Adsorption of Acid Blue 25 on Agricultural Wastes: Efficiency, Kinetics, Mechanism, and Regeneration

In this study, Acid Blue 25 (AB25), which is a negatively charged synthetic dye was removed from an aqueous solution by adsorption onto agricultural wastes, including banana (BP) and durian (DP) peels. The adsorption performances of AB25 were related to surface characteristics of the agricultural wastes, including their chemical functional groups, net surface charge, surface morphology, surface area, and pore volume. Parameters affecting the adsorption, including contact times, initial concentration, pH, and temperature were investigated. The results revealed that the adsorption of AB25 followed pseudo-second order kinetics, and that the adsorption process was controlled by a combination of intraparticle and film diffusion with a two-step mechanism. The equilibrium data could be simulated by the Langmuir isotherm model, suggesting that AB25 molecules are adsorbed on active sites with a uniform binding energy as a monolayer on the adsorbent surface. The adsorption process was spontaneous and exothermic, and the adsorption capacity decreased with the pH of the medium. The spent adsorbents were best regenerated by acid treatment (pH 2), and could be recycled for several adsorption-desorption processes. Under ambient conditions, the maximum adsorption capacities of AB25 on BP and DP were 70.0 and 89.7 mg g−1, respectively, which is much higher than on a large variety of reported adsorbents derived from other agricultural wastes.