Reduction of U(VI) in goethite (alpha-FeOOH) suspensions by a dissimilatory metal-reducing bacterium

Dissimilatory metal-reducing bacteria (DMRB) can utilize Fe(III) associated with aqueous complexes or solid phases, such as oxide and oxyhydroxide min erals, as a terminal electron acceptor coupled to the oxidation of H-2 or o rganic 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 e ffect on their solubility and overall geochemical behavior. In spite of con siderable study on an individual basis, the biogeochemical behavior of mult iple metals subject to microbial reduction is poorly understood. To probe t hese 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 sp eciation 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 [alpha-FeOOH(s)] present. Uranium(VI) in 1,4-piperazin ediethhane-sulfonic acid (PIPES) buffer that was estimated to be present pr edominantly as the U(VI) mineral metaschoepite [UO3. 2H(2)O((s))] was also reduced by the bacteria with or without goethite. In contrast, only similar to 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 electr on donor. This may have been due to the formation of a layer of UO2(s) or F e(OH)(3(s)) on the surface of the metaschoepite that physically obstructed further bioreduction. Increasing the lactate to a non-limiting concentratio n (10 mM) increased the reduction of U(VI) from metaschoepite to greater th an 80% indicating that the hypothesized surface-veneering effect was electr on donor dependent. Uranium(VI) was also reduced by bacterially reduced ant hraquinone-2,6-disulfonate(AQDS) in the absence of cells, and by Fe(II) sor bed to goethite in abiotic control experiments. In the absence of goethite, uraninite was a major product of direct microbial reduction and reduction by AH(2)DS. These results indicate that DMRB, via a combination of direct e nzymatic or indirect mechanisms, can reduce U(VI) to insoluble U(IV) in the presence of solid Fe oxides. Copyright (C) 2000 Elsevier Science Ltd.