Mechanistic Insights into the Interaction between Strätlingite and I – /IO ₃ – : Implications on the Iodine Migration in Alkali-Activated Slag Cement

Alkali-activated slag cement (AAS) is a sustainable and promising matrix for immobilizing radioactive wastes. Strätlingite (siliceous AFm-type phase), a typical hydration product in AAS, plays a crucial role in stabilizing anionic species. However, the immobilization mechanisms of iodine species within strätlingite remain unclear. In this study, strätlingite demonstrated a greater affinity for IO₃⁻ than I⁻, with adsorption capacities of 0.291 and 0.0016 mmol/g, respectively. Approximate site energy distribution revealed a more diverse energy partitioning of adsorption sites for IO₃⁻ than I⁻, and strong interactions between IO₃⁻ and Ca sites through O-Ca bonds forming. Conversely, the differential charge density between I⁻ and Ca sites was weak. The surface adsorption energy of IO₃⁻ to Ca sites was -5.05 eV, substantially higher than the -1.08 eV for I⁻ adsorption. A stronger overlap between the 2p orbitals of O atoms and the 3d orbitals of Ca was observed upon IO₃⁻ adsorption than I⁻ adsorption. Moreover, the anion selectivity of strätlingite followed the order: CO₃²⁻ > SO₄²⁻ ≥ IO₃⁻ > OH⁻ > Cl⁻ > NO₃⁻ ≥ I⁻. These findings provide valuable insights into the migration behavior of iodine species in repositories and alkaline environments.