table 4.9: Model parameters for y-alumina-goethite system.

Model Parameters

This

Studyy-alumina

This Studygoethite


Solid Concentration (g/L) 0.26 2-y

Site Density (sites/nm ) site 1 1.49 2



58

0% pC02 2 g/L goethite, 0.2598 g/L y-alumina

9 10 11

♦ ExperimentalData

DLM

Figure 4.11: Model predictions using y-alumina and goethite DLMs as compared toexperimental data from this study for 0% pC02 conditions.

59

T5

<

U

5% pC02 2 g/L goethite, 0.2598 g/L y-alumina

Experimental

Data

DLM

Figure 4.12: Model predictions using y-alumina and goethite DLMs as compared toexperimental data from this study for>5% pC02 conditions.

4.2.2 Cr(VI) Sorption on Goethite-HFO Mixtures

The adsorption behavior ofCr(VI) on HFO and goethite mixtures as a function of

pH, ionic strength, and pC02 is detailed in Section 3.2.4. The surface complexation

reactions and stability constants for the HFO (Table 4.2) and goethite DLM (protonation,

deprotonation and Cr(VI) adsorption reactions and stability constants in Table 4.8 andcarbonate adsorption reactions and stability constants in Table 4.9) DLMs were used inMINTEQ, along with the experimental conditions in Table 4.11 to calculate Cr(VI) and

carbonate adsorption onthe physical mixture ofthese two solids. At 0% pC02 the

combined HFO and goethite DLMs overpredict adsorption ascompared to the

60

experimental data(Figure 4.13). The combined HFO and goethite DLMs produce better

predictions of Cr(VI) adsorption at elevated pC02 (>5%) below pH-6.8, but adsorption

is slightly underpredicted at pH >6.8 (Figure 4.14).

Table 4.10: Model parameters for goethite-HFO systems.

Model Parameters

This

StudyHFO

This

Studygoethite


Solid Concentration (g/L) 0.2 2

Site Density (sites/nm2) site 1(Fe(s)=strong) 2.26 2

SiteDensity (sites/nm2) site2 (Fe(w) =weak) 0.056 _



61

0% pC02 2g/L goethite, 0.2 g/L HFO

ExperimentalData

DLM

Figure 4.13: Model predictions using combined goethite-HFO DLMs ascompared toexperimental data from this study at 0% pC02 conditions.

5%pC02 2g/L goethite, 0.2 g/L HFO


DLM

Figure 4.14: Model predictions using combined goethite-HFO DLMs as compared toexperimental data from this study at >5% pC02 conditions.

62

4.2.3 Cr(VI) Sorption on Goethite-Kaolinite Mixtures

The adsorption behavior of Cr(VI) on goethite-kaolinite as a function of pH, ionic

strength, andpC02 is detailed in Section 3.2.2. The surface complexation reactions and

stability constants for the kaolinite (Table 4.5) and goethite (protonation, deprotonation

and Cr(VI) adsorption reactions and stabilityconstants in Table 4.8 and carbonate

adsorption reactions and stability constants in Table 4.9) DLMs were used in MINTEQ to

calculate Cr(VI) and carbonate sorption in the mixed solid system using the experimental

parameters in Table 4.12. Combining the DLMs for goethite and kaolinite yields an

acceptable prediction of Cr(VI) adsorption in relation to theexperimental data for both

the 0%pC02 (Figure 4.15) and the atmospheric pC02 (Figure 4.16)conditions.

Table 4.11: Model parameters for goethite-kaolinite systems.

Model Parameters

This

Studykaolinite

This

Studygoethite

Surface Area(m2/g) 13.6a 35


Variable Charge Site (SOH) Density (sites/nm2) 4.8184D 2

Fixed Charge Site (XH) Density (sites/nm2) 0.6386c -

Cr(VI) Concentration (M) 10"'


aLandry etal. (2009) b Sverjensky and Sahai (1996), c. Exchangecapacity30 meq/lOOg Bordon and Giese (2001)

63

0% pC02 2 g/L Goethite &4.15 g/LKaolinite


DLM

Figure4.15: Model predictions using combined goethite and kaolinite DLMs ascompared to experimentaldata from this study for 0% pC02 conditions.

Atmospheric pC02 2 g/L Goethite &4.15 g/LKaolinite

. ♦ Experimental

Data

DLM

Figure 4.16: Model predictions using combined goethite and kaolinite DLMs ascompared to experimental data from this study for atmospheric pC02.

64

4.2.4 Cr(VI) Sorption on Goethite-Montmorillonite Mixtures

The adsorption behavior of Cr(VI) on goethite-montmorillonite as a function of

pH, ionic strength, and pC02 isdetailed in Section 3.2.1. The surface complexation

reactions and stability constants for the montmorillonite (Table 4.7) and goethite

(protonation, deprotonation, and Cradsorption reactions and stability constants from

Table 4.8 and carbonate adsorption reactions and stability constants from Table 4.9)

DLMs were entered in MINTEQ and used to calculate Cr(VI) and carbonate adsorption

on themineral mixtures using theexperimental parameters shown in Table 4.13. The

combined DLMs for montmorillonite and goethite overpredict Cr(VI) adsorption on the

mineral mixtures at pH 5-8, and slightly underpredict adsorption at higher pH, for both

0% pC02 (Figure 4.17) and atmospheric (Figure 4.18) conditions.

Table 4.12: Model parameters for goethite-montmorillonite system.

Model ParametersThis Study

montmorillonite

This

Studygoethite

Surface Area(m2/g) 32a 35


Variable Charge Site (SOH) Density (sites/nmz) 10b 2

Fixed Charge Site (XH) Density (sites/nmz) 1.11° -

Cr(VI) Concentration (M) io-'


a. Surface area Clay Minerals Society, b. Sverjensky and Sahai (1996), c.Exchange capacity 85 meq/lOOg Bordon and Giese (2001)

65

0% pC02 2 g/L Goethite & 1.89 g/L Montmorillonite

♦ Experimental

Data

Figure 4.17: Model predictions usingcombined goethite-montmorillonite DLMs ascompared to experimental data from this study for 0% pC02 experiment.

66

100

90

80

70

60

50

40

30

20

10

0

Atmopheric pC02 2 g/L Goethite & 1.89 g/LMontmorillonite

♦ \

,...._ ...._.. *... V _♦ V

I 1I 1

♦ V

!" 4 V '

v #^♦

5 6 7 8 9 10 11

PH

Experimental

Data

-DLM

Figure 4.18: Model predictions using combined goethite-montmorillonite DLMs ascomparedto experimental data from this study for atmospheric pC02 conditions.

