Spectroscopic Evidence for Uranium Bearing Precipitates in Vadose Zone Sediments at the Hanford 300-Area Site
Dates
Year
2007
Citation
Arai, Yuji, Marcus, M A, Tamura, N, Davis, J A, and Zachara, J M, 2007, Spectroscopic Evidence for Uranium Bearing Precipitates in Vadose Zone Sediments at the Hanford 300-Area Site: Environmental Science & Technology, v. 41, iss. 13, p. 4633-4639.
Summary
Uranium (U) solid-state speciation in vadose zone sediments collected beneath the former North Process Pond (NPP) in the 300 Area of the Hanford site (Washington) was investigated using multi-scale techniques. In 30 day batch experiments, only a small fraction of total U (?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]2·8H2O) and uranophane (Ca[UO2]2[SiO3(OH)]2·5H2O) at some U discrete grains, and muscovite at U intermediate [...]
Summary
Uranium (U) solid-state speciation in vadose zone sediments collected beneath the former North Process Pond (NPP) in the 300 Area of the Hanford site (Washington) was investigated using multi-scale techniques. In 30 day batch experiments, only a small fraction of total U (?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]2·8H2O) and uranophane (Ca[UO2]2[SiO3(OH)]2·5H2O) 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 ?m size metatorbernite particles that were embedded in C-, Al-, and Si-rich coatings on quartz and albite grains. In ?- and bulk-X-ray absorption structure (?-XAS and XAS) spectroscopy analyses, the structure of metatorbernite with additional U?C and U?U coordination environments was consistently observed at U discrete grains with high U concentrations. The consistency of the ?- 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.
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