CHROMATE REDUCTION BY HYDROXYCARBONATE GREEN

A.G.B. Williams and M.M. Scherer Department of Civil and Environmental Engineering, University of Iowa, 4016 Seamans Center, Iowa City, IA 52245; Phone: (319) 335-5654; Fax: (319) 335-5660. ABSTRACT To address the potential for abiotic natural attenuation of chromate, we have studied the kinetics and mechanism of chromate removal in the presence of hydroxycarbonate green rust. The kinetics of chromate removal were studied in batch systems to evaluate the effect of green rust surface area and pH. The rate of chromate removal was dependent on the green rust surface area concentration, and a surface area normalized -2 -1 rate constant (kSA = 0.044 L m min ) was determined. The rate of chromate removal increased with decreased pH. The effect of pH, however, was modest, with only a five-fold increase in rate observed over five pH values (from pH 9.0 to 5.0). Of the three potential pathways hypothesized for chromate removal (reduction, adsorption, or interlayer exchange), reduction was found to be the dominant pathway controlling chromate loss from solution. Negligible adsorption or interlayer exchange of chromate was found to occur independently of

reduction. The rapid reduction of chromate appears to result in precipitation of Cr(III) phases, such as Cr(OH)3 or [Fe,Cr](OH)3. Key words: mineral/water interface, reduced Fe minerals, natural attenuation INTRODUCTION for monovalent interlayer anions (Refait et al., Reduction by reduced minerals has 1997). Green rust compounds are divided into recently received a great deal of attention as a two types based on the interlayer anion. Type I potential pathway for the degradation of oxi- contains monotonic ions (e.g., Cl- and Br-) and -2 dized environmental contaminants, such as planar molecules (e.g., CO3 ). Type II contains -2 chlorinated solvents, nitroaromatics, and heavy three-dimensional molecules (e.g., SO4 ). The metals. The ubiquitous presence of iron in the presence of the interlayer anions is necessary to environment as both dissolved species and maintain electroneutrality in the green rust mineral-bearing phases (e.g. magnetite, iron structure. For hydroxycarbonate green silicates, and green ) suggests that reduction rust, the proposed chemical formula is II III -2 by iron may be a significant abiotic pathway in [Fe 4Fe 2(OH)12]·[CO3 ·2H2O] the natural attenuation of environmental con- (Hansen, 1989). taminants. Of these reduced iron minerals, green Although there is little data available on the rust compounds are some of the strongest reactivity of carbonate green rust, both organic reductants, presumably due to their high Fe(II) and inorganic contaminants have been shown to content. Green rust compounds are composed react with green rust. Chlorinated sol-

of positively charged Fe(II)-Fe(III) octahedral vents, such as carbon tetrachloride (CCl4) and

sheets and interlayers containing water and chloroform (CHCl3), were reduced by sulfate -2 -2 - anions such as CO3 , SO4 , and Cl . An green rust in batch reactors containing saturated

Fe(II)-Fe(III) ratio of approximately 2:1 has concentrations of CCl4 (Erbs et al., 1999). The

been found for divalent interlayer anions and 3:1 two major end products were CHCl3 and

