Oxidized multiwalled carbon nanotubes for the removal of methyl red (MR): kinetics and equilibrium study

Oxidized multiwalled carbon nanotubes (MWCNT) were used for the removal of methyl red (MR) from aqueous solutions. The effects of variables, such as initial solution pH, initial dye concentration, temperature, and sorption time, on MR removal were studied and optimized. Fitting the experimental equilibrium data to various investigated isotherm models, such as Langmuir, Freundlich, Tempkin, and Dubinin-Radushkevich (D-R) models, showed the suitability of the Langmuir model with the highest correlation coefficients. The calculated thermodynamic parameters indicated that the removal of MR by MWCNT is an entropy-driven and endothermic process. The experimental data were analyzed by different kinetic models, such as pseudo-first order, pseudo-second order, and Elovich and intraparticle diffusion models. It was found that the second-order equation and intraparticle diffusion models are the rate-limiting factor and control the kinetic of the adsorption process.