Adsorption of diclofenac onto Fe2O3-pillared montmorillonite: Equilibrium, kinetics and thermodynamic studies

Abstract

The kinetic regularities and thermodynamics of adsorption of one of the most common pharmaceutical pollutants diclofenac (DCF) on Fe2O3-pillared montmorillonites (Fe-PMts) have been first studied. Fe-PMts were synthesized by intercalation of natural montmorillonite with iron polyoxocations, followed by heat treatment at 623 and 723 K. The structural and textural characteristics have been studied by XRD, IR, SEM and low temperature N2 adsorption. It has been established that the pillaring lead to an increase in the specific surface of the natural clay by 2.4 times and an increase in the sorption capacity with respect to DCF up to 80 times. The main physicochemical parameters affecting the DCF adsorption were the pH, sorbent content, DCF concentration, and temperature. The specific DCF adsorption increased with decreasing pH, sorbent content, temperature, and with an increase in the initial DCF concentration. An analysis of the kinetic data and the adsorption isotherm was carried out using pseudo-first and pseudo-second order kinetic models, intraparticle diffusion model, and Langmuir and Freundlich adsorption models. The kinetic data of DCF adsorption were in agreement with the pseudo-second order kinetics model, and the adsorption isotherm was in good agreement with the Langmuir model with maximum adsorption capacity of 103.1 mg·g−1 at pH 5.9. DCF adsorption on Fe-PMt was a spontaneous and exothermic process based on the values of the thermodynamic parameters. The results of the study showed that Fe-PMt was an effective sorbent for removing DCF from aqueous solutions and could be used in pharmaceutical wastewater treatment as a cheap and environmentally friendly sorbent.