Strengthened Glass Network Reduces Ru Dissolution in Calcium Aluminosilicate Glasses

Abstract

Ruthenium (Ru), a platinum-group element produced in considerable amounts during nuclear fuel reprocessing, shows extremely low solubility in silicate melts. Consequently, ruthenium oxide precipitation can cause operational instability during vitrification. To clarify the structural origin of the solubility limits of Ru, the dissolution behavior of Ru in CaO–Al₂O₃–SiO₂ (CAS) glass melts with different Al₂O₃/SiO₂ (A/S) mole ratios (A/S  =  0.05–0.18) was investigated. Dissolved Ru contents were measured by inductively coupled plasma mass spectrometry (ICP–MS), and the structural evolution of the glass network was examined using Raman, FTIR, ²⁷Al MAS, and MQMAS NMR spectroscopies. UV–Vis spectroscopy and density functional theory (DFT) simulations were used to examine the oxidation state and local coordination environment of dissolved Ru, respectively. Increasing the A/S ratio from 0.05 to 0.18 resulted in a systematic reduction in dissolved Ru content. This decline was related to clear structural modifications within the glass network. With increasing A/S ratio, the structural replacement of Si⁴⁺ by Al³⁺ was achieved through the incorporation of [AlO₄] units into the network (from 0.03 to 0.08 mole fraction), whereas the content of [AlO₅] units remained nearly unchanged (∼0.004 ± 0.002 molfraction), promoting stronger cross-linking between [AlO₄] and [SiO₄], stabilized by Ca²⁺ cations acting as charge compensators. This decreased the concentration of non-bridging oxygens (NBO) and reduced the availability of Ca²⁺ for the stabilization of Ru (mainly [RuO₄]), thereby markedly suppressing the dissolved Ru content in these glass melts.