Effect of temperature on the complexation of triscarbonatouranyl(vi) with calcium and magnesium in NaCl aqueous solution

Abstract

The complex formation of triscarbonatouranyl(VI) UO₂(CO₃)₃⁴⁻ with the alkaline earth metal ions Mg²⁺ and Ca²⁺ in 0.10 mol kg_w⁻¹ NaCl was studied at variable temperatures: 5–30 °C for Mg²⁺ and 10–50 °C for Ca²⁺. Under appropriate conditions, the ternary complexes (M_nUO₂(CO₃)₃^(4−2n)− with n = 1 for Mg, n = {1; 2} for Ca) were identified by time-resolved laser-induced luminescence spectrometry. Their pure spectral components at 50 °C for Ca_nUO₂(CO₃)₃^(4−2n)− and 30 °C for MgUO₂(CO₃)₃²⁻ were recovered by multivariate curve resolution alternating least-squares analysis. Approximation models were tested to fit the experimental data—the equilibrium constants of complexation measured at different temperatures—and deduce the thermodynamic functions, i.e., enthalpy, entropy, and heat capacity. The weak influence of temperature on complexation constants induces large uncertainties in terms of thermodynamic functions. Assuming the enthalpy is constant with temperature using the Van't Hoff equation, the first stepwise complexation of UO₂(CO₃)₃⁴⁻ by Ca²⁺ is estimated to be slightly endothermic, with , while the second stepwise complexation of CaUO₂(CO₃)₃²⁻ by Ca²⁺ with is slightly exothermic, . In contrast to Ca²⁺, the complexation of UO₂(CO₃)₃⁴⁻ by Mg²⁺ is slightly exothermic, with . These values are not significantly different from zero inasmuch as the uncertainties are important due to a weak dependence of log₁₀ K° values. The entropic character of the complexation is verified as for the first stepwise complexation of UO₂(CO₃)₃⁴⁻ by Ca²⁺, for the second stepwise complexation of CaUO₂(CO₃)₃²⁻ by Ca²⁺, and for the complexation of UO₂(CO₃)₃⁴⁻ by Mg²⁺. The energetics of complexation and sensitivity analysis of the model estimates with temperature are discussed. The uranium speciation in the case of the safety of nuclear waste management, using the present thermodynamic functions, provides support to the assessment of underground nuclear waste repositories.