4.2.5 Cr(VI) Sorption on Goethite-HMO Mixtures

The adsorption behaviorof Cr(VI)on goethite-HMO as a function of pH, ionic

strength, and pC02 is detailed in Section 3.2.5. The surface complexation reactions and

stability constants for the HMO (Table 4.3)and goethite (protonation, deprotonation, and

Cr(VI) adsorption reactions and stability constants in Table 4.8 and carbonate adsorption

reactions and stability constants in Table 4.9) DLMs were used to calculate the sorption

of Cr(VI) and carbonate for the mixed solid system using the experimental parameters

shown in Table 4.14. The combination of HMO and goethite DLMs describes Cr(VI)

67

adsorption to the 0% pC02 data accurately at pH<8, however, at pH >8 the model

underpredicts Cr(VI) adsorption (Figure 4.19). Similarly, for the 5% pC02 conditions,

the combined DLMs produce a very close match to the experimental data at pH<6.5, but

at higher pH the model underpredicts the amount of Cr(VI) adsorbed by the solid mixture

(Figure 4.20).

Table 4.13: Model parameters for goethite-HMO system

Model ParametersThis Study

HMO

This

Studygoethite

Surface Area (m /g) 746 35


XOH Site Density (sites/nm2) 1.0848 2

YOH Site Density (sites/nm2) 0.6102 -



68

0% pC022g/L goethite, 0.093 g/L HMO

Experimental!

Data

DLM

Figure 4.19: Model predictions using combined goethite-HMO DLMs as compared toexperimental data from this study for 0% pC02 experiment.

5% pC02 2 g/L goethite, 0.093 g/L HMO

•oCD

T5

<

U

10 11

♦ Experimental!Data

—•DLM

Figure 4.20: Model predictions usingcombined goethite-HMO DLMs as compared toexperimental data from this study for >5% pC02 conditions.

69

4.3 Multiple Mineral Mixtures

The DLMs developed for Cr(VI) adsorption on pure solids (Section 4.1) were

used to compare calculated Cr(VI)adsorption for binary systemswith measured edges

(Section 4.2). Taking the next step to assess the applicability of single sorbate/sorbent

SCMs to more complex systems, Cr(VI) adsorption edges were measured for multiple

mineral mixtures. Enteringa separate DLM for each solid into MINTEQ (up to five

surfaces), allowed use of individual solid models to predict Cr(VI) adsorption in the

presence of multiple solids. Because the physical experiments were conducted with

combinations of six solids and MINTEQ only accommodates five surfaces, the respective

contribution to sorption of chromate surface complexes for each solid were determined

(using MINTEQ binary predictions) to eliminate thesolid expected to be least active in

Cr(VI) sorption.

4.3.1 Contribution of Individual Solid Chromate Surface Complexes in Binary

Mixtures

The contribution of Cr(VI) adsorption ony-alumina in they-alumina-goethite mix

was highest at pH 5-6 (-12%) but itwas far exceeded by the adsorption on the goethite

species (Figure 4.21). Within the pH range ofthis study (-3-10), inthe goethite-HFO

mixture, goethite only outcompetes HFO for Cr(VI) adsorption atpH<4 above which

HFO is the dominant surface complex (Figure 4.22). In the goethite-kaolinite DLM

prediction, kaolinite only slightly (<2%) contributes to adsorption inthe pH 6.8-8 range

(Figure 4.23). Although montmorillonite has over double the surface area ofkaolinite, inthe goethite-montmorillonite mix the contribution ofmontmorillonite to total Cr

70

adsorption reaches only 0.16% atpH 7.2 (Figure 4.24). Lastly, for the HMO-goethite

mix, the model prediction indicates that adsorption ofCr(VI) on HMO is negligible, only

reaching 0.01% at pH7-8 (Figure 4.25). Based on the lack of chromate adsorption on

HMO predicted for the binary mixtures, this was the solid eliminated from the multiple

mineral mixture modeling in MINTEQ.

Adsorbed Cr(CVI) Speciation0% pC02 2 g/L goethite, 0.2598g/L y-alumina

Total Sorbed on

Surface Sites

SOH2Cr04-2(GAI)l

- SOHCr04-2(Goe)

SHCr04(Goe)

SCr04-(goe)

Figure 4.21: Model predictions ofadsorbed Cr(VI) speciation using combined goethite-y-alumina DLMs (10-5 MCr, 0.01 MNaN03).

71

Adsorbed Cr(CVI) Speciation0% pC02 2g/L goethite, 0.2 g/L HFO Total Sorbed on

Surface Sites

FeCr04-(HFOs)

FeOHCr04-

2(HFOs)

FehCr04-(HFOw)

FehOHCr04-

2(HFOw)

SCr04-(goe)

SHCr04(goe)

- SOHCr04-2(goe)

Figure 4.22: Model predictions ofadsorbed Cr(VI) speciation using combined goethite-HFO DLMs (10"5 MCr, 0.01 MNaN03).

72

Adsorbed Cr(CVI) Speciation0% pC02 2g/L goethite, 4.15 g/L kaolinite

9 10

Total Sorbed on

Surface Sites

SCr04-(goe)

SHCr04(goe)

SOHCr04-2(goe)lJ

•SCr04-(kao)

SOHCr04-2(kao)|

Figure 4.23: Model predictions ofadsorbed Cr(VI) speciation using combined goethite-kaolinite DLMs (10'* MCr, 0.01 MNaN03).

73

T5<VSi

•o

<

<5^

Adsorbed Cr(CVI) Speciation0% pC022g/L goethite, 1.89 g/L montmorillonite

9 10

Total Sorbed on

Surface Sites

SHCr04(goe)

SOHCr04-2(goe)

SCr04-(goe)

•SCr04-(mont)

SOHCr04-2(mont)

Figure 4.24: Model predictions of adsorbed Cr(VI) speciation using combined goethite-montmorillonite DLMs (10"5 MCr, 0.01 MNaN03).

74

Adsorbed Cr(CVI) Speciation0% pC02 2g/L goethite, 0.093 g/L HMO

Total Sorbed

on Surface

Sites

• SHCr04(goe)

SOHCr04-

2(goe)

XOHCr04[-

2](HMO)

SCr04-(geo)

Figure 4.25: Model predictions ofadsorbed Cr(VI) speciation using combined goethite-HMO DLMs (10"5 M Cr, 0.01 MNaN03).

4.3.2 Cr(VI) Sorption on Multiple Solid Mixtures

The adsorption behavior ofCr(VI) on mixtures of multiple solids as a function of

pH, ionic strength, and pC02 is detailed inSection 3.3. The surface complexation

reactions, stability constants and experimental parameters for goethite, montmorillonite,

kaolinite, y-alumina, and hydrous ferric oxide were entered into MINTEQ (Table 4.15),

75

together with relevant experimental parameters (10"5 MCr; 0.01 MNaN03; 0,

atmospheric or 5% PCO2). Adsorbed Cr(VI) speciation was also run to determine the

distribution of each complex. For simplicity of the graphs any complex contributing less

than 2% to the overall adsorption was not plotted. The combined DLMs overpredict

Cr(VI) adsoiption as compared to the measured adsorption edge for the 0 pC02

experiment with HFO and goethite complexes dominating Cr(VI) adsorption (Figure

4.26). For experiments completed under atmospheric pC02, the combined DLMs

overpredict Cr(VI) adsorption from pH 5.5 - 8.5 and underpredict adsorption at pH > 8.5

again with HFO and goethite dominant (Figure 4.27). Interestingly, the predicted edge

from the combined DLMs matches best to the measured experimental edge at 5% pC02

only slightly overpredicting at pH < 7 and overpredicting at pH > 8. There are only four

dominant complexes involved in the Cr(VI) adsorption (Figure 4.28).

