## 3 Results and Discussion  
### Sorbent Characterization  
The surface physical characteristics of the samples are shown in Table 1. The BET specific surface area of CNFs(OX) was higher than that of the raw commercial nanofibers (89.512 m\({}^{2}\)/g). This suggested that the ultrasonic oxidation treatment of the commercial nanofibers resulted in an increase of the specific surface area. This may be owing to the removal of ash on the commercial nanofibers' surface during the ultrasonic oxidation treatment process. Meanwhile, the treatment process may also enrich surface defects. As shown in Table 1, the BET surface area and average pore volume decreased continuously as the loading value increased from 6% to 18%. This phenomenon indicates that metals entered the pore channels of CNFs(OX). The Hg\({}^{0}\) removal process over the transition metal oxides consists of typical heterogeneous redox reactions, so the details of the surface properties of the sorbents are very significant. The surface atomic concentrations of the different samples are summarized in Table 2. The surface oxygen concentration of CNFs was 2.59%. As shown in Table 2, the surface oxygen concentration increased to 3.24% after ultrasonic oxidation treatment. Six different oxygen species were fitted to O 1s XPS spectra, as shown in Figure 1a-c. As seen in Figure 1a,b, the proportion of oxygen-containing functional groups C=O or -OH in esters and anhydrides (532.3-532.8 eV) [26] increased from 34.01% to 44.15% after the ultrasonic oxidation treatment of the CNFs. Meanwhile, the proportion of -COOH (534.3-535.4 eV) [26] increased from 4.46% to 4.80%. The increase in oxygen-containing functional groups, especially-COOH, would facilitate the dispersion and anchoring of active metals. Besides this, as seen from Table 2, the surface oxygen atom concentration increased constantly with increasing MnO\({}_{\mathrm{x}}\) and CeO\({}_{2}\) loading values, which was assigned to oxygen species in the metal oxides. \begin{table}
\begin{tabular}{c c c} \hline \(\alpha\) & **Specific Surface Area (m\({}^{2}\)/g)** & **Pore Volume (cm\({}^{3}\)/g)** \\ \hline
0 & 96.532 & 0.461 \\
6\% & 97.020 & 0.350 \\
9\% & 93.853 & 0.299 \\
12\% & 88.948 & 0.307 \\
15\% & 87.158 & 0.293 \\
18\% & 86.729 & 0.286 \\ \hline \end{tabular}
\end{table} 
Table 1: The surface physical characteristics of different adsorbents. The surface Mn and Ce atomic concentrations also increased constantly with increasing Mn and Ce loading values. As shown in Figure 1d, two orbits of the Mn 2p XPS spectra belong to Mn 2p1/2 (\(\sim\)653.8 eV) and Mn 2p3/2 (\(\sim\)642.2 eV). Mn 2p3/2 can be fitted to three subpeaks: Mn\({}^{3+}\) (\(\sim\)641.8 eV), Mn\({}^{4+}\) (\(\sim\)643.5 eV) [16], and satellites (\(\sim\)646.1 eV) [27]. Mn\({}^{4+}\) species are recognized as the most important active component for Hg\({}^{0}\) oxidation. Although the ratio of Mn\({}^{4+}\)/Mn\({}^{3+}\) decreased slightly with increasing loading value, the absolute amount of Mn\({}^{4+}\) ions still increased with increasing loading value except for 18%MnO\({}_{\rm x}\)-18%CeO\({}_{2}\)/CNFs(OX). Figure 1e shows the Ce 3d XPS spectra of fresh 15%MnO\({}_{\rm x}\)-CeO\({}_{2}\)/CNFs(OX). The Ce 3d XPS spectra could be fitted into eight subpeaks [28]. Among them, v/u, v2/u2, and v3/u3 belong to Ce\({}^{4+}\), while v1/u1 belongs to Ce\({}^{3+}\). Ce\({}^{3+}\) species are considered to be important for the catalytic oxidation and adsorption of Hg\({}^{0}\)[29], because Ce\({}^{3+}\) could lead to a charge imbalance, which facilitates Hg\({}^{0}\) oxidation process as more active oxygen species yield. As shown in Table 3, the absolute amount of Ce\({}^{3+}\) cations also increased with increasing CeO\({}_{2}\) loading value. From the XPS characterization results, with the increase of the highly dispersed Mn and Ce loading, MnO\({}_{\rm x}\)-CeO\({}_{2}\)/CNFs(OX) sorbents offered abundant active oxygen, Mn\({}^{4+}\), and Ce\({}^{3+}\) species. They play important roles in promoting the Hg\({}^{0}\) removal activity.