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Western Michigan University Western Michigan University
ScholarWorks at WMU ScholarWorks at WMU
Master's Theses Graduate College
8-2013
Surface Complexation Modeling of CR(VI) Absorption on Mineral Surface Complexation Modeling of CR(VI) Absorption on Mineral
Assemblages Assemblages
Ann M. Gilchrist
Follow this and additional works at: https://scholarworks.wmich.edu/masters_theses
Part of the Environmental Health and Protection Commons, and the Geochemistry Commons
Recommended Citation Recommended Citation Gilchrist, Ann M., "Surface Complexation Modeling of CR(VI) Absorption on Mineral Assemblages" (2013). Master's Theses. 179. https://scholarworks.wmich.edu/masters_theses/179
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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
LIST OF FIGURES - CONTINUED
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
LIST OF FIGURES - CONTINUED
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
LIST OF FIGURES - CONTINUED
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
LIST OF FIGURES - CONTINUED
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
LIST OF FIGURES - CONTINUED
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
LIST OF FIGURES - CONTINUED
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
LIST OF FIGURES - CONTINUED
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
NaN03 Concentration (M) 0.1
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
Solid Concentration (g/L) 20
XOH Site Density (sites/nm2) 1.0848
YOH Site Density (sites/nm2) 0.6102
Cr(VI) Concentration (M) 10"5
NaN03 Concentration (M) 0.01
Surface Complexation Reactions
Log StabilityConstant (K)
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
Solid Concentration (g/L) 30
Variable Charge Site (SOH) Density (sites/nm2) 4.8184b
Fixed Charge Site (XH) Density (sites/nm2) 0.099c
Cr(VI) Concentration (M) io-5
NaN03 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
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.
Surface Complexation Reactions
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
Solid Concentration (g/L) 7
Variable Charge Site (SOH) Density (sites/nm2) 10b
Fixed Charge Site (XH) Density (sites/nm2) l.llc
Cr(VI) Concentration (M) io-5
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
Surface Complexation Reactions
Log StabilityConstant (K)
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
Log StabilityConstant (K)
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
Surface Area (m2/g) 233 35
Solid Concentration (g/L) 0.26 2-y
Site Density (sites/nm ) site 1 1.49 2
Cr(VI) Concentration (M) 10"5
NaN03 Concentration (M) 0.01
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
Surface Area (m2/g) 300 35
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 _
Cr(VI) Concentration (M) 10"5
NaN03 Concentration (M) 0.01
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
♦ ExperimentalData
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
Solid Concentration (g/L) 4.15 2
Variable Charge Site (SOH) Density (sites/nm2) 4.8184D 2
Fixed Charge Site (XH) Density (sites/nm2) 0.6386c -
Cr(VI) Concentration (M) 10"'
NaN03 Concentration (M) 0.01
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
♦ ExperimentalData
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
Solid Concentration (g/L) 1.89 2
Variable Charge Site (SOH) Density (sites/nmz) 10b 2
Fixed Charge Site (XH) Density (sites/nmz) 1.11° -
Cr(VI) Concentration (M) io-'
NaN03 Concentration (M) 0.01
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
Solid Concentration (g/L) 0.093 2
XOH Site Density (sites/nm2) 1.0848 2
YOH Site Density (sites/nm2) 0.6102 -
Cr(VI) Concentration (M) io-5
NaN03 Concentration (M) 0.01
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
♦ ExperimentalData
— • ~>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
♦ ExperimentalData
-- >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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