Electrokinetic Study of Migration of Anions, Cations, and Water in Water-Saturated Compacted Sodium Montmorillonite

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Electromigration studies of 36Cl− ions and 22Na+ ions, and electro-osmosis of water traced with HTO and H218O 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/m3), suggesting that Cl− ions migrate in the region distant from the montmorillonite sheets. In contrast, for Na+ ions, the obtained Da 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.