76

Table 4.14: Reaction stoichiometries, stability constants, and experimental parametersfor Cr(VI) adsorption on multiple solid mixtures.

Model Parameters

This Study

HFO

DLM

This Study

Goethite

DLM

This Study

y-alumina

DLM

This Study

Kaolinite

DLM

This Study

Montmorillonite

DLM

Surface Area (m"/g) 300 35 233 I3.6a 32dSolid Concentration (g/L) 0.2 3.25 0.26 4.15 1.89

Variable Charge Site (SOH|) Density(sites/nm ) (s) 2.26 2 1.49 4.8l84b 10b

ariable Charge Site (SOH2) Density(sites/nm ) (w) 0.056 . . - -

Fixed Charge Site (XH) Density(sites/nm ) . . . 0.099c I.llc

Surface Complexation Reactions Log K Stability Constants

SOH + H+ = SOH2+ (s) 7.29 6.93 7.3 2.1b 2.1b

SOH + H+ = SOH2+ (w) 7.29 - - - -

SO' + H+ - SOH+ (s) -8.93 -9.65 -8.6 -8.1b -8.1b

SO" + H+ = SOH+ (w) -8.93 . . - -

SCO3- +H20 = SOH + C03'2 + H+ (s) 12.78 12.71 - - -

SCO3- + H20 = SOH + C03"2 + H+ (w) 12.78 - - - -

SC03H+H20 = SOH+CO3'2 +2H+ (s) 20.37 20.78 - - -

SCO3H + H20 = SOH +CO3'2 + 2H+ (vv) 20.37 - - - -

(SOH2)2C03 = 2SOH +CO3"2 +2H+ - - 24.3 - -

SOH2Cr04' = SOH+Cr04"2 + H+ - - 10.3 - -

SC1O4- + H20 - SOH +Cr04'2 + H+ (s) 10.85 10.98 -

9.7 8.04

SCr04- +H20 = SOH +Cr04'2 + H+ (w) 10.85 - - - -

SHCr04- + H20 = SOH +Cr04'2 + 2H+ (s) - - - - -

SHCr04- + H20 = SOH +Cr04'2 + 2H+ (w) - - - -

SOHCr04'2 = SOH +Cr04'2 (s) 3.9 2.05e-

4.2 3.25

SOHCr04"2 = SOH +Cr04'2 (w) 3.9 - - - -

XH + Na+ = XNa + H+ . . - -8.2a -8.2a

aLandry et aL (2009). bSverjensky and Sahai (1996),c Exchange capacity Bordon and Giese (2001), dSurface areaClay Minerals Society,e Log Kvalue modified in MINTEQ to match this experimenal data from this study. *This study.

77

Adsorbed Cr(VI) Speciation0%pCO2 Mineral Mix (60m2/g)

— Combined

DLMs


— • ~>FeCr04-

(HFOs)--- >FeOHCr04-

2(HFOs)— • >SCr04-(goe)

>SHCr04(goe)

10

Figure 4.26: Prediction combined DLMs (experimental conditions Table 4.17) comparedto experimental data this study for a mixture of six solids of equal surface area (60 m /gfor each).

78

Adsorbed Cr(VI) SpeciationAtomospheric pC02 Mineral Mix (60m2/g)

——Combined

DLMs

♦ Experimental

Data

• «>FeCr04-(HF0s)|

>FeOHCr04-

2(HFOs)

- >SCr04-(goe)

->SHCr04(goe)

Figure 4.27: Prediction combined DLMs (experimental conditions Table 4.17) comparedto experimental data this study for atmospheric pCC>2 conditions with a mixture of sixsolids ofequal surface area (60 m2/g for each).

79

Adsorbed Cr(VI) Speciation5% pC02 Mineral Mix (60m2/g)

Combined DLMs

ExperimentalData

->FeCr04-(HFOs)

>FeOHCr04-

2(HFOs)

- >SCr04-(goe)

Figure 4.28: Prediction usingDLMs (experimental conditions Table4.17)compared toexperimental data from this study for 5%pC02 conditions with a mixture of sixsolids ofequal surface area (60 m2/g for each).

Multiple mineral mixture edges were also measured with 0.1 M NaNC>3 and at

increased solid concentrations. In these mixtures an equal surface area of 150 m /g of

each solid was used, requiring 4.68 g/L montmorillonite, 11.03 g/L kaolinite, 0.64 g/L y-

alumina, 5.3 g/L goethite, 0.5 g/L HFO and 0.37 g/L HMO (total solid = 22.5 g/L).

Predicted sorption edges for these conditions were calculated from the combined DLMs

and compared to the measured edges at 0%, atmospheric and 5% pC02. Interestingly, the

combined DLMs produce better fits to the experimental data under these conditions as

compared to the lower ionic strength and solid concentration experiments. The combined

DLMs applied to 0% pC02 only slightly overpredict Cr(VI) adsorption ascompared to

80

the measured adsorptionedge (Figure 4.29). Applying the combined DLMs to

atmospheric and 5%pC02 provide a good fit to the measured data (Figures 4.30 and

4.31).

Adsorbed Cr(VI) Speciaton0% pC02, Mineral Mix 150 m2/g, 0.1 M NaN03

——Combined DLMs


-- >FeCr04-(HFOs)

• »>FeOHCr04-

2(HFOs)>FehOHCr04-

2(HF0w)• - >SCr04-(goe)

Figure 4.29: Prediction using combined single solid DLMs as compared toexperimentaldata for experiments with 0.1 MNaN03,10"5 MCr(VI) and 150 m7g surface area ofallsix solids at 0% pC02.

81

T5

Si

<

u

Adsorbed Cr(VI) SpeciatonAtmospheric, Mineral Mix 150 m2/g, 0.1 M NaN03

Combined DLMs

^ Experimental Data

— • ->FeCr04-(HFOs)

—»- >FeOHCr04-

2(HFOs)—- «>FehOHCr04-

2(HF0w)— — >SCr04-(goe)

— - ->SHCr04(goe)

7

PH10 11

Figure 4.30: Model prediction using combined single solid DLMs as compared toexperimental data for experiments with 0.1 MNaN03, 10"5 MCr(VI) and 150 m2/gsurface area of all six solids at atmospheric pC02.

82

Adsorbed Cr(VI) Speciaton5% pC02, Mineral Mix 150 m2/g, 0.1 M NaN03

-Combined

DLMs

Experimental

Data

• >FeCr04-

(HFOs)

* >FeOHCr04-

2(HFOs)

>SCr04-(goe)

>SHCr04(goe)

Figure 4.31: Model prediction using combined single solid DLMs as compared toexperimental data for experiments with 0.1 MNaN03, 10"5 MCr(VI) and 150 m2/gsurface area of all six solids at 5% pC02.

