A thermodynamic model for the solubility of UO2(am) in the aqueous K+-Na+-HCO3--CO32--OH--H2O system

Solubility data for UO2(am) were determined in the aqueous K+-Na+-HCO3--CO3 2--OH--H2O system extending to high concentrations of carbonate, bicarbonat e, and mixed carbonate-hydroxide. Several precautions were taken, including the use of reductants (Fe powder and Na2S2O4), to maintain uranium in the tetravalent state. A combination of techniques (UV-Vis-IR, X-ray absorption spectroscopy [XAS], and solvent extraction) were used to identify species and oxidation states in the solid and aqueous phases. The most important sp ecies in CO32- and relatively concentrated HCO3- solutions was U(CO3)(5)(6- ), which was identified by W-Vis-IR and XAS techniques. The predicted minim um set of dominant species that reasonably described all of the experimenta l data were 1) U(CO3)(5)(6-) in low to high concentrations of carbonate and hydroxide and in high concentrations of bicarbonate, and 2) U(OH)(2)(CO3)( 2)(2-) in low concentrations of bicarbonate. Because U(CO3)(5)(6-) is highl y charged, the values of formation constants involving this species depend on the choice of ion-interaction parameters. The values of beta((0)) and be ta((1)), Fitter modeling parameters, for U(CO3)(5)(6-) with Na+ or K+ that best described the experimental data were 1.5 and 31.3, respectively, and a re similar to the recently determined values (beta((0)) = 1.31 and beta((1) ) = 30.0) for analogous Th(IV) species. The logarithm of the thermodynamic equilibrium constant for the UO2(am) dissolution reaction (UO2(am) + 5CO(3) (2-) + 4H(+) reversible arrow U(CO3)(5)(6-) + 2H(2)O) was found to be 33.8, and for (UO2(am) + 2HCO(3)(-) reversible arrow U(OH)(2)(CO3)(2)(2-)). This value was found to be -4.8. These values, when combined with the solubilit y product of UO2(am) (log = -53.44), provided the following values of the l ogarithm of the equilibrium constant: for (U4+ + 5CO(3)(2-) reversible arro w U(CO3)(5)(6-)) the value was 31.29 and for (U4+ + 2CO(3)(2-) + 2OH(-) rev ersible arrow U(OH)(2)(CO3)(2)(2-)) the value was 41.33.