Adsorption of reactive dye onto carbon nanotubes: Equilibrium, kinetics and thermodynamics

The adsorption efficiency of carbon nanotubes for Procion Red MX-5B at various pHs and temperatures was examined. The amount adsorbed increased with the CNTs dosage; however, the adsorption capacity initially increased with the CNTs dosage (<0.25 g/l) and then declined as the CNTs dosage increased further (>0.25 g/l). The linear correlation coefficients and standard deviations of Langmuir and Freundlich isotherms were determined and the results revealed that Langmuir isotherm fitted 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 a pseudo second-order model. Changes in the free energy of adsorption (ΔG°), enthalpy (ΔH°) and entropy (ΔS°), as well as the activation energy (Ea) were determined. ΔH° and ΔS° were 31.55 kJ/mol and 216.99 J/mol K, respectively, at pH 6.5 and 41.47 kJ/mol and 244.64 J/mol K at pH 10. The activation energy was 33.35 kJ/mol at pH 6.5. ΔH°, ΔG° and Ea all suggested that the adsorption of Procion Red MX-5B onto CNTs was by physisorption.