Modeling Cation Diffusion in Compacted Water-Saturated Sodium Bentonite at Low Ionic Strength

Abstract

23Sodium bentonites are used as barrier materials for the isolation of landfills and are under 24 consideration for a similar use in the subsurface storage of high-level radioactive waste. The 25 performance of these barriers is determined in large part by molecular diffusion in the bentonite 26 pore space. We tested two current models of cation diffusion in bentonite against experimental 27 data on the relative apparent diffusion coefficients of two representative cations, sodium and 28 strontium. On the 'macropore/nanopore' model, solute molecules are divided into two categories, 29 with unequal pore-scale diffusion coefficients, based on location: in macropores or in interlayer 30 nanopores. On the 'surface diffusion' model, solute molecules are divided into categories based 31 on chemical speciation: dissolved or adsorbed. The macropore/nanopore model agrees with all 32 experimental data at partial montmorillonite dry densities ranging from 0.2 (a dilute bentonite 33 gel) to 1.7 kg dm -3 (a highly compacted bentonite with most of its pore space located in 34 interlayer nanopores), whereas the surface diffusion model fails at partial montmorillonite dry 35 densities greater than about 1.2 kg dm -3 . 36 37

Modeling the diffusion of Na+ in compacted water-saturated Na-bentonite as a function of pore water ionic strength

“…For sodium, " interlayer = 0.32 ± 0.06 and $ macropore,I + $ interlayer,I = 1 (Bourg et al, 2007). If positive 152 anion adsorption in the interlayer is negligible and if 22 Na + and the background Na + ions have the 153 same affinity for the montmorillonite surface, the mole fraction $ interlayer for 22 Na + in Na-154 bentonite can be calculated from a simple charge balance consideration: 155…”

Removal of cobalt(II) from aqueous solution by local Saudi bentonite: Kinetic and equilibrium investigations

“…[25]. Specific surface areas of 23.9 and 18.6 m 2 g −1 for untreated montmorillonite were reported by Bourg et al [29] and Goldberg [30], respectively. Such small values of surface area were attributed to the limited penetration of N 2 gas molecules through the interlayer regions between the layer sheets [31].…”

Hydraulic Flow through Engineering Bentonite-Based Containment Barriers

“…The model considers a steady state condition with constant influent flux. An aqueous phase effective diffusion coefficients, D* of 1×10 −9 m 2 /s was adopted (Bourg et al, 2007;García-Gutiérrez et al, 2004;Shackelford and Daniel, 1991a;1991b;Iversen and Jorgensen, 1993). The advective and diffusive equivalency was then investigated separately.…”