Effect of solid phase transformation on the solubility product of thorium hydrous oxide at 363 K

Abstract

The solid phases and solubilities of thorium(IV) hydroxide after aging at 363 K were investigated in the pHc range of 2–8 in NaClO4 or NaCl solutions having a 0.1–3.0 mol/dm3 (M) ionic strength. Sample solutions containing solid amorphous thorium hydroxide (Th(OH)4(am)) were prepared by the oversaturation method, and stored in a temperature chamber kept at an aging temperature (Ta) of 363 K for specified periods. The sample solutions were then cooled to 298 K for measurement of the apparent solubilities. The apparent solubilities, after aging at Ta = 363 K, decreased about 3 orders of magnitude from the solubility of Th(OH)4(am) at Ta = 298 K. The solid phases were investigated by X-ray diffraction (XRD) in order to determine the particle sizes in the thorium hydroxide solid phases. A slight growth of the particle size of the solid phases after aging at Ta = 363 K was suggested from the obtained XRD spectrum and was correlated to the decrease of the apparent solubility at Ta = 363 K based on the particle size effect of the solubility product.