Heavy metal adsorption on iron oxide and iron oxide-coated silica ...

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CHAPTER 11CONCLUSIONS AND FUTURE WORKSorption of trace elements onto iron oxide is an important process regulating contaminantmobility and bioavailability in natural systems. The purpose of this research is tounderstand the effect of iron oxides, as discrete particles and coatings, on contaminant<strong>adsorption</strong> and develop surface complexation models to predict heavy <strong>metal</strong> <strong>adsorption</strong>and competition.A model system for soils and sediments, iron oxide-coated silica, was synthesizedusing <strong>adsorption</strong> and precipitation methods. The degree of coating was highly sensitiveto the particle size of silica and ranged between 0.59 and 21.36 mg Fe g -1 solid. Theinteraction between goethite and silica potentially involves chemical forces. Iron oxidecoatings on the silica surface are non-uniform increasing surface area and introducingsmall pores. The pHpzc of coated silica is between that of pure silica and iron oxidesuggesting both silica and goethite surfaces are important for <strong>adsorption</strong>. Ni <strong>adsorption</strong>on the coated silica is greater than that for pure silica and the sub-micrometer goethitecoatings exhibit a larger capacity than discrete goethite impacting interactions at theaqueous interface.EXAFS analysis of Pb(II)-sorbed to freshly precipitated HFO suggest that Pb(II)ions form mononuclear bidentate edge-sharing surface complexes on FeO6 octahedra.This mechanism appears to be invariant of pH, ionic strength, Pb loading, and reactiontime. The internal sites on micropore walls appear to be no different than the externalones. Intraparticle surface diffusion modeling described the two-step Pb/HFO sorptionprocess reasonably well. The rate-limiting diffusion process may occur for decades156
157accounting for approximately 46% of total Pb uptake. XAS also revealed that the freshlyprecipitated HFO exhibits short-range order possibly forming edge and face-sharingpolymers. By binding to the edges of octahedra polymers, Pb(II) ions inhibitcrystallization of HFO which is stable up to 21 months of aging, thus maintaining itslarge capacity for heavy <strong>metal</strong>s.Similarly, Ni(II) forms mononuclear bidentate edge-sharing surface complexesupon <strong>adsorption</strong> to HFO. This mechanism was observed as a function of ionic strength2.8x10-3 to 10-1 , pH 6 to 7, loading 8 x10 -4 to 8.1x10-3 mole Ni g-1 HFO, and reaction time4 hours to 8 months. The presence of Ni(II) potentially inhibited HFO transformationinto a more crystalline iron oxide; therefore the large sorption capacity of HFO wasmaintained. Metastable a-Ni(OH)2 appears to have formed when coprecipitated withHFO. Ni2+ was not found to substitute for Fe 3+ up to 8 months at room temperaturepossibly because HFO did not undergo phase transformation into a more crystalline form.Constrained with spectroscopic information, the 2-pK TLM can successfullypredict Ni or Zn <strong>adsorption</strong> in single adsorbate systems. The curvature in <strong>adsorption</strong>isotherms was accurately described using two types of sites — high affinity and lowaffinity ones. A unique set of parameters was found for each <strong>metal</strong> ion that cansuccessfully describe <strong>adsorption</strong> over a large range of experimental conditions, coveringsix to seven orders of magnitude in concentration, ionic strength from 10 -3 to 10-2, and anenvironmentally relevant pH range. Competition was also predicted quantitatively withparameters calibrated using single adsorbate data.GACS has a higher affinity for Ni and Zn than silica due to the presence ofgoethite. However, the electrostatic effect is controlled by silica. Therefore, both
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Copyright Warning & RestrictionsThe
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ABSTRACTHEAVY METAL ADSORPTION ON I
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HEAVY METAL ADSORPTION ON IRON OXID
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APPROVAL PAGEHEAVY METAL ADSORPTION
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Fan, M.; Boonfueng, T.; Xu, Y.; Axe
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ACKNOWLEDGMENTI would like to expre
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TABLE OF CONTENTS(Continued)Chapter
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TABLE OF CONTENTS(Continued)Chapter
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LIST OF TABLES(Continued)TablePageB
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LIST OF FIGURES(Continued)FigurePag
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LIST OF FIGURES(Continued)FigurePag
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CHAPTER 1INTRODUCTIONHeavy<
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3and Sigg, 1992; Gunneriusson et al
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CHAPTER 2OXIDES AND THEIR EFFECT ON
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7et al. 1996; Green-Pedersen et al.
