Molecular Dynamics Simulations of Anion Exclusion in Clay Interlayer Nanopores

Abstract

Abstract-The aqueous chemistry of water films confined between clay mineral surfaces remains an important unknown in predictions of radioelement migration from radioactive waste repositories. This issue is particularly important in the case of long-lived anionic radioisotopes ( 129 I À , 99 TcO 4 À , 36 Cl À ) which interact with clay minerals primarily by anion exclusion. For example, models of ion migration in clayey media do not agree as to whether anions are completely or partially excluded from clay interlayer nanopores. In the present study, this key issue was addressed for Cl À using MD simulations for a range of nanopore widths (6 to 15 Å ) overlapping the range of average pore widths that exists in engineered clay barriers. The MD simulation results were compared with the predictions of a thermodynamic model (Donnan Equilibrium model) and two pore-scale models based on the Poisson-Boltzmann equation under the assumption that interlayer water behaves as bulk liquid water. The simulations confirmed that anion exclusion from clay interlayers is greater than predicted by the pore-scale models, particularly at the smallest pore size examined. This greater anion exclusion stems from Cl À being more weakly solvated in nano-confined water than it is in bulk liquid water. Anion exclusion predictions based on the PoissonBoltzmann equation were consistent with the MD simulation results, however, if the predictions included an ion closest approach distance to the clay mineral surface on the order of 2.0 Ô 0.8 Å . These findings suggest that clay interlayers approach a state of complete anion exclusion (hence, ideal semi-permeable membrane properties) at a pore width of 4.2 Ô 1.5 Å .

Molecular dynamics simulations of cesium adsorption on illite nanoparticles

“…Adsorption on clay edge surfaces remains almost entirely unexamined, in part because the CLAYFF model was not parameterized to simulate clay edge surfaces. The few existing MD simulations of clay edges focused on smectite [78,43,67,99] or pyrophyllite [59,66] rather than illite. Only three of these studies used fully flexible clay structures [59,66,67].…”

“…This procedure ensures net charge neutrality and captures the expected dependence of the partial charge of edge O atoms on their degree of over-or under-saturation from a bond-valence perspective [99]. A list of edge oxygen sites, their coordination, and calculated partial charges is provided in Table 2.…”

atom: A Matlab Package for Manipulation of Molecular Systems

“…On a separate note, because the original CLAYFF (nor INTERFACE) does not contain all different oxygen atom types needed to model all minerals or, for instance, custom clay edge models, the atom library can also be used to derive partial charges and molecular topologies for new oxygen atom types not part of the original CLAYFF forcefield. This is accomplished by equation 1, which in fact can be used to calculate the partial charge of any new or original oxygen atom type in CLAYFF (Tournassat et al 2016;Lammers et al 2017).…”

Measurement of Cl− : Br−Ratios in the Porewater of Clay-Rich Rocks—A Comparison of the Crush-and-Leach and the Paper-Absorption Methods

“…The distribution of dissolved anions is limited, predominantly, to the mobile fraction [39,[77][78][79][80], but very little is known about the distribution of specific anions in the pore space. It is understood that anionic species are affected by anion exclusion in clay-rich media [81][82][83] and that not all anions are affected equally [12,24,84,85].…”