URANIUM(IV) HYDROLYSIS CONSTANTS AND SOLUBILITY PRODUCT OF UO2.XH2O(AM)

The solubility of UO2·xH2O was examined in dilute solutions at room temperature and in the pH range from 2 to 12. Iron powder and Eu2+were used to effectively eliminate O2and to maintain reducing conditions so as to minimize the possibility of oxidation of U(IV) to U(VI) during the experiments. The UO2·xtH2O was found to be amorphous to X-rays, and its solubility, especially at pH values >4, was found to be 3–4 orders of magnitude lower than previously reported. A thermodynamic analysis of our solubility data yielded a log K at zero ionic strength of 3.5 ± 0.8 for the solubility reaction UO2·xH2O(am) + 3H+= UOH3++ (x + 1)H2O. A recalculation of the first hydrolysis constant from spectrophotometric data reported in the literature in conjunction with our solubility data yielded a log K at zero ionic strength of −0.50 ± 0.03 for the reaction U4++ H2O ⇋ UOH3++ H+and a log K of −52.0 ± 0.8 for the reaction UO2·xH2O(am) ⇋ U4++ 40H−+ (x − 2)H2O. These values are consistent with both the reliable solubility and spectrophotometric data and the thermodynamic properties of other actinides. Although from our data it was not possible to obtain precise values of constants (Kln) for other hydrolysis reactions, U4++ nH2O ⇋ U(OH)n4−* + nH+, the upper limits that can be calculated for log K12through log K15(<−4.0, <−8.0, <−12.0, and <−26, respectively) are several orders of magnitude lower than those previously reported. © 1990, American Chemical Society. All rights reserved.