DISSOLUTION BEHAVIOR OF U60 NANOCLUSTERS IN AQUEOUS SOLUTION (Invited Presentation)

Uranyl peroxide nanoclusters represent a newly-discovered class of uranium compound whose presence could dramatically alter the mobility and partitioning of uranium in environmental systems and industrial applications. Although the structures of a number of uranyl peroxide nanocluster crystals are well-constrained, the nature and behavior of the isolated nanoclusters in aqueous solution remain unknown, and it is unclear whether the compounds behave as bulk solid phases or as large aqueous complexes in solution. Clearly, these fundamental properties must be determined in order to model the environmental behavior of these compounds.

In this study, we determined the dissolution behavior of isolated U60 nanoclusters in solution. We measured the dissolution of U60 nanoclusters as a function of pH and nanocluster concentration, using a molecular weight filter to separate isolated nanoclusters from aqueous species. We use the measurements to calculate the ion activity product (IAP) at each of the experimental conditions, and we use the behavior of the calculated IAP values as a function of pH and nanocluster concentration to define the nature of the nanoclusters in solution.

The measured aqueous U concentration increases with increasing nanocluster concentration in the experiment, clearly indicating that the aqueous U concentration is not buffered by the solubility of a bulk solid phase. Rather, the nanoclusters appear to behave more like aqueous complexes, with higher concentrations of aqueous U species in steady-state with higher concentrations of nanoclusters. The behavior of the calculated IAP values is also consistent with this conclusion, with less of a dependence of the IAP value on nanocluster concentration when the IAP is calculated with a non-unit activity for the isolated U60 nanocluster.

Our results strongly suggest that isolated U60 nanoclusters in solution do not behave as bulk solid phases and do not buffer the aqueous metal concentrations. However, the experimental results do suggest that a steady-state is attained between the nanoclusters and component aqueous species. We need many more studies of the dissolution behavior, hydrated composition, and overall charge in solution in order to model the fate and mobility of similar nanoclusters in aqueous environments.