Ion equilibrium in Na-montmorillonite - Insights from molecular dynamics simulations

Document available in extended abstract form only. Bentonite clay is proposed as buffer material in several concepts of High Level Radioactive Waste repositories, and a truthful description of ion diffusion in this material is of vital importance for any quantification of the chemical evolution of the repository near field. This study investigates the importance of ion equilibrium between montmorillonite interlayer space and an external solution for the diffusional behaviour of bentonite. We present molecular dynamics simulations of compacted montmorillonite with three hydration layers of water in the interlayer. The montmorillonite is contacted to a reservoir of NaCl solution as shown in Figure 1, and the calculations are performed at three different concentrations. Overall the calculations are performed in the canonical (NVT) ensemble but from the perspective of the interlayer the calculations are done in the grand canonical (μVT) ensemble. However, both the limited volume of the bulk solution and that of the interlayer poses problem for the simulations. A criterion, that must be fulfilled in order to obtain physically meaningful results, is derived using the Poisson-Boltzmann equation and standard Donnan theory. The validity of this criterion is tested in the simulations by choosing conditions where the criterion is either fulfilled or violated. The calculations show that excess salt, i.e., both anions and cations enter interlayer space to the extent predicted by Donnan equilibrium. Thus the excess salt concentration is reduced in the interlayer in comparison to the external electrolyte but not totally excluded. This draws attention to a misconception, often seen in the literature, stating that anions cannot enter interlayer space due to electrostatic repulsion forces. Taking into account that anions can and will enter the interlayer space a general theoretical framework for describing through-diffusion in montmorillonite is developed. By using a single pore space, the interlayers, cations and anions are treated symmetrically as opposed to conventional multi-porosity models where anions and cations follow different pathways. The validity of the presented approach is further justified in comparisons to experimental chloride tracer diffusion data. By means of Taylor expansion it is also shown that the standard Donnan expression follows from the Poisson-Boltzmann equation. From the Taylor expansion one can also derive corrections to the Donnan theory to be used when the layer separation is large. (authors)