# Removal of Phosphate from Aqueous Solution by
## 3 Results and Discussion
### Adsorption Mechanisms and Analysis
#### 3.4.1. Material Microstructural Characterization after Phosphate Adsorption  
Compared with the samples before adsorption, FTIR spectra of zeolite, biochar, and PZC 7:3 after phosphate adsorption exhibited significant changes, such as enhancement and shifting of the characteristic peaks and the appearance of new characteristics peaks (Figure 9). This indicates that phosphate reacted with functional groups during the adsorption processes. After phosphate adsorption, all three materials showed a weak characteristic peak near 2320 cm\({}^{-1}\), which was caused by P-H stretching vibrations. Biochar showed vibration peaks at 1377 and 574 cm\({}^{-1}\) due to P=O stretching [74]. This demonstrates that phosphate could be absorbed on the surface of the material, most likely through hydrogen bonding or metal bond bridges, etc. Biochar and PZC 7:3 showed a significant shift in the position of the O-H stretching vibration peak at 3200-3600 cm\({}^{-1}\). This indicates that the functional groups in the material chemically interacted with phosphate, possibly by ligand exchange between O-H and phosphate [43]. Figure 10 depicts the TEM and EDS images of zeolite, biochar, and PZC 7:3 after phosphate adsorption. As depicted in Figure 10A, biochar-adsorbed phosphate formed a sheet-like thin-film substance, and the diffuse ring was observed by the selected area electron diffractogram as characteristic diffraction of the amorphous state. This demonstrates that the thin-film substance is an amorphous mass. As shown in Figure 10a, when biochar adsorbed phosphate, P content increased significantly, proving that P was successfully loaded on the biochar. The contents of Mg, Ca, and P in this substance were positively correlated, which may be an amorphous thin-film-like calcium and magnesium phosphate compound, formed by Ca and Mg oxides in biochar and phosphate (Cu is the analytical background component of the instrument). As represented in Figure 10B, after adsorption of phosphate, zeolite had an uneven and irregular distribution with almost no change in morphology, in which diffraction spots were observed through the electron diffraction pattern of the selected area, indicating that the substance has a crystalline structure. The EDS analysis revealed that the material contained a large amount of O, Si, Al, and Mg (Cu is the analytical background component of the instrument) and a small amount of K, Ca, and P (Figure 10b). In this case, zeolite only adsorbed a small amount of phosphate, and there was no phosphorus-enriched phase, reflecting that the phosphorus may be dispersed as ions on the surface or within the pores of the zeolite. As shown in Figure 10C, some thin-film-like substances were attached to the edge of the zeolite-biochar composite, which is similar to \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Samples**} & \multicolumn{2}{c}{**Langmuir Adsorption Model**} & \multicolumn{2}{c}{**Freundlich Adsorption Model**} \\ \cline{2-7}  & \(\mathbf{K_{L}}\) & \(\mathbf{q_{m}}\) & \(\mathbf{R^{2}}\) & \(\mathbf{K_{F}}\) & \(\mathbf{1/n}\) & \(\mathbf{R^{2}}\) \\ \hline zeolite & 0.0064 & 0.6874 & 0.9540 & 0.0165 & 0.5775 & 0.8910 \\ biochar & 0.0552 & 3.6048 & 0.9437 & 0.5834 & 0.3366 & 0.9899 \\ PZC 7:3 & 0.0594 & 2.4052 & 0.9364 & 0.4146 & 0.3196 & 0.9430 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Isotherm parameters of phosphate adsorption. that displayed in Figure 10A. As shown in Figure 10c, EDS analysis showed a great amount of C, O, Ca, Mg, and P in the substance (Cu is the analytical background component of the instrument). This indicates that the Ca and Mg oxides in the zeolite-biocarbon composite were formed with P as calcium and Mg phosphate compounds and attached to the zeolite surface, demonstrating that the composite of zeolite and biochar improved the adsorption of phosphate.