Formation of ternary CaUO2(CO3)32- and Ca2UO2(CO3)3(aq) complexes under neutral to weakly alkaline conditions

The chemical behavior of ternary Ca-UO2-CO3 complexes was investigated by using time-resolved laser fluorescence spectroscopy (TRLFS) in combination with EDTA complexation at pH 7-9. A novel TRLFS revealed two distinct fluorescence lifetimes of 12.7 +/- 0.2 ns and 29.2 +/- 0.4 ns for uranyl complexes which were formed increasingly dependent upon the calcium ion concentration, even though nearly indistinguishable fluorescence peak shapes and positions were measured for both Ca-UO2-CO3 complexes. For identifying the stoichiometric number of complexed calcium ions, slope analysis in terms of relative fluorescence intensity versus calcium concentration was employed in a combination with the complexation reaction of CaEDTA(2-) by adding EDTA. The formation of CaUO2(CO3)(3)(2-) and Ca2UO2(CO3)(3)(aq) was identified under given conditions and their formation constants were determined at I = 0.1 M Na/HClO4 medium, and extrapolated to infinitely dilute solution using specific ion interaction theory (SIT). As a result, the formation constants for CaUO2(CO3)(3)(2-) and Ca2UO2(CO3)(3)(aq) were found to be log beta(0)(113) = 27.27 +/- 0.14 and log beta(0)(213) = 29.81 +/- 0.19, respectively, providing that the ternary Ca-UO2-CO3 complexes were predominant uranium(VI) species at neutral to weakly alkaline pH in the presence of Ca2+ and CO32- ions.