title: Inhibition of bacterial U(VI) reduction by calcium.
abstract: The rapid kinetics of bacterial U(VI) reduction and low solubility of uraninite (UO2,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-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)-C03 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 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 a less energetically favorable electron acceptor when the Ca-UO2-CO3 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)O4- under identical conditions was unaffected by the presence of Ca.
authors.affiliation: Environmental Sciences Division, P.O. Box 2008, MS 6038, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6038, USA. brookssc@ornl.gov
authors.forename: Scott C
authors.initials: SC
authors.lastname: Brooks
authors.forename: James K
authors.initials: JK
authors.lastname: Fredrickson
authors.forename: Sue L
authors.initials: SL
authors.lastname: Carroll
authors.forename: David W
authors.initials: DW
authors.lastname: Kennedy
authors.forename: John M
authors.initials: JM
authors.lastname: Zachara
authors.forename: Andrew E
authors.initials: AE
authors.lastname: Plymale
authors.forename: Shelly D
authors.initials: SD
authors.lastname: Kelly
authors.forename: Kenneth M
authors.initials: KM
authors.lastname: Kemner
authors.forename: Scott
authors.initials: S
authors.lastname: Fendorf
mesh_terms: D001421:Bacteria, Anaerobic / Q000502:physiology; D002118:Calcium / Q000737:chemistry* / Q000494:pharmacology; D012995:Solubility; D014501:Uranium / Q000737:chemistry*; D017974:Uranium Compounds / Q000737:chemistry*
chemical_list: D017974:Uranium Compounds; D014501:Uranium; C012597:uranium dioxide; D002118:Calcium