Characteristic and Mechanism of Cr(VI) Biosorption by Buckwheat Hull from Aqueous Solutions

Buckwheat hull, an agricultural industry waste, was used as a novel biosorbent to remove hexavalent chromium from water. The absorption kinetics was investigated by measuring the change of Cr(VI) and total chromium concentration with time. It showed that the removal of Cr(VI) by buckwheat hull followed the pseudo-second-order adsorption kinetics. Effects of pH., biosorbent dosage and initial concentration of Cr(VI) on its adsorption removal were also examined in the batch mode. The results showed that the buckwheat hull was very effective for removing Cr(VI) from water. At pH 2.0, load of 5.0 g.L-1, buckwheat hull could almost completely remove Cr(VI) (99.87% of 100 mg.L-1) from water. The removal of Cr(VI) increased with the decrease of solution pH values, and the maximum percentage removal was attained at pH 2.0. Increase of adsorbent dosage would raise the removal efficiency, while it would simultaneously reduce the adsorption capacity. Moreover, the removal of Cr(VI) was found to decrease with rising the initial concentration of Cr(VI). Whereas, the Cr(VI) uptake capacity increased with the rise of Cr(VI) concentration until reaching saturation, which was found to be about 36.4 mg.g(-1) at pH 2.0, temperature of 20 degrees C and adsorbent dose of 5.0 g.L-1. In addition, Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDX) analyses were employed to characterize buckwheat hull, and further to elucidate the mechanisms involved at the molecular level. SEM revealed that the buckwheat hull is a porous material, while FT-IR and XPS displayed that the biomass possess many active groups such as hydroxyl, carboxyl, and amino groups. The removal of Cr(VI) by the buckwheat hull was found to be a adsorption-coupled reduction process based on XPS analysis and batch experiments. Finally, the mechanisms of chromate anions removal from water by the buckwheat hull were proposed, which included adsorption of Cr(VI) onto buckwheat hull followed by the partial reduction to Cr(III). The reduced Cr(III) was rebound to the biomass mainly through the complexation and ion-exchange mechanisms.