## 3 Results and Discussion
### Sorbent Characterization
In addition, the electron pairs of Mn\({}^{4+}\)/Mn\({}^{3+}\) and Ce\({}^{3+}\)/Ce\({}^{4+}\) would also increase with increasing Mn and Ce loading value. A cloud of electrons would shift between the redox cycles of Mn\({}^{4+}\)\(\leftrightarrow\)Mn\({}^{3+}\) and Ce\({}^{3+}\)\(\leftrightarrow\)Ce\({}^{4+}\) through the CNFs carbon-based framework, which benefits the Hg\({}^{0}\) oxidation process. The XRD patterns of the different sorbents are shown in Figure 2. The raw CNFs contained two kinds of carbon structures [30]: graphite 2H (20 = 25.91\({}^{\circ}\)) and common carbon (20 = 42.49\({}^{\circ}\)), of which the graphite 2H was formed by the convolution of two layers of graphene. Graphene has a high degree of graphitization, which facilitates electron transferal through the CNF framework. In addition, Ni crystallization peaks were observed in raw CNFs due to residual Ni catalyst during CNF production. However, the Hg\({}^{0}\) removal efficiency of raw CNFs was only 16.02% (seen from Figure 3), which indicated that the residual Ni has a very limited impact on the Hg\({}^{0}\) removal efficiency. Hence, the impacts of residual Ni on the Hg\({}^{0}\) removal performance can be ignored in this study. It is worth noting that the XRD peak intensity changed significantly after ultrasonic oxidation treatment. The peak intensity of graphite 2H of the CNFs(OX) was much higher than that of raw CNFs, which was owing to the removal of ash content on the raw CNFs' surface during the ultrasonic purification treatment process and the exposure of the graphene phase. For the metal-oxide-loaded samples, the crystal phase ascribed to MnO\({}_{\rm x}\) or CeO\({}_{\rm x}\) was not observed when Mn and Ce loading increased from 6% to 15%, suggesting that MnO\({}_{\rm x}\) and CeO\({}_{2}\) species were well dispersed on the CNFs(OX) surface in amorphous states [22]. A slight crystal phase ascribed to MnO\({}_{2}\) was observed from 18%MnO\({}_{\rm x}\)-18%CeO\({}_{2}\)/CNFs(OX), indicating the aggregation of MnO\({}_{\rm x}\). The results also demonstrated that the highly dispersed threshold value of the metal oxides on the CNFs(OX) support was about 30%, which is higher than those on other supports. The specific surface area, oxygen-containing functional groups, and the intensity of graphite 2H peak were all enhanced after the ultrasonic oxidation treatment of CNFs, which also facilitated the dispersion and anchoring of active metals. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{\(\alpha\)} & \multicolumn{6}{c}{**Surface Atomic Concentration (\%)**} \\ \cline{2-7}  & **C 1s** & **O 1s** & **Mn 2p** & **Mn\({}^{4+}\)/Mn\({}^{3+}\)** & **Ce 3d** & **Ce\({}^{3+}\)/Ce\({}^{4+}\)** \\ \hline
0 & 96.76 & 3.24 & & & & \\
6\% & 96.31 & 2.98 & 0.48 & 0.4277 & 0.23 & 0.5523 \\
9\% & 91.28 & 6.28 & 1.61 & 0.4212 & 0.83 & 0.3174 \\
12\% & 90.24 & 6.94 & 1.88 & 0.4171 & 0.94 & 0.2900 \\
15\% & 85.98 & 9.74 & 2.85 & 0.4140 & 1.43 & 0.2859 \\
18\% & 79.95 & 13.12 & 3.64 & 0.3218 & 1.85 & 0.2734 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Surface atomic concentration of the adsorbents based on XPS results. The micromorphology of the CNFs and the dispersion of MnO\({}_{x}\)-CeO\({}_{2}\) on the surface of the support were evaluated, as shown in Figure 3. As shown in Figure 3a,b, the SEM image of the raw CNFs and 15%MnO\({}_{x}\)-15%CeO\({}_{2}\)/CNFs(OX) showed a homogeneous morphology of cross-linked nanofibers of about 100 nm in diameter. It is clearly observed that the MnO\({}_{x}\) and CeO\({}_{2}\) have been homogeneously dispersed on the support, as shown in Figure 3c. Figure 1: O 1s XPS spectra of (**a**) carbon nanofibers (CNFs) and (**b**) CNFs(OX). XPS spectra of (**c**) O 1s, (**d**) Mn 2p, and (**e**) Ce 3d for 15%MnO\({}_{x}\)-15%CeO\({}_{2}\)/CNFs(OX).