(723d) Micro-Scale Chemistry Influences Uranium Immobilization in Subsurface Sediment Biofilms From Hanford 300 Area

Micro-scale chemistry of sediment biofilms is one of the crucial aspects necessary for a comprehensive understanding of uranium immobilization/remobilization in the subsurface environment. In this study, we characterized the local chemistry of subsurface sediment using microsensors from a uranium contaminated site (Hanford 300 Area). We measured redox potential, dissolved oxygen (DO), hydrogen, and pH profiles in sediment biofilms. Our results showed that local chemistry inside the sediment biofilms was heterogeneous both vertically and horizontally in terms of redox potential, dissolved oxygen concentration, hydrogen concentration, and pH. Under oxic conditions, redox potentials were higher than those in anoxic conditions. DO in the oxic condition decreased when the microelectrodes were moved from the bulk phase into the sediment. Hydrogen concentration was undetectable in both the aqueous bulk phase and the sediment under oxic conditions. Under anoxic conditions, although hydrogen concentration in the bulk phase was undetectable, significant amounts of hydrogen were found in certain locations inside the sediment biofilms. When U(VI) was introduced, hydrogen concentrations in the sediment biofilms decreased significantly for the anoxic case. There were no significant changes in pH inside the sediment in all cases of oxic, anoxic with and without U conditions (pH 7.8-8.0). Micro-scale local chemistry in the sediment was heterogeneous and significantly different from those at the macro-scale, suggesting the importance of micro-scale studies in understanding U mobility in the subsurface. Microscale heterogeneous chemistry inside sediment is expected to result in heterogeneity in U immobilization in the sediment. Redox potential, DO and hydrogen concentrations are the main factors contributing to the heterogeneity, while pH is relatively constant inside the sediment.