Preparation, Characterization, and Application of Graphene–Zinc Oxide Composites (G–ZnO) for the Adsorption of Cu(II), Pb(II), and Cr(III)

In the present study, graphene–zinc oxide composites (G–ZnO) were prepared and applied to the adsorption of Cu(II), Pb(II), and Cr(III) from aqueous solutions. The synthesized composites were characterized by X-ray diffraction and transmission electron microscopy. Effects of pH and contact time were studied in batch experiments. Kinetics, adsorption isotherms, and thermodynamics were also examined. Kinetics experiments indicated that the processes could be simulated by a pseudo-second-order model. The experimental results of equilibrium adsorption were tested by Langmuir and Freundlich isotherms, indicating that the former fit the data better. Thermodynamic parameters, the Gibbs free energy change (ΔG), enthalpy change (ΔH), and entropy change (ΔS), were calculated, indicating that the adsorption of heavy metals onto G–ZnO was spontaneous and endothermic in nature.