Molecular dynamics simulation on adsorption performances of phenol by oxygenic functional groups on adsorbent surface

Coal, carbon and graphene oxide are good adsorbents for phenolic compounds.They are often used as adsorbents to remove harmful substances from phenolic wastewater.However, the adsorption performances of phenolic compounds by oxygenic functional groups, which are complex and diverse on the adsorbent surface, are not clear.In this study, graphene substrates were used as models of coal and carbon adsorbents, and carboxyl, hydroxyl and carbonyl groups were used to modify the substrates.The adsorption processes of phenol on the four different adsorbent substrates were studied by molecular dynamics simulation.The effects of different oxygenic functional groups on the adsorption of phenol were discussed from the aspects of van der Waals interaction, electrostatic interaction, π-π interaction, hydrogen bond interaction and interaction energy.The van der Waals interaction, electrostatic interaction, π-π interaction and hydrogen bonding between the substrates and phenol were affected by the modification of the substrates with three oxygenic functional groups.It was found that van der Waals interaction and π-π interaction were the main factors affecting the adsorption of phenol on the substrates, while electrostatic interaction and hydrogen bonding were the secondary factors.Two different effects, which are promoting and hindering, on the adsorption of phenol were generated depending on the position of hydrogen bonding.The hydrogen bond between phenol and oxygenic functional groups on the substrate promoted the adsorption of phenol, whereas the hydrogen bond between water molecules and water molecules or between water molecules and phenol hindered the migration of phenol in solution.The interaction energy reflected the spontaneity of the adsorption process and the stability of the adsorption system with an equilibrium state.A large absolute value of interaction energy meant a stable adsorption equilibrium system.Carboxyl group showed a strong effect in enhancing the adsorbing stability of phenol to adsorbent substrate.The hydrophilicity of the substrates can be enhanced to various degrees by modifying the substrates with different oxygenic functional groups.Specially, carboxyl group had the strongest effect followed by hydroxyl group, whereas carbonyl group hardly increased the hydrophilicity of the substrate.