Role of biotite content and structure in controlling Cesium diffusion in granitic rocks

Matrix diffusion and sorption in crystalline rocks are key processes for predicting radionuclide transport in the safety assessments of geological repositories for radioactive waste. This study investigated the diffusion and sorption behavior of Cs⁺ in granitic rocks by conducting through-diffusion experiments using tonalite, granodiorite, and granite samples. The effective diffusion coefficients (D_e) and distribution coefficients (K_d) of Cs⁺ under various KCl concentrations ranged from 10^-12 to 10^-11 m² s^-1 and from 10^-4 to 10^-2 m³ kg^-1, respectively. Under high KCl conditions (1 × 10^-3 mol L^-1), where a dual profile characterized by localized Cs enrichment near the inlet boundary, was suppressed, both D_e and K_d showed an increase with biotite content. This suppression is inferred to result from the disappearance of a hydrobiotite peak observed in XRD spectra, suggesting modification of the interlayer structure of hydrobiotite. As a result, this reduction in localized Cs enrichment near the inlet improved the reliability of D_e and K_d estimates. Furthermore, in all rock types, the D_e values for Cs⁺ exceeded those of deuterated water, indicating cation excess diffusion, in which Cs⁺ diffusivity is enhanced by its accumulation in the electrical double layer adjacent to negatively charged surfaces of biotite. Although structural modifications of biotite depending on KCl concentration may influence Cs⁺ transport, the systematic correlation between biotite content and Cs⁺ diffusivity provides a consistent explanation for matrix diffusion and sorption behavior among various granitic rock types. These findings demonstrate that cation excess diffusion associated with biotite is a critical mechanism controlling Cs⁺ transport in the matrix of crystalline rocks.