272 Proceedings of the 2000 Conference on Hazardous Waste Research hexachloroethane (C2Cl6), indicating that 1980; Abdelmoula et al., 1988; Koch and complete dechlorination did not occur. Sulfate Morup, 1991; Genin et al., 1998) and field- green rust has also been shown to reduce scale permeable reactive barriers (PRBs) inorganic compounds, such as nitrate (Hansen et containing iron metal (Fe0) (Vogan et al., 1998). al., 1996; Hansen and Koch, 1998), selenium To evaluate the role of green rust compounds in (Myneni et al., 1997), and chromate (Loyaux- these systems, we have studied the transforma- - -2 Lawniczak et al., 2000). Sulfate green rust was tion of chromate (HCrO4 and CrO4 ) in the found to reduce chromate to Cr(III), which presence of hydroxycarbonate green rust. The precipitated as a poorly crystalline Cr(III)- objective of this work is to develop an under- Fe(III) oxyhydroxide. Unlike chromate reduc- standing of both the pathway and kinetics of tion, which resulted in a completely oxidized chromate removal by hydroxycarbonate green , the end product of sulfate green rust rust. The ability to assess the significance of this and CCl4, selenate, and nitrate was magnetite (a transformation pathway relative to other path- partially oxidized iron oxide). Nitrate was ways, such as biotic transformation, is limited by completely reduced to ammonium at rates (i) a lack of kinetic data for the transformation similar in magnitude to those observed for biotic contaminants, and (ii) a lack of understanding of degradation (Hansen and Koch, 1998). The the mechanisms controlling the reactivity of expected stoichiometry of eight Fe(II) con- green rusts. The kinetic data derived from this sumed for every nitrate reduced was observed work provides the first set of rate constants for (Hansen and Koch, 1998). Selenium was the transformation of chromate by reduced from Se(VI) to Se(O), and similar to hydroxycarbonate green rust. The insight nitrate, the degradation rates were comparable derived will provide an improved basis for to those found in sediments where biological predicting the fate of chromate in anoxic soils, activity was presumed to mediate the degrada- sediments, and aquifers and improving the tion of selenium (Myneni et al., 1997). Despite design and implementation of remediation these findings, the relative importance of abiotic technologies based on Fe0. reactions mediated by green rusts compared to EXPERIMENTAL biotic reactions in natural systems is still unre- Hydroxycarbonate green rust was synthe- solved (Oremland et al., 1998). sized in an anoxic chamber with a nitrogen and The most likely environment for green rust hydrogen atmosphere (95%-5%). All solutions compounds to form is where Fe(II) and Fe(III) were prepared in nitrogen- purged 18 MW-cm can coexist. Examples of environments that deionized water and were magnetically stirred at may support these conditions include all times. Separate solutions of anhydrous FeCl reductomorphic soils and anoxic groundwater 2 (125 mM) and FeCl ·7H O (25 mM) were systems. Previous studies have identified green 3 2 prepared in the anoxic chamber. Both solutions rust compounds in both natural systems (Taylor,

Proceedings of the 2000 Conference on Hazardous Waste Research 273 were maintained at a pH of 7.0 with a Metrohm 736 GP Titrino by automatic addition of 1 M

Na2CO3. The iron salt solutions were prepared to give a ferrous to ferric iron ratio of 5:1 (Hansen, 1989). The ferric iron solution was added to the ferrous iron solution and main-

tained at pH 8.0 by titration with 1 M Na2CO3. The solution was magnetically stirred for ap- proximately two hours until a blue-green pre- cipitate formed and consumption of the 1 M Na CO ceased. The blue-green precipitate 2 3 Figure 1. Comparison of XRD patterns for was vacuum filtered under anoxic conditions freshly precipitated and freeze-dried and rinsed with nitrogen-purged deionized hydroxycarbonate green rust. XRD scans were water. The amassed precipitate was freeze dried made between 5º and 80º with a step of 0.02º per second. with a custom vacuum valve to ensure no exposure to the atmosphere during the freeze- Quantachrome cell-seal assembly (to prevent drying process. The freeze-dried the oxidation of green rust) on a Quantachrome hydroxycarbonate green rust was sieved (0.15 Nova analyzer. BET analysis of a freeze-dried mm) to obtain a uniform particle size. hydroxycarbonate green rust sample yields a To ensure purity and crystallinity of the surface area of 33.3 m2g-1. synthesized hydroxycarbonate green rust, x-ray Batch experiments were conducted in an

diffraction (XRD) analysis was performed on a anoxic chamber, with an N2/H2 atmosphere to Siemens Diffractometer utilizing CuKα radia- prevent the rapid oxidation of green rust. Batch tion. Samples for XRD analysis were prepared reactors were performed by addition of specific by admixing with glycerol to prevent oxidation amounts of freeze-dried hydroxycarbonate during measurement (Hansen, 1989). The blue- green rust to nitrogen- purged deionized water, green color of hydroxycarbonate green rust was adjusted to desired pH with 0.2 M HCl or 0.1 maintained for the duration of the XRD analysis M NaOH, and spiked with potassium chromate

and no oxidation was observed. In addition, an (K2CrO4) to achieve a specific initial concentra- XRD pattern of freeze-dried hydroxycarbonate tion of Cr(VI). To avoid possible buffer interac- green rust was compared to freshly precipitated tions, all batch experiments were performed hydroxycarbonate green rust to ensure that the individually with pH control achieved by addi- freeze-drying process had no effect on the tion of 0.2 M HCl with a Metrohm 736 GP crystal structure (Figure 1). Surface area was Titrino. Samples for chromium analysis were analyzed anoxically by nitrogen Brunauer- withdrawn from the reactor and filtered through Emmett-Teller (BET) analysis with a a Gelman 0.22 mm PTFE filter for dissolved

