A density functional study of the swelling behaviors of Na-bentonite

Document available in extended abstract form only. The density functional theory has been long recognized as one powerful tool of the statistical physics, due to its numerical simplicity and physical clarity, to investigate the structural and thermodynamic properties of condensed matter in many modern disciplines. However, among its extensive applications to a wide range of natural and industrial processes, the study of structures and thermodynamics of clay-water system is, to our interest, of considerable importance. In Sweden, the natural Na-bentonite of Wyoming type (MX-80) is taken as an ideal candidate in the KBS-3 disposal concept for the buffer and backfill material in deep repository for spent nuclear fuels. The Na-bentonite, consisting mainly of montmorillonite, has a swelling capacity when it is in contact with the groundwater. As a matter of fact, the swelling behavior and the stability of the Na bentonite would be different with different salinities; however, the clay buffer itself must be able to sustain such a change in the environment so that its function as a hydraulic barrier does not fail. Hence, a theoretical approach that can predict the swelling pressure between these clay particles is needed, to not only understand the swelling behaviors of the Na-Bentonite but also assess the performance of the KBS-3 repository design. The Derjaguin-Landau-Verwey-Overbeek (DLVO) theory establishes the cornerstone in the study of colloidal stability, where the net interaction is resulting from the competition between the repulsive diffuse double layer force and the attractive Van der Waals force. It is noticed that, however, the DLVO theory relies on a model of point charges only, and therefore the important ion-ion correlation effects have been entirely neglected. To avoid the DLVO defect, we have formulated a novel pressure expression based on the density functional theory to include the ion-ion correlations, and it can be used to calculate the interaction pressure between plate-like colloidal particles. This expression is a sum of distinct physical contributions to the pressure, and it evolves evaluations to the hard-sphere contribution and the electric residual contribution to the single-particle direct correlation function. In this study, the expression is used together with the so called FMT/WCA density functional approach to simulate the swelling pressure of the Na-bentonite in a range of NaCl solutions. The results are compared with both the experiment data and the predictions from a newly developed Donnan equilibrium- based DLVO model, which takes the hydration force into account. To exemplify our simulation results together with their comparisons with experimental data, we present in Fig. 1, the swelling pressures of a Na-bentonite in the solutions of 0.1 and 0.3 M NaCl, as a function of the examined clay dry densities. The experiment demonstrates that the Na-bentonite swells much more strongly as the dry density increases. The comparisons also show that both the Donnan-equilibrium-based DLVO model and the FMT/WCA approach are in good agreement with the experimental data. This possibly indicates that the water molecular nature is not important in predicting Na-bentonite swelling pressure in ionic solutions when the ion-ion correlation effects have been considered. Such statement is also confirmed by a lately Monte Carlo study where the forces between particles are the same magnitude in both conditions with and without accounting for the water molecular solvent. To summarize, a recently developed pressure formulation, together with the FMT/WCA approach, is applied to predict the swelling pressures of the Na-bentonite in NaCl solutions. The results compare well with the experimental data. In addition, its good agreement with the prediction of the Donnan equilibrium- based DLVO model, in a range of high dry clay densities, suggests that the influence of the hydration force is not decisive in determining the forces between clay particles when the ion-ion correlations are included. (authors)