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: logβ 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 logβ 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.,