Experimental measurements of U24Py nanocluster behavior in aqueous solution.

Uranyl peroxide nanoclusters may impact the mobility and partitioning of uranium at contaminated sites and could be used in the isolation of uranium during the reprocessing of nuclear waste. Their behavior in aqueous systems must be better understood to predict the environmental fate of uranyl peroxide nanoclusters and for their use in engineered systems. The aqueous stability of only one uranyl peroxide nanocluster, U60 (K₁₆Li₄₄[UO₂(O₂)OH]₆₀), has been studied to date [Flynn, S. L., Szymanowski, J. E. S., Gao, Y., Liu, T., Burns, P. C., Fein, J. B.: Experimental measurements of U60 nanocluster stability in aqueous solution. Geochemica et Cosmochimica Acta 156, 94-105 (2015)]. In this study, we measured the aqueous stability of a second uranyl peroxide nanocluster, U24Py (Na₃₀[(UO₂)₂₄(O₂)₂₄(HP₂O₇)₆(H₂ P₂O₇)₆]), in batch systems as a function of time, pH, and nanocluster concentration, and then compared the aqueous behavior of U24Py to U60 to determine whether the size and morphology differences result in differences in their aqueous behaviors. Systems containing U24Py nanoclusters took over 30 days to achieve steady-state concentrations of monomeric U, Na, and P, illustrating slower reaction kinetics than parallel U60 systems. Furthermore, U24Py exhibited lower stability in solution than U60, with an average of 72% of the total mass in each nanocluster suspension being associated with the U24Py nanocluster, whereas 97% was associated with the U60 nanocluster in parallel experiments [Flynn, S. L., Szymanowski, J. E. S., Gao, Y., Liu, T., Burns, P. C., Fein, J. B.: Experimental measurements of U60 nanocluster stability in aqueous solution. Geochemica et Cosmochimica Acta 156, 94-105 (2015)]. The measurements from the batch experiments were used to calculate ion activity product (IAP) values for the reaction between the U24Py nanocluster and its constituent monomeric aqueous species. The IAP values, calculated assuming the activity of the U24Py nanocluster is equal to its concentration in solution, exhibit a significantly lower nanocluster concentration dependence than those IAP values calculated assuming an activity of 1 for the nanocluster. The inclusion of a deprotonation reaction for U24Py minimizes the pH dependence of the calculated IAP values. The modeling results suggest that the U24Py nanocluster experiences sequential deprotonation. Taken together, the results indicate that the aqueous behavior of the U24Py nanocluster, like that of U60, is best described as that of an aqueous complex. [ABSTRACT FROM AUTHOR]