| Abstract
| - The rapid kinetics of bacterial U(VI) reduction and lowsolubility of uraninite (UO2,cr) make this process an attractiveoption for removing uranium from groundwater. Nevertheless, conditions that may promote or inhibit U(VI) reductionare not well-defined. Recent descriptions of Ca−UO2−CO3 complexes indicate that these species may dominatethe aqueous speciation of U(VI) in many environments.We monitored the bacterial reduction of U(VI) in bicarbonate-buffered solution in the presence and absence of Ca.XAFS measurements confirmed the presence of a Ca−U(VI)−CO3 complex in the initial solutions containing calcium.Calcium, at millimolar concentrations (0.45−5 mM),caused a significant decrease in the rate and extent ofbacterial U(VI) reduction. Both facultative (Shewanellaputrefaciens strain CN32) and obligate (Desulfovibriodesulfuricans, Geobacter sulfurreducens) anaerobic bacteriawere affected by the presence of calcium. Reduction ofU(VI) ceased when the calculated system Eh reached −0.046± 0.001 V, based on the Ca2UO2(CO3)3 → UO2,cr couple.The results are consistent with the hypothesis that U is aless energetically favorable electron acceptor when the Ca−UO2−CO3 complexes are present. The results do not supportCa inhibition caused by direct interactions with the cellsor with the electron donor as the reduction of fumarate orTc(VII)O4- under identical conditions was unaffected bythe presence of Ca.
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