83

V. CONCLUSIONS

5.1 Cr(VI) Sorption Experiments

In all goethite-containing systems Cr(VI) adsorption is near 100% below pH~6.

The sorption edges typically span from pH 6-9 with <10% Cr(VI) adsorbed at pH>9.

Adsorption is rapid in all goethite binary mineral combinations; in general no significant

differences are observed for equilibration times of 24 hrs to 2 weeks. The pH at which

50% of Cr(VI) is adsorbed varies from pH 6-7.5 among the different mineral-mineral

combinations. No evidence of Cr(III) in solution is observed for any of the studied

systems. Atmospheric pC02 levels do not significantly affect Cr(VI) adsorption

behavior, but adsorption decreases in experiments with >5% pC02.

In multi-solid systems containing 60 m2/g surface area of each solid with 0.01 M

NaN03, Cr(VI) adsorption below pH ~6 is near 100%. Adsorption reaches a steady state

within 24 hours, and remains unchanged for up to 2 weeks for multiple solid systems.

The edges typically span a pH range of 6-9 with <10% Cr(VI) adsorbed at pH >9. The

pH50 varies from 7.5 with 0 pC02 to 7 under atmospheric conditions, and drops to -6.3

under >5% pC02. This indicates that carbonate competition for adsorption sites is

negligible under low pC02 conditions, but is likely important at >5 % pC02. For

systems with higher solid concentrations (150 m2/g each solid = 22.5 g/L total solid) and

background electrolyte concentration (0.1 M NaN03) adsorption remains rapid, with

little change from 24 hr to 2 week equilibration times. Adsorption increases significantly

for the higher ionic strength and solid concentrations under atmospheric pC02 (Figure

84

5.1). However, the increase in Cr(VI)adsorption is much smaller under the 0 or >5%

pC02 (Figures 5.2 and 5.3).

All Minerals Atmosphere150 m2/g, 0.1 M NaNQ3 vs. 60 m2/g, 0.01M NaN03

Si

100

80

60

<w 400

20 j—

0 j3 4 5 6 7

PH

<.-•

9 10 11

24 hr UV-VIS 0.1M

NaN03

24 hr UV/VIS 0.01M

NaN03

Figure 5.1: Comparison ofCr(VI) adsorption edges atvarying ionic strength and solidconcentrations.

85

All Minerals 0% pC02150 m2/g;0.1 M NaN03vs 60 m2/g, 0.01M NaN03

100

80"O

"§ 6013

<40 r

20

V

• ^

• * ♦

PH

11

♦ 24 hr UV-VIS 0.1M

NaN03

• 24 hrUV/VIS O.OIM

NaN03

Figure 5.2: Comparison of adsorption edges at varying ionic strength and solidconcentrations.

All Minerals 5% pC02150 m2/g, 0.1 M NaN03 vs. 60 m2/g 0.01 M NaN03

100

90

80

| 70| 60•8 50

S 40£ 30

20

10

0

• »

V

7

PH

*• Jt

11

♦ 24hrUV-VIS0.1M

NaN03

• 24 hr UV/VIS 0.01 M

NaN03

Figure 5.3: Comparison ofadsorption edges at varying ionic strength and solidconcentrations.

86

The change of pH50 for both the 0 and >5% pC02 experiments between the 0.01

MNaN03 and 60 m2/g solid and the 0.1 MMNaN03 and 150 m2/g solid conditions areslight. The small differences between these edges could be attributed to experimental

error ormay indicate enhanced adsorption due to the increased available surface area for

adsorption in the 150 m2/g experiments. Increased carbonate concentrations and

increased ionic strength are expected to reduce Cr(VI) adsorption, so these are unlikely to

account for the increase in adsorption with increasing ionic strength, pC02 and solid

concentration. HFO and goethite are expected to bethe dominant surfaces for Cr(VI)

adsorption, with little contribution predicted from the DLM for the other four solids in

the mixtures. Prior studies have suggested that coagulation or heterocoagulation can lead

to the formation of strong "hybrid sites". If coagulation increases at the higher ionic

strength and solid concentrations, leading to the formation ofsuch a site, this could lead

toenhanced Cr(VI) adsorption. It should also benoted that for theexperiments

conducted under atmospheric pC02 edge, the large shift in pH5o (from pH 7 to pH 8.1)

cannot be readily attributed to any ofthese factors, because a similar increase would also

be expected for the experiments conducted under 0pC02. More likely, experimental error

accounts for this large shift.

5.2 Surface Complexation Modeling

DLMs for single sorbate/sorbent systems for Cr(VI) adsorption on y-alumina

(Reich and Koretsky, 2011), hydrous ferric oxide (Dzombak and Morel, 1990), hydrousmanganese oxide (Tonkin et al., 2004 as expanded by MacLeod, 2013 to includechromium) were entered into Visual MINTEQ. DLMs for kaolinite (Landry et al., 2008),and montmorillonite (Akafia et al., 2011) were expanded to include chromium adsorption

87

reactions and stability constants. A goethite DLM (Mathur and Dzombak, 2006) was

recalibrated to fit the end member data for single solid experiments from this study.

These DLMs individually resulted in calculated adsorption edges that compared well to

the experimental data for single sorbate/sorbent systems. However, combining the DLMs

to describe Cr(VI) adsorption in systems containing two solids, with the exception of the

y-alumina-goethite and the HMO-goethite systems, Cr(VI) adsorption was systematically

over predicted in the other three-solid systems.

Perhaps unsurprisingly, the tendency for overprediction of adsorption was also

observed for the Cr(VI) adsorption edges measured for the six mineral mixture with 0.01

MNaN03 and 60 m2/g equal surface area of each solid, particularly at pH >6. Surface

complexation was slightly underpredicted at pH <6 for this system. The DLMpredicts

that the surface complexes in this solid mixtureare dominatedby >FeOHCr04" (HFO(s)),

>FeCr04*(goethite), and >FeCr04" (HFO(S)). Predictions were more accurate for

experiments with increased ionic strength (0.1 M NaN03) and higher solid concentrations

(150 m2/g each solid). Predicted speciation varies with the lower ionic strength and

lower surface area having a stronger formation of HFO complexes whereas the higher

ionic strength and larger surface area have a more dominant formation of goethite surface

complexes.

The accurate prediction of the adsorption edges for the mixed mineral assemblage

experiment conducted with increased ionic strength and solid concentrations, suggests

that the simple component additivity approach of Davis et al. (1998) may provide a

useful approach for quantifying adsorption in natural systems. However, the

overprediction compared to measured edges for the lower ionic strength experiments with

the decreased solid surface area suggests that this approach should be used withcaution

88

and that more testing is still required to determine under what conditions predications are

likely to be accurate.

