Complexation of UO2(CO3)34− with Mg2+ at varying temperatures and its effect on U(vi) speciation in groundwater and seawater

Abstract

The ternary alkaline earth metal uranyl tricarbonate complexes, M_nUO₂(CO₃)₃²ⁿ⁻⁴ (M = Mg and Ca), have been considered to be the major U(VI) species contributing to uranium mobility in natural water. Although MgUO₂(CO₃)₃²⁻ can account for a substantial portion of U(VI) in a Mg²⁺-rich aqueous system and most processes regarding uranium are subjected to variable temperatures, chemical thermodynamic data for the prediction of the formation of MgUO₂(CO₃)₃²⁻ at variable temperatures are still unknown. To fill the knowledge gap in the current chemical thermodynamic database, ultraviolet/visible (UV/Vis) absorption spectroscopy was employed to determine the formation constants (log K′) of MgUO₂(CO₃)₃²⁻ at varying temperatures of 10–85 °C in 0.5 mol kg⁻¹ NaCl. The formation constants at infinite dilution, log K°, were obtained with specific ion interaction theory (SIT), and an increasing tendency of log K° with temperature was observed. Using calorimetric titration, the endothermic molar enthalpy of reaction (Δ_rH_m) of Mg²⁺ complexation with UO₂(CO₃)₃⁴⁻ was determined at 25 °C. According to the chemical thermodynamic data obtained in this work, approximation models for the prediction of the temperature-dependent formation constant at a given temperature were examined and the constant enthalpy approximation with modification to the isoelectric reaction showed a satisfactory agreement with our experimental results. Finally, the effects of temperature on U(VI) speciation in Mg²⁺-rich groundwater and U(VI) extraction from seawater by amidoxime derivatives were examined. For the first time, this work provides important chemical thermodynamic data of MgUO₂(CO₃)₃²ⁿ⁻⁴ to assess the impact of temperature on U(VI) behaviour in groundwater and seawater.