## 3 Results and Discussion
### Pure Water Permeation and Membrane Recyclability Evaluation  
The pure water permeation and recyclability of the membranes are key aspects of the membranes for large-scale and real-life applications. The water permeation of the fabricated membranes per minute of filtration was assessed with and without visible light irradiation for five cycles. These measurements were done as it has been shown that the presence of light may result in the nanoparticles being affected in ways that may influence the membrane's permeability [1; 51; 52]. It is important to ascertain if changes that occur  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Membrane ID** & **Water Uptake/\%** & **Porosity/\%** & **Shrinkage/\%** \\ \hline
**CA** & 72.00 & 79.42 & 31.85 \\
**CTG–100** & 76.93 & 83.37 & 18.65 \\
**CTG–300** & 78.06 & 84.25 & 17.68 \\
**CTG–500** & 80.53 & 86.14 & 13.83 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Evaluation of membrane water uptake, porosity and shrinkage.  
Figure 6: The plot of porosity and shrinkage ratio concerning the content of the dopant nanocomposite (1% CuO/TiO\({}_{2}\)@GCN (1:9)).  
to the photocatalytic membrane are due to the contaminant or experimental conditions, such as light. The membranes were flushed with water in between the cycles. The results are plotted in Figure 7. Here, it was found that the membranes showed good stability with minor reductions in pure water permeation when experiments were conducted in the presence of visible light. Additionally, it was observed that the CA membrane had a lower water permeation flux and filtration rate, which could be attributed to the hydrophobic nature of the membrane.  
Under visible light, the CA, CTG-100, CTG-300, and CTG-500 membranes exhibited average pure water permeability of 72.9, 329.6, 351.2 and 470.9 mL/m\({}^{2}\).min, respectively. It was noticeable that the water permeation flux increased slightly with the increasing concentration of 1% CuO/TiO\({}_{2}\)@GCN (1:9) loading onto the CA matrix due to the increasing hydrophilicity of the membranes. This observation was consistent with the water contact angle measurements as shown in Figure 5. In the dark conditions and only flushing, the membrane with water before the next cycle, the CA, CTG-100, CTG-300, and CTG-500 membranes exhibited continuous declination in permeation flux with almost similar performance at the first cycle. The average water permeability was 67.7, 289.6, 319.6, and 365.3 mL/m\({}^{2}\).min for CA, CTG-100, CTG-300 and CTG-500, respectively.  
In a study conducted by [PERSON] et al., it was reported that photocatalytic membranes containing TiO\({}_{2}\) exhibited elevated pure water fluxes under light irradiation as compared to dark surroundings [51]. Under light irradiation, the TiO\({}_{2}\) membranes absorb more water molecules due to the hydrophilic properties of the membranes [51; 53; 54]. This is because the radiation yields photoexcited electron-hole pairs, which are capable of reducing Ti\({}^{4+}\) to Ti\({}^{3+}\) and oxidizing O\({}_{2}\)\({}^{-}\) anions through photoexcited holes. Then, the oxygen vacancies are produced on the membrane surface for the occupation of water molecules on these empty vacancies. Therefore, chemically bonded water molecules on the membrane surface can be adsorbed and passed through the membrane matrices continuously [51; 53; 54].