Adsorption Thermodynamics and Kinetics of Malachite Green onto Ca(OH)2-Treated Fly Ash

Fly ash, an industrial by-product abundant in India, was treated with alkali and tested as a low-cost adsorbent for the removal of malachite green from an aqueous solution in a batch adsorption procedure. Effects of stirring rate, temperature, pH, initial dye concentration, contact time, and adsorbent dose were investigated. The adsorption was found to be strongly dependent on pH of the medium and the adsorption capacity decreased with an increase in temperature. The Langmuir isotherm model showed a good fit to the equilibrium adsorption data at all temperatures. The mean free energy (E) estimated from the Dubinin-Radushkevich model indicated that the adsorption mechanism was chemical ion exchange. The kinetic data were found to follow the pseudo second-order kinetic model. The rate constant decreased with the increase in temperature indicating the exothermic nature of adsorption. Intraparticle diffusion was not the sole rate-controlling factor. The Arrhenius and Eyring equations were used to evaluate the activation parameters. The activation energy (Ea) was estimated to be 56.08 kJ mol-1. Gibbs free energy (ΔG0) was spontaneous for all interactions, and the adsorption process exhibited exothermic enthalpy values. Results suggest that alkali-treated fly ash is a potential low-cost adsorbent for removal of malachite green from an aqueous solution.