# Enhanced Performance of nZVI/MXene@CNTs for Rapid As(III) Removal from Aqueous Solutions
## 3 Results and Discussion
### Mechanism Analysis  
To explore the changes in the functional groups, the FTIR analysis was utilized before and after the adsorption of As(III) by nZVI/MXene@CNTs (Figure 6a). The peaks at 3385.74, 1623.91 and 1383.82 cm\({}^{-1}\) after adsorption corresponded to the -OH stretching, O-C=O symmetric and asymmetric stretching vibrations, respectively [61]. The functional group species were changed slightly before and after the adsorption, while the wave number of absorption peaks were changed, indicating that the aforementioned functional groups might be involved in the removal of As(III). The intensity of the O-C=O peak had increased, which indicated that the ligand process combined As(III) with the carboxylate, possibly producing the free -COOH. The intensity of the -OH peak had dropped, probably due to the Fe-OH breaking, implying that the hydroxyl group was engaged in the reaction and gradually decreased as the reaction progressed. The typical peak of Fe\({}_{3}\)O\({}_{4}\) was shifted to 448.47 eV, which might be attributed to the electrostatic gravitational effect between Fe and As(III). The peak at 815.14 eV was corresponded to the Fe-O-As vibrational peak, indicating that As(III) removal process occurred through surface complexation, as well as solid-phase precipitation by bonding As(III) to Fe [62]. The results suggested that the nZVI/MXene@CNTs retained a good magnetic characteristic, making it suitable for recycling [63; 64]. Figure 5: (**a**) Pseudo-first-order and (**b**) pseudo-second-order models of nZVI/MXene@CNTs; (**c**) Langmuir and (**d**) Freundlich isotherms models of nZVI/MXene@CNTs; (**e**) thermodynamics of nZVI/MXene@CNTs at different temperatures. In order to further investigate the adsorption mechanism of nZVI/MXene@CNTs, the XPS analysis was performed before and after the adsorption of As(III). The obtained XPS full and fine spectrum of each element are shown in Figure 6b-f. There was an additional characteristic peak of As 3d at 44.71 eV, suggesting that As(III) was successfully adsorbed by the nZVI/MXene@CNTs, in addition to the typical peaks of C 1s, Ti 2p, O 1s and Fe 2p at 284.45, 458.37, 530.85 and 711.33 eV, respectively. The characteristic peaks of Fe 2p were unchanged, indicating that the nZVI/MXene@CNTs retained good magnetic properties after adsorption. Furthermore, in Figure 6c, three convex peaks appeared at 284.80, 287.15 and 288.80 eV, corresponding to C-C, C-O-C, and O-C=O, respectively, suggesting that the various groups on the surface remained after adsorption. Thus, the structure was stable and could be utilized repeatedly. As shown in Figure 6d, the binding energy of the associated Fe\({}^{3+}\) peak decreased from 714.99 to 713.97 eV, implying that the charge density increased while the binding energy decreased, owing to the migration of the outer electrons. Therefore, the Fe\({}^{3+}\) was involved in the removal process [65]. The change of the Fe\({}^{0}\) characteristic peak indicated that it was oxidized to form hydrated iron oxide during the removal process. It was clearly observed that the characteristic peaks at 532.78, 531.53, 530.64 and 530.48 eV were attributed to C=O, C-OH, Ti-O and Fe-O, respectively (Figure 6e). However, the C-OH content was reduced after the reaction, which might imply that the hydroxyl groups were consumed during the adsorption process, while the spots were occupied by As(III) and As(V). The results were compatible with the FTIR data. As shown in Figure 6f, the characteristic peak of As 3d appeared in the 42-48 eV interval after the adsorption. The characteristic electron-binding energy of As(III) was lower than that of As(V), as the research reported [66]. As a result, the peak at 44.10 eV might be denoted as As(III), whereas the peaks at 45.19 eV and 45.58 eV might be indicated as As(V), with the atomic percentages of 32.02% and 67.98%, respectively. It was suggested that a significant portion of As(III) was oxidized to As(V) during the adsorption process, due to the nZVI and the abundant oxygen-containing groups.