Sodium hydroxide-activated Casuarina empty fruit: Isotherm, kinetics and thermodynamics of methylene blue and congo red adsorption

Abstract

This work was aimed to evaluate the adsorptive properties of sodium hydroxide-activated Casuarina empty fruit carbons for methylene blue and congo red removal. The activated carbons were prepared at varying NaOH ratios and characterized for specific surface, functional groups, thermal stability, and the pH at the point of zero charge. Batch adsorption was performed at varying concentrations, contact times, and temperatures. The specific surface of activated carbons decreased with increasing ratio, but the maximum capacity increased. The methylene blue capacity onto AC2.5 is 530 mg/g, while for congo red onto AC2.0 is 232 mg/g. The equilibrium data fitted well with the Langmuir model, while the kinetics could be described by the pseudo-second-order model. The thermodynamics indicate that the adsorption is endothermic and spontaneous at high solution temperature. • Conversion of Casuarina empty fruit into effective activated carbon via NaOH activation. • Excellent textural and adsorptive properties for dyes removal. • Exceptional removal of methylene blue at 530 mg/g, and congo red at 232 mg/g. • Microporous activated carbon rich in acidic groups with surface area of 1511 m 2 /g.