Structure of Cl and F Incorporation in Aluminoborosilicate Glass: Sulfate Retention, Salt Formation, and Other Properties

Volatile anions such as chloride (Cl⁻), fluoride (F⁻), and sulfate (SO₄²⁻) play crucial roles in determining the behavior of silicate melts across geologic, industrial, and nuclear waste vitrification systems. Their incorporation affects the melt structure, phase stability, and processing properties, often through complex interactions with modifying cations. In nuclear waste vitrification specifically, volatiles play a significant role in inducing salt formation, a waste-loading-limiting phenomenon deleterious to melter operations and waste form durability. Here, model Hanford Site Low Activity Waste peralkaline aluminoborosilicate glasses were synthesized with Cl or F additions, alongside fixed SO₄²⁻ content. These glasses were examined with a combined spectroscopy-microscopy approach that included electron microscopy and dispersive X-ray spectroscopy, Raman spectroscopy, NMR spectroscopy, and X-ray nano-Computed Tomography. Cl incorporation saturated at ∼3 mol %, beyond which excess halide and SO₄²⁻ partitioned to a NaCl-rich molten salt. This salt formation reduced Na₂O and SO₄²⁻ content in the glass and increased the glass transition temperature (Tg), molar volume, 4-coordinated boron (N₄), and network polymerization. By contrast, F was retained up to ∼12 mol % before liquid-liquid phase separation, leading to submicron-scale crystallization of CaF₂, and with subsequent F addition, villiaumite (NaF) and cryolite (Na₃AlF₆). Before phase separation, F addition decreased Tg and increased N₄. F had minimal effect on SO₄²⁻ retention prior to crystallization, although SO₄²⁻ preferably segregated into F-rich droplets after fluoride saturation.