REACTIVITY AND STRUCTURAL CHEMISTRY OF BIOGENIC URANINITE NANOPARTICLES IN GEOCHEMICAL MATRICES

The observation that microbes can reduce U(VI) to uraninite has stimulated interest in manipulating the biological precipitation of this solid as a sparingly soluble sink phase to remediate uranium-contaminated aquifers. This idea is supported by the observation that uraninite is a stable end-product of biogeochemical redox reactions in low-temperature sedimentary ore deposits. Biologically precipitated uraninite is extremely small, occurring as 2 to 3 nm particles, raising the expectation that it should exhibit significantly elevated solubility and corrosion rates as compared to coarse grained UO₂. This tendency may be counter-balanced in part by the ability of groundwater solutes to retard uraninite corrosion via substitution and sorption reactions. Further complicating this picture is the fact that uraninite displays compositions ranging from UO₂ to UO_2.25. Compositionally pure UO₂ is extremely rare in nature. Biogenic nano-uraninite is thus likely to be subject to rich and diverse chemical influences in geochemical matrices that will significantly moderate its reactivity. We have studied the reactivity and local, intermediate, and long-range structure of biogenic uraninite nanoparticles under controlled laboratory conditions and in aquifers at the Old Rifle IFRC site to elucidate the key factors that control its behavior in the subsurface. This work suggests that nano-biogenic uraninite is relatively stable in groundwater, even under oxidizing conditions, and that diffusion limitations imposed by sediments will be particularly critical to its longevity. This work sheds light not only on the usefulness of biogenic uraninite for subsurface uranium remediation, but also on the corrosion of nuclear fuel/waste in holding and disposal sites, and factors governing the formation and migration of uranium ore deposits.