Modelling the sulfate capacity of simulated radioactive waste borosilicate glasses

The capacity of simulated high-level radioactive waste borosilicate glasses to incorporate sulfate has been studied as a function of glass composition. Combined Raman, ⁵⁷Fe Mössbauer and literature evidence supports the attribution of coordination numbers and oxidation states of constituent cations for the purposes of modelling, and results confirm the validity of correlating sulfate incorporation in multicomponent borosilicate radioactive waste glasses with different models. A strong compositional dependency is observed and this can be described by an inverse linear relationship between incorporated sulfate (mol% SO4²⁻) and total cation field strength index of the glass, Σ(z/a²), with a high goodness-of-fit (R² ≈ 0.950). Similar relationships are also obtained if theoretical optical basicity, Λth (R² ≈ 0.930) or non-bridging oxygen per tetrahedron ratio, NBO/T (R² ≈ 0.919), are used. Results support the application of these models, and in particular Σ(z/a²), as predictive tools to aid the development of new glass compositions with enhanced sulfate capacities.