## 3 Results and Discussion
### Proposed Adsorption Mechanism of Cr(VI) Removal  
The FT-IR spectra of FeBC before and after Cr(VI) adsorption are shown in Figure 10. After adsorption, the intensity of the peak at 1095 cm\({}^{-1}\) (C-O-C) increased. The peak at 1624 cm\({}^{-1}\) (C=O) shifted to 1619 cm\({}^{-1}\), with an increase in peak height, indicating the complexation of the carboxylic group with Cr. It was also observed that the peak at 1383 cm\({}^{-1}\) (-COOC-) was no longer significant; the intensity of the peak at 785 cm\({}^{-1}\) (C-H) increased, indicating that C-H bonds changed after adsorption. In addition, new peaks appeared at 672, 663 and 560 cm\({}^{-1}\), indicating the formation of Fe-O-Cr complexes between the Fe-O characteristic peak and Cr. These results indicated that C-O-C, C=O, -COOC- and C-H groups were involved in the adsorption, and C-O-C and C=O were the main groups on FeBC for the Cr(VI) removal from aqueous solutions [35]. After adsorption, the surface of FeBC became rough due to the coating of adsorbed Cr(VI) ions (Figure 1b,c). The EDX results (Figure 2) show that after the Cr(VI) removal process, the carbon losses of FeBC was 18.8%, while the content of Fe and O increased by 12.5% and 5.9%, respectively. A new peak characteristic of Cr (0.3%) was observed, indicating the successful combination of Cr and FeBC and the formation of more iron oxides during adsorption [36]. Therefore, surface adsorption or ion exchange may be responsible for the Cr(VI) removal process. XPS was used to examine surface structural information and elemental composition of FeBC before and after the Cr(VI) removal process (Figure 11). A new typical peak at the binding energy of 578.0 eV corresponding to Cr(VI) ions appeared in the total survey spectra, indicating the occurrence of Cr adsorption/precipitation onto FeBC. In addition, three peaks at 576.8, 578.0 and 579.2 eV in the high-resolution spectra of Cr2p were assigned to Cr(III)-O, Cr (III)-OH and Cr(VI)-O of Cr\({}_{2}\)O\({}_{3}\), Cr(OH)\({}_{3}\) and Cr\({}_{2}\)O\({}_{7}\)\({}^{2-}\), accounting for 44.52, 29.22 and 26.26%, respectively [35]. The relatively high Cr(III) content (73.74%) indicated that Cr(VI) contributed significantly to the reduction rather than direct Cr(VI) adsorption [37]. The Fe spectra was also analyzed to further clarify the Cr(VI) removal process (Figure 11); it illustrated the peaks at 710.6 and 713.9 eV, the representative contents of Fe(II) (61.0%) and Fe(III) (39.0%) [38], and the nascent Fe(II) produced from the reduction of Fe(III) by BC. Fe(III) loaded on BC surface was reduced to Fe(II) in water, which directly reduced Cr(VI) to Cr(III), and then Cr(III) settled onto the FeBC surface. The resulting Fe(III) could be re-adsorbed onto BC and participate in the adsorption. Therefore, the reduction process could be the main mechanism of the Cr(VI) removal by FeBC and Fe played a strengthening role in the reduction process.  
Figure 9: Pseudo-first-order model (**a**) and pseudo-second-order model (**b**) for the adsorption of Cr(VI) onto FeBC.  
Figure 11: XPS spectra of FeBC. (**a**) The total survey spectra before and after Cr(VI) adsorption, and high resolution of (**b**) Cr2p and (**c**) Fe2p after Cr(VI) adsorption.  
Figure 10: FT-IR spectra of FeBC before and after Cr(VI) adsorption.