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

Abstract

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 CaEDTA²⁻ by adding EDTA. The formation of CaUO₂(CO₃)₃²⁻ 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)₃²⁻ and Ca₂UO₂(CO₃)₃(aq) were found to be log β⁰₁₁₃ = 27.27 ± 0.14 and log β⁰₂₁₃ = 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.