Abstract

The solubility product of NaUO₂PO₄· x H₂O was determined in phosphate containing solutions at low pC_H+ values in the absence of carbonate and at higher pC_H+ values in the presence of carbonate. NaUO₂PO₄· x H₂O exhibited very low solubilities (∼10⁻⁷ M in U) over a broad range of hydrogen ion concentrations, NaNO₃ concentrations and in the absence of added carbonate. Time Resolved Laser Fluorescence Spectroscopy (TRLFS) analysis of non-carbonate solutions outside of the acidic region revealed the presence of complex mixtures of aqueous U(VI) hydroxyl or phosphate species and uranium phosphate nanoparticles. The presence of the nanoparticles made it impossible to accurately calculate a solubility product for NaUO₂PO₄· x H₂O in the absence of carbonate and at higher pC_H+ values. Therefore in order to increase the concentration of U(VI) in solution and thereby verify the solubility product calculated from the most acidic samples, we systematically introduced know concentrations of carbonate, which resulted in the formation of U(VI) carbonate complexes. Development of an accurate aqueous thermodynamic model for the aqueous U(VI) carbonate complexes then allowed calculation of a solubility product for NaUO₂PO₄· x H₂O in the higher pH samples which was in good agreement with the values for the more acidic samples.