Adsorption of hexavalent chromium ions from aqueous solution by graphene nanosheets: kinetic and thermodynamic studies

In the present study, a batch system was used to investigate the adsorption of chromium (VI) ions from an aqueous solution by graphene nanosheets. Graphene is decorated with functional groups containing oxygen such as epoxy and hydroxyl on the basal plane. The large negative charge density available on graphene causes effective adsorption of chromium (VI) ions from aqueous solutions. The adsorption capacity and rate of chromium (VI) ions at different temperatures, adsorbent dosages, initial concentrations, and contact times were evaluated. The kinetic study illustrated that the adsorption of chromium (VI) ions onto graphene obeys the pseudo-second-order model with activation energy of 21.91 kJ mol^-1. The chromium (VI) ions adsorption was well explained using Dubinin–Radushkevich isotherm model. The values of standard enthalpy, entropy, and Gibbs free energy changes at 25 °C were calculated as 686.07 kJ mol^-1, 2.38 kJ mol^-1K^-1, and -22.43 kJ mol^-1, respectively. In this work, graphene was prepared via a green method. Transmission electron microscopy, Fourier transform-infrared spectroscopy, energy-dispersive X-ray analysis, powder X-ray diffraction, Boehm’s titration, and N₂ adsorption–desorption techniques revealed a high-quality few-layer nanosheets of graphene with surface area and inter-planar spacing of 594.7 m² g^-1 and 3.6 Å, respectively.

Keywords
Chromium (VI) removal; Graphene; Dubinin–Radushkevich isotherm; Thermodynamics