Mineralization of contaminant uranium and leach rates in sediments from Hanford, Washington

Five U-contaminated sediment samples collected near nuclear waste storage and processing facilities at Hanford, Washington, were subjected to characterization and leach tests. Discrete U-bearing particles and U-rich areas were identified using SEM analysis, and energy-dispersive spectroscopy revealed that the U-bearing phases contained U, Si, Ca, and Fe. The presence of Al in the spectra was consistent with the observation of complex intergrowths between U-bearing and aluminosilicate solids. Analysis of U-rich regions by XANES spectroscopy showed that all the U was present as the uranyl cation (UO22+). EXAFS spectroscopy indicated the presence of adsorbed uranyl and uranyl silicate precipitates. Flow-through column leach tests revealed that U release from the contaminated sediments was non-linear and U solution concentrations failed to reach steady-state conditions. In sum, the data are in accord with a model of U mineralization caused by precipitation in geochemically-isolated pore spaces. Further, the release of U from these sediments is limited by very slow dissolution kinetics of uranyl phases coupled with slow transport of uranyl from the site of removal to bulk pore solution. The data reported in this investigation call into question the efficacy of using equilibrium-based Kd models to describe the behavior of U in oxidizing environments.