# Preparation and Physicochemical Properties of Tannin-Imobilized Membrane Adsorbent
## 3 Results and Discussion
This is because when the tannin is immobilized by the dip coating method, the pH value of the solution is adjusted to alkaline. At this time, the phenolic hydroxyl groups on tannins are oxidized to quinones [27], and the tannins containing hydrophobic benzene rings are mainly hydrophobic due to the loss of phenolic hydroxyl groups, so the hydrophilicity of CELL-BWT was reduced. The pure water fluxes of the four organic filter membranes and their cured tannin membranes are shown in Figure 4b. After immobilizing the tannins, the pure water fluxes of both PA and PVDF membranes increased. The pure water flux of the PA membrane was 70.25 L m\({}^{-2}\) h\({}^{-1}\), while the pure water flux of PA-BWT and PA-AA-BWT after the tannin was immobilized increased to 110.52 and 108.45 L m\({}^{-2}\) h\({}^{-1}\), respectively. Although immobilized tannin reduces the pore size of the membrane and may slightly reduce the flux of pure water, the rich phenolic hydroxyl groups on the tannin greatly increase the hydrophilicity of the membrane, which is beneficial for the flux of pure water. The pure water flux of PA-BWT was greater than that of PA-AA-BWT due to the former's larger tannin grafting degree. The pure water flux of the hydrophobic PVDF membrane is basically 0 without external pressure, while the pure water flux of PVDF-BWT can reach 54.65 L m\({}^{-2}\) h\({}^{-1}\) without external pressure. Such a significant change is also caused by the immobilization of tannins. This shows that the immobilization of tannin on the membrane not only enables the membrane to have adsorption properties but can even change the hydrophilicity and hydrophobicity of the membrane, expanding the application of tannin in membrane surface modification. The flux of pure water of CELL and CELL-BWT did not change much before and after tannin immobilization, and the flux of pure water of CELL-BWT even decreased slightly due to its slight reduction of hydrophilicity and pore size. In order to investigate the adsorption performance of different M-TANs, UO\({}_{2}\)\({}^{2+}\) was used as the adsorption object to determine the adsorption capacity of these four M-TANs. The adsorption capacities of the four M-TANs for UO\({}_{2}\)\({}^{2+}\) are shown in Figure S2. The adsorption capacities of PA-BWT, PA-AA-BWT, PVDF-BWT, and CELL-BWT for UO\({}_{2}\)\({}^{2+}\) are 0.209, 0.197, 0.186, and 0.102 mmol g\({}^{-1}\), respectively. It can be seen that, except for CELL-BWT, the adsorption capacities of other M-TANs for UO\({}_{2}\)\({}^{2+}\) are relatively large, and the PA-BWT > PA-AA-BWT > PVDF-BWT > CELL-BWT is arranged according to the adsorption capacity, which is in direct proportion to the grafting degree of tannin. It is known that the original PA, PVDF, and CELL membranes have no adsorption capacity for UO\({}_{2}\)\({}^{2+}\), indicating that the adsorption of UO\({}_{2}\)\({}^{2+}\) by the M-TANs is mainly dependent on the tannins. Therefore, the adsorption capacity for UO\({}_{2}\)\({}^{2+}\) increased with the increase of tannin grafting degree. The adsorption performance of UO\({}_{2}\)\({}^{2+}\) by PA-BWT was further studied. PA-BWT shows a considerable adsorption capacity to UO\({}_{2}\)\({}^{2+}\), reaching 0.398 mmol g\({}^{-1}\) at 333 K with an initial UO\({}_{2}\)\({}^{2+}\) concentration of 2 mmol L\({}^{-1}\), as shown in Figure 5a. In addition, the adsorption capacity was increased with the increase of temperature, suggesting that the process of UO\({}_{2}\)\({}^{2+}\) adsorption of PA-BWT is endothermic in nature.