Anion Diffusion in Compacted Clays by Pore‐Scale Simulation and Experiments

Abstract

Accurate understanding of diffusion of anionic radionuclides in different clays is significant to predict long-term performance of high-level radioactive waste (HLW) repositories. The importance of electrical double layer (EDL) on anionic tracer diffusion in different clays has been studied by both through-diffusion experiments and pore-scale simulations in this work. The through-diffusion experiments measured the effective diffusion coefficient and the accessible porosity of Re (VII) in compacted montmorillonite, Na-bentonite, and illite/smectite mixed layer (I/S) at different salinities. The results showed that the accessible porosity and the effective diffusion coefficient of Re (VII) in montmorillonite and Na-bentonite were similar but lower than those in I/S under the same conditions. For mechanism analysis and predictions, a pore-scale modeling was implemented to simulate the diffusion of anions in compacted clays. In the simulations, the characteristics (porosity, density, and total surface area) of microstructures of montmorillonite and I/S were used to regenerate three-dimensional pore structures numerically by a Quartet Structure Generation Set method. The Re (VII) diffusion was then simulated by directly solving coupled Poisson-Nernst-Planck equations via the lattice Boltzmann method. The diminished effect of EDL was therefore calculated and compared with Donnan and multiporosity models. As EDLs overlap in compacted clays, the Donnan model overestimates the influence of EDL on Re (VII) diffusion, while the multiporosity model underestimates it. The pore-scale modeling, which captures the structure of overlapping EDLs automatically, can simulate the diffusion of anionic radionuclides in compacted clays without any fitting parameters.

Pore‐Scale Modeling of Water and Ion Diffusion in Partially Saturated Clays

“…This study extends our previous numerical framework for saturated porous media (Wu et al., 2020; Yang & Wang, 2019; Yuan et al., 2022) to partially saturated conditions by adding (a) the equilibrium water/vapor distribution in pore geometries; (b) a stable method to capture the water transport through the water/vapor interface; (c) the electrokinetic boundary condition of liquid/gas interfaces; and (d) the local solute mobility relating to the distance away from the solid/liquid interface. The commonly used numerical method for pore‐scale simulations is the Lattice Boltzmann Method (LBM) benefitting from its efficient parallelization and flexible handling of various liquid‐solid boundary conditions.…”

“…The regenerated nanostructures are not absolutely the same as the real structures of clays. This method has been utilized in previous pore-scale studies on diffusion in compacted clays and is detailed inter alia in Wu et al (2020) and Yang and Wang (2019). Savoye et al (2014) measured the effective diffusion coefficients of HTO and iodide in variably water-saturated illite/sand mixtures that exhibited porosities ranging from 0.24 to 0.36; the specific surface area (SSA) of the solids was estimated to range from 50 m 2 /g to 80 m 2 /g (corresponding volume-specific surface area: 0.09 1/ nm -0.15 1/nm).…”

“…This method has been utilized in previous pore‐scale studies on diffusion in compacted clays and is detailed inter alia in Wu et al. (2020) and Yang and Wang (2019). Savoye et al.…”

Integrating Process‐Based Reactive Transport Modeling and Machine Learning for Electrokinetic Remediation of Contaminated Groundwater

“…The flux of charged species in a porous medium can be described with the Nernst-Planck equation (Alt-Epping et al, 2015;Rasouli et al, 2015;Rolle et al, 2018;Steefel & Tournassat, 2020;Tournassat & Steefel, 2019;Wu et al, 2020), which accounts for the contribution of diffusion, migration, and electroosmosis:…”

“…The flux of charged species in a porous medium can be described with the Nernst‐Planck equation (Alt‐Epping et al., 2015; Rasouli et al., 2015; Rolle et al., 2018; Steefel & Tournassat, 2020; Tournassat & Steefel, 2019; Wu et al., 2020), which accounts for the contribution of diffusion, migration, and electroosmosis: where is the accessible porosity, is the pore diffusion/dispersion coefficient in which is the aqueous diffusion coefficient of the species , is the tortuosity, the molar concentration, is the concentration gradient, is the charge, is the Faraday constant, is the gas constant, is the temperature, is the electric potential gradient, and is the average velocity resulting from electroosmotic flow, calculated as …”