Effect of temperature on the complexation of triscarbonatouranyl(VI) with calcium and magnesium in NaCl aqueous solution
Résumé
The complex formation of triscarbonatouranyl(VI) UO$_2$(CO$_3$)$_3$$^{4−}$ with the alkaline earth metal ions Mg$^{2+}$ and Ca$^{2+}$ in 0.10 mol kg$_w$$^{-1}$ NaCl was studied at variable temperatures: 5–30 °C for Mg$^{2+}$ and 10–50 °C for Ca$^{2+}$. Under appropriate conditions, the ternary complexes (M$_n$UO$_2$(CO$_3$)$_3$$^{(4−2n)−}$ with n = 1 for Mg, n = {1; 2} for Ca) were identified by time-resolved laser-induced luminescence spectrometry. Their pure spectral components at 50 °C for Ca$_n$UO$_2$(CO$_3$)$_3$$^{(4−2n)−}$ and 30 °C for MgUO$_2$(CO$_3$)$_3$$^{2−}$ were recovered by multivariate curve resolution alternating least-squares analysis. Approximation models were tested to fit the experimental data—the equilibrium constants of complexation measured at different temperatures —and deduce the thermodynamic functions, i.e., enthalpy, entropy, and heat capacity. The weak influence of temperature on complexation constants induces large uncertainties in terms of thermodynamic functions. Assuming the enthalpy is constant with temperature using the Van’t Hoff equation, the first stepwise complexation of UO$_2$(CO$_3$)$_3$$^{4−}$ by Ca$^{2+}$ is estimated to be slightly endothermic, with $Δ_r$$^{VH}$ $H°_m$ = (4.9 ± 5.9)kJ mol$^{-1}$, while the second stepwise complexation of CaUO$_2$(CO$_3$)$_3$$^{2−}$ by Ca$^{2+}$ with is slightly exothermic, $Δ_r$$^{VH}$ $H°_m$ = -(6.2 ± 13.3) kJ mol$^{-1}$. In contrast to Ca$^{2+}$, the complexation of UO$_2$(CO$_3$)$_3$$^{4-}$ by Mg$^{2+}$ is slightly exothermic, with $Δ_r$$^{VH}$ $H°_m$ = -(11.7 ± 10.3) kJ mol$^{-1}$. These values are not significantly different from zero inasmuch as the uncertainties are important due to a weak dependence of log$_{10}$ K° values. The entropic character of the complexation is verified as $TΔ_rS°_m$ = (36.5 ± 6.0) kJ mol$^{-1}$ for the first stepwise complexation of UO$_2$(CO$_3$)$_3$$^{4−}$ by Ca$^{2+}$, $TΔ_rS°_m$ = (13.8 ± 13.3) kJ mol$^{-1}$ for the second stepwise complexation of CaUO$_2$(CO$_3$)$_3$$^{2−}$ by Ca$^{2+}$, and $TΔ_rS°_m$ = (14.2 ± 10.3)= kJ mol$^{-1}$ for the complexation of UO$_2$(CO$_3$)$_3$$^{4−}$ by Mg$^{2+}$. 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.
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