## 3 Results and Discussion
### Adsorption Studies  
Under the conditions of T = 25 \({}^{\circ}\)C, pH = 6, the absorption rate of the as-grown SWCNTs and SWCNTs/Fe\({}_{\text{x}}\)O\({}_{\text{y}}\) on BPA was studied. As depicted in Figure 6, the adsorption rate of BPA by both nanocomposites increased with time. Both adsorbents reached equilibrium within 15 min, with the SWCNTs/Fe\({}_{\text{x}}\)O\({}_{\text{y}}\) exhibiting an equilibrium adsorption rate of 97%, which represents a 50% improvement over the as-grown SWCNTs, indicating that the heat-treated composites exhibited a superior adsorption performance. Consequently, further experiments will utilize SWCNTs/Fe\({}_{\text{x}}\)O\({}_{\text{y}}\) as the adsorbent for research purposes. \begin{table}
\begin{tabular}{c c c c} \hline \multirow{2}{*}{**Sample**} & **BET** & **Pore Volume** & **Pore Diameter** \\  & **m\({}^{2}\cdot\)g\({}^{-1}\)** & **cm\({}^{3}\cdot\)g\({}^{-1}\)** & **nm** \\ \hline SWCNTs/Fe\({}_{\text{x}}\)O\({}_{\text{y}}\) & 191 & 0.389 & 4.314 \\ As-grown SWCNTs & 115 & 0.299 & 3.828 \\ \hline \end{tabular}
\end{table} 
Table 1: Pore structure analysis of SWCNTs/Fe\({}_{\text{x}}\)O\({}_{\text{y}}\) and as-grown SWCNTs. Figure 4: (**a**) N\({}_{2}\) adsorption–desorption curves and (**b**) pore size distribution of as-grown SWCNTs and SWCNTs/Fe\({}_{\text{x}}\)O\({}_{\text{y}}\). Figure 5: (**a**) Hysteresis loop of SWCNTs/Fe\({}_{\text{x}}\)O\({}_{\text{y}}\) and (**b**) a photo of a small experiment. Figure 7 illustrates the relationship between the rate of adsorption of BPA and the absorption time under different starting concentrations of BPA. Solutions of varying concentrations reached adsorption equilibrium within 15 min, and the starting concentration significantly influenced the BPA equilibrium adsorption rate. The adsorption rate decreased as the initial BPA concentration increased. When the concentration of the BPA solution increased from 0.025 to 0.15 mmol/L, the equilibrium adsorption rate decreased from 98% to 51%. This reduction is primarily due to the limited adsorption capacity of the SWCNTs/Fe\({}_{x}\)O\({}_{y}\) towards BPA. Exceeding the adsorption capacity of the SWCNTs/Fe\({}_{x}\)O\({}_{y}\) with a high initial concentration of BPA solution results in the adsorption of merely a fraction of the BPA molecules. Within the initial 0-10 min, the adsorption rate of the SWCNTs/Fe\({}_{x}\)O\({}_{y}\) increased rapidly, followed by a slower progression. This phenomenon can be attributed to the initial abundance of adsorption sites on the adsorbent, which allowed for the rapid adsorption of BPA from the solution. As adsorption continued, these sites became gradually saturated with BPA molecules, reducing the pore transport capacity and leading to a gradual decrease in the adsorption efficiency until equilibrium was reached. Furthermore, it was observed that for BPA at low concentrations, such as concentrations of 0.025 and 0.05 mmol/L, an adsorption rate of 90% can be achieved after 5 min. Hence, the SWCNT composite loaded with iron oxide nanoparticles can be considered a rapid and effective adsorbent for BPA, especially at lower concentrations. The relationship between the removal rate of BPA and the dosage of the SWCNTs/Fe\({}_{x}\)O\({}_{y}\) is depicted in Figure 8, with the dosage of ranging from 0.05 to 0.35 g/L. An adsorbent dosage of 0.05 g/L resulted in a BPA adsorption rate of 34%. As the adsorbent dosage increased to 0.3 g/L, the adsorption rate reached 96.8% and, upon further increment to 0.35 g/L, exhibited little change, remaining at approximately 97%. Nonetheless, an increment in the adsorbent dosage resulted in a diminution of the adsorption capacity. This phenomenon can be attributed to the fact that while the adsorbent dosage increased,  
Figure 6: As-grown SWCNTs and SWCNTs/Fe\({}_{x}\)O\({}_{y}\) adsorption curves (T = 25 \({}^{\circ}\)C, pH = 6, adsorbent dosage = 0.2 g/L, C\({}_{\text{BPA}}\) = 0.025 mmol/L). Figure 7: Effect of different initial concentrations (T = 25 \({}^{\circ}\)C, pH = 6, adsorbent dosage = 0.2 g/L).