The solubility of Th(IV) hydrous oxide in concentrated NaCl and MgCl2 solutions

The solubility of Th(IV) hydrous oxide was studied in concentrated 4m and 6m NaCl solutions as well as in MgCl2 solutions ranging in concentration from 1m to 3m over a broad range of hydrogen ion concentrations. The observed solubilities in all solutions showed the same trend as observed previously of higher solubilities at early equilibration times, usually 7 to 8 days, followed by decreases in solubility with time as the precipitates aged. The trend of decreasing solubility with time was more pronounced in NaCl solutions than in MgCl2 solutions. The observed ThO2(am) solubilities in concentrated NaCl solutions (i.e., 4m and 6m) were lower than previously reported solubilities in more dilute NaCl solutions (i.e., < 3M NaCl). The results in MgCl2 were similar in all solutions regardless of the MgCl2 concentration. Current thermodynamic models for the solubility of hydrous thorium, oxide in chloride solutions, which primarily describe only aqueous Th4+-Cl- ion-interactions, predicted higher solubilities than observed in 4 and 6m NaCl as well as in all MgCl2 solutions. An improved aqueous thermodynamic model, which includes ion-interaction parameters for like charged species, is proposed to explain these results