Isotopic study of uranium: determining the isotopic fractionation of uranium during abiotic reduction with iron(II)

Uranium is a contaminant of interest due to its toxicity and mobility in groundwater. It is a redox active element in which the oxidation state can change by the addition or removal of electrons. In one method of groundwater remediation, mobile, hexavalent uranium (U(VI)) can be reduced to immobile, tetravalent uranium (U(IV)) by microbial or abiotic chemical reactions. Unfortunately, due to uranium’s complex geochemistry, measuring concentrations does not reliably give an accurate indicator of the efficiency of removal. However, uranium isotope ratios can provide a direct way to track the extent of reduction. At the Old Rifle site (Colorado, USA), reduction of U(VI) to U(IV) was induced via injections of acetate to stimulate microbial activity. A decrease of ~1‰ in the 238U/235U ratio of the dissolved U(VI) was observed and correlated closely to the decrease in concentration (Bopp et al., 2010). However, the primary mechanism of chemical reduction, microbial or abiotic, is still not well understood. Microbial laboratory experiments observed a shift of 1-2‰ in 238U/235U during reduction experiments with multiple bacterial strains (Basu et al., 2014). Little work has been done to determine the isotopic fractionation induced by abiotic reduction. This study focuses on determining the isotopic fractionation of uranium during reduction with Fe(II). Time series experiments with aqueous Fe(II) and magnetite (Fe3O4) were conducted with samples analyzed by a MC-ICP-MS. We find aqueous Fe(II) has a limited ability to reduce U(VI) and no isotopic data is reported. Stoichiometric magnetite (χ=0.49) reduces U(VI) to U(VI) completely and rapidly as confirmed by XANES analysis, but induces no shift in 238U/235U. This finding matches results observed in three other studies (Rademacher et al, 2006; Stirling et al., 2007; Stylo et al., 2014) where abiotic reduction does not induce isotopic fractionation