Formation, stability and structural characterization of ternary MgUO₂(CO₃)₃^2- and Mg₂UO₂(CO₃)₃(aq) complexes

The formation of ternary Mg-UO₂-CO₃ complexes under weakly alkaline pH conditions was investigated by time-resolved laser fluorescence spectroscopy (TRLFS) and extended X-ray absorption fine structure (EXAFS) and compared to Ca-UO₂-CO₃ complexes. The presence of two different Mg-UO₂-C₃ complexes was identified by means of two distinct fluorescence lifetimes of 17±2 ns and 51±2 ns derived from the multi-exponential decay of the fluorescence signal. Slope analysis in terms of fluorescence intensity coupled with fluorescence intensity factor as a function of log [Mg(II)] was conducted for the identification of the Mg-UO₂-CO₃ complexes forming. For the first time, the formation of both MgUO₂(CO₃)₃^2- and Mg₂UO₂(CO₃)₃(aq) species was confirmed and the corresponding equilibrium constants were determined as log β ₁₁₃=25.8±0.3 and β ₂₁₃=27.1±0.6, respectively. Complementarily, fundamental structural information for both Ca-UO₂-CO₃ and Mg-UO₂-CO₃ complexes was gained by extended EXAFS revealing very similar structures between these two species, except for the clearly shorter U-Mg distance (3.83 Aa) compared with U-Ca distance (4.15 Aa). These results confirmed the inner-sphere character of the Ca/Mg-UO₂-CO₃ complexes. The formation constants determined for MgUO₂(CO₃)₃^2- and Mg₂UO₂(CO₃)₃(aq) species indicate that ternary Mg-UO₂-CO₃ complexes contribute to the relevant uranium species in carbonate saturated solutions under neutral to weakly alkaline pH conditions in the presence of Mg(II) ions, which will induce notable influences on the U(VI) chemical species under seawater conditions.