## 3 Results and Discussion
### Ni Renoval Mechanism
#### 3.6.4 XPS  
To analyze the elemental valence and relative content of the material before and after the reaction, the material was characterized via XPS; the results are shown in Figure 13. Before the reaction, the main elements of the material were Fe, O, C, and Si (Figure 13a), and, after the reaction (Figure 13b), the fractional spectrum of Ni\({}^{2+}\) appeared in the material, indicating that nZVI@ATP exerted an adsorption effect on Ni\({}^{2+}\). Figure 13c shows the characteristic peaks of O1s before the reaction; the O1s characteristic orbitals can be divided into four peaks: Fe-O (529.79 eV), C-O (530.95 eV), -OH (532.05 eV), and C=O (533.73 eV) [42; 43]. After the reaction, the Fe-O characteristic peaks and its relative content considerably increased (Figure 13d and Table 5). In addition, the relative contents of -OH and C=O decreased after the reaction (Table 6), indicating the coordination reaction between the oxygen-containing functional groups in the nZVI@ATP and Ni\({}^{2+}\)[44]. Figure 13e,f shows the characteristic peaks of Fe2p before and after the nZVI@ATP reaction. Fe and O were the main elements present on the material surface, mainly in the form of FeO (709.51 eV), Fe\({}_{2}\)O\({}_{3}\) (713.58 eV), Fe\({}_{3}\)O\({}_{4}\) (711.45 eV), and FeOOH (724.23 eV) [45; 46]. The appearance of the characteristic peak of Fe\({}^{0}\) before the reaction indicated the successful preparation of the nZVI@ATP, and the disappearance of this peak after the reaction confirmed that the Fe\({}^{0}\) present on the nZVI@ATP surface was involved in the reaction and largely consumed. Figure 12: XRD diagram before and after the nZVI@ATP reaction. Notably, the iron oxide content on the material surfaces increased to different degrees after the reaction (see Table 7 and the semiquantitative analysis of the XPS spectra), indicating that the Fe\({}^{0}\) on the nZVI@ATP surface underwent continuous oxidation to produce more iron oxides during the reaction. Figure 13g shows the Ni2p fractional spectrum and the five characteristic peaks obtained after fitting. In this spectrum, the binding energies of 879.53 and 861.31 eV corresponded to Ni(II)-O, those of 873.31 and 855.71 eV corresponded to Ni(II)-OH, and that of 852.23 eV corresponded to Ni\({}^{0}\), indicating that the reaction products were mainly in the form of nickel oxides, nickel hydroxides, and nickel monomeric forms [31]. The standard electrode potential of Ni\({}^{2+}\) is \(-0.257\) V, which slightly exceeds the standard potential of Fe\({}^{2+}\)/Fe\({}^{0}\) (\(-0.44\) V) [47], indicating that Ni\({}^{2+}\) removal is partly due to the redox effect of nZVI. In summary, the mechanism of Ni\({}^{2+}\) removal by nZVI@ATP involved the complexation of Ni\({}^{2+}\) by oxygen-containing functional groups on the nZVI@ATP surface, the reduction of Ni\({}^{2+}\) to Ni\({}^{0}\) by Fe\({}^{2+}\)/Fe\({}^{0}\), precipitation via the reaction of Ni\({}^{2+}\) with OH\({}^{-}\) due to water corrosion of Fe\({}^{0}\) on the nZVI@ATP surface [31], and the partial dehydration of the hydroxide to produce Ni oxide. The data presented in Table 7 indicate that the main removal mechanism involves the generation of a large amount of OH\({}^{-}\) through the reaction between nZVI@ATP and water, inducing Ni\({}^{2+}\) to generate hydroxide precipitates. Figure 13: _Cont._ \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Status** & **Fe\({}^{\mathbf{0}}\)** & **FeO** & **Fe\({}_{\mathbf{2}}\)O\({}_{\mathbf{3}}\)** & **Fe\({}_{\mathbf{3}}\)O\({}_{\mathbf{4}}\)** & **FeOOH** \\ \hline Before Reaction & 7.23\% & 13.66\% & 18.24\% & 20.79\% & 40.08\% \\ After Reaction & - & 14.67\% & 21.91\% & 20.57\% & 42.84\% \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Changes in the relative content ratio of Fe elements before and after adsorption.