Temperature-dependent luminescence spectroscopic and mass spectrometric investigations of U(VI) complexation with aqueous silicates in the acidic pH-range

In this study the complexation of U(VI) with orthosilicic acid (H₄SiO₄) was investigated between pH 3.5 and 5 by combining electrospray ionization mass spectrometry (ESI-MS) and laser-induced luminescence spectroscopy. The ESI-MS experiments performed at a total silicon concentration of 5 · 10^-3M (exceeding the solubility of amorphous silica at both pH-values) revealed the formation of oligomeric sodium-silicates in addition to the UO₂OSi(OH)₃⁺ species. For the luminescence spectroscopic experiments (25 °C), the U(VI) concentration was fixed at 5 · 10^-6M, the silicon concentration was varied between 1.3 · 10^-4-1.3 · 10^-3M (reducing the formation of silicon oligomers) and the ionic strength was kept constant at 0.2 M NaClO₄. The results confirmed the formation of the aqueous UO₂OSi(OH)₃⁺ complex. The conditional complexation constant at 25 °C, log *β = -(0.31 ± 0.24), was extrapolated to infinite dilution using the Davies equation, which led to log *β⁰ = -(0.06 ± 0.24). Further experiments at different temperatures (1-25 °C) allowed the calculation of the molal enthalpy of reaction Δ_rH_m⁰ = 45.8 ± 22.5 kJ·mol^-1 and molal entropy of reaction Δ_rS_m⁰ = 152.5 ± 78.8 J·K^-1·mol^-1 using the integrated van't Hoff equation, corroborating an endothermic and entropy driven complexation process.