## 3 Results and Discussion
### Characterization of the Zeolite Samples
#### 3.1.5 XPS Analysis  
The XPS data of fresh Z-A, fresh Iron-Z-A, P-adsorbed Iron-Z-A, and P desorbed Iron-Z-A shows the peaks of Fe 2p appeared in the region between 706-730 eV binding energy. As shown in Figure 5, for fresh iron-Z-A, the centers of electron binding energy of Fe 2p\({}^{1/2}\) and Fe 2p\({}^{3/2}\) at 724.48 and 711.08 respectively are quite closer to the standard data of Fe\({}_{2}\)O\({}_{3}\). At the binding energy 722.18 eV (Fe-2p\({}^{3/2}\)), the shakeup satellite peak is a fingerprint of the electronic structure of Fe\({}^{3+}\) and tells that Fe\({}^{2+}\) is absent [35]. No peak was observed for Z-A as it did not contain iron in the zeolite frame. The peak of Fe-2p appeared in the region 706-718 eV of binding energy. The Fe 2p peaks of P-adsorbed Iron-Z-A appeared at a higher energy side (711.48 eV) than fresh Iron-Z-A, whereas it shifted to the lower energy side (711.28 eV) for P desorbed Iron-Z-A. The peak shift trend after adsorption and desorption suggests a reversible pattern. Phosphate adsorbed and desorbed Iron-Z-A exhibits a characteristic peak at about 711.1 eV, indicating the binding energy of Fe\({}_{2}\)O\({}_{3}\) (Fe\({}^{3+}\)) [51]. The similar peaks of Fe in adsorbed and desorbed Iron-Z-A also signifies that the bond strength between Fe and phosphate is high. The Si 2p peak of Iron-Z-A with iron in the IR spectrum slightly shifted as compared to Z-A's Si 2p peak. The peak of Si 2p shifted when Si-O-Fe bonds were formed because the bond-lengths in Si-O-Fe are bigger than the bond-lengths in Si-O-Al [51]. Therefore, the shifting in the peak indicated that iron was substituted into the zeolite structure. It was supported by the findings of SEM-EDS, XRD,  
Figure 4: Illustration of XRD patterns of fresh Z-A, fresh Iron-Z-A, P-adsorbed Iron-Z-A, and P-desorbed Iron-Z-A. and FTIR. The Si 2p peak appeared in the region between 99 and 105 eV of binding energy, as shown in Figure 5. The Si 2p peak of iron-Z-A also slightly shifted in both phosphate adsorbed iron-Z-A as well as phosphate desorbed iron-Z-A. Si was not directly combined with phosphate; however, the crowding of phosphate atoms in its vicinity may have shifted the Si peak in the case of both; phosphate adsorbed iron-Z-A and phosphate desorbed iron-Z-A. The change in the intensity of the Al 2p peak of Iron-Z-A than Al 2p of Z-A suggested the weaker interaction of aluminum in the zeolite backbone. The XPS spectra in the Al 2p region of Iron-Z-A, as shown in Figure 5, appeared in the region 71-78 eV of binding energy. It can be observed that the Al 2p peak of phosphate adsorbed Iron-Z-A slightly shifted to a higher energy side as compared to fresh Iron-Z-A. This shifting of the Al 2p peak indicated that Al interacted with phosphate. However, Al peak of phosphate desorbed Iron-Z-A moved towards lower energy in comparison to Al of fresh Iron-Z-A, and binding energy was similar to that of fresh Iron-Z-A. Hence, it confirmed that Al in the Iron-Z-A is also contributing to the adsorption. Figure 5: Illustration of XPS spectra of Na 1s, Si 2p, O 1s, Al 2p, Fe 2p, and P 2p in the framework of fresh Z-A, fresh iron-zeolite-A (Iron-Z-A), P-adsorbed Iron-Z-A, and P-desorbed Iron-Z-A. The P 2p peak appeared in the region 132-137 eV of binding energy, as shown in Figure 5. The phosphate adsorbed Iron-Z-A showed a peak at 134.39 eV for P 2p, which clarified the adsorption of phosphate on the zeolite's surface. Phosphate desorbed iron-Z-A did not show any peak for P 2p, which confirms that the phosphate is well desorbed. The fall in peak intensity of O 1s spectra of fresh Iron-Z-A than fresh Z-A confirmed the accommodation of Fe in its vicinity. The change in peak intensity and peak shift of O 1s of in the P-adsorbed Iron-Z-A and P desorbed Iron-Z-A than fresh iron-Z-A suggested the interaction of oxygen in the adsorption-desorption process.