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

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2 LANIE - Laboratoire de développement Analytique Nucléaire Isotopique et Elémentaire
SEARS - Service d'études analytiques et de réactivité des surfaces : DEN/DPC
Abstract : 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_nUO_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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Submitted on : Thursday, December 16, 2021 - 4:42:19 PM
Last modification on : Friday, December 17, 2021 - 10:54:15 AM

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### Citation

Chengming Shang, Pascal E. Reiller. Effect of temperature on the complexation of triscarbonatouranyl( vi ) with calcium and magnesium in NaCl aqueous solution. Dalton Transactions, Royal Society of Chemistry, 2021, 50 (46), pp.17165-17180. ⟨10.1039/D1DT03204F⟩. ⟨cea-03472460⟩

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