Solubility of crystalline thorium dioxide

Abstract

Summary The solubility of thorium oxides of different crystallinity is investigated at 25°C by different experimental approaches. The dissolution of bulk crystalline ThO2(cr) is a very slow process and the Th(IV) concentrations measured after one year at pH 1-3 in 0.1 and 0.5M HCl-NaCl solutions do not represent equilibrium data. Coulometric titration of thorium nitrate solutions in the low pH range of 1.5-2.5 leads to the formation of microcrystalline ThO2·xH2O(mcr) particles which subsequently agglomerate to a precipitate. The solubility of this solid, in equilibrium with Th4+(aq), is measured from the oversaturation direction. The solubility product is determined to be log K´sp=-49.9±0.4 in 0.5M NaCl corresponding to log K°sp=-53.2±0.4 (converted to I=0 with the SIT). It is close to the thermochemical value for ThO2(cr) and about 6 orders of magnitude lower than that of X-ray amorphous Th(IV) hydroxide or hydrous oxide. The differences in the solubility products are discussed with regard to the particle size and compared with analogous data for U(IV), Np(IV) and Pu(IV). Above the threshold of hydrolysis of Th4+ at pH>2.5, the dissolution of microcrystalline ThO2·xH2O(mcr) is found to be irreversible. In near-neutral to alkaline solutions, the measured thorium concentrations approach those of amorphous Th(OH)4(am). Similar results are obtained with crystalline ThO2(cr) in 0.5M NaCl-NaOH solutions. The solubility is not controlled by the bulk crystalline solid but by amorphous fractions on the surface.