## 3 Results and Discussion  
### Characterization of BC and FeBC  
The surface area, pore volume and average pore size of BC and FeBC are listed in Table 1. As shown in Table 1, the BET surface area of FeBC (44.69 m\({}^{2}\cdot\)g\({}^{-1}\)) was higher than that of BC (26.75 m\({}^{2}\cdot\)g\({}^{-1}\)). The pore volume and average pore size of FeBC (i.e., 0.0351 cm\({}^{3}\cdot\)g\({}^{-1}\) and 5.50 nm) were also higher than those of BC (i.e., 0.0072 cm\({}^{3}\cdot\)g\({}^{-1}\) and 5.11 nm). This indicated that the Fe modification process allowed BC to provide a more developed pore structure and more active adsorption sites, thus reducing the mass transfer resistance in the adsorption process and facilitating the adsorption [17].  
The surface morphologies of BC and FeBC through SEM images are shown in Figure 1. It can be found that, compared with BC, FeBC had a rougher surface and slightly damaged pore structure. Many fine flocculent solid particles with different sizes were observed on the surface and in the pores, which were dispersed, disorderly and partially overlapping (Figure 1b). The EDS spectra of BC and FeBC are shown in Figure 2. The absorption peak of Fe was found in the spectrum in FeBC (Figure 2b); it can be concluded that these particles may be iron oxides, indicating that Fe was successfully loaded onto BC.  
XRD patterns of BC and FeBC samples are shown in Figure 3. The wide peak of BC and FeBC at 2\(\theta\) of 26.8512\({}^{\circ}\) were the characteristic peak of C with the highest intensity. No significant peaks were detected in the pattern of FeBC except for some relatively narrow peaks, suggesting that the crystalline Fe content was low and the amorphous structure was mainly loaded on the FeBC [18].  
FT-IR was used to investigate the functional groups in BC and FeBC, as shown in Figure 4. The peaks detected in BC at about 3434, 2921, 1619, 1384, 1097 and 797 cm\({}^{-1}\) were attributed to the stretching vibration of the -OH, C-H, C=O in aromatic rings, -COO-, C-O-C and C-H groups, respectively. The Fe modification resulted in a shift of FT-IR absorption peaks and a decrease of the peak intensity, indicating that surface complexes of some groups (e.g., -OH and -COO-) formed with Fe (III) led to changes in the chemical environment around the groups. In addition, the decrease of the peak intensity was mainly attributed to the physical coverage of functional groups on the BC surface by iron oxides [19]. Nevertheless, the surface of FeBC still had abundant hydroxyl and carboxyl groups, which promoted the adsorption of heavy metals [20]. Furthermore, the abundant aromatic carbon provided electrons and enhanced the potential redox activity [21].  
\begin{table}
\begin{tabular}{c c c c} \hline \multirow{2}{*}{**Sample**} & **Specific Surface Area** & **Pore Volume** & **Average Pore Diameter** \\  & **/m\({}^{2}\cdot\)g\({}^{-1}\)** & **/cm\({}^{3}\cdot\)g\({}^{-1}\)** & **/nm** \\ \hline BC & 26.75 & 0.0072 & 5.11 \\ FeBC & 44.69 & 0.0351 & 5.50 \\ \hline \end{tabular}
\end{table}
Table 1: Pore characteristics of BC and FeBC.  
Figure 1: SEM images of (**a**) BC, (**b**) FeBC and (**c**) Cr-FeBC.  
Figure 2: EDX spectra of (**a**) BC, (**b**) FeBC and (**c**) Cr-FeBC.