5.3 Future Work

The DLM predicts that for mixtures of HFO, goethite, HMO, kaolinite,

montmorillonite, and y-alumina, Cr(VI) preferentially adsorbs to sites onthe HFO and

goethite. Experimental edges should be measured with mixtures containing only HFO or

goethite and compared to component additivity DLM predictions to further assess the

possibility of iron oxides colliding. Though beyond the scope of this study, x-ray

adsorption fine structure (XAFS) spectroscopy on the experimental mixture could

provide a spectra ofthe chemical bonds formed. Comparing the XAFS spectra to the

combined DLM speciation would provide verification of the accuracy of the model. The

Lawrence Berkeley National Laboratory has a molecular environmental science beamline

at the Advanced Light Source that isable to perform the XAFS. Because contaminated

sites seldom have only one heavy metal present, future work should also include mixtures

ofmultiple sorbates and sorbents to truly advance understanding ofmultiple interactions.

Ionic strength plays an important role that could not be readily distinguished from

the effects of increased solidconcentrations for the mixed solid systems, experimental

edges should be measured using equal surface areas ofeach mineral with 60 m/g with10"5 MCr(VI) at 0% pC02, atmospheric and 5% pC02 conditions and only increasing thebackground electrolyte concentration to 0.1 MNaN03 Likewise, comparisons could alsobe obtained by conducting adsorption edge measurements at the increased solidconcentrations (e.g., 150 m2/g equal surface area), while holding the background

electrolyte concentration constant at 0.01 MNaN03. Performing these experiments and

89

comparing the results to the predictions from component additivity with the end-member

DLMs will provide more evidence regarding the efficacy of this approach.

The ability ofa component additivity approach to accurately predict adsorption

behavior is strongly influenced by the robustness of the end-member (single solid)

DLMs. More exploration should be done regarding the goethite DLM developed by

Mathur (1995) and used by Mathur and Dzombak, 2006 to determine why this model

failed to predict experimental data for the goethite only adsorption edges measured in this

study. It should be noted that an attempt was made to retrieve the M.S. Thesis ofMathur

(1995) referenced by Mathur and Dzombak (2006) for details ofthe development ofthe

goethite DLM. However, Carnegie Mellon University is no longer in possession ofa

copy ofthis thesis. Thus, it is impossible to know what range ofexperimental data was

used to calibrate the model. Since the original document is notavailable a comparison to

published studies ofCr(VI) adsorption on goethite at avariety ofsorbate/sorbent ratiosand various ionic strengths should be conducted toverify the robustness of this DLM.

Although montmorillonite and kaolinite are not expected to contribute

significantly to Cr(VI) adsorption when iron oxides are present, the DLMs for Cr(VI) onthese solids should be more fully developed, using much broader ranges ofexperimental

data, so that they can be used under conditions where clays may be more significant

sorbents. Both of these models were fit to oneexperimental edge andshould byno

means be considered calibrated for use under any other experimental conditions without

calibration to several more edges conducted under a variety ofconditions. Lastly, to truly

advance understanding ofadsorption behavior ofmixed contaminants on mixed solids,the geochemical community needs to develop modeling software that is capable ofcombining multiple sorbate/sorbent mixes and that, together with robust thermodynamicdata, can be readily shared among researchers.

90

APPENDICIES

91

Appendix A:

Single Solid Cr(VI) Adsorption Edge Data

92

Al. Cr(VI) Adsorption on Kaolinite

Wyman Data: 30 g/l kaolinite, 0.01 M NaN03, 10-5 M Cr04

PH24hr

%ads

24hr

PH48hr

%ads

48hr

PHlwk

%ads 1

wk

PH2wk

%ads

2wk

3.18 13.141 3.17 9.2652 3.46 42.056 3.52 56.209

3.99 19.231 3.92 12.78 4.22 31.153 4.24 43.137

4.33 16.667 4.3 11.502 4.47 31.776 4.49 39.869

4.63 18.269 4.64 15.016 4.77 30.841 4.79 36.928

4.79 18.59 4.8 15.016 4.93 30.218 4.95 35.621

5.19 17.949 5.22 17.252 5.37 28.66 5.38 34.967

5.8 25.641 5.78 21.725 5.92 31.464 5.87 37.908

6.24 25 6.36 22.045 6.28 36.449 6.24 45.752

6.53 23.718 6.64 22.045 6.45 36.137 6.33 45.425

6.91 21.795 7.1 23.323 6.85 29.595 6.58 38.889

7.53 16.987 7.71 15.974 7.44 21.807 7 33.333

8.02 13.782 8.16 13.099 8.07 18.38 7.37 22.549

8.89 7.3718 8.95 8.6262 8.86 9.9688 8.52 15.686

9.2 8.0128 9.27 8.9457 9.15 8.4112 8.82 7.5163

10 2.2436 10.01 1.9169 10.15 3.4268 10.08 4.2484

A2. Cr(VI) Adsorption on HMOMacLeod Data: 20 g/l HMO, 0.01 M NaN03,10-5 M Cr(VI)

pH

24hr

%ads

24hr

PH48hr

%ads 48

hr

pH72

hr

%ads 72

hr

3.13 88.571 3.19 88.065 3.13 86.392

3.67 82.54 3.73 79.032 3.58 81.013

4.21 76.19 4.24 75.161 4.15 75.633

4.83 66.349 4.88 65.806 4.8 66.456

5.25 55.556 5.3 55.484 5.24 55.696

5.62 46.984 5.67 43.226 5.63 45.253

6.21 16.825 6.2 19.032 6.18 25.316

6.93 9.5238 6.91 5.8065 6.94 12.025

8.17 2.5397 8.19 2.2581 8.04 5.3797

9.43 4.127 9.32 3.871 9.19 11.076

93

A3. Cr(VI) Adsorption on -/-alumina

Reich Data: 5 q/L GA, 10-5 M Cr, 0.01 M NaN03

PH %Ads PH %Ads

4.03 95.46 4.28 100.00

3.89 98.49 4.41 99.12

4.19 97.73 4.52 100.00

4.28 99.24 5.01 100.00

5.15 99.24 5.53 100.00

5.50 98.49 5.92 98.25

5.93 98.49 6.28 92.11

6.31 96.21 6.57 81.58

6.82 92.42 6.79 71.93

7.23 81.82 6.85 66.67

7.67 59.09 7.01 56.14

8.19 31.06 7.19 48.25

8.56 15.91 7.41 37.72

9.16 3.79 7.93 18.42

9.80 0.00 8.94 6.14

94

A4. Cr(VI) Adsorption on montmorillonite and HFO

Reich Data:

7 g/i montmoriilonite, 0.01NaN03, 10-5 M Cr(VI)

This Study: HFO

.045 g/L 0.1 MN3N03, 10-5.3 M

Cr(VI)

pH 48 hr48 hr UV

%ads CrpH 24 hr

24 hr UV

%ads Cr

2.4165.23

3.97 96.62

3.38 15.69 4.53 90.54

6.61 14.77 4.89 93.92

7,04 6.46 5.37 87.16

7.88 4.92 5.5 78.38

8.32 3.38 6.25 44.59

9.07 -3.08 6.79 45.27

9.72 -0.92 7.02 33.11

7.24 34.46

8.02 13.51

9.7 14.19

95

A5. Cr(VI) Adsorption and kinetics goethite (3.25 g/L)

