Taguchi L9 (34) orthogonal array study based on methylene blue removal by single-walled carbon nanotubes-amine: Adsorption optimization using the experimental design method, kinetics, equilibrium and thermodynamics

Taguchi orthogonal array (L9 OA) experimental design was used to determine the optimum conditions for the removal of methylene blue (MB) dye using single-walled carbon nanotubes functionalized with an amine group (SWCNT-NH2). The effects of the experimental parameters (MB initial concentration, SWCNT-NH2 dosage, temperature and pH) on the removal efficiency were studied and optimized in terms of the standard average (SA) and signal-to-noise ratio (S/N) in order to efficiently improve the process. Results showed that the initial concentration has the most significant impact on the adsorption of MB onto SWCNT-NH2 and the best conditions were measured to be 20 mg/L initial MB concentration, 0.05 g SWCNT-NH2 dosage, pH 9 and 50 °C. Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and X-ray diffraction (XRD) techniques were used to characterize the adsorbed surface before and after adsorption. Adsorption kinetics of SWCNT-NH2 was studied and modelled using the pseudo-first-order, pseudo-second-order and intraparticle diffusion models and it was best adjusted to the pseudo-second-order model. Moreover, Langmuir isotherm model with regression coefficient R2 of 0.9479, was the best fit for the equilibrium adsorption study and the adsorption capacity was found to be 13.62 mg g−1. The thermodynamic study showed that the adsorption of MB dye is spontaneous and endothermic.