274 Proceedings of the 2000 Conference on Hazardous Waste Research Cr(VI) measurements. For total chromium

measurements (Cr(TOT)soln,GR), the sample was not filtered. Total chromium, Cr(TOT), is the sum of both Cr(III) and Cr(VI). Hexavalent chromium was measured colorimetrically by the diphenylcarbazide method at 540 nm by either a Dionex HPLC, Dionex method TN24, or with a Spectronic Genesys 5 spectrophotometer according to the method outlined in Standard Methods (AWWA, 1995). Total chromium was Figure 2. Effect of aging on the rate of chro- measured by oxidation of Cr(III) by potassium mate reduction by two separate batches of permanganate and measured colorimetrically by freeze-dried hydroxycarbonate green rust. All diphenylcarbazide as outlined by Standard reactors contained 0.25 g L-1 (8.3 m2 L-1) of Methods (AWWA, 1995). Dissolved iron was green rust and an initial concentration of 10 mg L-1 as chromate. measured colorimetrically with the 1,10- Phenathroline method (AWWA, 1995). green rust is shown in Figure 3. Chromate disappears rapidly in the presence of RESULTS AND DISCUSSION hydroxycarbonate green rust with half-lives (t ) A preliminary set of experiments was 1/2 of less than four minutes. The rate of removal is performed with the freeze-dried directly related to the concentration of green hydroxycarbonate green rust to determine if (i) rust in the reactor, with about a four-fold faster the green rust synthesis was reproducible and removal rate observed for a two-fold increase in (ii) the reactivity of the freeze-dried green rust green rust concentration. The rate of chromate was stable over the time period needed to removal is significantly faster than removal rates conduct experiments. Figure 2 shows that the

observed for carbon tetrachloride (t1/2 > 10 first-order reaction rate constant (kobs) for hours) (Erbs et al., 1999), nitrate (t = 3.9 ± chromate reduction by freeze-dried 1/2 hydroxycarbonate green rust was reasonably 0.3 hours) (Hansen et al., 1996), and selenate (t = 60.3 ± 16.2 hours) (Myneni et al., 1997) reproducible between separate batches of 1/2 in the presence of sulfate green rust. A direct synthesized green rust (average kobs: batch A = 0.24 min-1, batch B = 0.21 min-1). In addition, comparison among these rates, however, is not possible because the specific surface area of kobs remained relatively consistent over a two- week period with a relative standard deviation green rust used in these studies is not known. A

plot of kobs versus green rust surface area of less than 40 percent (kobs = 0.25 ± 0.1) concentration reveals a linear relationship among 27 experiments. (Figure 3 inset). A surface area normalized rate The removal of chromate in the presence constant (k ) of 0.044 L m-2 min-1 for the of different concentrations of hyroxycarbonate SA

Proceedings of the 2000 Conference on Hazardous Waste Research 275 accumulation of precipitates on the green rust surface. Surface passivation occurs due to the oxidized precipitate physically covering other- wise available Fe(II) sites on the green rust surface. Surface passivation has previously been shown to limit heterogeneous reactions involving the reduction of chromate by magnetite surfaces (Peterson et al., 1997). Based on the properties of chromate (i.e., oxyanion and strong oxidant), we hypothesize Figure 3. Effect of hydroxycarbonate green rust three potential pathways for chromate removal surface area concentration on the rate of chro- in the presence of green rust: (i) reduction of mate removal. All experiments were performed chromate to Cr(III), (ii) adsorption of chromate in triplicate. Batch reactors were maintained at pH 7.0 with a pH-STAT by addition of hydro- to the green rust surface, and (iii) interlayer chloric acid (0.2 M). Initial chromate concentra- exchange of chromate with the interlayer anion -1 tion was 10 mg L as chromate. Solid lines (CO -2) in hydroxycarbonate green rust. Previ- represent a first-order kinetic model. Inset: 3 ous studies have identified the nature of the end Effect of hydroxycarbonate green rust surface area concentration on first-order rate constants product to be a solid-solution precipitate of for chromate disappearance. Points represent reduced chromium and oxidized iron, resulting in the average observed rate constant, and error a solid-solution precipitate of [Fe,Cr](OH) bars represent one standard deviation based on 3 (Loyaux-Lawniczak et al., 2000). A mass triplicate experiments. balance on chromate, however, was not deter- reduction of chromate by hydroxycarbonate mined, making it difficult to determine if all of the green rust was determined from the slope of chromate was reduced. To quantify the loss of this line. chromate to each pathway, a series of four At higher green rust surface area concen- measurements were made at each sampling time trations (8.3 m2 L-1 and 16.7 m2 L-1), there is a to determine the phase (i.e., aqueous or solid) strong adherence to first-order kinetics with and oxidation state (i.e., III or VI) of chromium. respect to the chromate concentration. At a The four measurements include (i) chromate in a