This Study: Goethite 3.2559 g/L; Cr0410-S M; NaN03 0.01 M

1/25

24hr

pH

1/25

24hr

%ads

24 hr

ICP

%ads

1/26

48hr

pH

1/26

48hr

%ads

48 hr

Icp

%ads

2/11

wkpH

2/11

wk

%ads

lwk

ICP

%ads

2/8 2

wk pH

2/8 2

wk

%ads

2wk

ICP

%ads

3.19 101.09 99.50 3.22 97.90 96.43 3.23 100.00 98.21 3.23 100,00 96.04

3.95 100.36 100.28 3.94 97.20 102.37 4.04 100.00 100.30 4.05 100.00 99.28

4.36 100.36 100.32 4,39 98.95 102.57 4.50 100.00 100,45 4.53 100.00 99.29

4.83 99.28 100.34 4.87 96.15 102.49 4.95 100.00 100.44 4.86 100.00 99.33

5.31 101.09 100.32 5.29 96.15 102.63 5.33 100.00 100.36 5.26 100.00 99.29

5.59 99.64 100.20 5.62 95.80 101.38 5.54 100.00 100.35 5.57 100.00 99.25

6.16 98.91 99.81 6.10 95.10 98.53 5.98 100.00 100.26 5.83 100.00 99.17

6.53 97.46 98.25 6.46 93.71 92.38 6.24 100.00 99.99 6.19 100.00 99.20

7.08 92.03 94.28 6.81 91.26 64.01 6.56 100.00 98.44 6.35 100.00 98.18

7.54 64.49 68.97 6.89 94.14 93.97 6.62 97.30 97.29

8.06 26.09 0.05 7.33 83.59 77.29 6.64 98.99 97.33

8.55 2.54 26.86 8.04 30.08 1.69 7.03 90.54 90.85

9.16 0.00 28.19 8.84 -6.64 4.51 8.09 37.84 50.50

10.09 -9.78 12.04 10.04 -20.70 -9.28 9.84 -11.82 13.01

96

A6. Cr(VI) Adsorption goethite (2 g/L)

2 g/L goethite, 10-5 M Cr, 0.01 M NaN03

0% pC02 atm pC02 5% pC02

PH

%Cr

Adsorbed PH

%Cr

Adsorbed PH

% Cr

Adsorbed

3.32 100.00 3.56 97.88 3.28 98.83

3.61 98.47 4.02 97.27 3.88 95.61

4.22 98.47 5.04 94.55 4.41 95.32

4.62 98.77 5.55 93.03 4.79 93.57

5.19 97.55 5.98 92.73 5.15 86.84

5.57 96.93 6.23 90.61 5.64 76.90

6.39 92.64 6.48 84.55 6.01 63.45

6.68 79.45 6.48 83.03 6.15 54.97

7.06 70.86 6.85 71.52 6.32 46.78

7.62 37.12 7.04 66.97 6.47 36.26

8.05 15.03 8.11 19.39 6.74 29.24

8.63 3.99 9.46 5.15 7.29 16.96

9.89 1.84 10.45 4.55 8.95 3.51

10.27 0.00 9.54 7.60

97

Appendix B:

Binary Solids Cr(VI) Adsorption Edge Data

98

Bl. Cr(VI) Adsorptionand kinetics on goethite and kaolinite in 0 pC02

Data This Study: Goethite 2g/L; Cr04 10-5 M;NaN03 0.01 M; Kaolinite 4.15 g/L

0% pC02

12/29

24hr

pH

12/29

24hr

%ads

12/30

48hr

pH

12/30

48hr

%ads

1/51 wk

pH

1/5

1 wk

%ads

1/12

2 wk

pH

1/12

2 wk

%ads

3,07 100 3.14 100 3.07 97.22 3.11 100

3.67 98.93 3.64 100 3.69 95.14 3.72 100

4.13 89.32 4.10 100 5.35 89.24 4.15 94.34

4.88 96.09 4.87 99.39 5.78 73.96 4.85 96,54

S.36 86.83 5.42 100 6.24 58.68 5.41 94.03

5.92 77.58 5.91 99.09 6.59 30.56 5.84 91.82

6.29 92.17 6.60 92.68 6.94 21.53 6.37 88.99

7.01 85.77 6.81 85.06 7.32 19.79 6.56 84,91

7.01 61.92 7.37 59.76 7.64 21.88 6.89 65.41

7.85 29.89 7.70 36.89 8.28 3.47 7.38 37.74

8.23 7.83 8.45 13.11 9.20 5.21 7.88 14.15

8.66 0.71 8.84 2.74 9.93 2.78 8.42 0.31

9.28 1.78 9.33 8.23 9.32 0.00

9.64 0.00 9.98 0.30 10.03 0.00

99

B2. Cr(VI) Adsorption and kinetics goethite and kaolinite atmospheric pC02

Data This Study: Goethite 2g/L; Cr04 10-5 M; NaN030.01 M; Kaolinite 4.15 g/L

Atmospheric pC02

12/30

24hr

pH

12/30

24hr

%ads

12/31

48hr

pH

12/31

48hr

%ads

1/6

1 wk

pH

1/6

1 wk

%ads

1/13

2 wk

pH

1/13

2 wk

%ads

3.25 100 3.45 95.69 3.39 100 3.49 100

4,08 100 4.25 95.69 4.22 100 4,27 100

4.54 100 4.64 96 4.6 100 4.64 100

4,92 99.34 4.98 95.69 4.96 100 4.99 100

5.37 97.7 5.43 97.23 5.4 100 5,45 100

5.74 95.41 5.76 94.15 5.7 100 5.74 99.11

6.14 93.44 6.11 92.92 5.99 100 6.02 97.62

6.55 77.7 6.46 91.38 6.29 98,38 6.23 95.54

6.87 74.43 6.71 86.15 6.46 94.82 6.4 91.67

6,37 47.87 7.4 55.38 6,78 81.55 6.62 87.2

8.04 9.508 7.85 24.62 7.07 59.87 6.87 70.54

8.8 0 8.65 4 7.64 24.92 7.08 52.38

9.49 0 9.45 0.308 9.19 0 8.88 2.679

10.02 0 10 0.615 9.92 1.294 9.84 °l

100

B3. Cr(VI) Adsorption and kinetics goethite and montmorillonite 0% pCO2

Data This Study: Goethite 1.9608g/L; Cr04 10A-5; NaN03 0.01 M;Montmorillonite 1.875 g/L

