Effect of pH and Temperature on Adsorption of Dimethyl Phthalate on Carbon Nanotubes in Aqueous Phase

Adsorptions of dimethyl phthalate (DMP) on carbon nanotubes (CNTs) in aqueous phase at various pH and temperatures were studied. The increase in pH results in the increase in adsorption coefficient. The adsorption is governed by the π-π electron interaction which is affected by the changes in pH of the medium. The outer diameter of the CNTs greatly influences the adsorption behavior of CNT for DMP. Under the same working temperature, the adsorption capacity of CNTs for DMP is inversely related to the average outer diameter of the CNT: single-walled SWCNT (1.4 nm)>multi-walled MWCNT10 (9.4 nm)>MWCNT30 (27.8 nm)>MWCNT40 (42.7 nm). The larger surface area of CNTs provides many active sites for adsorption of DMP molecules. The Freundlich model can describe well the adsorption isotherms of DMP on CNTs. The thermodynamic parameters of standard free energy, standard enthalpy (ΔH), and standard entropy changes are determined, showing that the adsorption of DMP on CNTs is an endothermic and spontaneous reaction. The ΔH value of 27.8 nm-sized MWCNT (22.69 kJ/mol) is higher than 1.4 nm-sized SWCNT (6.05 kJ/mol), inferring that the adsorption process becomes more endothermic with the increase in the outer diameter of CNTs.