Equilibrium and kinetic studies of methyl orange adsorption on multiwalled carbon nanotubes

The adsorption of methyl orange onto multiwalled carbon nanotubes (MWCNTs) from aqueous solutions was studied, in which the influence of contact time, dosage, temperature, pH, and methyl orange concentration in the solution were investigated. The equilibrium adsorption data were analyzed using three common adsorption models: Langmuir, Freundlich and Temkin. The results revealed that Langmuir isotherm fit the experimental results well. Kinetic analyses were conducted using pseudo-first and second-order models and the intra-particle diffusion model. The regression results showed that the adsorption kinetics was more accurately represented by pseudo-second-order model. Values of activation parameters such as Standard free energy changes (ΔG0), standard enthalpy change (ΔH0), and standard entropy change (ΔS0) were calculated using adsorption equilibrium constants obtained from the Langmuir isotherm at different temperatures. All ΔG0 values were negative; the (ΔH0) values and (ΔS0) values of MWCNTs were 19.39 kJ/mol and 0.1015 kJ/mol K, respectively, indicating that the adsorption was feasible, spontaneous and endothermic process in nature.