Uranyl peroxide nanoclusters represent a newly-discovered class of uranium compounds whose presence in aqueous solution could dramatically alter the mobility and partitioning of uranium. In previous work, we studied the equilibration between U60, a uranyl peroxide nanocluster with a diameter of approximately 2.4 nm and mass of 19,640 Da, and aqueous U(VI) species. The concentration of aqueous U, K, and Li was found to increase with increasing nanocluster concentration. The solubility of bulk solid phases does not depend on the concentration of that phase in suspension, so the observed U60 behavior in our previous experiments suggests that U60 behaves like aqueous complexes rather than a bulk solid phase. In this study, we measured the equilibration behavior of two other uranyl nanoclusters in order to determine the effect of nanocluster size and composition on the equilibration behavior.We investigated one nanocluster that is smaller than U60 and one that is larger. The smaller nanocluster was U24Py12, a uranyl peroxide pyrophosphate which is 10,566 Da; the larger nanocluster was U60 oxalate, a uranyl peroxide oxalate nanocluster which is 23,900 Da. Batch equilibration experiments were conducted as a function of equilibration time, nanocluster concentration and pH. Separation of the nanoclusters in each experiment from other aqueous species was accomplished using molecular weight filtration. Aqueous U concentrations were measured at each sampling point and were found to be in steady state with each of the nanoclusters studied after several days. The experimental results were used calculate ion activity products (IAP) using aqueous speciation for each condition studied using two approaches: 1) assuming unit activity for the nanocluster 'solid', and 2) treating the nanocluster as an 'aqueous' complex. The calculated IAP values were used to compare the equilibration behaviors between nanoclusters and to constrain the size boundary between aqueous complex and solid phase behaviors.

Experimental measurements of the effect of size on uranyl nanocluster stability in aqueous solution

246th National Meeting (2013)

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