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 UO3.2H2O 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 (Ksp ), 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 Ksp ° value at the standard state (I=0) was determined to be log Ksp °=—22.46±0.10. Possible differences in the log Ksp° values due to different solid phases were recognized.