Electrokinetic study of migration of anions, cations, and water in water-saturated compacted sodium montmorillonite

Electromigration studies of ³⁶Cl^- ions and ²²Na⁺ ions, and electro-osmosis of water traced with HTO and H₂ ¹⁸O were conducted with water-saturated compacted sodium montmorillonite having dry densities from 0.8 to 1.6 Mg/m³. The mobilities and dispersivities for each species were obtained from the apparent electromigration velocities and hydrodynamic dispersion coefficients, respectively. When corrected by water flow, the apparent diffusion coefficients of Cl^- ions obtained from the Einstein relation are in good agreement with the coefficients obtained from an earlier conventional diffusion experiment in the high dry densities (above 1.3 Mg/m³). This result suggests that the migration pathways of Cl^- ions are nearly identical to those of water above 1.3 Mg/m³. The former diffusion coefficient is smaller than the latter in the low dry densities (below 1.3 Mg/m ³), suggesting that Cl^- ions migrate in the region distant from the montmorillonite sheets. In contrast, for Na⁺ ions, the obtained D_a values without water flow correction are in good agreement with conventional values at all dry densities. This suggests that Na⁺ ions migrate mainly in the interlayer and the vicinity of montmorillonite sheets. The dispersivities increase with increasing dry densities for all species. This corresponds to a geometrical complexity that increases with increasing dry density.