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9identify components, distribution,
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11oxides. Because extraction method
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13Table 2.2 Some Coating Work and C
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15increasing the sorbate concentrat
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17mole Cu g-1 goethite (-1-7% of th
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19Overall, macroscopic adso
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21hematite (up to 9.9 limo' Pb ril-
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23concentration in their system was
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25Surface complexation modeling (SC
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27Overall, surface complexation mod
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29Spectroscopic techniques, especia
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31• Understand the effect of comp
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33existing in almost all soils and
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CHAPTER 4EXPERIMENTAL METHODSIn thi
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37included coating temperature (T),
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39generated at 45 kV and 40 mA, sca
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41PZC include electrophoretic mobil
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43Lin-edge, from 8,133 to 8,884 eV
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CHAPTER 5CHARACTERIZATION OF IRON O
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47measured by potentiometric titrat
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Table 5.2 XRD Data of Alfa Aesar (A
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51Table 5.3 XRF Results for Alfa Ae
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Figure 5.4 Particle size distributi
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55Figure 5.5 ESEM micrograph for Al
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585.2.5 Surface Charge Distribution
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Figure 5.10 Potentiometric titratio
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CHAPTER 6SYNTHESIS AND CHARACTERIZA
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Figure 6.1 Comparison between Fecon
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Figure 6.2 XRD patterns for goethit
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68iron oxide coatings at 60 °C, go
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70substrate diameter of 0.2 mm, an
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72goethite and silica results in on
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Figure 6.7 ESEM micrograph (9000x)
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76Furthermore, because discrete goe
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78Table 6.2 Surface Area and Porosi
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Figure 6.10 Surface charge distribu
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Figure 6.11 Ni adsorption</
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84that for pure silica, and the goe
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Table 7.1 Sample Preparation Condit
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Figure 7.2 EXAFS spectra of Pb/HFO
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Figure 7.3 Fourier transform and fi
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Figure 7.4 Fourier transform and fi
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Table 7.3 EXAFS Results of Pb/HFO S
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967.2 Fe XAS of Iron Oxides and Pb/
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Figure 7.7 EXAFS spectra of iron ox
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100and Venturelli, 1999). Overall t
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102(hematite, goethite, akaganeite,
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104adsorption proc
Page 128 and 129: 106of pH, ionic strength, Pb loadin
Page 130 and 131: Figure 8.1 Ni adsorption</s
Page 132 and 133: 110high metal load
Page 134 and 135: Figure 8.3 x(k)•k3 spectra and Fo
Page 136 and 137: Figure 8.4 (k)•k3 spectra of Ni-H
Page 138 and 139: Figure 8.5 Fourier transforms (magn
Page 140 and 141: 118be fit using either Fe or Ni ato
Page 142 and 143: 1208.3 EXAFS Analysis of FeThe x(k)
Page 144 and 145: Table 8.4 EXAFS Results of HFO and
Page 146 and 147: CHAPTER 9SURFACE COMPLEXATION MODEL
Page 148 and 149: Figure 9.1 Potentiometric titration
Page 150 and 151: 128Table 9.1 Surface Reactions and
Page 154 and 155: 132Table 9.2 Model Parameters for N
Page 158 and 159: 1369.3 Zn Adsorption on GoethiteReg
Page 160 and 161: Figure 9.6 Zn adsorption</s
Page 162 and 163: 140data over a broad range of condi
Page 164 and 165: 142Figure 9.8 Ni adsorption
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Page 168 and 169: Figure 10.1 Ni adsorption</
Page 170 and 171: Figure 10.2 Zn adsorption</
Page 172 and 173: Figure 10.4 Ni adsorption</
Page 174 and 175: 152one for GACS (2.77x10 -6 mole si
Page 176 and 177: Figure 10.6 Zn adsorption</
Page 180 and 181: 158surfaces are needed for successf
Page 182 and 183: Figure A.2 Ni speciation in 1x10 -5
Page 184 and 185: Figure A.4 Pb speciation in 5x10 -8
Page 186 and 187: Figure A.6 Zn speciation in 1x10 -5
Page 188 and 189: 166Table B.2 Potentiometric Titrati
Page 190 and 191: 168Table BA Potentiometric Titratio
Page 192 and 193: 170Table C.3 Zn Adsorption Edges on
Page 194 and 195: APPENDIX DADSORPTION STUDIES ON SIL
Page 196 and 197: 174Table D.5 Zn Adsorption Isotherm
Page 198 and 199: 176Table E.3 Pb CBC Study on 0.3 g
Page 200 and 201: APPENDIX GINTRAPARTICLE DIFFUSION M
Page 202 and 203: 180DofA = flier ^ 2fA = Exp(-fD * f
Page 204 and 205: REFERENCES1. Adamson A. W. (1982) P
Page 206 and 207: 18426. Blake R. L., Hessevick R. E.
Page 208 and 209: 18650. Combes J. M., Manceau A. and
Page 210 and 211: 18876. Elzinga E. J. and Sparks D.
Page 212 and 213: 190102. Gupta V. K. (1998) Equilibr
Page 214 and 215: 192126. Joshi A. and Chaudhuri M. (
Page 216 and 217: 194150. Liu C. and Huang P. M. (200
Page 218 and 219: 196175. Muller B. and Sigg L. (1992
Page 220 and 221: 200. Perry R. H. and Green D. W. (1
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304. Zhong Z. Y., Prozorov T., Feln
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