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) between pH 3.5 and 5 with electrospray ionization mass spectrometry (ESI‒MS) and laser‒induced luminescence spectroscopy was comprehensively characterized. The ESI‒MS experiments performed at a total silicon concentration of 5∙10^‒5 M (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^‒6 M, the silicon concentration was varied between 1.3∙10^‒4 ‒ 1.3∙10^‒3 M (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 van’t Hoff equation, corroborating an endothermic and entropy driven complexation process.