Solubility of (UO2)3(PO4)2.4H2O in H+-Na+-OH--H2PO-4-HPO2-4-PO3-4-H2O and its Comparison to the Analogous PuO2+2 System

The objectives of this study were to address uncertainties in the solubility product of (UO2)3(PO4)2?4H2O(c) and in the phosphate complexes of U(VI), and more importantly to develop needed thermodynamic data for the Pu(VI)-phosphate system in order to ascertain the extent to which U(VI) and Pu(VI) behave in an analogous fashion. Thus studies were conducted on (UO2)3(PO4)2?4H2O(c) and (PuO2)3(PO4)2?4H2O(lc) solubilities for long equilibration periods (up to 870 days) in a wide range of pH values (2.5 to 10.5) at fixed phosphate concentrations of 0.001 and 0.01 M, and in a range of phosphate concentrations (0.0001 to 1.0 M) at fixed pH values of about 3.5. A combination of techniques (XRD, DTA/TG, XAS, and thermodynamic analyses) was used to characterize the reaction products. The U(VI)-phosphate data for the most part agree closely with thermodynamic data presented in Guillaumont et al., (1) although we cannot verify the existence of several U(VI) hydrolyses and phosphate species and we find the reported value for formation constant of UO2PO-4 is in error by more than two orders of magnitude. A comprehensive thermodynamic model for (PuO2)3(PO4)2?4H2O(am) solubility in the H+-Na+-OH--Cl--H2PO-4-HPO2-4-PO3-4-H2O system, previously unavailable, is presented and the data shows that the U(VI)-phosphate system is an excellent analog for the Pu(VI)-phosphate system