## 3 Results and Discussion
### Proposed Mechanism for Adsorption and Desorption of Phosphate by Iron-Z-A  
The XPS spectra of P 2p, SEM image and EDS data of P-adsorbed Iron-Z-A and P-desorbed Z-A confirmed the complete adsorption and desorption of PO4\({}^{3-}\) from the surface of Iron-Z-A. The following explanation claims the domination of the ligand exchange mechanism in the adsorption-desorption process: (1) The P 2p XPS spectra of P-adsorbed iron-Z-A demonstrated a characteristic peak at 134.38 eV, whereas fresh iron-Z-A and P-desorbed iron-Z-A's XPS spectra did not show any characteristic P 2p peak. (2) The peak of Fe 2p of fresh iron-Z-A moved from 711.08 eV to a higher energy side (711.48 eV) after adsorption of phosphate, and the peak shifted back to the lower energy side (711.28 eV) after desorption of phosphate. So, the XPS spectra of Fe 2p exhibited a reversible pattern. (3) The intensity of FT-IR spectra of the fresh Iron-Z-A backbone corresponding to -OH bending and stretching at 1652.89 fell from 0.23 to 0.07 for P-adsorbed Iron-Z-A and raised to 0.08 for P-desorbed Iron-Z-A. So, the peak intensity pattern of -OH bond showed a reversible behavior for P-adsorbed Iron-Z-A and P-desorbed Iron-Z-A. The above points confirmed the involvement of the ligand exchange mechanism for phosphate adsorption as well as desorption by iron-Z-A. The above discussion has shown a reasonable correlation for the adsorption of phosphate to the iron-Z-A as a result of the presence of active iron sites on Z-A's surface. Although it is evident that Z-A (without iron substitution), can remove anions (including PO4\({}^{3-}\)) from water in our study, Iron-Z-A presented a much faster removal capacity than Z-A (Supplementary Figure S2). It indicates that not only positive charge bearing species Na\({}^{+}\) is active for adsorption of phosphate but also a modest amount of removal may be attributed by the interactions (hydrogen bond) between the framework of oxygen and derivative of phosphate [57]. It was confirmed by XPS results, as there was no peak detected corresponding to sodium phosphate in contrast to the peak for iron phosphate (Figure 5). In Iron-Z-A, the Fe was incorporated in the zeolite backbone in the place of aluminum and chelation on the -O-Al-O- site by replacing Na\({}^{+}\), which increased the positive charge density centers. When Fe was substituted with Al, it created a hole on the surface (increasing the positive charge density), which ultimately favors anion adsorption. In the reaction mixture for removal of phosphate, the positive charge of Iron-Z-A was balanced by OH\({}^{-}\) ions; when phosphate was added, it was attracted to the positive centers following the ligand-exchanged principle. As, at pH 5, phosphate exists as H\({}_{2}\)PO4\({}^{-}\) during the adsorption phase (as a derivative of phosphate) and as HPO4\({}^{2-}\) at pH 10 during desorption, where both acts as a ligand. In aqueous phase at pH 5, the OH\({}^{-}\) ion on the Fe atom of Iron-Z-A backbone gets replaced with the ligand H\({}_{2}\)PO4\({}^{-}\) (Figure 8) through ligand exchange mechanism to form \"Iron Zeolite (oxy) hydroxide bound phosphate.\" As zeolites are more selective to H\({}^{+}\) ion, it exchanged its Na\({}^{+}\) ion crowding from its surface with H\({}^{+}\) ion in the phosphoric acid medium due to reaction with an acid (HCl). The adsorption of PO4\({}^{3-}\) on Iron-Z-A can be confirmed through the lump of deposition visible in the SEM images, spiking of P 2p peak in the XPS spectra, and EDS data of Iron-Z-A. In alkaline pH, the OH\({}^{-}\) ion acts as a ligand and replaces the derivative of phosphate (H\({}_{2}\)PO4\({}^{-}\)) from Fe. In the desorption phase, the H\({}^{+}\) ion  
Figure 7: Pseudo-first-order and pseudo-second-order kinetics for adsorption of phosphate. in the zeolite gets exchanged with Na\({}^{+}\) ion due to the addition of adequate NaOH to maintain alkalinity.