Diffusion of iodide in compacted Beishan Granite: Impact of saturation duration, ionic strength, and dry density

The migration behavior of ¹²⁹I in host rocks constitutes a critical parameter for long-term safety assessments of a high-level radioactive waste disposal repository. Although iodide diffusion in granite has been extensively characterized, the impact of temporal water-rock interactions on transport pathways remains inadequately understood. This study systematically investigated I^- diffusion in compacted Beishan granite using through-diffusion experiments, specifically evaluating the effects of water saturation duration (7 d and 60 d), dry density (2000-2400 kg m^-3), and ionic strength. Experimental results indicate that prolonging the water saturation period significantly enhances the effective diffusion coefficient (D_e) of I^-. BET and pore structure analyses reveal that this enhancement is driven by extended water-rock interactions, which promote mineral dissolution and reprecipitation. These processes restructure the pore network, effectively improving connectivity and potentially increasing accessible surface area. Regarding other physicochemical controls, elevated ionic strength increases the apparent diffusion coefficient (D_a) by compressing the electrical double layer and mobilizing additional free pore water. Conversely, higher dry density significantly retarded I^- diffusion, attributable to reduced total porosity and intensified anion exclusion within the compacted matrix. These findings demonstrate that time-dependent pore evolution must be explicitly incorporated into long-term radionuclide migration assessments for geological disposal safety cases.