Modeling of Adsorption Isotherms for Methylene Blue Sorption onto Fly Ash Modified with Hydrochloric Acid

Batch adsorption experiments were conducted to investigate equilibrium, kinetic and thermodynamics of the sorption of Methylene Blue onto Fly Ash modified with various concentrations of hydrochloric acid (FAHC-4, FAHC-3, FAHC-2 and FAHC-1). Equilibrium data was best described by the Freundlich isotherm model which suggests adsorbents had heterogeneous surfaces therefore multilayer adsorption occurred in all cases. Fly Ash modified with 1M HCl exhibited the highest adsorption capacity (9.8 mg/L) and Fly Ash modified with 4M HCl had the least adsorption capacity (4.4mg/L). Kinetic data was best described by the pseudo-second-order kinetic model which means adsorption processes were controlled by chemisorption. Boyd kinetic model plots revealed that the rate determining step of each sorption process followed the film diffusion mechanism in which adsorbate ions travelled towards the external surfaces of adsorbents due to electrostatic interactions between chemically modified Fly Ash and adsorbate ions. All adsorption processes were spontaneous because Gibbs free energy of adsorption was negative in all adsorption processes. KeywordsAdsorption; Langmuir; Freundlich; Intra-particle Diffusion; Boyd