Removal of Cr(VI) from Aqueous Solutions Using Buckwheat (Fagopyrum esculentum Moench) Hull through Adsorption-Reduction: Affecting Factors, Isotherm, and Mechanisms

Abstract

Cr(VI) is a highly toxic pollutant and its contamination frequently occurs in aqueous environment. As an agricultural waste byproduct, buckwheat hull was used in this study as a low-cost biomaterial to remove Cr(VI) from aqueous solutions. The biomass was characterized using scanning electron microscopy, X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FT-IR) spectroscopy. The performance of buckwheat hull on Cr(VI) removal was investigated using the batch experiments. The effect of affecting factors, such as solution pH, initial concentration of Cr(VI), adsorbent dose, temperature, and contact time, were carefully examined. The obtained results revealed that pH had significant effect on Cr(VI) removal and the optimal pH was about 2.0. Increase of adsorbent dose was favorable to the increase of removal ratio, but it decreased the adsorption capacity. Among Langmuir, Freundlich, and Dubinin-Radushkevich models, the Langmuir model was found to best fit the adsorption isotherms of Cr(VI) on buckwheat hull. The maximum adsorption capacity at 45°C was 63.61mgg-1. The thermodynamic parameters (ΔG°, ΔH°, and E) analysis indicated that the adsorption process was a spontaneous, endothermic and chemisorption process. In addition, XPS analysis revealed that the adsorbed Cr(VI) was partially reduced to Cr(III). FT-IR spectra indicated that the carboxyl and amino groups were involved in chromium adsorption. From these results, the most probable mechanisms for removal of Cr(VI) using buckwheat hull were found to be electrostatic attraction, chemical reduction, and complex interaction.