Determination of the formation constants of ternary complexes of uranyl and carbonate with alkaline earth metals (Mg²⁺, Ca²⁺, Sr ²⁺, and Ba²⁺) using anion exchange method

The formation constants of ternary complexes (MUO₂(CO ₃)₃^2- and M₂UO₂(CO ₃I₃⁰) of uranyl and carbonate with alkaline earth metals (M²⁺ denotes Mg²⁺, Ca²⁺, Sr ²⁺, 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 MUO ₂(CO₃)₃^2- and M₂UO ₂(CO₃)₃ are simultaneously formed for Ca ²⁺ 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 / = 0 are as follows: logβ₁₁₃ = 26.11 ± 0.04, 27.18 ± 0.06, 26.86 ± 0.04, and 26.68 ± 0.04 for Mg²⁺, Ca ²⁺, Sr²⁺, and Ba²⁺, respectively. For M ₂UO₂(CO₃)₃⁰, the value of logβ₂₁₃ at / = 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 Ca ₂UO₂(CO₃)₃ distribution increased with increasing Ca²⁺ concentration. Uranium sorption onto anion-exchange resins is inhibited by the formation of the neutral Ca ₂UO₂(CO₃)₃ species.