SEQUESTRATION OF ARSENIC AND OTHER TOXIC METALS BY NATURAL BACTERIAL SULFATE REDUCTION UNDER VARIOUS GEOLOGICAL AND HYDROGEOCHEMICAL CONDITIONS

Arsenic is a common metal contaminant found in groundwaters from both natural and anthropogenic sources. This study investigated the contaminant assimilative capacity of sulfide biominerals formed in various natural geological settings. Biogenic pyrite samples were collected from alluvial sediments, coastal saltmarshes, and sedimentary shales for XRF and ICP-MS geochemical analysis. The results showed abnormally high levels of arsenic (up to >1 weight %) and other toxic metals incorporated in the structure of the sulfide biominerals formed naturally in alluvial sediments. Initial S-isotopic investigations showed an average δ34S value of about -25‰ (CDT), strongly suggesting that bacterial sulfate metabolism led to pyrite formation. Laser ablation ICP-MS analysis of biogenic pyrite extracted from heavily oiled Louisiana saltmarsh sediments (contaminated by the Deepwater Horizon oil spill) also shows high concentrations of arsenic, along with other trace metals (e.g., Ni, Cu, Pb, Zn, Co, V, Ba, Hg). Pore-waters extracted from oiled sediments are characterized by very high levels of reduced sulfur (up to 80 mg/kg), indicating that sulfate-reducing bacteria outcompete other microorganisms in oiled saltmarshes. Elevated concentrations of arsenic and lead were also found in pyrite extracted from three deep gas shale units (i.e., Conasauga, Chattanooga, and Floyd (Neal) Shale) in the Black Warrior basin, Alabama, confirming that fine-grained sulfide minerals can serve as the major sinks for toxic metals under reducing environments. The assimilative capability of biogenic pyrite to removing arsenic and other metal(loid)s from groundwater serves the basis for ongoing field-scale bioremediation experiments for immobilization of groundwater arsenic in Holocene alluvial aquifers in Bangladesh and an industrial site in Florida. Preliminary results indicate that dissolved arsenic concentrations may be effectively lowered by sulfate reducers by amending groundwater with appropriate organic carbon and electron acceptors.