Multicomponent gases (CH4/CO2/C6H6) diffusion and adsorption in unsaturated bentonite: A molecular insight

Diffusion and adsorption are the key to describe the behavior of gas pollutant species in engineered clay barrier. In this study, the diffusion and adsorption properties of multicomponent gases (C6H6/CH4/CO2) in montmorillonite interlayer were investigated at the molecular level based on molecular dynamics (MD) simulations considering the interaction among gases, water, and montmorillonite. Moisture content has great effect both on diffusion coefficient and adsorption properties of multicomponent gases. The diffusion coefficients firstly increase with increasing water content due to increasing interaction energy between gases and water. When moisture content reaches 10 % or more, the diffusion coefficients decrease slightly or remain unchanged because the interlayer is saturated with water. The interaction force between different gas species accelerates the diffusion of gases and indicates the phenomenon of cross diffusion, which should be considered when studying the transport of multicomponent gases. With moisture content increasing, adsorption capacity of water molecules on montmorillonite increases, while the adsorption capacity of C6H6/CH4/CO2 decreases. With increasing moisture content, H2O molecules first occupy the sorption sites near the surface of montmorillonite as the interaction energy between H2O and montmorillonite increases. Gases are sandwiched between water molecule layers when moisture content reaches 10 %. The results found in this study can be used to predict mutual diffusion coefficients of multicomponent gases and to explain the experimental results or engineering issues at molecular scale.