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

Data are extremely limited on the effects of temperature on crystallinity and the resulting changes in solubility products of thermally transformed thorium oxide phases. Such data are required to reliably predict thorium behavior in high-level waste repositories where higher than ambient temperatures are expected. Solubility studies were conducted as a function of pH and time and at 0.1 M NaCl for 1) ThO₂(am) at 23°C, 2) ThO₂ (am→c), a thermally transformed amorphous thorium hydrous oxide at 90°C, and 3) ThO₂(c) at 23°C and 90°C. Results show that when ThO₂(am) is heated to 90°C, it transforms to a relatively insoluble and crystalline solid [ThO₂(am→c)]. At a fixed pH, the observed solubility of ThO₂(am) at 23°C is more than 11 orders of magnitude greater than those for ThO₂(c) at 23°C or of ThO₂(am→c) and ThO₂(c) at 90°C. Solubility data were interpreted using the Pitzer ion-interaction model. The log of the solubility product for the thorium dioxide dissolution reaction [ThO₂(s) + 2 H₂O ↔ Th⁴⁺ + 4 OH^-] was determined to be -44.9 for ThO₂(am) at 23°C, ≥-56.9 for ThO₂(c) at 23°C, and -51.4 for ThO₂(c) at 90°C. At 90°C, a relatively less crystalline phase, ThO₂(am→c), showed slightly higher solubility (log K_sp = -49.2) than crystalline ThO₂(c).