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: CO₃^2- > SO₄^2- ≥ IO₃^- > OH^- > Cl^- > NO₃^- ≥ I^-. These findings provide valuable insights into the migration behavior of iodine species in repositories and alkaline environments.