0% pC02

1/624hr

PH

1/624hr

%ads

24 hr

ICP %

ads

1/748hr

PH

1/748hr

%ads

48 hr

ICP %

ads

1/131 wk

PH

1/13

1 wk

%ads

lwk

ICP %

ads

1/20

2 wk

PH

1/20

2 wk

%ads

2wk

ICP %

ads

4.00 99.68 99.07 5.05 100 98.95 5.08 100 98.26 5.49 95.60 96.65

4.90 97.15 95.24 5.94 93.07 93.62 6.11 87.50 85.90 6.09 80.50 83.10

5.85 90.82 91.61 6.24 87.95 87.01 6.40 73.72 73.53 6.35 71.70 74.54

6.13 82.59 84.71 6.55 74.40 75.70 6.48 73.08 70.80 6.42 65.72 70.01

6.45 56.65 60.99 6.88 58.73 57.49 7.26 32.69 34.06 6.65 51.89 56.03

6.99 39.56 42.30 7.24 35.84 36.14 7.10 34.94 39.20 6.93 35.22 39.04

7.30 26.58 28.15 7.75 21.39 19.80 7.33 3.21 28.90 7.12 26.73 32.17

7.59 16.46 20.04 7.90 16.87 17.20 7.63 16.35 19.58 7.21 22.64 26.52

7.90 19.94 13.90 8.27 14.16 14.21 7.85 15.71 9.03 7.34 16.04 20.33

8.33 7.28 12.56 8.38 13.86 10.74 8.37 8.97 7.52 12.26 16.01

8.57 6.01 9.46 8.59 9.64 9.70 8.47 5.13 11.70 8.12 11.64 10.00

8.89 5.06 8.37 9.38 4.52 9.95 8.85 0.64 8.47 8.58 8.18 8.59

9.39 0.00 7.89 9.92 8.43 6.75 9.30 16.67 8.88 9.25 11.01 5.82

9.95 1.58 5.56 6.06 9.84 1.60 6.03 9.82 2.52 5.97

101

B4. Cr(VI) Adsorption and kinetics goethite and montmorillonite atmospheric pC02

Data This Study: Goethite 1.9608g/L; Cr04 10A-5; NaN03 0.01 M;Montmorillonite 1.875 g/L

Atmospheric pC02

1/b24hr

PH

1/b24hr

%ads

1/648hr

PH

1/6

48hr

%ads

48 hr

ICP

%ads

1/12

1 wk

PH

1/1Z 1

wk

%ads

IWK

ICP

%ads

i/iy

2 wk

PH

i/iy z

wk

%ads

ZWK

ICP

%ads

4.49 100 4.41 98.14 99.55 4.60 99.09 99.11 4.66 100 98.88

5.71 98.21 5.77 94.88 96.46 5.97 89.70 90.64 6.17 86.08 85.34

6.10 98.21 5.94 93.49 94.02 6.14 84.55 85.74 6.30 79.61 79.13

6.27 97.85 6.24 87.91 90.01 6.37 76.97 71.06 6.42 70.23 70.41

6.55 97.13 6.52 76.74 79.47 6.59 65.76 65.47 6.63 64.72 64.17

6.73 97.13 7.14 54.42 56.11 6.87 46.06 46.64 6.83 50.81 51.60

6.82 97.13 7.28 35.81 35.23 6.99 36.36 37.77 6.85 48.54 47.98

6.93 92.47 7.55 24.65 22.49 7.09 30.00 30.48 6.86 44.01 45.01

6.95 79.21 7.94 16.28 15.45 7.38 16.97 17.95 7.10 28.16 27.44

7.00 57.71 8.19 11.16 10.75 7.72 10.91 10.78 8.58 8.41 5.52

7.07 28.32 8.76 11.63 5.57 8.58 3.94 5.84 9.48 10.36 3.60

8.20 2.15 8.63 6.98 4.12 8.68 5.45 6.65

9.36 0.36 9.16 6.98 5.92 8.96 3.03 4.02

9.87 6.05 7.36 9.63 2.73 2.78

102

B5. HMO, y-alumina, and HFO binary mixes with 2 g/L goethite, 0 pC02

ThisStudy: 24hr, Binary geothite mixes (60m2/g each solid);

10-5 Cr{Vl);0.01 M NaN03; 0% pC02

Goethite 2.0 g/L

HMO 0.093 g/L

Goethite 2.0 g/L

gamma-alumina

0.2598 g/L

Goethite 2.0 g/L

HFO 0.222 g/L

pH

%Cr

Adsorbed pH

%Cr

Adsorbed PH

%Cr

Adsorbed

3.48 100-00 3.72 100.00 4.16 98.25

3.94 100.00 4.25 100,00 4.35 98.54

4.38 100.00 4.62 100.00 4,70 97.37

4.60 99.73 5.24 100.00 5.09 96.49

5.12 98.64 5.73 98.42 5.33 96.49

5.27 99.18 6.04 96.84 5.70 96.49

5.70 98.91 6.29 95.89 5.99 96.20

6.23 97.55 6.62 91.14 6.34 94.74

6.18 96.74 7.25 45.89 6.82 89.77

6.46 93.21 8.00 10.44 7.25 73.10

6,84 79.08 8.60 2.85 7.96 23.68

8.07 22.01 S.94 1.27 8.97 0.00

9.12 16.03 9.48 0.00 10.46 0.00

10.05 14.13 9.S9 0.00

103

B6. HMO, y-alumina, and HFO binary mixes with 2 g/L goethite, 5% pCO;

This Study: 24 hr, Bl

10-5 Cr

narygeothite mixes (60m2/g each solid);VI); 0.01 M NaN03; 5% pC02

Goethite 2.0 g/L

HMO0.093 g/L

Goethite 2.0 g/L

gamma-alumina

0.2598 g/L

Goethite 2.0 g/L

HFO 0.222 g/L

PH

%Cr

Adsorbed PH

%Cr

Adsorbed pH

%Cr

Adsorbed

3.55 98.18 3.53 100.00 3.23 100.00

4.06 96.10 3.92 99.40 4.07 99.41

4.50 95.58 4.34 98.20 4.48 9S.S2

4.95 91.43 4.75 94.59 4.92 96.76

5.42 86.23 5.21 87.39 5.39 92.92

5.74 76.36 5.61 75.38 5.78 85.55

6.14 58.18 5.86 63,06 6.03 74.93

6.33 48.31 6.13 48.05 6,53 63.13

6.49 45.97 6.25 40.54 6.64 55.75

6.63 41.04 6.40 34.83 6.89 48.08

6.85 32.47 6.46 32.13 7.10 38.94

7.11 26.75 6.66 26.73 8.64 10.32

8.82 12.99 8.12 9.31 9.40 8.55

9.55 12.99 8.98 6.91 9.76 9.44

9.38 2.40

104

Appendix C:

Multiple Solids Mix Cr(VI) Adsorption Edge Data

105

CI. Cr(VI) Adsorption and kinetics on mixed solids at 5% pC02 (150 m2/g surface area)

This Study: 5% pC02 ; 6 Solid Mixture Equal Surface Area 150 m2/g;Cr04 10-5 M; NaN03 0.1 M

