Removal of sodium diclofenac from aqueous medium using layered double hydroxide: a thermodynamic and theoretical approach

Abstract

In this study, an adsorbent based on layered double hydroxide (Co–Al–NO3]-LDH) was synthesized by the co-precipitation method at constant pH 8.0 ± 0.5. This new material was used for the removal of diclofenac from water. The X-ray diffraction pattern of [Co–Al–NO3]-LDH revealed a basal spacing of 0.859 nm. Equilibrium time was reached after 120 min for an initial concentration (C0) of diclofenac of 500 mg L−1, and the pseudo-second order model best fitted the kinetic data obtained at C0 values of 100, 250, and 500 mg L−1. The isotherms performed at 15, 25, 35, and 45 °C showed an increase in the maximum adsorption capacity (Qmax = 494.9 mg g−1) up to 25 °C, but at temperatures above 25 ºC, the Qmax value was not increased. Equilibrium data were fitted using the Langmuir, Freundlich, and Sips models, and the change in standard free energy of adsorption was estimated from the Langmuir constant, corrected for the equilibrium activity coefficient, while the changes in standard enthalpy and entropy of adsorption were calculated from the van’t Hoff equation. Adsorption studies as a function of nitrate concentration at two C0 values (50 and 500 mg L−1) showed that the increase in nitrate concentration led to a decrease in the Qmax of diclofenac, showing that nitrate competes with diclofenac for the adsorption sites. Theoretical studies were carried out using four different configurations of the diclofenac molecule approaching the surface of [Co–Al–NO3]-LDH. The interaction distance between diclofenac and [Co–Al–NO3]-LDH of 2.0 Å presented the lowest energy.