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

Abstract

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).

Solubility of Thorium(IV) in the Presence of Oxalic and Malonic Acids

“…It is interesting to note that Th(IV) hydroxide is considered to be stable in a wide variety of electrolyte solutions, and the solubility equilibrium has been observed within at least a few days. 11) The present solubility data obtained by 3 kDa filtration are in agreement with the literature data obtained by filtration with 5 9) and 2.5 kDa 10) NMWL membranes, and are slightly lower than the data obtained with 30 kDa NMWL membranes. 11) The apparent solubility increased slightly with an increase in filter pore size, indicating the possible presence of a colloidal species (Table 1).…”

“…11) The present solubility data obtained by 3 kDa filtration are in agreement with the literature data obtained by filtration with 5 9) and 2.5 kDa 10) NMWL membranes, and are slightly lower than the data obtained with 30 kDa NMWL membranes. 11) The apparent solubility increased slightly with an increase in filter pore size, indicating the possible presence of a colloidal species (Table 1). In the present study, the colloidal species were assumed to be different from the wellordered polynuclear species, the contribution of which to the apparent solubility was considered to be not dominant in the previous study.…”

“…Thorium Solubility in the Absence of Carboxylic Acid Figure 1 shows the Th(IV) solubility in 0.2-2.0 M NaClO 4 solutions containing no carboxylic acid, obtained by 3 kDa NMWL filtration, together with previously reported data. [9][10][11] No significant change in the Th(IV) concentrations was observed between 89 and 104 days, so that the solubility equilibrium was considered to be achieved at least after 89 days. It is interesting to note that Th(IV) hydroxide is considered to be stable in a wide variety of electrolyte solutions, and the solubility equilibrium has been observed within at least a few days.…”

Thermodynamic Model for ThO2(am) Solubility in Isosaccharinate Solutions

“…Although previous studies [9,16] higher for the pH 6 set and lower for the pH 12 set ( Figure 2). It should be noted that these differences are up to several orders of magnitude greater than what might be expected from uncertainties in the estimated log 10 β n 0 values caused by using estimated ion-interaction parameters.…”

Applicability of Thermodynamic Database of Radioactive Elements Developed for the Japanese Performance Assessment of HLW Repository

“…1 14) ). Figure 1 also shows recent our data for U, 15) Th 16) and Pu described in the following section. Although the thermodynamic data dictates that the crystalline phases will be more stable than the amorphous phases, it does not indicate the time required for the conversion of amorphous phases to crystalline phases.…”