Systematic Investigation of the Product of Microbial U(VI) Reduction by Different Bacteria

Bioremediation of U contaminated groundwater is based on the microbial in situ reduction of soluble, and hence mobile, oxidized uranium, U(VI), to a comparatively insoluble reduced species, U(IV). A number of microorganisms have been identified as capable of reducing U(VI), including metalreducing bacteria (MRB) and sulfate-reducing bacteria (SRB). While the product of microbial U(VI) reduction is often reported as the mineral uraninite, UO2, there is increasing evidence that other compounds may be produced. In this study, we evaluated the product of U(VI) reduction by six microorganisms under near-identical chemical conditions to determine the variability in the speciation of the reduced U product. The microorganisms considered included SRB and MRB, Gram negative and Gram positive bacteria: Shewanella oneidensis MR-1, Desulfovibrio vulgaris, S. putrefaciens CN32, Geobacter sulfurreducens, G. metallireducens and Desulfotomaculum reducens MI-1. The products were characterized by XAS (X-ray Absorption Spectroscopy). The majority of the bacteria produced a reduced mineral structurally homologous to stoichiometric UO2. However, in three cases, the product was a reduced U species lacking the U second-shell that is typical of UO2. Those products were determined to be molecular U(IV) complexes adsorbed on iron precipitates (Geobacter spp.) or spores (D. reducens). Thus, we conclude that the chemical reduction of U(VI) by a reduced Fe phase or biological reduction by spores leads to the formation of sorbed U(IV) whereas direct enzymatic reduction of U(VI) produces UO2. In order to confirm the above conclusion for field remediation efforts, U(VI) reduction was carried out in columns packed with sediment from a U contaminated site and seeded with S. oneidensis MR-1. The reduction of added U(VI) and endogenous Fe(III) was observed and the uranium product characterized using XAS