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, Da, were obtained. The profiles from the migration experiments under the potential gradient were well reproduced by an advection-dispersion equation, and hydraulic dispersion coefficients, Dh, and migration velocities, V, were obtained. From the quantities, Da, Dh, and V, mechanical dispersion coefficients, Dm, 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.