Kinetic behavior of water as migration media in compacted montmorillonite using H₂¹⁸O and applying electric potential gradient

As migration media for radionuclides in water-saturated compacted sodium montmorillonite (Kunipia-F) at dry densities from 0.8 to 1.6 Mg/m³, water migration under electric potential gradient was investigated as well as water diffusion, using H₂¹⁸O as a tracer. The concentration of H₂¹⁸O was measured in terms of δ¹⁸O, which corresponds to a relative difference of (¹⁸O/¹⁶O) between a specimen and a standard, by a mass spectrometric method coupled with isotope exchange. The concentration profiles from the diffusion experiments were well described by the Fick's second law. By least-squares fits of the profile, diffusion coefficients, D_a, were obtained. The profiles from the migration experiments under the potential gradient were well reproduced by an advection-dispersion equation, and hydraulic dispersion coefficients, D_h, and migration velocities, V, were obtained. From the quantities, D_a, D_h, and V, mechanical dispersion coefficients, D_m, and dispersivities, α, were also obtained. The dispersivity is considered as a geometrical property of medium for chemical species. They were used to discuss migration pathways in water-saturated compacted montmorillonite, compared to available data of α for helium, ²²Na⁺, ⁸⁵Sr²⁺, and ³⁶Cl^-. It is found that the species are classified into three regions by the value of α; the positively charged ions in the smallest region of α, H₂¹⁸O in intermediate region, and electrically neutral helium in the largest region. Based on these results, migration behavior under electric potential gradient was interpreted for water and the species.