Solubility Product of Hexavalent Uranium Hydrous Oxide

The solubility of U(VI) hydrous oxide was measured in the hydrogen ion concentration (pHc) range from 4 to 6 at 25 ± 1° C in a NaClO4 solution. The experiment was carried out by oversaturation method at ionic strength I=0.1, 0.5 and 1.0 and by undersaturation method at I=1.0. The solid phase was found to be composed of a mixture of UO(3)(.)2H(2)O and UO2(OH)(2) by XRD. The concentration of U(VI) was measured by inductively coupled plasma mass spectrometry (ICP-MS), and was analyzed to obtain the solubility products (K-sp), by taking into account the participation of the U(VI) hydrolysis species such as (UO2)(2)(OH)(2)(2+). By using the literature values of the hydrolysis species and by using the specific interaction theory (SIT), the log K-sp° value at the standard state (I=0) was determined to be log K-sp°=-22.46 ± 0.10. Possible differences in the log K-sp° values due to different solid phases were recognized.