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The rapid kinetics of bacterial U(VI) reduction and low solubility of uraninite (UO_2,cr) make this process an attractive option for removing uranium from groundwater. Neverthe less, conditions that may promote or inhibit U(VI) reduction are not well-defined. Recent descriptions of Ca−UO₂−CO₃ complexes indicate that these species may dominate the 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 of bacterial U(VI) reduction. Both facultative (Shewanella putrefaciens strain CN32) and obligate (Desulfovibrio desulfuricans, Geobacter sulfurreducens) anaerobic bacteria were affected …