Thermodynamic model for the solubility of thorium dioxide in the Na+-Cl--OH--H2O system at 23 degrees C and 90 degrees C

Data are extremely limited on the effects of temperature on crystallinity a nd the resulting changes in solubility products of thermally transformed th orium oxide phases. Such data are required to reliably predict thorium beha vior in high-level waste repositories where higher than ambient temperature s are expected. Solubility studies were conducted as a function of pH and t ime and at 0.1 M NaCl for 1) ThO2(am) at 23 degrees C, 2) ThO2(am --> c), a thermally transformed amorphous thorium hydrous oxide at 90 degrees C, and 3) ThO2(c) at 23 degrees C and 90 degrees C, Results show that when ThO2(a m) is heated to 90 degrees C, it transforms to a relatively insoluble and c rystalline solid [ThO2(am --> c)]. At a fixed pH, the observed solubility o f ThO2(am) at 23 degrees C is more than 11 orders of magnitude greater than those for ThO2(c) at 23 degrees C or of ThO2(am --> c) and ThO2(c) at 90 d egrees C. Solubility data were interpreted using the Pitzer ion-interaction model. The log of the solubility product for the thorium dioxide dissoluti on reaction [ThO2(s) + 2 H2O = Th4+ + 4 OH-] was determined to be -44.9 for ThO,(am) at 23 degrees C, greater than or equal to -56.9 for ThO2(c) at 23 degrees C, and -51.4 for ThO2(c) at 90 degrees C. At 90 degrees C, a relat ively less crystalline phase, ThO2(am --> c), showed slightly higher solubi lity (log K-sp = -49.2) than crystalline ThO2(c).