Molecular dynamics simulation study of benzene adsorption to montmorillonite: Influence of the hydration status

Abstract

Clay minerals are ubiquitous in soil and sediments and they have been considered as important geosorbents for organic contaminants. However, the adsorption capacity of clay minerals strongly depends on their hydration status. In this work, the adsorptive characteristics of benzene on a set of hydrated montmorillonites which contain different counterions (Na+, K+, Cs+) and water content were studied using classical molecular dynamics (MD) simulations. On the outer surface of unhydrated montmorillonites the adsorption energy of benzene decreases in the order: Na–montmorillonite (−0.46eV)>K–montmorillonite (−0.39eV)>Cs–montmorillonite (−0.36eV). After hydration benzene molecule will be gradually expelled away from the siloxane surface by water molecules and be adsorbed at the water–vacuum interface. Within the interlayer space of unhydrated montmorillonite benzene molecule can only lay flat on the siloxane surface, and a large positive adsorption energy is obtained due to the energy consumption which is needed to expand the interlayers. Hydration causes the expansion of montmorillonite interlayers, which then can help the adsorption of benzene to the interlayer spaces. As a result, the adsorption energy gradually evolves from large positive to negative as the water content increases to monolayer arrangement, and after that the adsorption energy remains negative. Hydration also significantly influences the interaction between counterions and benzene molecule, and benzene can bind closer to the counterbalance cation with weaker hydration capacity. Compared to the hydrated counterions, the hydrated siloxane surface is more likely to be the adsorption sites for benzene molecule. The above findings indicate that the hydration of montmorillonite has versatile effects on the adsorption of organic contaminants.