Abiotic U(VI) reduction by sorbed Fe(II) on natural sediments

Laboratory experiments were performed as a function of aqueous Fe(II) concentration to determine the uptake and oxidation of Fe(II), and Fe(II)-mediated abiotic reduction of U(VI) by aquifer sediments from the DOE Rifle field research site in Colorado, USA. Mössbauer analysis of the sediments spiked with aqueous 57Fe(II) showed that 57Fe(II) was oxidized on the mineral surfaces to 57Fe(III) and most likely formed a nano-particulate Fe(III)-oxide or ferrihydrite-like phase. The extent of 57Fe oxidation decreased with increasing 57Fe(II) uptake, such that 98% was oxidized at 7.3 μmol/g Fe and 41% at 39.6 μmol/g Fe, indicating that the sediments had a limited capacity for oxidation of Fe(II). Abiotic U(VI) reduction was observed by XANES spectroscopy only when the Fe(II) uptake was greater than approximately 20 μmol/g and surface-bound Fe(II) was present, possibly as oligomeric Fe(II) surface species. The degree of U(VI) reduction increased with increasing Fe(II)-loading above this level to a maximum of 18% and 36% U(IV) at pH 7.2 (40.7 μmol/g Fe) and 8.3 (56.1 μmol/g Fe), respectively in the presence of 400 ppm CO2. Greater U(VI) reduction was observed in CO2-free systems [up to 44% and 54% at pH 7.2 (17.3 μmol/g Fe) and 8.3 (54.8 μmol/g Fe), respectively] compared to 400 ppm CO2 systems, presumably due to differences in aqueous U(VI) speciation. While pH affects the amount of Fe(II) uptake onto the solid phase, with greater Fe(II) uptake at higher pH, similar amounts of U(VI) reduction were observed at pH 7.2 and 8.3 for a similar Fe(II) uptake. Thus, it appears that abiotic U(VI) reduction is controlled primarily by sorbed Fe(II) concentration and aqueous U(VI) speciation. The range of Fe(II) loadings tested in this study are within the range observed in biostimulation experiments at the Rifle site, suggesting that Fe(II)-mediated abiotic U(VI) reduction could play a significant role in field settings.