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 involving uranyl and carbonate with alkaline earth metals (specifically Mg2+, Ca2+, Sr2+, and Ba2+) were investigated using an anion exchange method. The objective was to quantify the stability of complexes MUO2(CO3)32- and M2UO2(CO3)30 by varying metal concentrations (0.1-5 mmol/L) at a fixed pH of 8.1, while maintaining a constant ionic strength of 0.1 mol/L NaNO3 under equilibrium with atmospheric CO2. The results reveal that for Ca2+ and Ba2+, both complexes are formed simultaneously, while Mg2+ and Sr2+ only form the MUO2(CO3)32- complex under the established experimental conditions. Cumulative stability constants for MUO2(CO3)32- varied slightly with different metals: values were found to be 26.11 (Mg2+), 27.18 (Ca2+), 26.86 (Sr2+), and 26.68 (Ba2+). For M2UO2(CO3)30, log /betatwo213 values were 30.70 (Ca2+) and 29.75 (Ba2+). Speciation calculations indicated that at low Ca2+ concentrations, the CaUO2(CO3)32- complex predominated, while at higher concentrations, the distribution of Ca2UO2(CO3)3 increased. The findings also suggest that uranium sorption onto anion-exchange resins is hindered by the formation of neutral Ca2UO2(CO3)30 species.