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)32- and M2UO2(CO3)30) 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)32- and M2UO2(CO3)3 are simultaneously formed for Ca2+ and Ba2+, while Mg2+ and Sr2+ form only the MUO2(CO3)32- complex under our experimental conditions. The cumulative stability constants for the MUO2(CO3)32- complex obtained at I = 0 are as follows: logβ113 = 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)30, the value of logβ213 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)32- 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)30 species.