Iron-Induced Structural Rearrangements and Their Impact on Sulfur Solubility in Borosilicate-Based Nuclear Waste Glasses

The work presented in this review combines the strength of experimental and computational materials science to understand the iron-induced structural rearrangements in peralkaline aluminoborosilicate glasses and their impact on the solubility of sulfur (as SO42-) in model nuclear waste glasses. The majority (>= 96%) of iron exists as tetrahedrally coordinated Fe3+ (both isolated and clustered) in the investigated glasses, thus acting as a network former being charge-compensated by Na+, while the remaining iron exists as tetrahedrally (distorted) coordinated Fe2+. Increasing Fe2O3 concentration (from 0 to 3 mol %) leads to glass network repolymerization through two mechanisms: (1) scavenging of Na+ by FeO4- units for charge compensation, likely from the nonbridging oxygen (NBO) sites, and (2) formation of Fe-O-(Si, B) linkages. Due to the decreased availability of NBO-associated Na+ in the glassy matrix with increasing Fe2O3 content, sulfur solubility diminishes as SO42- struggles to compete effectively with AlO4-, BO4-, and FeO4- for preferential charge compensation by Na+. Mossbauer spectroscopy reveals an insignificant impact of sulfur on the redox behavior or environment of iron in the glass structure, and SO42- prefers to be charge-compensated by Na+ over Fe2+