Molecular Insights on the Adsorption of Polycyclic Aromatic Hydrocarbons on Soil Clay Minerals

The role of soil clay minerals is often ignored in studying the polycyclic aromatic hydrocarbons (PAHs) adsorption in soil. In this work, Monte Carlo and molecular dynamics simulations are used to study the adsorption of PAHs on several typical soil clay mineral surfaces, such as (0 0 1) and (0 0 - 1) of kaolinite, pyrophyllite, mica, and montmorillonite. Results show that van der Waals and cation-pi interactions dominate the interactions between PAHs and mineral surfaces. Bridging oxygen atom on the mineral surface structure is the main site of hydrophobic interaction with PAHs. The cation-pi effect can significantly enhance the PAHs adsorption energy. In addition, the adsorption on clay minerals is enhanced with the increase in the number of PAHs aromatic rings. In the presence of water, different minerals exhibit different adsorption properties. A layer of water molecules is formed close to the kaolinite (0 0 1) surface, reducing PAHs adsorption. The self-diffusion coefficient (D) of PAHs in Ca2+-montmorillonite is more significant than that in K+-mica, due to the different hydration properties of mineral cations. These findings reveal the adsorption mechanism of PAHs on the surfaces of typical clay minerals at a molecular scale and provide a new perspective for soil adsorption of PAHs.