## 4 Results and Discussion
### Performance under Light Conditions  
#### 4.3.1 Effect of Photocatalysis on Filtration Performance  
The membrane modified by TiO\({}_{2}\) using the polydopamine binding method has a remarkable anti-fouling performance in dark conditions--as described above. However, since TiO\({}_{2}\) modified by dopamine can be excited by visible light, the performance of this visible light-induced photocatalysis is evaluated here. The membrane modified by polydopamine-TiO\({}_{2}\) was tested before and after visible light illumination. The pure water flux and salt rejections are shown in Figure 6a. It was found that the flux was significantly higher (by 61.2%), and the salt rejections of MgSO\({}_{4}\) and NaCl decreased by 43.9% and by 11.1%, respectively, after 24 h of illumination. To make sure that these changes were caused by the polydopamine-modified TiO\({}_{2}\) instead of the bare TiO\({}_{2}\), the membrane modified by bare TiO\({}_{2}\) using the self-assembly method was tested as a reference. The filtration performance of this self-assembly method was not significantly different (<2%) either before or after the illumination (Figure 6b). When comparing the filtration performance of these two methods after illumination (Figure 6a,b), the results indicate that the functional top layers (polydopamine and/or polyamide) of the membrane modified by polydopamine-TiO\({}_{2}\) was probably degraded by the visible light-induced photocatalysis of the polydopamine-modified TiO\({}_{2}\).  
Figure 5: Relative flux of the unmodified (heat) and modified membranes as a function of time during the filtration of BSA (bovine serum albumin) (0.5 g\(\cdot\)L\({}^{-1}\)) in dark conditions.  
Figure 6: Filtration performance of the modified membranes before and after the visible light illumination for 24 h: (**a**) the membrane modified by TiO\({}_{2}\) using polydopamine binding method; (**b**) the membrane modified by TiO\({}_{2}\) using self-assembly method.  
The mechanism of this visible light-induced photocatalytic degradation is as follows. Modification of TiO\({}_{2}\) nanoparticles using dopamine or polydopamine promotes spatial separation charges. Since holes localize on dopamine and electrons localize in the lattice of TiO\({}_{2}\), the recombination of charges is suppressed [29]. This dopamine (or polydopamine) modified TiO\({}_{2}\) enables adsorption in the visible light spectrum, and can be excited by visible light energies below 3.2 eV (\(\sim\)387.5 nm) [29]. In this spectrum range, bare TiO\({}_{2}\) cannot be excited, because the energy of these photons is not sufficient to excite electrons from the valence band to the conduction band of TiO\({}_{2}\). In this situation, the well-known redox chemical reactions generated by photoexcitation of bare TiO\({}_{2}\) cannot happen. For dopamine-(or polydopamine) modified TiO\({}_{2}\), the formation of reactive oxygen species can be produced by multiple redox chemistries under visible light illumination. [PERSON] et al. [29] reported that the spatial separation caused by the presence of dopamine does not affect the properties of photogenerated electrons. The reduction of oxygen molecules and formation of superoxide anions as precursors for some reactive oxygen species is not influenced. All of these reactive oxygen species produced by the visible light excitation of dopamine- (or polydopamine) modified TiO\({}_{2}\) results in the degradation of the membrane surface. This degradation suggests that the TiO\({}_{2}\)-modified membranes using polydopamine should be stored in dark conditions in order to maintain the filtration performance. Alternatively, the membrane surface should be covered by a protective layer. If this polydopamine binding method is utilized under light conditions (e.g., in photo-reactors), antioxidativeing reagents should be added inside and/or under the polydopamine layer.