Efficient Adsorption of Methyl Orange on Nanoporous Carbon from Agricultural Wastes: Characterization, Kinetics, Thermodynamics, Regeneration and Adsorption Mechanism

Nanoporous carbon derived from Moringa oleifera seed waste was synthesized by an original process of flash pyrolysis followed by zinc chloride impregnation. The N2-adsorption–desorption results of the optimized sample revealed a BET surface area of 699.6 m2/g and a pore size of 2 nm. It was evaluated for the adsorption of a mono azo dye, methyl orange (MeO), from aqueous solution. Four isothermal models (Langmuir, Freundlich, Dubinin–Radushkevic and Temkin) were applied to fit the experimental data. The results revealed that Langmuir is the most appropriate isothermal adsorption model to describe the adsorption process (X2 = 1.16); with an adsorption capacity 367.83 mg/g at 298 K, the interaction of MeO dye with the nanoporous carbon surface is a localized monolayer adsorption. The adsorption kinetics was consistent with the pseudo-second-order model and found to correlate well with the experimental data (X2 = 9.06). The thermodynamic study revealed a spontaneous and endothermic adsorption process, and the substances are adsorbed in a random manner. The desorption of MeO dye from MOC-ZnCl2 by sodium hydroxide solution was achieved to a level of about 84%, and the nanoporous carbon was recycled and reused at the fifth cycle. This work demonstrates that MOC-Zncl2 could be employed as an alternative to commercially available activated carbon in the removal of dyes from wastewater.