The environmental transport of uranium is often controlled by its oxidation state, with UVI (uranyl) species being more soluble and mobile than reduced UIV species such as uraninite (UO2). Because of the lower solubility of UIV, reductive transformation to UIV is often considered for remediation of contaminated sediments and groundwater. A general concern with reductive immobilization is the susceptibility of UIV to subsequent oxidation. Here, we determine the oxidation rates of two environmentally relevant UIV phases, nanoparticulate UO2 and Ca-UIV-phosphate, using X-ray absorption spectroscopy measurements of U speciation in the solid-phase. Reactions were carried out in 15 mM NaClO4 solutions buffered to pH 7.0 with 5 mM MOPS. The rates of nano-UO2 and UIV-phosphate oxidation by dissolved O2 differ by less than an order of magnitude on a molar or surface-area normalized basis. We also studied the effect of dissolved Ca and phosphate (PO43-) on the oxidation of UIV. Calcium alone had little effect on UO2 oxidation under our experimental conditions, whereas phosphate slowed the initial oxidation but promoted the extent of oxidation between 24 and 72 hours. In combination, Ca and PO43- further decreased the rate and extent of UO2 oxidation relative to what was observed for separate treatments. Additions of dissolved Ca and PO43- also decreased the rate and extent of Ca-UIV-phosphate oxidation, both in the presence and absence of 10 mM bicarbonate. The oxidative release of U in our systems was mitigated by incorporation of UVI in Na-autunite in the presence of PO43- or in Na-uranates in the absence of PO43-. The results of our study confirm that UIV precipitated as nano-UO2 and UIV-phosphate are highly reactive towards re-oxidation. Long-term strategies for UIV immobilization may need to consider a defense-in-depth strategy that includes both reductive precipitation of UIV as well as amendments with PO43- and Ca.

Calcium and phosphate decrease the oxidation rates of UO<sub>2</sub> and U<sup>IV</sup>-phosphate

248th National Meeting (2014)

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Calcium and phosphate decrease the oxidation rates of UO2 and UIV-phosphate

Calcium and phosphate decrease the oxidation rates of UO₂ and U^IV-phosphate