Uranium is both a legacy and an emerging contaminant resulting from past enrichment activities and the ongoing accumulation of spent nuclear fuel. Unintended release can produce plumes which propagate at rates that are controlled by hydrologic factors and the interactions of the dissolved uranium species (predominantly UVI) with geomedia components such as bacteria and minerals. Understanding the transformations of U in reducing subsurface zones is of particular interest because U(IV) is significantly less soluble than U(VI). Here, we have studied the interactions of U(VI) with environmentally-relevant metal oxides and clay minerals (magnetite, TiO2, montmorillonite, nontronite) under reducing groundwater conditions (2 mM bicarbonate, pH 7.2, presence or absence of a reduced electron shuttle). The speciation of U in the solid phase was determined using a combination of spectroscopic and imaging techniques (XANES, EXAFS, TEM, EDX). We find that the reduction of U(VI) to U(IV) in the presence of magnetite and TiO2 results in the predominance of adsorbed U(IV) complexes or the formation of 2-5 nm uraninite particles (UO2), depending on whether a mineral-specific surface coverage threshold is exceeded. In contrast, the studied clay minerals were unable to stabilize adsorbed U(IV) species under the same conditions. Instead, the speciation of uranium depended on the reducing capacity in the system, with uraninite being predominant in systems with high loadings of reduced Fe-containing clays and a mixed valence U(V)-U(VI) mineral forming at low clay loading. The observed adsorption of U(IV) to high-affinity sites on metal oxide surfaces and the stabilization of U(V)-U(VI) species by clays present new challenges to the inclusion of appropriate reactions for reduced U in the reactive transport models that are used to model U migration at the field-scale.

Influence of mineral surfaces on the speciation of uranium under reducing conditions

251st National Meeting (2016)

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