lower green rust surface area concentration (4.2 filtered sample (Cr(VI)soln), (ii) chromate in an m2 L-1), however, a shift from first-order kinetics unfiltered sample that is dissolved with sulfuric

to zero-order kinetics is observed. This phe- acid (Cr(VI)soln,GR), (iii) total chromium in a

nomena is typical of heterogeneous systems and filtered sample (Cr(TOT)soln), and (iv) total may be due to surface-site saturation (Zepp and chromium in an unfiltered sample dissolved in

Wolfe, 1987) of the hydroxycarbonate green sulfuric acid (Cr(TOT)soln,GR). The filtered rust with chromate or surface passivation by samples provide information on aqueous con-

276 Proceedings of the 2000 Conference on Hazardous Waste Research hydroxycarbonate green rust. It is important to note that the processes of adsorption or interlayer exchange may be occurring; however, as the chromate adsorbs to the surface or exchanges with the interlayer carbonate ion, it is quickly reduced. The similarity between

Cr(VI)soln and Cr(TOT)soln measurements shown in Figure 4 suggests that the reduced chromium is not remaining in solution. If Cr(III)

remained in solution, Cr(TOT)soln would be Figure 4. Disappearance of Cr(VI) and total greater than Cr(VI) . The loss of Cr(III) from chromium from solution and hydroxycarbonate soln green rust with time. Reactor contained 0.25 g solution is most likely due to the low solubility of -1 L green rust and an initial concentration of 10 Cr(OH)3 and [Fe,Cr](OH)3 at near-neutral pH mg L-1 as chromate. values. [Fe,Cr](OH)3 has been shown to limit centrations of chromium, whereas the unfiltered the solubility of Cr(III) in systems with ferric samples include both aqueous chromium and (Sass and Rai, 1986), removing adsorbed, precipitated, or exchanged chro- chromium to levels below the U.S. EPA Maxi- mium. From the difference between the filtered mum Contaminant Level of 100 parts per billion and unfiltered samples, the amount of chromium (U.S. EPA, 2000). bound to the surface (i.e., adsorbed, precipi- The solubility of [Fe,Cr](OH)3, how- tated, or exchanged) can be calculated. ever, is dependent on solution pH, suggesting Based on a series of these measurements that removal of chromate in the presence of taken over time, it is clear that chromate is not hydroxycarbonte green rust may be dependent conserved in the system (Figure 4). The mea- on the solution pH. The reaction of chromate sured quantities of chromium, as Cr(VI)soln and and hydroxycarbonate green rust was studied at

Cr(VI)soln,GR, are almost identical, indicating that five different pH values ranging from 5.0 to 9.0 chromate is being transformed and that adsorp- (Figure 5). The rate of chromate reduction tion or interlayer exchange are not significant increases as pH decreases, with about a five- removal pathways on their own. If chromate fold increase in kobs as the pH is decreased from -1 was conserved in the system by either adsorp- 9.0 to 5.0 (kobs range = 0.1 to 0.55 min ). The tion or interlayer exchange, the measurement for increase in the kobs at lower pH values may be

Cr(VI)soln,GR would be greater than Cr(VI)soln. due to an increase in affinity of the negatively The total chromium in the unfiltered sample charged chromate oxyanion for the

(Cr(TOT)soln,GR) remains constant throughout hydroxycarbonate green rust surface. Typically the experiment showing that chromium mass is at lower pH values, oxide surfaces become conserved and that chromate is reduced by protonated and the surface charge becomes

Proceedings of the 2000 Conference on Hazardous Waste Research 277 Figure 5. Effect of pH on the reduction of Figure 6. Disappearance of Cr(VI) and Fe(II) chromate by hydroxycarbonate green rust. All in the presence of hydroxycarbonate green rust. reactors contained 0.25 g L-1 hydroxycarbonate Reactors contained 10 mg L-1 chromate and green rust and an initial chromate concentration of 3.33 m2 L-1 hydroxycarbonate green rust and 10 mg L-1. Reactors were maintained at specified were maintained at pH 7.0 by automatic addi- pH value by automatic addition of 0.2 M HCl. tion of 0.2 M HCl.

