Abstract:
Dissimilatory metal-reducing bacteria (DMRB) can utilize Fe(III) associated with aqueous complexes or solid phases, such as oxide and oxyhydroxide minerals, as a terminal electron acceptor coupled to the oxidation of H2 or organic substrates. These bacteria are also capable of reducing other metal ions including Mn(IV), Cr(VI), and U(VI), a process that has a pronounced effect on their solubility and overall geochemical behavior. In spite of considerable study on an individual basis, the biogeochemical behavior of multiple metals subject to microbial reduction is poorly understood. To probe these complex processes, the reduction of U(VI) by the subsurface bacterium, Shewanella putrefaciens CN32, was investigated in the presence of goethite under conditions where the aqueous composition was controlled to vary U speciation and solubility. Uranium(VI), as the carbonate complexes UO2(CO3)3(aq)4- and UO2(CO3)2(aq)2-, was reduced by the bacteria to U(IV) with or without goethite [α-FeOOH(s)] present. Uranium(VI) in 1,4-piperazinediethhanesulfonic acid (PIPES) buffer that was estimated to be present predominantly as the U(VI) mineral metaschoepite [UO3 . 2H2O(s)], was also reduced by the bacteria with or without goethite. In contrast, only ~30% of the U(VI) associated with a synthetic metaschoepite was reduced by the organism in the presence of goethite with 1 mM lactate as the electron donor. This may have been due to the formation of a layer of UO2(s) or Fe(OH)3(s) on the surface of the metaschoepite that physically obstructed further bioreduction. Increasing the lactate to a non-limiting concentration (10 mM) increased the reduction of U(VI) from metaschoepite to greater than 80% indicating that the hypothesized surface-veneering effect was electron donor dependent. Uranium(VI) was also reduced by bacterially reduced anthraquinone-2,6-disulfonate (AQDS) in the absence of cells, and by Fe(II) sorbed to goethite in abiotic control experiments. In the absence of goethite, uraninite was a major product of direct microbial reduction and reduction by AH2DS. These results indicate that DMRB, via a combination of direct enzymatic or indirect mechanisms, can reduce U(VI) to insoluble U(IV) in the presence of solid Fe oxides.