Adsorption behavior of methylene blue on carbon nanotubes

The effect of temperature on the equilibrium adsorption of methylene blue dye from aqueous solution using carbon nanotubes was investigated. The equilibrium adsorption data were analyzed using two widely applied isotherms: Langmuir and Freundlich. The results revealed that Langmuir isotherm fit the experimental results well. Kinetic analyses were conducted using pseudo-first and second-order models and the intraparticle diffusion model. The regression results showed that the adsorption kinetics were more accurately represented by pseudo-second-order model. The activation energy of system (Ea) was calculated as 18.54 kJ/mol. Standard free energy changes (ΔG0ΔG0), standard enthalpy change (ΔH0ΔH0), and standard entropy change (ΔS0ΔS0) were calculated using adsorption equilibrium constants obtained from the Langmuir isotherm at different temperatures. All ΔG0ΔG0 values were negative; the ΔH0ΔH0 values and ΔS0ΔS0 values of CNTs were 7.29 kJ/mol and 64.6 J/mol K, respectively. Results suggested that the methylene blue adsorption on CNTs was a spontaneous and endothermic process.