Equilibrium, kinetic, and thermodynamic study for the adsorption of methylene blue onto activated carbons prepared from the banana root through chemical activation with phosphoric acid

The main challenges in wastewater treatment using adsorbents are the selection, development, and characterization of the absorbents. In this study, activated carbons (ACs) were prepared through chemical activation with phosphoric acid, followed by carbonization of banana root (BR). Proximate, ultimate, BET, FTIR, and FE-SEM analyses were employed to characterize the precursor and prepared ACs. A maximum BET surface area of 989 $${m}^{2}/g$$ was observed for one of the ACs. The developed ACs were employed to remove methylene blue (MB) from synthetic wastewater at different initial concentrations (20–200 mg/L) and adsorption temperatures (25, 30, and 40 $$^\circ{\rm C}$$ ) for 24 h in a batch mode. Adsorption isotherm models in linear and non-linear forms of Langmuir, Freundlich, and Temkin were considered to analyze experimental equilibrium data. For evaluation of the optimum isotherm, six different error analysis functions were used. Results indicated that the non-linear form of the Temkin isotherm model was the most suitable to explain the adsorption of MB. The maximum adsorption of 152.83 $$mg/g$$ was observed for an adsorption temperature of 40 $$^\circ{\rm C}$$ and an initial concentration of 200 mg/L. The adsorption kinetic data were fitted with two models: pseudo-first-order and pseudo-second-order. The pseudo-second-order model was the best to explain the adsorption mechanism. The Weber and Morris intraparticle diffusion model was used to identify the rate-controlling step. The intraparticle diffusion was not only the rate-controlling step; another step like surface diffusion might also be involved in the adsorption process. Thermodynamic analyses were carried out, which revealed that the adsorption process was spontaneous and endothermic.