Isotherms, kinetics and thermodynamics of acid dye adsorption on activated palm ash

Removal of acid green 25 (AG25) dye onto activated palm ash from aqueous solutions was investigated. Experiments were carried out as function of contact time, initial concentration (50–600 mg/L), pH (2–12) and temperature (30–50 °C). The equilibrium adsorption data of AG25 dye on activated palm ash were analyzed by Langmuir and Freundlich models. The results indicate that the Freundlich model provides the best correlation of the experimental data. The adsorption capacities of the activated palm ash for removal of AG25 dye was determined with the Langmuir equation and found to be 123.4, 156.3 and 181.8 mg/g at 30, 40, and 50 °C, respectively. Adsorption data were modeled using the pseudo-first-order, pseudo-second-order and intra-particle diffusion kinetics equations. It was shown that pseudo-second-order kinetic equation could best describe the adsorption kinetics. Isotherms have also been used to obtain the thermodynamic parameters such as free energy, enthalpy and entropy of adsorption. The positive value of the enthalpy change (26.64 kJ/mol) indicates that the adsorption is endothermic process. The results indicate that activated palm ash is suitable as adsorbent material for adsorption of AG25 dye from aqueous solutions.