Simulating Adsorption Of Organic Pollutants On Single-walled Carbon Nanotube In Water

In recent years, carbon nanotubes (CNTs) attracted increasing attention in the field of environmental science and technology. The adsorption of organic compounds by CNTs may affect the fate, transformation, and transport of pollutants and CNTs in the environment. Understanding the adsorption mechanism and thermodynamics of chemicals on carbon nanotubes (CNTs) is important to risk assessment and pollution control of both CNTs and chemicals. We computed the adsorption of19different nonionic and ionic organic compounds (including cyclohexane, benzene derivatives and polycyclic aromatic hydrocarbon) on (8,0) single-walled carbon nanotubes (SWNTs) by the M05-2X functional of density functional theory. Adsorption parameters including adsorption energy (Ea) and the thermodynamic parameters (â–³H,â–³Så'Œâ–³G) were calculated to investigate the adsorption mechanisms.The computed adsorption energies (Ea) in aqueous phase are lower than those in gaseous phases, indicating the adsorption in aqueous phase is more favorable. The hydrophobic interaction and Ï€-Ï€ stacking were the main interaction forces for the adsorption of organic compounds on SWNT in water. For a hypothetic aromatic with the same hydrophobicity (logKow) to cyclohexane, Ï€-Ï€interactions contribute ca.25%of the total interactions as indicated by Ea.-NO2,-OH and-NH2in aromatics enhanced the Ï€-Ï€ stacking due to their electron withdrawing or donoring effects. Compared with a hypothetical aromatic with the same logKow of nitrobenzene and without-NO2,-NO2in nitrobenzene increase Ea by ca.34%. Phenol and aniline can be ionized in water. The absolute adsorption energy of C6H5O-and C6H5NH3+was lower than C6H5OH and C6H5NH2due to decrease the hydrophobic interaction.The thermodynamic parameters show that the adsorption of unionic organic compounds on SWNT was physical adsorption, which exothermic and spontaneous. The adsorption of C6H5O-and C3H5H3+was exothermic and nonspontaneous. The computed Gibbs free energy changes for the adsorption correlate with the experimental values significantly. The correlation and the computed thermodynamic parameters can be employed to predict the adsorption of other chemicals.