Uranium isotopic fractionation factors during U(VI) reduction by bacterial isolates

We experimentally determined the magnitude of uranium isotopic fractionation induced by U(VI) reduction by metal reducing bacterial isolates. Our results indicate that microbial U(VI) reduction induces isotopic fractionation; heavier isotopes (i.e., U-238) partition into the solid U(IV) products. The magnitudes of isotopic fractionation (expressed as epsilon = 1000 parts per thousand * (alpha-1)) for U-238/U-235 were 0.68 parts per thousand +/- 0.05 parts per thousand and 0.99 parts per thousand +/- 0.12 parts per thousand for Geobacter sulfurreducens strain PCA and strain IFRC-N, respectively. The epsilon values for Anaeromyxobacter dehalogenans strain FRC-W, strain FRC-R5, a novel Shewanella isolate, and Desulfitobacterium sp. strain Viet1 were 0.72 parts per thousand +/- 0.15 parts per thousand, 0.99 parts per thousand +/- 0.12 parts per thousand, 0.96 parts per thousand +/- 0.16 parts per thousand and 0.86 parts per thousand +/- 0.06 parts per thousand, respectively. Our results show that the maximum epsilon values of similar to 1.0 parts per thousand were obtained with low biomass (similar to 10(7) cells/mL) and low electron donor concentrations (similar to 500 mu M). These results provide an initial assessment of U-238/U-235 shifts induced by microbially-mediated U(VI) reduction, which is needed as U-238/U-235 data are increasingly applied as redox indicators in various geochemical settings. (C) 2014 Elsevier Ltd. All rights reserved.