THE SOLUBILITY OF TH(IV) AND U(IV) HYDROUS OXIDES IN CONCENTRATED NACL AND MGCL2 SOLUTIONS

The solubilities of Th(IV) and U(IV) hydrous oxides can be used to set upper limits on Th(IV) and U(IV) concentrations that can leach out of salt repositories which contain concentrated Na and Mg chloride brine s. Therefore, the solubilities of Th(IV) and U(IV) hydrous oxides were studied in concentrated solutions of NaCl (ranging up to 6.0 m) and M gCl2 (ranging up to 3.0 m) over a wide range of H+ concentrations and equilibration periods. Extreme precautions (such as the use of pure U( IV) stock solutions, EuCl2 reducing agent, and deoxygenated waters thr ough equilibrations with Fe powder) were taken to ensure that the diss olved U was present as U(IV). Similar trends of rapidly decreasing sol ubilities, 3 to 4 orders of magnitude with a unit change in pc(H)+, ov er very narrow ranges in pc(H)+ values, for both hydrous oxides and th e oxidation state analyses of aqueous U indicate that U was being main tained as U(IV). The differences in solubility between the two hydrous oxides did not change significantly over wide ranges of chloride conc entrations. These solubility data, along with other data available in the literature, are interpreted using Pitzer's ion-interaction model, which requires the use of 1) U4+, Th4+, and UOH3+ species and binary i nteractions of these species with Cl-, 2) cation-cation and cation-cat ion-anion interactions involving Th4+ with bulk electrolyte ions, and 3) log of the solubility products of UO2(am) and ThO2(am) of -53.45 an d -45.5, respectively. The model predictions agree well with the exper imental solubilities of U(IV) and Th hydrous oxides in dilute to conce ntrated NaCl and MgCl2 solutions.