Electromigration of sodium ions and electro-osmotic flow in water-saturated, compacted Na-montmorillonite

Abstract Sodium ions spiked with 22Na as a tracer were migrated by electromigration and electro-osmosis in the water-saturated compacted Na-montmorillonite at dry densities 1.0×103 kg m−3, under an electric potential gradient. Dissolved helium was also migrated by electro-osmosis in the montmorillonite. After migration, concentration profiles of the sodium ions and helium were obtained by γ-spectrometry and mass-spectrometric methods, respectively. From the profiles of both chemical species, not only migration due to electrokinetic phenomena but also mechanical dispersion was observed in the montmorillonite. The dispersion coefficients, Di, and apparent migration rates, Uia, of 22Na and helium were found in the compacted Na-montmorillonite at 1.0×103 kg m−3. The migration of helium in the montmorillonite under an electric potential gradient reflects that of water because helium migrates as an electrically neutral species. The parameters DHem, UHea, and αHe correspond to those of water. The mechanical dispersion coefficients, DNam, of 22Na+ ions are much smaller than those of water obtained by helium. The dispersivity parameters, αNa, for 22Na+ obtained from these DNa and UNaa values are 10−5 m and those for water (αHe) are 10−3 m. This indicates that 22Na+ ions migrate in different spaces than water in the compacted montmorillonite under a potential gradient. This finding suggests that the migration of Na+ ions occurs in the interlayer and/or on the outer surfaces of the montmorillonite; whereas dissolved helium migrates in the pore water.