Impact of microstructure on anion exclusion in compacted clay media

Abstract

The sensitivity of ion-concentration distribution models to three key model assumptions, the pore-size distribution of clay media, the 'distance of closest approach' of ions to the clay surface, and the accessibility of sub-nanometer-wide clay mineral interlayer spaces to anions, was explored by solving the Poisson-Boltzmann equation for swelling and nonswelling clay materials. The calculations show that all three model assumptions impact significantly on values predicted for the anion-accessible porosity. As a consequence, macroscopic measurements of anion exclusion in clay media cannot be used to test any of the three model assumptions independently of the other two. Information gained at the nanoscale, a detailed characterization of pore-size distribution in particular, is necessary to develop accurate predictive models of the anion accessible porosity of clay media.

Influence of Polarizability on the Prediction of the Electrical Double Layer Structure in a Clay Mesopore: A Molecular Dynamics Study

“…In the presence of added NaCl, Cl − anions were repelled from the negatively charged montmorillonite surface, in agreement with previous MD investigations results. [5][6][7]27,72,[74][75][76] The distance of closest approach to the surface was 6.8Å for ClayFF, 7Å for ClayFF MOD and 7.1Å for PIM, where the position z = 0 refers to the center of the clay layer (Fig. 4c).…”

Reactive Transport Modeling of Coupled Processes in Nanoporous Media

“…In practice, the information about ion concentrations in the diffuse layer must be upscaled so that calculations can be carried out at the continuum scale. A common upscaling approach relies on the use of a mean electrostatic potential model (Revil and Leroy 2004;Appelo and Wersin 2007;Appelo et al 2010;Revil et al 2011;Tournassat and Steefel 2015;Tournassat et al 2016b). This model averages ion concentrations in the diffuse layer ( c i DL , ) by scaling them to a mean electrostatic potential (MEP, ψ M ) that applies to the diffuse layer volume (V DL ):…”

4. Reactive Transport Modeling of Coupled Processes in Nanoporous Media

“…The smallest pores, i.e. the interlayer pores in swelling clay minerals, are logically usually described as slit-shaped pores (Birgersson and Karnland, 2009;Appelo et al, 2010;Tournassat and Appelo, 2011;Tournassat et al, 2016b;Appelo, 2017;Wigger and Van Loon, 2017). Cementitious materials are complex assemblages of a range of solid phases.…”