Inhibition of Bacterial U(VI) Reduction by Calcium.

The rapid kinetics of bacterial U(VI) reduction and low solubility of uraninite (UO[sub 2,cr]) make this process an attractive option for removing uranium from groundwater. Nevertheless, conditions that may promote or inhibit U(VI) reduction are not well-defined. Recent descriptions of Ca-UO[sub 2]CO[sub 3] 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 bicarbonatebuffered solution in the presence and absence of Ca. XAFS measurements confirmed the presence of a CaU(VI)-CO[sub 3] 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 by the presence of calcium. Reduction of U(VI) ceased when the calculated system E[sub h] reached -0.046 ± 0.001 V, based on the Ca[sub 2]UO[sub 2](CO[sub 3])[sub 3] → UO[sub 2,cr] couple. The results are consistent with the hypothesis that U is a less energetically favorable electron acceptor when the CaUO[sub 2]-CO[sub 3] complexes are present. The results do not support Ca inhibition caused by direct interactions with the cells or with the electron donor as the reduction of fumarate or Tc(VII)O[sub 4, sup -] under identical conditions was unaffected by the presence of Ca. [ABSTRACT FROM AUTHOR]