Dissolution of biogenic and synthetic UO2 under varied reducing conditions

The chemical stability of biogenic UO(2), a nanoparticulate product of environmental bioremediation, may be impacted by the particles' surface free energy, structural defects, and compositional variability,in analogy to abiotic UO(2+x) (0 <= x <= 0.25). This study quantifies and compares intrinsic solubility and dissolution rate constants of biogenic nano-UO(2) and synthetic bulk UO(2.00), taking molecular-scale structure into account. Rates were determined under anoxic conditions as a function of pH and dissolved inorganic carbon in continuous-flow experiments. The dissolution rates of biogenic and synthetic UO(2) solids were lowest at near neutral pH and increased with decreasing pH. Similar surface area-normalized rates of biogenic and synthetic UO(2) suggest comparable reactive surface site densities. This finding is consistent with the identified structural homology of biogenic UO(2) and stoichiometric UO(2.00). Compared to carbonate-free anoxic conditions, dissolved inorganic carbon accelerated the dissolution rate of biogenic UO(2) by 3 orders of magnitude. This phenomenon suggests continuous surface oxidation of U(IV) to U(VI), with detachment of U(VI) as the rate-determining step in dissolution. Although reducing conditions were maintained throughout the experiments, the UO(2) surface can be oxidized by water and radiogenic oxidants. Even in anoxic aquifers, UO(2) dissolution may be controlled by surface U(VI) rather than U(IV) phases.