ARSENIC SEQUESTRATION IN ARSENIAN PYRITE IN LOW TEMPERATURE IRON- BEARING GROUNDWATER CONDITIONS: FIELD BIOREMEDIATION, LABORATORY INVESTIGATION, AND GEOCHEMICAL MODELING APPROACHES

Low temperature arsenic-bearing pyrite occurs in several geologic settings, including: 1) anoxic marine sediments, 2) Holocene alluvial aquifers under sulfate-reducing conditions (e.g., US, Cambodia, Bangladesh, Nepal, and India), and 3) Gulf coast salt dome cap rocks made by sulfate-reducing bacteria. Our study on arsenic sequestration into sulfidic minerals, at low temperature, with soluble iron, shows that soluble arsenic that affix with sulfide in reduced aquifer sediments during microbial sulfate-reduction is mostly iron-sulfide minerals namely, iron monosulfide (FeS), biogenic pyrite (FeS₂), or arsenopyrite (FeAsS). We have analyzed the pyrite prepared in the laboratory and natural biogenic pyrite from a Holocene floodplain in Alabama using electron microprobe, LA-ICP-MS and X-ray diffraction. The arsenic bearing nature of biogenic pyrite prepared in the laboratory and those collected from the field revealed that arsenic apparently substitutes for sulfur in the pyrite and co-precipitation and sorption processes effectively removes arsenic from solution. This is in accord with observations from Bangladesh and West Bengal, where As-bearing pyrite occurs in As-enriched groundwater under sulfate-reducing conditions. In contrast, some studies of anthropogenic As-contaminated areas have suggested that the pure arsenic-sulfide minerals realgar (AsS) and orpiment (As₂S₃) are the predominant low-temperature As phases under sulfate-reducing conditions, although X-ray diffraction confirmation is lacking. We used Geochemist's Workbench to model the stability of arsenian pyrite using a new set of thermodynamic data containing Fe-S-As solid solution. Modeling was conducted in the presence of abundant soluble iron, and As geochemical behavior was evaluated under various Eh-pH conditions. Reported arsenic concentration in biogenic pyrite formed under sulfate-reducing condition range from 0.5 to 10 wt. % As. Considering the fact that that soluble iron is typically found in concentrations exceeding As in As-contaminated groundwater, it seems likely that the Fe-S geochemical cycles will control As geochemistry in most circumstances. This is supported by our modeling results that arsenian pyrite (containing 1 wt.% As), dominates the stability fields at circum-neutral pH and sulfate-reducing condition.