ARSENIC TRANSPORT AND FATE IN SULFIDIC ENVIRONMENTS: AS(III) - FES INTERACTIONS

Arsenic mobility in groundwater and retention in aquifer materials at contaminated sites is often linked to redox processes, especially iron and sulfur cycling at redox boundaries. Important processes include adsorption or co-precipitation reactions of arsenate, arsenite, or thioarsenite species with poorly crystalline iron (oxy)hydroxides, iron monosulfides, and pyrite. We report results of laboratory studies that evaluate sorption and co-precipitation processes of As(III) with freshly precipitated and aged iron monosulfides. Results indicate contrasting adsorption behavior of arsenic depending on Fe²⁺/HS^-. Arsenite sorption onto iron monosulfide surfaces readily occurs at high Fe²⁺/HS^- (5 mg/L>As). Saturation of available surface sites occurs at higher arsenic spikes and lower surface densities. In contrast, when free hydrogen sulfide or bisulfide is present (Fe²⁺/HS^- < 0.01), little adsorption occurs and arsenic remains in solution. The lack of sorption at low Fe/S apparently is related to the presence of thioarsenite species. Arsenic speciation measurements with IC-ICP-MS and IC-HG-AFS suggest a cross-over, at SH₂S of around 10^-4.6 mol/L (pH~7), from H₃AsO₃(aq) to a suite of thioarsenite species where OH^- is progressively replaced by HS^-. The experimental results suggest that thioarsenites are particle unreactive compared to H₃AsO₃(aq). An important implication is that in environments where iron monosulfide precipitates accumulate, arsenic uptake will occur preferentially in systems where reactive iron is available as compared to regions where free sulfide is present at levels greater than about 0.1 mM. Note: This is an abstract of a proposed presentation and does not necessarily reflect EPA policy.