positive, increasing the electrostatic attraction chromate reduced should be 3:1. The overall between the hydroxycarbonate green rust and stoichiometic ratio of Fe(II) oxidized to chro- the chromate oxyanion (Schindler, 1990). mate reduced in Figure 6 was found to be The effect of pH is based on the premise approximately 4:1 (Figure 7). The average ratio that the reductant in the batch reactor is Fe(II) of Fe(II):Cr(VI) for Trial A was 4.3 ± 1.54 and associated with the hydroxycarbonate green for Trial B, 4.0 ± 0.97. These ratios are not rust. Figure 6 shows that Fe(II) associated with consistent with the expected result of 3:1, based the hydroxycarbonate green rust was lost at a on the requirement of three Fe(II) atoms oxi- rate proportional to chromate loss. The mea- dized to Fe(III) for every chromate reduced to

surements for total Fe(II) (Fe(II)soln,GR) in the Cr(III). A potential explanation for the exces- reactor sample decreased as chromate was sive Fe(II) loss is the possible presence of other

removed. Fe(II) in solution (Fe(II)soln) was oxidants (e.g., aqueous O2 and CO2). It has found to be below measurable detection limits been demonstrated that iron metal can reduce

and is not a significant factor contributing to the CO2 to small chain hydrocarbons (Hardy and reduction of chromate. This indicates that Fe(II) Gillham, 1995), suggesting that it may be -2 associated with the green rust structure is the possible that the CO3 interlayer ion could be active reductant. Structural Fe(II) within the reduced by the Fe(II) associated with green rust has previously been shown to be the hydroxycarbonate green rust. A consistent ratio reductant of nitrate (Hansen and Koch, 1998) of approximately 4:1 is observed for both trials and carbon tetrachloride (Erbs et al., 1999). in Figure 7 for the first 20 minutes of the reac- Theoretically the change in total Fe(II) to tion; however, at times beyond 20 minutes, the

278 Proceedings of the 2000 Conference on Hazardous Waste Research of chromate and subsequent precipitation of

Cr(III) phases, such as Cr(OH)3 or

[Fe,Cr](OH)3, suggests that abiotic reduction of chromate may be an important transforma- tion process in natural systems and engi- neered remediation technologies based on iron metal (Fe0).

ACKNOWLEDGEMENTS We would like to acknowledge The Figure 7. Stoichiometric ratio of Fe(II) removal Petroleum Research Fund, administered by the to Cr(VI) reduced over time. Trial A and Trial B ACS, and the U.S. Environmental Protection were performed from separate synthesized Agency for partial support of this research. batches of hydroxycarbonate green rust. Batch Although this article has been funded in part by reactors contained 0.1 g L-1 carbonate green rust and an initial chromate concentration of 10 the U.S. Environmental Protection Agency mg L-1. under assistance agreements R-819653, R- system behaved erratically. The unusual behavior 825549, and R-825550 through the Great at later data points (>20 min) may be due to a Plains/Rocky Mountain Hazardous Substance decrease in the rate of chromate reduction while Research Center, headquartered at Kansas the Fe(II) continues to oxidize at variable rates, State University, it has not been subjected to the possibly by one of the aforementioned reactions. agency’s peer and administrative review and therefore may not necessarily reflect the views CONCLUSION of the agency, and no official endorsement The results of this study show that chro- should be inferred. mate is rapidly reduced by hydroxycarbonate green rust, and that there is negligible adsorption REFERENCES to the green rust surface or interlayer exchange Abdelmoula, M., F. Trolard,G. Bourrie, and J. M. R. Genin. 1988. Evidence for occurring independently of reduction. At low the Fe(II)-Fe(III) green rust surface area concentrations, the rate of chro- “” mineral occurrence in a mate reduction by hydroxycarbonate green rust hydromorphic soil and its transforma- appears to be limited by availability of Fe(II) tion with depth. Hyperfine Interactions 112(1-4): 235-238. sites by either a finite number of Fe(II) sites or surface blockage by [Fe,Cr](OH) precipitating Erbs, M., H. C. B. Hansen, and C. E. Olsen. 3 1999. Reductive dechlorination of on the hydroxycarbonate green rust. The carbon tetrachloride using iron(II) disappearance of structural Fe(II) in the carbon- iron(III) sulfate (green rust). ate green rust suggests that Fe(II) is responsible Environmental Science and Technology for the chromate reduction. The rapid reduction 33: 307-311.

