Enhanced iodide sorption onto montmorillonite via interlayer ion-pairing with calcium

Understanding geochemical interactions between iodide and montmorillonite, a key mineral in radioactive waste repository barriers, is essential for assessing iodine mobility and its radiological effects on the ecosystem. Herein, we present the first experimental evidence and thermodynamic data demonstrating that iodide sorption onto montmorillonite is significantly enhanced by a calcium-associated interlayer ion-pairing mechanism under repository-relevant and environmentally realistic conditions. Our investigation is based on multiple complementary approaches, including a series of batch sorption experiments supported by thermodynamic modeling, X-ray diffraction, X-ray photoelectron spectroscopy, infrared spectroscopy, and transmission electron microscopy. The interlayer ion-pairing (X₂Ca + Ca²⁺ + 2I^- ⇌ 2XCaI, log K = 4.58 ± 0.32) occurred throughout the pH range of 4-10, whereas surface complexation of iodide with the clay mineral's edge sites was observed only under acidic conditions. With excess calcium, iodide uptake approached the cation exchange capacity of montmorillonite, accompanied by slight interlayer expansion due to homogenization toward a higher hydration state, reflecting increased interlayer water retention. These findings challenge the conventional view that iodide behaves as a non-sorbing anion onto clay minerals and underscore the potential importance of divalent cations, omnipresent in natural systems, in the fate and transport of anionic species in subsurface environments.