## 4 Results and Discussion
### Performance under Dark Conditions  
#### 4.2.1 Filtration Performance under Dark Conditions  
Generally, any additional layer will change the hydraulic resistance of a membrane and further influence the filtration performance. The pure water permeability of the membranes before and after the modification is shown in Figure 4. The additional hydraulic resistance caused by each modification is summarized in Table 3. In order to assure that the differences in the performance for the modified membranes are caused by the modification, rather than by experimental errors, all of the tests are operated under exactly the same conditions.  
As can be seen in Figure 4a, after each modification, the membrane modified by polydopamine displays the most flux reduction, when compared to the other modified membranes, indicating that  
Figure 4: Filtration performance of the membranes before and after the modification. (**a**) Water permeability (L-m\({}^{2}\)-h\({}^{1}\)-bar\({}^{1}\)); (**b**) Rejection of MgSO\({}_{4}\) (%); (**c**) Rejection of NaCl (5).  
\begin{table}
\begin{tabular}{c c c c} \hline
**Membranes** & **pDA Modified \(R_{\text{pDA}}\)** & **pDA + TiO\({}_{2}\) Modified \(R_{\text{TIO2\_pDA}}\)** & **TiO\({}_{2}\) Modified \(R_{\text{TIO2\_self-assembly}}\)** \\ \hline Hydraulic Resistance & 40.28 & 4.47 & 25.27 \\ (\(\times 10^{10}\) m\({}^{-1}\)) & 40.28 & 4.47 & 25.27 \\ \hline \end{tabular}
\end{table}
Table 3: Hydraulic resistance of the additional layers produced by different modification procedures.  
the polydopamine layer produces the highest hydraulic resistance (Table 3). This high hydraulic resistance is probably caused by a pore blockage of the support membrane and transport hindrance from polydopamine. When the dopamine contacts the membrane in the base aqueous condition, the size of polydopamine oligomers increases with increasing deposition time. The nanosized polydopamine oligomers presumably block the large pores of the membrane surfaces at the initial stage. As the deposition time increases, the thickness of the polydopamine layer also increases. Therefore, the hydraulic resistance of the final membrane increases and the pure water permeability is reduced by 10.5%. In the case of TiO\({}_{2}\) modification, less flux decline (6.99%) is observed. The performance of TiO\({}_{2}\) is a competition between hydrophilicity and a shelter effect. The large TiO\({}_{2}\) clusters formed by self-assembly can block the pores on the membrane surface. Moreover, the efficient surface area of these coagulated particles was limited. Therefore, the shelter effect was dominant, which results in a modest hydraulic resistance (Table 3). For the polydopamine binding method, the final composite layer (polydopamine together with TiO\({}_{2}\)) yields a minimal flux decline (1.26%). When using this method, the nanoparticles are minimally coagulated because the dopamine monomers act as a surfactant at the initial stage. This good dispersion allows a relatively uniform layer of nanosized TiO\({}_{2}\) to bind on the membrane surfaces and provides a larger active surface area. This significantly larger surface area maximises the hydrophilic performance of TiO\({}_{2}\). In this case, the hydrophilicity of TiO\({}_{2}\) nanoparticles is dominant, and it greatly reduces the hydraulic resistance of the polydopamine layer (Table 3).  
The rejection of MgSO\({}_{4}\) and NaCl is presented in Figure 4b,c. The polydopamine coating slightly increases the rejection of both salts. As previously mentioned, the nanosized polydopamine oligomers may block or grow in the defects or larger pores of the TFC membrane surfaces at the early stage, which possibly leads to a slight increase in the salt rejection. However, considering the rejection range and the standard deviation, there seems to be no significant difference in salt rejections for the membranes modified by TiO\({}_{2}\) using both binding methods.