The solubility of microcrystalline ThO2 in phosphate media

The complexation of Th with phosphate in aqueous solution has been investigated in batch solubility experiments performed in the pH range 5-13 in 0.01 and 0.1 M total phosphate solutions and in the pH range 10.5-13 in the absence of phosphate. The solutions were mixed with NaClO4 to a total ionic strength of 1.0 M. Microcrystalline ThO2 was used as solid phase in the experiments. The solubility in the pH range 5-10 is around 10-6.4 M, which is close to what is predicted using published hydrolysis constants and solubility products. Significant deviation from the solubility of ThO2(act) compared to solutions without phosphates, is obtained in 0.1 M phosphate solutions in the pH range 10.5-13 where a small increase in the solubility is found. SEM microprobe spectra of the solid phase after use in the experiments show that it contains small amounts of P. Hence, it is possible that a solid thorium phosphate phase is forming. While the experimental results in the pH range 10.5-13 at the highest total phosphate concentrations could only be qualitatively explained, it is clear from the measurements that the influence of phosphate on the solubility of thorium in most natural waters will not be significant. (orig.)