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

花崗岩類におけるセシウム拡散挙動に対する黒雲母含有量と構造の影響

深津 勇太  ; Hu, Q.*; 舘 幸男  

Fukatsu, Yuta; Hu, Q.*; Tachi, Yukio

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 (De) and distribution coefficients (Kd) of Cs were evaluated under various KCl concentrations, as potassium is a key competing cation for Cs sorption. In addition, elemental mapping and mineralogical characterization of solid samples were conducted using laser ablation-inductively coupled plasma-mass spectrometry and X-ray diffraction, to support the interpretation of Cs transport. The results showed that rocks with higher biotite content exhibited higher De values and, in some cases, higher Kd values under high KCl conditions (110 mol L). At this concentration, formation of a dual profile, characterized by localized Cs enrichment near the inlet boundary, was suppressed. This suppression may be explained by the disappearance of a hydrobiotite peak observed in XRD spectra, suggesting that structural modification of biotite to hydrobiotite was inhibited. As a result, the number of high affinity of Cs sorption sites near the sample boundaries was reduced. Furthermore, in all rock types, the De values for Cs exceeded those of deuterated water as a neutral tracer. This trend indicates cation excess diffusion, in which Cs diffusivity is enhanced by its accumulation in the electrical double layer adjacent to negatively charged surfaces of biotite, occur. Although structural modifications of biotite depending on KCl concentration may have influenced Cs transport, the systematic correlation between biotite content and Cs diffusivity provides a consistent explanation in the understanding of matrix diffusion and sorption behaviors among various granitic rock types. These findings demonstrate that cation excess diffusion associated with biotite is a critical mechanism in the control of Cs transport in the matrix of crystalline rocks and should be considered in predictive models for repository safety assessments.