## 3 Results and Discussion
### Interaction between Modified Zeolite and Cu(II)  
To discuss the reaction mechanism of the Cu(II) adsorption onto the modified zeolite, we applied characterization methods (FT-IR and XPS analyses) by comparing the results before and after adsorption. We present the results in Figures 9-11.  
We found the FT-IR spectra of the modified-zeolite peaks in Figure 9 at 3646 cm\({}^{-1}\) (Si-OH), 3463 cm\({}^{-1}\) (OH) [38], and 1648 cm\({}^{-1}\) (for water molecules associated with the cations) [50]. We detected peaks for Si-O and Al-O bonds at 1052 cm\({}^{-1}\) and peaks for Si-O-Si and Al-O-Al bonds at 459 and 796 cm\({}^{-1}\), respectively. Moreover, we found peaks with low intensity at 2928 and 2858 cm\({}^{-1}\) (asymmetric and symmetric stretches of the CH bonds) [51], which indicated the presence of the propyl chains of APTES. The peaks appearing at around 1500 and 1600 cm\({}^{-1}\) belong to the NH\({}_{2}\) scissor vibrations, which indicated the presence of the NH\({}_{2}\) functional group of APTES [52]. These peaks correspond to the Si(CH\({}_{2}\))\({}_{3}\)NH\({}_{2}\), which we detected with low intensity due to the low-concentration solution of APTES that we used for obtaining the adsorbent. After the Cu(II) adsorption, we detected the following changes in the FT-IR spectra: The peaks at 2928 cm\({}^{-1}\), 2858 cm\({}^{-1}\)  
\begin{table}
\begin{tabular}{c c c} \hline
**adsorbent** & **adsorption Capacity (mg/g)** & **Reference** \\ \hline \multicolumn{2}{c}{Zeolite} \\ \hline Clinoptiolite zeolite & 4.9–15.4 & [47] \\ \hline Chifeng zeolite & 4.0 & [48] \\ \hline Synthetic zeolite & 9.5 & [49] \\ \hline Modified zeolite & & \\ \hline Natural alumosilicate modified with & & \\ \(N\),\(N^{\prime}\)-bis(3-triethoxysilypropy)|thiocarbamide & 29.7 & [25] \\ \hline Natural zeolite modified with 4-(3-triethoxysilypropy)| thiosemicarbazide & 29.5 & [33] \\ \hline Dopamine functionalized tannic-acid-templated mesoporous silica & 58.7 & [45] \\ \hline Silica and APTES–silica-modified NiFe\({}_{2}\)O\({}_{4}\) & 63.5 & [46] \\ \hline Zeolite modified with APTES & 4.5–20.6 & This study \\ \hline \end{tabular}
\end{table}
Table 7: Comparison of the adsorption capacities of modified zeolite with some other adsorbents for Cu(II) adsorption.  
and 1513 cm\({}^{-1}\) that we recorded before the adsorption disappeared. The intensities of the peaks around 3400-3600 cm\({}^{-1}\) and at 1648 cm\({}^{-1}\) decreased, while the peak at 1063 cm\({}^{-1}\) (Si-O-Si) shifted to 1073 cm\({}^{-1}\). The Cu(II) interacted with a functional group of C, N, and O atoms, and the Si-O-Si-bond spectrum shifted due to the formation of metallic organic compounds.  
Figure 11: Desorption of Cu(II) from the modified zeolite.  
Figure 9: FT-IR spectra of modified zeolite, before and after the adsorption of Cu(II).  
We present the X-ray photoelectron spectroscopy (XPS) spectra of the modified zeolite before and after the adsorption of Cu(II) in Figure 10. As shown in Figure 10, scanned XPS spectra of the modified recorded the following peaks, 77 (Al2p), 105 (Si2p), 287 (C1s), 402 (N1s), and 534 eV (O1s), respectively [53; 54; 55; 56].  
According to the XPS results, as shown in Table 8, the main elements of the modified zeolite were silicon (26.55%), carbon (17.52%), oxygen (45.43%), and nitrogen (4.23%).  
The changes in the atomic values of O1s and N1s were more significant than other atomics after Cu(II) adsorption. It suggests that the Cu(II) interacted with the oxygen and nitrogen on the surface of the modified zeolite [57; 58; 59; 60].  
The XPS analysis was consistent with the results of the FT-IR, XRD, and EDX analyses.  
Researchers [61; 62] have proven the coordination interaction between heavy metals and the amine (-NH\({}_{2}\)) and hydroxyl (-OH) groups. Based on all of the above mentioned results and previous studies on heavy-metal adsorption, the following reaction can be a controlled adsorption reaction in Scheme 2: