Adsorptive removal of methyl orange and methylene blue from aqueous solution with finger-citron-residue-based activated carbon

Activated carbon derived from finger citron residue (FAC) was tested as a new type of adsorbent for the removal of harmful dyes, namely, the anionic dye methyl orange (MO) and the cationic dye methylene blue (MB), from contaminated water. Liquid-phase adsorption experiments were conducted, and the maximum adsorption capacity was determined. Various conditions were evaluated, including initial dye concentration, adsorbent dosage, contact time, solution pH, and temperature. The Langmuir and Freundlich adsorption models were used to describe the equilibrium isotherm and to calculate the isotherm constants. It was found that the adsorption capacity of FAC is much higher than those of other types of activated carbons. Maximum equilibrium adsorption capacities of 934.58 and 581.40 mg/g for MO and MB, respectively, were achieved. Three simplified kinetic models, namely, pseudo-first-order, pseudo-second-order, and intraparticle diffusion equations, were used to investigate the adsorption process. The pseudo-second-order equation was followed for the adsorption of MO and MB on FAC. The temperature-dependent adsorption behaviors of MO and MB indicated that the adsorption is a spontaneous and endothermic process accompanying an entropy increases (the driving force for adsorption). This work demonstrates that FAC could be employed as a low-cost alternative to commercially available activated carbons in the removal of dyes from wastewater.