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A novel application of x-ray absorption spectroscopy has provided structural information for ions sorbed at oxide-water interfaces. As an example, in situ extended x-ray absorption fine structure (EXAFS) measurements of adsorbed selenate and selenite ions at ah alpha-FeOOH(goethite)-water interface have been performed; these measurements show that selenate forms a weakly bonded, outer-sphere complex and that selenite forms a strongly bonded, inner-sphere complex. The selenite ion is bonded directly to the goethite surface in a bidentate fashion with two iron atoms 3.38 angstroms from the selenium atom. Adsorbed selenate has no iron atom in the second coordination shell of selenium, which indicates retention of its hydration sphere upon sorption. This method provides direct structural information for adsorbed species at solid-liquid interfaces.

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The ability of surface complexation models (SCMs) to fit sets of titration data as a function of changes in model parameters was evaluated using FITEQL and acid-base titration data of a-FeOOH, a-AlzO3, and TiO2. Three SCMs were evaluated: the triple-layer model (TLM), the constant capacitance model (CCM), and the diffuse-layer model (DLM). For all models evaluated, increasing the model input value for the total number of surface sites caused a decrease in the best-fit Log K values of the surface protolysis constants. In the case of the CCM, the best-fit surface protolysis constants were relatively insensitive to changes in the value of the capacitance fitting parameter, G, particularly for values of C1 greater than 1.2 F/m 2. Similarly, the best-fit values of TLM surface electrolyte binding constants were less influenced by changes in the value of C~ when C~ was greater than 1.2 F/m 2. For a given C1 value, the best-fit TLM values of the electrolyte binding constants were sensitive to changes in ApK, up to ApKa values of 3. For ApKa values above 3, no changes in the best-fit electrolyte binding constants were observed. Effects of the quality and extent of titration data on the best-fit values for surface constants are discussed for each model. A method is suggested for choosing a unique set of parameter values for each of the models.

The transformation of trichloroethylene (TCE), tetrachloroethylene (PCE), and 1,1-dichloroethylene (1,1-DCE) by 10 g/L (0.5 m2/L) FeS in aqueous solution at pH 8.3 was studied in batch experiments. TCE and PCE were transformed by FeS with pseudo-first-order rate constants, corrected for partitioning to the sample headspace, of (1.49 ± 0.14) × 10-3 h-1 (TCE) and (5.7 ± 1.0) × 10-4 h-1 (PCE). A 17% decrease in the concentration of 1,1-DCE was observed over 120 days; however, no reaction products were detected. TCE and PCE transformation data were fit to a rate law assuming transformation of TCE via parallel reaction pathways to acetylene and cis-1,2-dichloroethylene (cis-DCE) and transformation of PCE via parallel reaction pathways to acetylene and TCE. Acetylene was the major reaction product for both TCE and PCE. Determination of rate constants for each reaction pathway indicated that TCE was transformed to acetylene 11.8 ± 1.1 times faster than to cis-DCE and that PCE was transformed to acetylene 8.2 ± 1.8 times faster than to TCE. Additional minor reaction products were vinyl chloride (VC) for TCE and cis-DCE for PCE. Detection of acetylene as the major product of both TCE and PCE transformation by FeS contrasts with the sequential hydrogenolysis products typically observed in the microbial transformation of these compounds, making acetylene a potential indicator of abiotic transformation of TCE and PCE by FeS in natural systems.

Transition metal sulfide minerals are under investigation as potentially important abiotic reductants for chlorinated organic pollutants in anaerobic environments. This paper describes parametric rate studies done to evaluate the influ ence of environmental variables such as pH and ionic and organic solution composition on the reductive dechlorination of hexachloroethane (HCA) by FeS (poorly crystal line mackinawite). Results indicate that the reaction takes place at the mineral surface and is strongly pH-dependent. The influence of pH was explained by an acid/base equilibrium between two FeS surface species with different reactivities. Tetrachloroethylene was the principal reaction product, with pentachloroethane (PCA) as a minor intermediate and trichloroethylene, cis-1,2-dichloroethylene, and acetylene as minor products. Detection of PCA and the insensitivity of the reaction to numerous inorganic and organic solution species is consistent with an outer-sphere HCA dechlorination pathway involving two successive one-electron transfers. 2,2‘-Bipyridine and 1,10-phenanthrolene significantly increased the rate of HCA dechlorination by FeS, which was explained by the participation of delocalized π* molecular orbitals in the electron-transfer reaction. Cysteine and methionine were found to slow, but not stop, the reaction rate, and this was attributed to adsorption of thiol and sulfide functional groups to FeS surface iron atoms, causing an energetic or steric barrier to electron transfer. Rapid dechlorination rates and the insensitivity of the dechlorination reaction to numerous ionic and organic species suggest that FeS-mediated reductive dechlorination may be an important transformation pathway in natural systems.

Batch experiments were performed to assess (i) the influence of pH, solution amendments, and mineral aging on the rates and products of trichloroethylene (TCE) transformation by iron sulfide (FeS) and (ii) the influence of pretreatment of iron metal with NaHS on TCE transformation rates. The relative rates of FeS-mediated transformation of TCE to different products were quantified by branching ratios. Both pseudo-first-order rate constants and branching ratios for TCE transformation by FeS were significantly influenced by pH, possibly due to a decrease in the reduction potential of reactive surface species with increasing pH. Neither Mn2+, expected to adsorb to FeS surface S atoms, nor 2,2'-bipyridine, expected to adsorb to surface Fe atoms, significantly influenced rate constants or branching ratios. FeS that had been aged at 76 degrees C for 3 days was completely unreactive with respect to TCE over 6.5 months, yet this aged FeS transformed hexachloroethane to tetrachloroethylene with a rate constant only slightly lower than that for nonaged FeS. This finding suggests that the oxidation state of iron sulfide minerals in the environment will strongly influence the potential for intrinsic remediation of pollutants such as TCE. Treatment of iron metal with bisulfide significantly increased the pseudo-first-order rate constant for TCE transformation at pH 8.3. This effect was attributed to formation of a reactive FeS coating or precipitate on the iron surface.

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