Solid-aqueous phase equilibria of uranium(VI) under ambient conditions

The solubility of uranium(VI) is investigated in the pH range 2.8 to 4.6 in equilibrium with 8.0% and 0.3% CO2 partial pressure and in the pH range 3.8 to 7.0 in equilibrium with 0.03% CO2 partial pressure at I=0.1 M NaClO4 and 25 C. UO2CO3(s) under 8.0% and UO3 x 2 H2O under 0.3% CO2 and 0.03% CO2 partial pressures at pH<7 are characterized as solubility limiting solid phases. Above pH 7, a spontaneous phase transformation of UO3 x 2 H2O(s) in a sodium containing poorly crystalline material is observed. Thermodynamic solubility products lg Ksp0=-14.91±0.10 for UO2CO3(s) and Ks0=4.68±0.14 and lg Ks0=5.14±0.05 for UO3 x 2 H2O(s) under 0.3% and 0.03% CO2 partial pressures, resp. are obtained. The evaluation of UO3 x 2 H2O(s) solubilities on basis of spectroscopic speciation by TRLFS are discussed and a formation constant lg K'35=-17.14±0.13 is obtained. (orig.)