Removal of hexavalent chromium from aqueous solutions by use of chemically modified sour cherry stones

A new adsorbent for Cr(VI) removal was prepared from sour cherry stones, by treatment with H2SO4 (12–96%). Adsorbent properties were characterized by means of Fourier transform infrared spectroscopy, scanning electron microscopy coupled with energy dispersive X-ray spectroscopy, specific surface area, point of zero charge, iodine number, and analysis of surface functional groups. Surface area increased from 225.5 to 484.7 m2 g−1, point of zero charge decreased from 6.0 to 3.2, iodine number increased from 4 to 80, total amount of surface acidic groups increased from 1.12 to 2.3 mmol g−1 and adsorption capacity increased from 0.53 to 5.27 mg g−1, with increasing H2SO4 concentration from 0 to 96%. The pseudo-first- and pseudo-second-order kinetic models have been used to analyze the kinetics of the adsorption; it was found that kinetic experimental data were successfully fitted by the pseudo-first-order model. Equilibrium data were mathematically interpreted by applying the Langmuir, Freundlich, and Dubinin–Radushkevich models; the results indicate that Freundlich model provides the best correlation. Negative values of Gibbs free energy suggest an endothermic and spontaneous adsorption process. The mechanism of Cr(VI) removal involves adsorption of Cr(VI) followed by its partial reduction to Cr(III). Analysis of thermodynamic parameters, coupled with modeling of experimental data with two different intraparticle diffusion models, revealed that binding of Cr(VI) occurred via physisorption, while the rate-limiting step was film diffusion. On the basis of present study, it can be concluded that sour cherry could be suitable adsorbents for the removal of Cr(VI) from polluted waters.