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| - Uranium Immobilization bySulfate-Reducing Biofilms
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| - Hexavalent uranium [U(VI)] was immobilized using biofilmsof the sulfate-reducing bacterium (SRB) Desulfovibriodesulfuricans G20. The biofilms were grown in flat-platecontinuous-flow reactors using lactate as the electron donorand sulfate as the electron acceptor. U(VI) was continuouslyfed into the reactor for 32 weeks at a concentration of126 μM. During this time, the soluble U(VI) was removed(between 88 and 96% of feed) from solution and immobilizedin the biofilms. The dynamics of U immobilization in thesulfate-reducing biofilms were quantified by estimating: (1)microbial activity in the SRB biofilm, defined as thehydrogen sulfide (H2S) production rate and estimatedfrom the H2S concentration profiles measured usingmicroelectrodes across the biofilms; (2) concentration ofdissolved U in the solution; and (3) the mass of U precipitatedin the biofilm. Results suggest that U was immobilized inthe biofilms as a result of two processes: (1) enzymaticallyand (2) chemically, by reacting with microbially generatedH2S. Visual inspection showed that the dissolved sulfidespecies reacted with U(VI) to produce a black precipitate.Synchrotron-based U L3-edge X-ray absorption nearedge structure (XANES) spectroscopy analysis of Uprecipitated abiotically by sodium sulfide indicated thatU(VI) had been reduced to U(IV). Selected-area electrondiffraction pattern and crystallographic analysis oftransmission electron microscope lattice-fringe imagesconfirmed the structure of precipitated U as being that ofuraninite.
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