Adsorption of heхavalent chromium on mechanically activated graphite

Abstract

Mechanical activation of spectrally pure graphite was carried out in a high-energy planetary mill. Changes in the structure and physicochemical properties caused by this modification are shown by SEM, XRD, Raman spectroscopy, XPS, N2 adsorption-desorption, and inert gas fusion methods. Because of the mechanical activation, a decrease in crystallite size of the graphite and the oxidation occurred; it began to exhibit sorption properties with respect to Cr(VI) ions. The optimal conditions were found for removing more than 99% of Cr(VI) from the 50 mg∙l−1 aqueous solutions. In an acetate buffer solution with pH 6.3 after 1 h of heating at 90°C, the Langmuir adsorption capacity for chromium on this sorbent is 14.7 mg∙g−1. Thermodynamic characteristics of the adsorption were determined; they show that the process is spontaneous and endothermic. It was found that only incomplete desorption of chromium from the surface of the modified graphite is possible using concentrated NaOH solutions. An assumption was made about a mixed mechanism of chromium adsorption, including partial reduction of Cr(VI) to Cr(III), anionic adsorption of Cr(VI) ions through electrostatic interaction, and the formation of an insoluble chemical compound of Cr(III) on the surface of the mechanically activated graphite. The sorbent was tested on a real sample of wastewater from bichromate production, after adsorption hexavalent chromium was not detected in the water.