Solid-Liquid Phase Equilibria of Pu(VI) and U(VI) in Aqueous Carbonate Systems. Determination of Stability Constants

Abstract

The carbonate complexation constants of hexavalent plutonyl and uranyl ions are determined by solubility study of PuO2CO3(s) and UO2CO3(s) under 100% and 1% CO2 atmosphere for Pu(VI) and 100% CO2 atmosphere for U(VI). pH is varied in the range from 3.5 to 8 in 0.1 M NaClO4 at room temperature (22±2°C). Based on the solubility products from the previous experiment, the following carbonate complexation constants are evaluated: logβ11(UO2CO3) = 8.93±0.05 logβ11(PuO2CO3) = 8.7±0.3 logβ12(UO2(CO3)2)2-= 15.3±0.2 logβ12(PuO2(CO)3)2 2-) = 14.1±0.5 logβ13(UO2(CO3)34-) = 21.0±0.3 logβ13(PuO2(CO3)34-) = 17.8±0.2. The ion-interaction approach (Pitzer equations) is parameterized for the U(VI) system on the basis of the literature data. The present solubility data are found to be in close agreement with calculated results based on the parameters. The same parameters appear also applicable to the Pu(VI) system for a good approximation.