An experimental study of the solubility and speciation of thorium in chloride-bearing aqueous solutions at temperatures up to 250 °C

The solubility and speciation of Th in Cl-bearing solutions saturated in ThO₂ was investigated experimentally at 150, 175, 200, 225 and 250 °C, and saturated water vapor pressure. At these experimental conditions (pH_25°C < 3.8, mNaCl < 2.5), Th-chloride complexation is insignificant, and the solubility of Th is controlled by hydrolysis. The two main species in solution were identified as Th(OH)₂²⁺ and Th(OH)₄° and formed via the reactions Th⁴⁺ + 2 OH^- ↔ Th_(OH)2²⁺ and Th⁴⁺ + 4 OH^- ↔ Th_(OH)4⁰. The logarithms of the formation constants determined for the first reaction are 23.91 ± 0.05, 24.79 ± 0.07, 25.78 ± 0.05, and 25.85 ± 0.05 for 175, 200, 225 and 250 °C, respectively, and 42.18 ± 0.06, 43.50 ± 0.07, 43.17 ± 0.08, and 43.74 ± 0.11 for the second reaction at the same temperatures. Formation constants could not be determined reliably for 150 °C. Extrapolations of our formation constants to 25 °C are in excellent agreement with the values determined at 25 °C by Neck and Kim (2001) and Ekberg et al. (2000). For natural systems buffered by thorianite (ThO₂), the solubility determined in our study likely reflects a minimum, as the presence of additional ligands would lead to higher solubility.