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/m3), suggesting that Cl− ions migrate in the region distant from the montmorillonite sheets. In contrast, for Na+ ions, the obtained Da values without...