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

Abstract The solubility and speciation of Th in Cl-bearing solutions saturated in ThO 2 was investigated experimentally at 150, 175, 200, 225 and 250 °C, and saturated water vapor pressure. At these experimental conditions (pH 25°C 2 2+ and Th(OH) 4 ° and formed via the reactions T h 4 + + 2 O H - ↔ T h ( O H ) 2 2 + and T h 4 + + 4 O H - ↔ T h ( O H ) 4 0 . 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 (ThO 2 ), the solubility determined in our study likely reflects a minimum, as the presence of additional ligands would lead to higher solubility.