Tetravalent uranium mobilization by complexation or oxidation and associated U isotope fractionation

Uranium (U) isotopes are suggested to monitor the success of (bio)remediation relying on the reduction of soluble and mobile U(VI) to less soluble U(IV)1. However, the subsurface stability of U(IV), typically present as solid-phase non-crystalline U, may be affected by complexation or oxidation. Understanding these processes and their impact on U isotope fractionation is important to correctly interpret field U isotope signatures. We investigated U mobilization by complexation and oxidation and measured the associated U isotope fractionation in laboratory batch experiments. Non-crystalline U(IV) was produced as the starting material by reducing a U(VI) isotope standard with Shewanella oneidensis MR-12. Subsequently, U(IV) was mobilized: 1) anoxically, with ligands (EDTA, citrate, or bicarbonate), 2) by oxidation with Fe(III), or 3) with molecular oxygen at low pH in the presence of the bacterium Acidithiobacillus ferrooxidans. All ligands mobilized U(IV) and enriched 238U in the complexed fraction (δ238U: 0.2 to 0.6 ‰). Oxidative U mobilization both, with Fe(III) or with At. ferrooxidans biomass, resulted in insignificant U isotope fractionation. Either isotope fractionation during all involved reaction steps was very small or cancelled eachother out. The latter may be indicated by the observation of high aqueous δ238U values (~0.8 ‰) in corresponding abiotic control experiments (without biomass), which may be the result of adsorption effects after oxidative U mobilization. (1) Bopp et. al. Environ. Sci. Technol. 2010, 44 (15), 5927–5933. (2) Stylo et al. Environ. Sci. Technol. 2013, 47 (21), 12351–12358.