24hr

PH

24hr

%ads

24 hr

ICP

%ads

48hr

PH

48hr

%ads

48 hr

ICP

%ads

1 wk

PH

1 wk

%ads

lwk

ICP

%ads

2 wk

PH

2 wk

%ads

2wk

ICP

%ads

4.39 99.62 100 4.63 96.46 99.84 4.75 99.02 99.08 4.80 95.68 99.16

4.85 98.85 99.80 4.93 96.46 99.63 4.97 98.36 99.49 5.03 95.02 98.78

5.06 99.23 99.48 5.10 99.04 99.14 5.21 96.39 98.42 5.20 93.69 97.99

5.32 99.62 99.19 5.38 99.36 98.75 5.34 95.08 97.53 5.35 93.02 97.53

5.67 97.31 97.90 5.69 98.71 97.52 5.61 94.10 97.08 5.55 92.69 96.50

6.12 94.23 93.97 6.05 89.07 93.30 5.95 85.25 92.60 5.84 86.05 92.58

6.60 78.85 80.51 6.46 76.53 78.97 6.27 72.79 79.29 6.19 76.74 80.78

7.04 53.08 53.14 6.85 50.16 51.75 6.63 50.16 51.61 6.53 57.14 59.09

7.49 26.15 28.41 7.17 24.76 30.19 6.83 19.34 42.93 6.71 40.53 46.51

7.73 20.38 15.87 7.44 11.58 17.54 6.90 21.31 33.72 6.83 36.88 35.90

8.12 11.54 9.34 7.64 10.61 10.28 6.97 22.95 1.21 6.92 35.22 30.13

9.31 3.46 0.77 8.89 7.40 0.00 7.31 8.52 13.32 7.06 19.60 23.69

9.73 8.08 0.30 9.34 0.00 0.00 7.63 1.31 13.10 7.27 13.29 12.46

10.87 0.77 0.88 10.34 0.00 0.00 9.70 0.00 -0.58 8.74 0.00 -2.24

106

C2. Cr(VI) Adsorption and kinetics on mixed solids atmospheric pC02 (150 m2/g surfacearea)

This Study: Atmospheric pC02 ; 6 Solid Mixture Equal Surface Area 150 m2/g;Cr04 10-5 M; NaN03 0.1 M

24hr

PH

24hr

%ads

24 hr

ICP

%ads

48hr

PH

48hr

%ads

48 hr

ICP

%ads

1 wk

PH

1 wk

%ads

lwk

ICP

%ads

2 wk

PH

2 wk

%ads

2wk

ICP

%ads

5.09 97.73 100 5.19 98.14 100 5.50 98.49 100 5.65 99.42 100

5.31 98.06 100 5.40 99.07 100 5.71 98.87 100 5.84 98.84 100

5.47 97.41 100 5.66 98.76 100 6.00 99.25 100 6.09 98.27 99.73

5.71 97.41 100 5.88 99.07 100 6.17 94.34 100 6.25 98.84 99.54

5.94 96.76 100 6.09 99.69 100 6.39 93.58 100 6.43 97.69 98.53

6.26 95.79 100 6.33 98.45 100 6.62 90.94 100 6.65 96.82 97.57

6.47 95.15 100 6.57 96.27 100 6.87 89.81 100 6.82 94.22 95.44

6.71 93.20 100 6.80 93.17 100 7.13 85.28 97.81 7.00 91.04 92.06

6.96 90.29 99.08 7.01 88.82 95.70 7.31 81.13 93.37 7.17 85.84 88.41

7.22 85.11 92.48 7.22 83.23 89.61 7.53 75.85 87.07 7.34 80.06 81.99

7.68 66.02 71.85 7.70 64.60 69.87 7.94 54.34 69.71 7.67 48.55 67.19

8.26 40.78 39.62 8.26 39.44 44.26 8.42 38.49 45.92 8.06 27.46 45.91

8.97 13.92 14.06 8.94 13.66 16.02 9.18 15.09 15.83 8.69 4.62 17.28

9.43 7.12 6.26 9.37 5.28 6.57 9.52 4.15 7.54 9.11 0.00 10.33

107

C3. Cr(VI) Adsorption and kinetics on mixed solids at 0% pC02 (150 m2/g surface area)

This Study: 0% pC02 ; 6 Solid Mixture Equal Surface Area 150 m2/g;Cr04 10-5 M; NaN03 0.1 M

24hr

PH

24hr

%ads

24 hr

ICP

%ads

48hr

PH

48hr

%ads

48 hr

ICP

%ads

1 wk

pH

1 wk

%ads

lwk

ICP

%ads

2 wk

PH

2 wk

%ads

2wk

ICP

%ads

4.20 98.40 100 4.33 100 100 4.43 100 100 4.81 99.36 99.80

4.70 99.04 100 4.79 98.75 100 5.14 100 100 5.33 100 99.84

5.03 98.40 100 5.11 98.44 99.99 5.82 100 95.95 5.58 96.14 99.77

5.35 98.08 100 5.35 98.44 99.82 6.13 100 98.14 5.74 95.18 99.48

5.71 97.44 99.54 5.73 97.82 99.27 6.44 100 99.27 6.07 95.18 99.00

5.96 96.47 98.93 5.94 97.82 98.76 8.18 32.88 34.91 6.30 91.96 98.31

6.29 95.83 97.44 6.30 95.33 96.89 8.76 21.69 19.47 6.57 91.64 96.33

6.53 92.95 95.77 6.46 93.15 95.00 9.43 13.56 8.93 6.85 88.10 92.45

6.85 87.50 90.21 6.84 85.67 87.95 7.11 79.42 84.15

7.25 73.72 77.06 7.16 74.45 77.00 7.42 70.42 75.58

7.69 50.64 51.06 7.58 53.89 54.28 7.72 54.66 55.30

8.30 25.64 25.80 8.20 28.35 31.86 8.23 30.55 34.60

8.98 14.74 15.61 8.90 19.00 17.86 8.88 15.43 21.82

9.70 11.86 9.63 9.56 10.59_____

10.05 9.58 8.36 8.04

108

C4. Cr(VI) Adsorption on mixed solids at 0% pC02 (60 m2/g each surface area)

This Study: 24 hr, 0% pC02 ; Cr04 10-5 M; NaN03 0.1 M, Goethite

2.0g/L, HFO 0.222g/L, HMO 0,09g/L, g-alumina 0.25S g/L, kaolinite4.412 g/L, montmorillonite 1.875 g/L

0 %pC02 Atmospheric pC02 5 % pC02

pH Adsorbed pH

%Cr

Adsorbed pH

%Cr

Adsorbed

4.38 100.00 3.99 100.00 3.94 99.69

4.53 100.00 4.33 99.68 4.06 99.69

5.21 97.38 4.53 100.00 4.29 99.07

5.56 98.69 4.82 98.71 4.61 97.53

5.95 96.39 5.22 97.41 4.95 95.06

6.2 93.77 5.69 93.20 5.35 87.96

6.21 91.48 6.1 84.14 5.7 79.01

6.51 85.57 6.44 70.55 5.95 70,06

6.83 73.11 6.75 50.49 6.19 55.86

6.98 66.56 7.03 3S.19 6.3 50.00

7.21 52.13 7.37 26.S6 6.4 45.99

7.7 28.20 7.86 15.86 6.49 41.05

8.34 10.16 8.57 9.71 6.7 32.72

8.86 4.92 9.11 9.39 7.13 19.14

10.08 0 9.68 7.77 8.66 5.8641975

10.35 7.44 9.63 6.4814815

109

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Surface Complexation Modeling of CR(VI) Absorption on