## 4 Conclusions  
In this work, high-quality SWCNTs containing an iron catalyst were prepared by a simple arc discharge method in a hydrogen-argon mixed atmosphere, and the iron nanoparticles were oxidized in situ into magnetic iron oxide nanoparticles by air heat treatment. Based on the unique structure of this nanocomposite material, the adsorption performance of BPA in water as an adsorbent was studied. The main findings are as follows:  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Target Adsorbate**} & \multirow{2}{*}{**Adsorbent**} & \multirow{2}{*}{**pH Value**} & **Maximum** & \\  & & & **Capacity** & **References** \\  & & & **(mg g\({}^{-1}\))** & \\ \hline BPA & N-NiO@N-Fe\({}_{\text{x}}\)O\({}_{\text{y}}\)@N-ZnO & 5 & 100 & [41] \\ BPA & activated carbon & 7 & 44.1 & [42] \\ BPA & MWCNTs/SiO\({}_{\text{y}}\)/CS & 6.22 & 46.2 & [43] \\ BPA & HM/WCNTs/Fe\({}_{\text{y}}\)O\({}_{\text{y}}\) & - & 113 & [44] \\ BPA & MWCNTs/Fe\({}_{\text{y}}\)O\({}_{\text{z}}\) & 6.2 & 45.31 & [27] \\ BPA & Fe\({}_{\text{y}}\)O\({}_{\text{z}}\)/GO & - & 72.8 & [45] \\ BPA & SWCNTs/Fe\({}_{\text{x}}\)O\({}_{\text{y}}\) & 3 & 117 & This work \\  & 4 & 96 & This work \\  & 5 & 93 & This work \\  & 7 & 91 & This work \\ \hline \hline \end{tabular}
\end{table}
Table 4: The adsorption capacity of BPA by SWCNTs observed in this work is compared with that reported for other adsorbents in the literature.  
Figure 13: The main mechanism of adsorption of SWCNTs/Fe\({}_{\text{x}}\)O\({}_{\text{y}}\) on BPA.  
1. At the same pH value, compared to other CNTs or carbon materials coated with different oxides, this composite material demonstrates a higher efficiency in adsorbing BPA. After air heat treatment, the SWCNT composite material possesses an increased surface area, which enhances its adsorption efficiency.
2. At low initial BPA concentrations, especially below 0.05 mmol/L, the adsorption efficiency quickly reaches 90% and quickly reaches equilibrium. Acidic conditions were more conducive to the adsorption of BPA by SWCNTs/Fe\({}_{\text{x}}\)O\({}_{\text{y}}\).
3. The kinetic calculations of adsorption indicate that the process follows a pseudo second-order kinetic model. The experimental data fit well with the Langmuir and D-R adsorption isotherm models. Therefore, the adsorption of BPA on the SWCNTs/Fe\({}_{\text{x}}\)O\({}_{\text{y}}\) is likely to be in the form of a uniform monolayer, and the adsorption process is more consistent with physical adsorption.  
The nanocomposite not only exhibits excellent adsorption capacity for hydrophobic phenols (BPA) but also exhibits effective magnetic separation performance, and is easy to recover from water, so it is a promising material for removing BPA from water. Its adsorption capacity is higher under acidic conditions, so the next step will be to study its adsorption performance in actual wastewater. We also need to pay attention to the effect of the Fe doping ratio on adsorption.  
Conceptualization, L.C.; methodology, L.C., J.J. and L.S.; software, L.C.; validation, L.C. and J.J.; formal analysis, L.C.; investigation, L.C. and J.J.; resources, L.S.; data curation, L.C. and J.J.; writing--original draft preparation, L.C.; writing--review and editing, L.S. and J.J.; visualization, L.C.; supervision, L.S.; project administration, L.S.; funding acquisition, L.S. All authors have read and agreed to the published version of the manuscript.  
This work was financially supported by the Foundation Strengthening Program Project Fund (No. 2020-JCJCJ-J17).  
Not applicable.  
The data presented in this study are available on request from the corresponding author.  
The authors declare no conflicts of interest.