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Solubility / Thermodynamics / Thorium dioxide / ThO 2 (c) / ThO2(am) / Temperature effects on ThO2 crystallinity / Solubility product / Solubility products at different temperatures Summary. 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 highlevel 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) ThO 2 (am) at 23°C, 2) ThO 2 (am → c), a thermally transformed amorphous thorium hydrous oxide at 90°C, and 3) ThO 2 (c) at 23°C and 90°C. Results show that when ThO 2 (am) is heated to 90°C, it transforms to a relatively insoluble and crystalline solid [ThO 2 (am → c)]. At a fixed pH, the observed solubility of ThO 2 (am) at 23°C is more than 11 orders of magnitude greater than those for ThO 2 (c) at 23°C or of ThO 2 (am → c) and ThO 2 (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 [ThO 2 (s) ϩ 2 H 2 O < Th 4ϩ ϩ 4 OH Ϫ ] was determined to be Ϫ44.9 for ThO 2 (am) at 23°C, Ն Ϫ56.9 for ThO 2(c) at 23°C, and Ϫ51.4 for ThO2(c) at 90°C. At 90°C, a relatively less crystalline phase, ThO 2 (am → c), showed slightly higher solubility (log K sp ϭ Ϫ49.2) than crystalline ThO 2(c).

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