SPECTROSCOPIC INSIGHTS ON U(IV) SPECIES IN BIOLOGICALLY REDUCED SEDIMENTS AT THE OLD RIFLE AQUIFER

The chemical and physical forms of U(IV), as well as the biogeochemical processes by which they form and transform, are believed to profoundly influence the behavior and persistence of uranium in contaminated groundwater. In this study, the reduction of U(IV) was examined during acetate-stimulated bioreduction in the Old Rifle Aquifer. We have developed an in-situ technique for studying U(IV) products of biological U(VI) reduction and their dynamics in aquifers over the scale of days to years. This technique uses in-well columns to obtain direct access to sediment U(IV) species, evolving microbial communities, and trace and major ion groundwater constituents. Whole sediments from these in-situ columns have been examined using x-ray and electron microscopy, and x-ray absorption spectroscopy. EXAFS measurements revealed that U(IV) was primarily oxygen coordinated and monomeric U(IV) complexes were the primary products. Furthermore, different reduced U(IV) products were observed during sulfate and Fe reducing regimes, showing the strong dependence of the reduced product on biogeochemical conditions. This work establishes the importance of non-uraninite forms in subsurface sediments at the Rifle site and provides a conceptual framework in which previously observed U(IV) reduction products can be related. These experiments also establish that U(IV) species are dynamic in aquifers and can undergo non-oxidative transformation reactions. These insights provide a starting point for understanding uranium behavior in naturally reduced zones.