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

Abstract

Electromigration studies of 36 Cl À ions and 22 Na þ ions, and electro-osmosis of water traced with HTO and H 2 18 O were conducted with water-saturated compacted sodium montmorillonite having dry densities from 0.8 to 1.6 Mg/m 3. 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 À 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/m 3). This result suggests that the migration pathways of Cl À ions are nearly identical to those of water above 1.3 Mg/m 3. The former diffusion coefficient is smaller than the latter in the low dry densities (below 1.3 Mg/m 3), suggesting that Cl À ions migrate in the region distant from the montmorillonite sheets. In contrast, for Na þ 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 þ 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.

Calcium carbonate precipitation in compacted bentonite using electromigration reaction method and its application to estimate the ion activity coefficient in the porewater

“…Electrokinetic method was used to enhance the migration of reacting ions because the hydraulic conductivity in compacted bentonite is very low (less than 10 -11 m/s). This method has been used to study radionuclides migration [31][32][33][34] and gypsum (CaSO4) precipitation in compacted bentonite [35]. Under the electrical potential gradient, Ca 2+ ions migrate towards the cathode side while HCO3 -/CO3 2ions migrate towards the anode side in the opposite direction.…”

Molecular Dynamics Simulations of Water and Sodium Diffusion in Smectite Interlayer Nanopores as a Function of Pore Size and Temperature

“…Because of their high surface area (about 800 m 2 g –1 ), large negative structural charge (0.7–1.7 mmol c g –1 ), and high swelling capacity, these minerals strongly influence the permeability, mechanics, and geochemistry of argillaceous porous media such as soils, rocks, and geologic faults. − The same properties make them valuable as catalysts or sorbents in many industrial processes and as barrier materials in the isolation of contaminated sites and proposed high-level radioactive waste (HLRW) repositories. , The design of HLRW repositories, in particular, requires a fundamental understanding of the self-diffusion coefficients ( D ) of water and solutes in compacted water-saturated smectite clay barriers. These D values have been extensively characterized at the macroscopic scale, but most of the experimental data were obtained at ambient temperatures. − …”

Modeling the transport of water and ionic tracers in a micrometric clay sample

“…dicates when the clay is saturated with pure water. Black symbols are used for the best available data obtained by standard measurement techniques such as TD or CC (Muurinen et al, 1988;Kim et al, 1993;Kozaki et al, 1998bKozaki et al, , 1999bKozaki et al, , 2001aMolera et al, 2003)) (full symbols) or by electromigration experiments (Tanaka et al, 2011) (open symbols); data obtained without account for the influence of filter-plates ( Choi and Oscarson, 1996;Garcia-Gutierrez et al, 2004) and with no sampling of concentration profiles in the clay (Van Loon et al, 2007;Glaus et al, 2010Glaus et al, , 2011 are shown in open and full gray symbols, respectively. The star indicates when the clay was saturated with pure water, the italics indicates that the ionic strength is unknown.…”