# Preparation and Physicochemical Properties of Tannin-Imobilized Membrane Adsorbent
## 3 Results and Discussion  
The tannin grafting degrees of the four kinds of M-TANs are shown in Figure 1. The tannin grafting degrees of the all M-TANs are relatively large, reaching more than 180 mg g\({}^{-1}\). However, the tannin grafting degrees of M-TANs prepared by different methods were diverse. Among them, PA-BWT had the maximum grafting degree of 215 mg g\({}^{-1}\), followed by PA-AA-BWT and PVDF-BWT. CELL-BWT had the smallest grafting degree. This may be because the PA membrane contains many amino groups, which can interact with tannins through hydrogen bonding so that more tannins can be adsorbed on the membrane. Then, through the cross-linking of glutaraldehyde, the tannin is covalently cross-linked and immobilized on the membrane. Compared with PA-BWT, the tannin grafting degree of PA-AA-BWT was not high. This is because the grafting rate of methacrylic acid is lower, so there is less tannin immobilized by the esterification reaction. For PVDF-BWT, benzophenone is used as the initiator in the light-initiated grafting process, and its molecular structure has two benzene rings, resulting in a large steric hindrance and low grafting degree. When CELL-BWT is prepared by the dipping method, tannin is fixed to the membrane through non-covalent bonding. The tannin on the M-TAN prepared by this method can easily fall off, so the grafting degree of CELL-BWT was the lowest. Figure 2 shows the scanning electron micrographs (FESEM) of the four membranes before and after immobilizing the tannins. As shown in Figure 2, no matter what kind of membrane or method was used to prepare the M-TANs, the membrane structure of the raw material was still maintained, while the membrane pore size and pore wall roughness changed. Compared with the original PA (Figure 2a), the pore size of PA-BWT (Figure 2b) was significantly reduced to 0.5 \(\sim\) 0.2 \(\upmu\)m. Similar to PA-BWT, the FESEM images of PA-AA-BWT show similar changes (Figure 2c,d). Figure 2e,f shows FESEM images of PVDF and PVDF-BWT. There were many micropores on the surface of the PVDF membrane with a wide pore size distribution ranging from 5.0 to 1.0 \(\upmu\)m, and the membrane pore walls were relatively smooth. Figure 2f shows the structure of the PVDF-BWT membrane; it can be seen that the pore size of the membrane was reduced due to the adhesion of tannin. The membrane pore wall of PVDF-BWT became rougher. Figure 2g,h shows FESEM images of  
CELL and CELL-BWT. Compared with the other three M-TANs, the structural changes of CELL-BWT before and after the immobilization of tannins were not obvious. The original CELL structure (Figure 2g) was densely packed with cotton fiber bundles, and the fiber surface was relatively smooth. Compared with CELL, the surface of CELL-BWT fiber became rough, but the pore structure did not change much (Figure 2h). This is because the tannin grafting degree of CELL-BWT prepared by the dip coating method was not large, and the effect on the pore size was not obvious. Figure 3a-d shows the XPS full spectrum and O 1s spectrum of PA and PA-BWT. The characteristic peaks of C 1s, N 1s, and O 1s appeared in the spectra of PA and PA-BWT, but after the tannin was immobilized, the peak intensity of C 1s and O 1s in PA-BWT was much higher than that of N 1s. This is because tannins are mainly C, H, and O elements, so when tannins are immobilized on the PA membrane, the relative content of N 1s decreases. In addition, as shown in Figure 3b, the O 1s in PA is mainly a C=O characteristic peak with a binding energy of 531.2 eV. However, except for the above C=O peak of O 1s of PA-BWT, the immobilization of tannin also introduced an additional C-O peak with a binding energy of 532.9 eV, indicating that PA-BWT was successfully prepared. The full XPS spectra of PVDF and PVDF-BWT and the partial spectra of O1s are shown in Figure 3e.f.