Proceedings of the 2000 Conference on Hazardous Waste Research 279 Genin, J.-M. R., G. Bourrie, A. Herbillon, M. Oremland, R. S., J. Stolz, D. Lovley, S. C. B. Abdelmoula, A. Jaffrezic, P. Refait, V. Myneni, T. K. Tokunaga, and G. E. J. Maitre, B. Humbert, and A. Herbillon. Brown. 1998. “Green rust” in the lab 1998. Thermodynamic equilibria in and in the soil. Science 281: 1111a. aqueous suspensions of synthetic and Peterson, M. L., A. F. White, G. E. Brown, and natural Fe(ll)-Fe(lll) green rusts: occur- G. A. Parks. 1997. Surface passivation rences of the mineral in hydromorphic of magnetite by reaction with aqueous soils. Environmental Science & Tech- Cr(VI): XAFS and TEM results. nology 32(8): 1058. Environmental Science & Technology Hansen, H. C., and C. B. Koch. 1998. Reduc- 31(5): 1573-1576. tion of nitrite to ammonium by sulphate Refait, P., S. H. Drissi, J. Pytkiewicz, and J.-M. green rust: activation energy and reaction R. Genin. 1997. The anioic species mechanism. Clay Mineral 33: 87-707. competition in iron aqueous : Hansen, H. C., and C. B. Koch, O. K. role of various green rust compounds. Borggaard, and J. Sorensen. 1996. Corrosion Science 39(9): 1699-1710. Abiotic nitrate reduction to ammonium: Sass, B. M., and D. Rai. 1986. Solubility of key role of green rust. Environmental amorphous chromium(III)-iron(III) Science and Technology 30(6): 1996. hydroxide solid solutions. Inorganic Hansen, H., and C. B. Koch 1989. Composi- Chemistry 26: 2228-2232. tion, stabilization, and light absorption of Schindler, P. W., 1990. Co-adsorption of metal iron(II) iron(III) hydroxy-carbonate ions and organic ligands: formation of (‘green rust’). Clay Miner 24(4): 663-9. ternary surface complexes. Mineral- Hardy, L. I., and R. W. Gillham (1995). Forma- water interface geochemistry. M. tion of hydrocarbons from the reduction Hochella Jr. and A. F. White. Washing- of aqueous CO2 by zero-valent iron. ton, D.C., Mineralogical Society of 209th National Meeting, Anaheim, CA, America. 23: 281-307. American Chemical Society. Standard Methods for the Examination of Water Koch, C. B., and S. Morup. 1991. Identifica- and Wastewater. 1995. Washington tion of green rust in an ochre sludge. D.C., American Public Health Associa- Clay Miner. 26(4): 577-82. tion, American Water Works Associa- tion Water Environment Federation. Loyaux-Lawniczak, S., P. Refait, J.-J. Ehrhardt, P. Lecomte, and J.-M. R. Genin. 2000. Taylor, R. M., 1980. Formation and properties Trapping of Cr by formation of of Fe(II)Fe(III) hydroxy-carbonate and during the reduction of its possible significance in soil formation. chromate ions by Fe(II)-Fe(III) Clay Miner. 15: 369-382. hydroxysalt green rusts. Environmental U.S. Environmental Protection Agency. Office Science and Technology 34: 438-443. of Water. Summer 2000. Drinking Myneni, S. C. B., T. K. Tokunaga, and G. E. Water Standards and Health Adviso- Brown, Jr. 1997. Abiotic selenium ries. EPA 822-B-00-001. transformations in the presence Vogan, J. L., B. J. Butler, M. S. Odziemkowski, of Fe(II,III) oxides. Science 278: G. Friday, and R. W. Gillham. 1998. 1106-1109.

280 Proceedings of the 2000 Conference on Hazardous Waste Research Inorganic and biological evaluation of Zepp, R. G., and N. L. Wolfe. 1987. Abiotic cores from permeable iron-reactive transformation of organic chemicals at barriers. Designing and applying treat- the particle-water interface. Aquatic ment technologies: remediation of Surface Chemistry: Chemical Processes chlorinated and recalcitrant compounds. at the Particle-Water Interface. W. G. B. Wickramanayake and R. E. Stumm. New York, Wiley: 423-455. Hinchee. Columbus, Ohio, Batelle Memorial Institute: 163-8.

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