The Effect Of Temperature On The Interlayer Structure Of Na-,Cs-Montmorillonite Using Molecular Simulation

Bentonite which is widely distributed in the nature is a kind of claystone, and it is a very important non-metallic mineral. It is regarded as the buffer/backfilling material in handling nuclear waste for the deep geological disposal because of its excellent adsorption performance and low permeability. Because bentonite is mainly composed of montmorillonite clay minerals, the basic properties of the bentonite depend on the characteristics of the montmorillonite. And the quality of bentonite depends on the content of montmorillonite. Therefore, it is particularly necessary to discuss the relationship between montmorillonite structure and its properties, the diffusion and the migration of water molecules and radionuclide ions in montmorillonite interlayer and the mechanism of the interaction among the clay in the deep geological disposal. The conventional experimental methods which were used to study the application of bentonite as a buffer/backfill material is very restricted due to the special nature of the deep geological disposal and nuclear waste.With the rapid development of computer technology and the gradual improvement of algorithms and the force field theory, as a theoretical study in combination with the experimental determination means, molecular simulation has a tremendous advantage in micro-areas of the research system and it has been widely applied in the research of montmorillonite.Firstly, this paper discusses the changes of layer spacing, density, volume and other parameters of Na-,Cs-montmorillonite with the changes of the content of interlayer water using molecular mechanics. The results show that Na-, Cs-montmorillonite have a different water swelling properties, and the layer spacing, volume and density of Cs-montmorillonite are greater than Na-montmorillonite with the increase of the interlayer water content.Secondly, based on the stable configurations obtained by molecular mechanics, molecular dynamics is applied to study the changes of interlayer structure and the diffusion and migration of interlayer species in different degree of hydration of the Na-, Cs-montmorillonite in the range of300k-600k The concentration profiles of Na+and Cs+show that Na+and Cs+present different distribution characteristics under the different temperatures and hydration, and the distributions of the interlayer Na+and Cs+in the dry state are less influenced by the temperature than that the hydrated state.Concentration profiles of interlayer water oxygens show that the distribution of oxygen atoms in the Na-montmorillonite is more concentrated than the Cs-montmorillonite in one-layer hydrate. However, the results are similar in two-, three-layer hydrates. It does not form a third layer water molecules layer in three-layer hydrates, and the oxygen atoms gradually become dispersive with the temperature rising. Combined with the O-O RDF and H-O RDF analysis of interlayer water, it can be found that the structure of the interlayer water is different from liquid water, the degree of polymerization and the ordering of interlayer water are higher than the liquid water in different temperature and hydration.Interlayer cation-clay tetrahedral oxygens RDF demonstrate that the interaction among Na+, Cs+and the clay layer will be weakened with increasing temperature and hydration. Not only the interaction between Cs+and clay layer is stronger than that of Na+and the structure is more compact, but also the interaction between cations and the clay layer is less affected by temperature with increasing hydration.The RDF of the interlayer cations and water molecules show that cations will be hydrated when the interlayer includes water molecules. Due to the solvation of ions, the water molecules which surround the Na+and Cs+will form the first and second hydration shell in one-, two-layer hydrates and three-layer hydrates respectively, and the structure of the former is more compact than the latter. As the temperature rises, ions solvation is weakened.According to the self-diffusion coefficients of the interlayer species, the diffusion of interlayer species in montmorillonite is slower than the liquid phase; although the diffusion coefficients of Na+and Cs+and interlayer water molecules will increase with the rising of temperature and hydration, the diffusion coefficients of water molecules are greater than that of the interlayer cations, and the diffusion coefficients of Cs+is greater than that of Na+.