Experimentally determined uranium isotope Fractionation during reduction of hexavalent U by bacteria and zero valent iron

摘要

Variations in stable isotope ratios of redox sensitive elements are often used to understand redox processes occurring near the Earth's surface. Presented here are measurements of mass-dependent U isotope fractionation induced by U(VI) reduction by zerovalent iron (Fe-0) and bacteria under controlled pH and HCO3- conditions. In abiotic experiments, Fe-0 reduced U(VI), but the reaction failed to induce an analytically significant isotopic fractionation. Bacterial reduction experiments using Geobacter sulfurreducens and Anaeromyxobacter dehalogenans reduced dissolved U(VI) and caused enrichment of U-238 relative to U-235 in the remaining U(VI). Enrichment factors (epsilon) calculated using a Rayleigh distillation model are -0.31 parts per thousand and -0.34 parts per thousand for G. sulfurreducens and A. dehalogenans, respectively, under identical experimental conditions. Further studies are required to determine the range of possible values for U-238/U-235 fractionation factors under a variety of experimental conditions before broad application of these results is possible. However, the measurable variations in delta U-238 show promise as indicators of reduction for future studies of groundwater contamination, geochronology, U ore deposit formation, and U biogeochemical cycling.