Uranium speciation in acid waste-weathered sediments: The role of aging and phosphate amendments

Abstract

Uranium speciation and lability are strongly coupled to mineral transformations in silicate sediments, particularly for sediments subjected to weathering in acidic, high-level radioactive waste, as occurred at the Department of Energy's Hanford (WA) site. In this study, uncontaminated Hanford sediments were reacted for 365 days with acidic (pH 3), uranium-bearing waste solutions, with and without phosphate in batch experiments, prior to detailed characterizations using electron microscopy, x-ray diffraction and x-ray absorption spectroscopy. In PO4-reactant free systems, uranium speciation was controlled initially by precipitation of compreignacite [K2(UO2)6O4(OH)6·8H2O]- and becquerelite [Ca(UO2)6O4(OH)6·8H2O]-like species. Subsequent further removal of uranium coincided with that of Si and accumulation of boltwoodite, [(K, Na)(UO2)2O4(HSiO4)2•0.5(H2O)]-like species of uranium at 180 and 365 days. When present, PO4 exerted a direct and strong control over U speciation. The detection of meta-ankoleite, [K2(UO2)2O4(PO4)2·6H2O] at all reaction times when U was present emphasizes the importance of dissolved phosphate as a control on U speciation. Here, meta-ankoleite appears well crystallized and when it occurs as the principal product of sediment weathering, its low solubility is expected to limit dissolved U(VI) concentrations in groundwater. Although boltwoodite solubility is also low, it is formed more slowly (and only when PO4 is absent), after initial precipitation of more soluble, less crystalline uranyl hydroxides. In the context of Hanford crib waste our results suggest that with PO4 present, nearly all uranium would have precipitated in the upper soil.