Interfacial chemistry is profoundly affected by redox conditions in complex subsurface systems, as well as by the physical and electronic structure of mineral-water interfaces. Uranium mobility and bioavailability are largely moderated by redox conditions; U(VI) is highly mobile in groundwater in the presence of complexing ligands such as bicarbonate, whereas U(IV) is relatively insoluble and therefore typically immobile. Redox conditions within natural aquifers can vary spatially and temporally, giving rise to a range of transport properties. Our group is studying interfacial processes in redox-variable and organic-rich sediments that influence the mobility of uranium in contaminated aquifers. Uraninite (UO2) is a desirable product of bioreduction and a major component of US spent nuclear fuel. In sediments containing UO2, long-term uranium release is controlled by surface-mediated oxidative dissolution. Dissolution rates are profoundly influenced by the concentrations of dissolved oxygen (DO), carbonate, and solutes such as Ca2+, which retards UO2 oxidation. We show that Ca2+ reacts strongly with UO2 in the aquifer at Rifle, CO, a result of surface binding at uranium structural sites. Overall oxidative uranium loss rates in the aquifer were highly sensitive to DO concentrations, with relatively rapid loss when DO was &gt; 0.6 mg/L, but no measureable loss at lower concentrations. We conclude that the spatial and temporal dynamics of DO in many shallow aquifers will drive periodic oxidative release of uranium from UO2 and other forms of U(IV). Whereas a relatively large body of literature has examined sorption of U(VI) on aquifer materials, relatively little attention has focused on U(IV) sorption. U(IV) exhibiting local molecular structure consistent with sorbed complexes has been observed in natural sediments, suggesting that such complexes are important. It is unknown which materials will adsorb U(IV) in complex aquifer solids, or under what geochemical conditions adsorption will outcompete UO2 precipitation to dominate U(IV) speciation. We will show examples to illustrate the importance of U(IV) adsorption to uranium mobility in contaminated sediments in the upper Colorado River Basin, USA. Together, these results highlight the tightly intertwined roles of interfacial and redox chemistry in controlling contaminant mobilization in complex natural subsurface systems.

Impact of redox conditions on interfacial uranium chemistry in complex natural sediments

251st National Meeting (2016)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.