Quantitative analysis on the redox conversion mechanism of Cr(VI) and As (III) by iron carbide based biochar composites

Fe-based materials have been widely used for removing Cr(VI) and As(III) in wastewater, however, the quantitative understanding of their removal mechanism and functions of Fe is lacking. In this study, three types of iron carbide based biochar composites (Fe3C@BC-A, Fe3C@BC-B, and Fe3C@BC-C) with different Fe dosages of 19.7%, 24.0%, and 26.5%, respectively, were prepared for Cr(VI) and As(III) removal. The removal of Cr(VI) and As(III) was found to increase as the Fe dosage increased. Fe3C@BC-C with the highest Fe content showed the greatest reduction and oxidation capacities for Cr(VI) and As(III). X-ray absorption near-edge structure analysis indicated that the reduction of Cr(VI) afforded FeCr2O4, (CrxFe1-x)(OH)(3), Cr3+, Cr(OH)(3), and Cr2O3, whereas AsO43-was the oxidation product of As(III). Results of a pickling experiment revealed that Fe accounted for 18.9%-47.4% and 98.1%-99.4% of the removed Cr(VI) and As(III), respectively. The adsorption mechanism revealed by Fourier transform infrared spectrometry and X-ray photoelectron spectroscopy (XPS) suggested that Cr(VI) was adsorbed by the OH groups of biochar, whereas As(III) was bonded to gamma-FeOOH of the reacted Fe3C particles. The quenching experiment, electron spin resonance analysis, and XPS suggested that Fe-0, Fe2+, atomic H, and O-containing groups contributed to the reduction of Cr(VI), while (OH)-O-center dot, center dot O-2(-), O-1(2), and Fe(OH)3 were responsible for the oxidation of As(III). The quantitative mechanisms contributed to an improved understanding for the removal of Cr(VI) and As(III) by Fe/C composites, and the results may guide further preparation and application of Fe/C composites for redox-active contaminants removal.