Experimental and computational evidence of U(VI)-OH-Si(OH)₄ complexes under alkaline conditions: Implications for...

The 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 kg_w^-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 kg_w^-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)₂^2- 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)₂^2-). 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.