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

The solubility and speciation of Th in Cl-bearing solutions saturated in ThO2 was investigated experimentally at 150, 175, 200, 225 and 250 degrees C, and saturated water vapor pressure. At these experimental conditions (pH(25 degrees 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)(2)(2+) and Th(OH)(4)(0) and formed via the reactions Th4+ + 2OH(-) <--> Th(OH)(2)(2+) and Th4+ + 2OH(-) <--> Th(OH)(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 degrees 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 degrees C. Extrapolations of our formation constants to 25 degrees C are in excellent agreement with the values determined at 25 degrees 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. (C) 2018 Elsevier Ltd. All rights reserved.