Solubility of crystalline thorium dioxide

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.