Speciation and Reactivity of Biogenic UO2 in Water

Biogenic UO2 is being investigated as a stable waste form of uranium under anoxic conditions in contaminated aquifers. Flow-through dissolution experiments performed under reducing conditions showed that the presence of carbonate accelerated UO2 dissolution. Concomitant XPS analyses detected penta- and hexavalent uranium in a roughly 2:1 ratio on the UO2 surface. Calculations suggest water radiolysis products were a source of oxidants. Carbonate promoted the UO2 dissolution by forming water-soluble complex species with uranyl(VI) and possibly U(V). Under more oxidizing conditions, dissolution rates increased because direct oxidation of U(IV) to U(VI) became more favorable. Although biogenic UO2 nanoparticles dissolved at higher rates than larger particles of abiotic UO2, surface area-normalized rate constants were similar. In carbonate-limited systems, U(VI) may accumulate on the UO2 surface and eventually form (meta)schoepite as the dissolution rate-controlling phase. These surface-specific redox and sorption/desorption reactions may play an important role in determining the stability of uranium in aqueous and remediated environments