Ion equilibrium between montmorillonite interlayer space and an external solution—Consequences for diffusional transport

Molecular Dynamics Simulations of Anion Exclusion in Clay Interlayer Nanopores

“…Because of this similarity, the terms ''Donnan model,'' or ''Donnan volume,'' are sometimes erroneously used to describe the MEP model despite its molecular-scale assumptions (Appelo and Wersin, 2007;Schoch et al, 2008;Birgersson and Karnland, 2009;Tertre et al, 2011;Glaus et al, 2013). The DE model, however, is based on chemical thermodynamics and, therefore, is fundamentally different from the MGC and MEP models, or any other model involving molecular-scale concepts.…”

“…However, three recent studies that quantified anion exclusion in clay interlayers by MD simulation have yielded contradictory results: one concluded that interlayer water can be treated as having the same aqueous chemistry as bulk liquid water (Hedström and Karnland, 2012), whereas two others reached the opposite conclusion, that anion exclusion in clay interlayers is much greater than would be expected based on the aqueous chemistry of bulk liquid water (Rotenberg et al, 2007b;Hsiao and Hedströ m, 2015). Consequently, the correct way in which to account for anion exclusion in macroscopic models of ion migration in clay formations and engineered clay barriers remains uncertain (Muurinen et al, 2007;Birgersson and Karnland, 2009;Appelo et al, 2010;Tournassat and Appelo, 2011). The present study aimed to resolve the differences among previous MD simulations.…”

“…The answer to this question has important implications for the migration of salts and groundwater through clay-rich rocks (Fritz, 1986;Leroy et al, 2006), the design of engineered nanofluidic systems (Schoch et al, 2008;Bocquet and Charlaix, 2010), the performance of desalination membranes (Richards et al, 2012), and the transport of ions through channels in cell membranes (Nikaido and Rosenberg, 1981;Nikaido, 2003). It also is of importance to highlevel radioactive waste (HLRW) management, where the performance of geologic repositories is determined, in many cases, by the migration of anionic, long-lived fission products ( 129 I À , 99 TcO 4 À , 36 Cl À ) through nanoporous clay barriers (Tournassat et al, 2007;Altmann, 2008;Claret et al, 2010;Glaus et al, 2011;Altmann et al, 2012).…”

“…At present, extant models of anion adsorption and diffusion in nanoporous clayey media postulate either that anions are completely excluded from clay interlayer nanopores (Sposito, 1992;Wersin et al, 2004;Bourg et al, 2008;Churakov and Gimmi, 2011;Tournassat and Appelo, 2011) or that anions are partially excluded from these nanopores in a manner that can be predicted accurately by assuming nanopore water is the same as bulk liquid water, albeit with a lower electrostatic potential imposed by negatively charged clay surfaces (Leroy et al, 2006;Birgersson and Karnland, 2009;Glaus et al, 2013;Tachi et al, 2014). Efforts to test these two competing hypotheses using anion exclusion measurements have been inconclusive because the results of such measurements depend not only on the mechanistic details of anion exclusion in individual nanopores, but also on the pore-size distribution in saturated, compacted clayey media (Tournassat and Appelo, 2011;Tournassat et al, 2016;Tinnacher et al, 2016), which is not yet fully understood (Holmboe et al, 2012).…”

Electrochemistry and capacitive charging of porous electrodes in asymmetric multicomponent electrolytes

“…13,41,42,43 In that case we can assume that across the micropore the potential is constant (the typical approach in Donnanbased models), different from the potential in the macropores by the Donnan potential, ∆φ D . 52 We modify this standard Donnan concept by including a Stern layer between the electrolyte-filled micropore volume and the electrically-conducting matrix phase, with a concommitant Stern layer voltage, ∆φ S . These two potentials together bridge the difference in potential between that in the macropore, φ, and that in the electron-conducting matrix, φ 1 .…”