Experimental and computational evidence of U(VI)–OH–Si(OH)4_4 complexes under alkaline conditions: Implications for cement systems

International audienceThe complexation of uranyl hydroxides with orthosilicic acid was investigated by experimental and theoretical methods. Spectroluminescence titration was performed in a glovebox under argon atmosphere at pH 9.2, 10.5 and 11.5, with [U(VI)] = 10−6 and 5 × 10−6 mol kgw−1. The polymerization effects of silicic acid were minimized by ruling out samples with less than 90 % monomeric silicic acid present, identified via UV–Vis spectrometry using the molybdate blue method. Linear regression analysis based on time-resolved laser-induced fluorescence spectroscopy (TRLFS) results yielded the conditional stepwise formation constants of U(VI)–OH–Si(OH)4 complexes at 0.05 mol kgw−1 NaNO3. The main spectroscopic features – characteristic peak positions and decay-time – are reported for the first time for the UO2(OH)2SiO(OH)3− species observed at pH 9.2 and 10.5 and UO2(OH)2SiO2(OH)22− predominant at pH 11.5. Quantum chemical calculations successfully computed the theoretical luminescence spectrum of the complex UO2(OH)2SiO(OH)3− species, thus underpinning the proposed chemical model for weakly alkaline systems. The conditional stability constants were extrapolated to infinite dilution using the Davies equation, resulting in log10β°(UO2(OH)2SiO(OH)3−) and log10β°(UO2(OH)2SiO2(OH)22−). Implications for U(VI) speciation in the presence and absence of competing carbonate are discussed for silicate-rich environments expected in certain repository concepts for nuclear waste disposal

Coordination and Thermodynamics of Trivalent Curium with Malonate at Increased Temperatures: A Spectroscopic and Quantum Chemical Study

The complexation of Cm­(III) with malonate is studied by time-resolved laser fluorescence spectroscopy (TRLFS) in the temperature range from 25 to 90 °C. Three complexes ([Cm­(Mal)_<i>n</i>]^{3–2<i>n</i>}, <i>n</i> = 1, 2, 3) are identified and their molar fractions are determined as a function of the ligand concentration, the ionic strength, and the temperature. A general shift of the chemical equilibrium toward higher complexes with increasing temperature is observed, with the [CmMal₃]^3– complex forming only at <i>T</i> > 40 °C. The conditional stability constants (log <i>K</i>′_<i>n</i>(<i>T</i>)) are calculated and extrapolated to <i>I</i>_m = 0 with the specific ion interaction theory (SIT). The log <i>K</i>_<i>n</i>⁰(<i>T</i>) values increase by 0.25 to 0.5 logarithmic unit in the studied temperature range. The temperature dependency of the log <i>K</i>°_<i>n</i>(<i>T</i>) is fitted by the integrated Van’t Hoff equation, yielding the thermodynamic functions Δ_r<i>H</i>°_m and Δ_r<i>S</i>°_m. The results show positive reaction enthalpies and entropies for each complexation step. While the Δ_r<i>H</i>°_<i>n</i> values are constant within their error range, the Δ_r<i>S</i>°_<i>n</i> values decrease successively with each ligand added. To explain this effect, quantum chemical calculations of binding energies and bond lengths of the different Cm­(III) malonate species are performed. The results show that malonate is capable of stabilizing its end-on coordination mode to some extent by forming hydrogen bonds to first-shell water molecules. As a result, an equilibrium between side-on and end-on coordinated malonate ligands is present, with the latter becoming more pronounced for the higher complexes due to steric reasons