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

Abstract

The work presented in this review combines the strength of experimental and computational materials science to understand the ironinduced structural rearrangements in peralkaline aluminoborosilicate glasses and their impact on the solubility of sulfur (as SO 4 2−) in model nuclear waste glasses. The majority (≥96%) of iron exists as tetrahedrally coordinated Fe 3+ (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 Fe 2+ . Increasing Fe 2 O 3 concentration (from 0 to 3 mol %) leads to glass network repolymerization through two mechanisms: (1) scavenging of Na + by FeO 4 − 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 Fe 2 O 3 content, sulfur solubility diminishes as SO 4 2− struggles to compete effectively with AlO 4 − , BO 4 − , and FeO 4 − 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 SO 4 2− prefers to be charge-compensated by Na + over Fe 2+ .