Spectroscopic evidence for uranium bearing precipitates in vadose zone sediments at the Hanford 300-area site

Uranium (U) solid-state speciation in vadose zone sediments collected beneath the former North Process Pond (NPP) in the 300 Aroa of the Hanford site (Washington) was investigated using multi-scale techniques. In 30 day batch experiments, only a small fraction of total U (similar to 7.4%) was released to artificial groundwater solutions equilibrated with 1% PCO2- Synchrotron-based micro-X-ray fluorescence spectroscopy analyses showed that U was distributed among at least two types of species: (i) U discrete grains associated with Cu and (ii) areas with intermediate U concentrations on grains and grain coatings. Metatorbernite (CU[UO2](2)[PO4](2 center dot)8H(2)O) and uranophane (Ca[UO2](2)[SiO3(OH)](2 center dot) 5H(2)O) at some U discrete grains, and muscovite at U intermediate concentration areas, were identified in synchrotron-based micro-X-ray diffraction. Scanning electron microscopy/energy dispersive X-ray analyses revealed 8-10 mu m size metatorbemite particles that were embedded in C-, Al-, and Si-rich coatings on quartz and albite grains. In mu- and bulk-X-ray absorption structure (mu-XAS and XAS) spectroscopy analyses, the structure of metatorbernite with addi-ional U-C and U-U coordination environments was consistently observed at U discrete grains with high U concentrations. The consistency of the mu- and bulk-XAS analyses suggests that metatorbernite may comprise a significant fraction of the total U in the sample. The entrapped, micrometer-sized metatorbernite particles in C-, Al-, and Si-rich coatings, along with the more soluble precipitated uranyl carbonates and uranophane, likely control the long-term release of U to water associated with the vadose zone sediments.