PNNL

Abstract

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 (WA, USA) 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 (?XRF) analyses showed that U was distributed among at least two types of species: 1) U discrete grains associated with Cu, and 2) 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 (?XRD). SEM/EDS analyses revealed 8-10 µm 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 were 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, micron-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.