MICROBIOLOGICAL REDUCTION OF SB AND AS IN ANOXIC SEDIMENTS FROM A CONTAMINATED MINE SITE

Antimony (Sb) is a toxic element that is ubiquitous in the environment and is considered to be a pollutant of priority interest by the USEPA and the EU. Antimony occurs just below As in group 15 of the periodic table, and as a result the two elements share many chemical and toxicological properties. Like As, Sb can exist in a variety of oxidation states (-III, 0, III, V) but in the environment it is mainly present as the oxyanions antimonate [Sb(V)] and antimonite [Sb(III)]. The pentavalent Sb(V) form predominates in oxygenated environments, while the more-toxic trivalent Sb(III) ion is stable in reducing settings such as interstitial pore waters. In contrast with thermodynamic equilibrium predictions, however, significant proportions of Sb(V) are often reported in nature under anoxic conditions, and Sb(III) is sometimes detected in oxic settings. These discrepancies may be attributable to biological activity, however unlike microbe-As interactions the geomicrobiology of Sb is still largely undefined. In this study we report bacterially-mediated reduction of Sb(V) to Sb(III) in anoxic sediments collected from a Sb- and As-contaminated mine tailings site located near Yellow Pine, Idaho. Live sediment microcosms as well as enrichment cultures grown from sediment inoculants readily reduced millimolar amendments of Sb(V) to Sb(III) when incubated under anoxic conditions, while no activity was observed in autoclaved control sediments. The reduction of Sb(V) was stimulated by amendment with lactate and was quantitatively coupled with the oxidation of lactate to acetate. The microbial community in these sediments also actively reduces sulfate, and the subsequent production of sulfide appears to exert a profound effect on the precipitation and sequestration of Sb as insoluble thiocomplexes. The precipitation of Sb(III)-sulfide was verified as a major sink for reduced Sb in these microcosms using X-ray spectroscopy. Amendment with equimolar As(V) inhibited Sb(V) reduction by acting as a preferred electron acceptor for the sedimentary microbial community. We are currently working to isolate and identify bacterial strain(s) from these sediments that are capable of dissimilatory growth linked to Sb(V) reduction.