Determination of the formation constants of ternary complexes of uranyl and carbonate with alkaline earth metals (Mg2+, Ca2+, Sr2+, and Ba2+) using anion exchange method

The formation constants of ternary complexes (MUO2(CO3)(3)(2-) and M2UO2(CO3)(3)(0)) of uranyl and carbonate with alkaline earth metals (M2+ denotes Mg2+, Ca2+, Sr2+, and Ba2+) were determined with an anion exchange method by varying the metal concentrations (0.1-5 mmol/L) at pH 8.1 and a constant ionic strength (0.1 mol/L NaNO3) under equilibrium with atmospheric CO2. The results indicate that the complexes of MUO2(CO3)(3)(2-) and M2UO2(CO3)(3) are simultaneously formed for Ca2+ and Ba2+, while Mg2+ and Sr2+ form only the MUO2(CO3)(3)(2-) complex under our experimental conditions. The cumulative stability constants for the MUO2(CO3)(3)(2-) complex obtained at I = 0 are as follows: log beta(113) = 26.11 +/- 0.04, 27.18 +/- 0.06, 26.86 +/- 0.04, and 26.68 +/- 0.04 for Mg2+, Ca2+, Sr2+, and Ba2+, respectively. For M2UO2(CO3)(3)(0), the value of log beta(213) at I = 0 was measured to be 30.70 +/- 0.05 and 29.75 +/- 0.07 for Ca2+ and Ba2+, respectively. Based on the formation constants obtained in this study, speciation calculations indicate that at low Ca2+ concentration (e.g., < 2.2 mmol/ L), CaUO2(CO3)(3)(2-) is more important than Ca2UO2(CO3)(3) and that the Ca2UO2(CO3)(3) distribution increased with increasing Ca2+ concentration. Uranium sorption onto anion- exchange resins is inhibited by the formation of the neutral Ca2UO2(CO3)(3)(0) species.