Reduction of Earth Alkaline Metal Salts in THF Solution Studied by Picosecond Pulse Radiolysis

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

The complex formation of triscarbonatouranyl(VI) UO₂(CO₃)₃^4- 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)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 UO₂(CO₃)₃^4- by Ca²⁺ is estimated to be slightly endothermic, with , while the second stepwise complexation of CaUO₂(CO₃)₃^2- by Ca²⁺ with is slightly exothermic, . In contrast to Ca²⁺, the complexation of UO₂(CO₃)₃^4- 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)₃^4- by Ca²⁺, for the second stepwise complexation of CaUO₂(CO₃)₃^2- by Ca²⁺, and for the complexation of UO₂(CO₃)₃^4- 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.

Direct Evidence for Transient Pair Formation between a Solvated Electron and H3O(+) Observed by Picosecond Pulse Radiolysis

The reaction between the solvated electron and hydronium cation H3O(+) in water constitutes a fundamental reaction in chemistry. Due to significant rearrangement of solvent molecules around both the electron and H3O(+), the reaction rate of this process is not controlled by diffusion. The presence of a reaction barrier suggests the formation of an intermediate that has so far not been observed. Here, the time-resolved visible absorption spectra in three concentrated acid solutions, perchloric, sulfuric, and phosphoric, at various concentrations are recorded by the picosecond pulse radiolysis method. In contrast to previous reports, a strong blue shift of the absorption band of the solvated electron in acidic solutions compared to neat water is clearly observed, consistent with formation of a pair between the solvated electron and hydronium cation.