## 3 Results and Discussion
### Performance of PVDF Composite Ultrafiltration Membrane  
Figure 5 shows water fluxes, BSA rejection rates, flux recovery rates and a schematic diagram of PVDF, GO/PVDF and CGO/PVDF ultrafiltration membranes. The water flux of ultrafiltration membrane is an important parameter to characterize membrane permeability [29]. Figure 5a shows the water flux of PVDF, GO/PVDF and CGO/PVDF ultrafiltration membranes. It can be seen that the water flux of the modified composite ultrafiltration membrane is higher than that of the pure PVDF ultrafiltration membrane. The water flux of the composite membrane with GO is increased by about 20 L-m\({}^{-2}\cdot\)h\({}^{-1}\), while that of the membrane with CGO is increased by nearly 40 L-m\({}^{-2}\cdot\)h\({}^{-1}\) and both show the trend of first increasing and then decreasing. When the GO dosage was 0.75 wt%, the water flux reached the maximum, which was 68 L-m\({}^{-2}\cdot\)h\({}^{-1}\). When the GO dosage was 1.0 wt%, the water flux decreased to 67 L-m\({}^{-2}\cdot\)h\({}^{-1}\), indicating that the maximum dosage was between 0.75 and 1.0 wt%. The water flux of the composite membrane with CGO reached the maximum at 1.0 wt%, which was 92 L-m\({}^{-2}\cdot\)h\({}^{-1}\) and then decreased. The reason for this result is that CGO has better hydrophilicity, which makes the composite membrane form more holes, which is conducive for more water to pass through. In addition, the hydrophilic groups on the membrane make it easier to adsorb water molecules, which means higher water molecule transfer efficiency in the membrane pores, so the water flux is greater. At the same time, the maximum addition of CGO is slightly larger than that of GO, which also reflects that CGO has better dispersion and can be more dispersed on PVDF substrate. The water flux of the two ultrafiltration membranes increased at first and then decreased, indicating that both GO and CGO had the tendency of aggregating when the dosage was too large, so that the hydrophilic groups exposed on  
Figure 5: (**a**) Water fluxes, (**b**) BSA rejection rates, (**c**) flux recovery rates and (**d**) a schematic diagram of a composite ultrafiltration membrane. the surface of the membrane did not increase, but decreased instead. This is also a problem to be considered when GO is used as an additive for ultrafiltration membrane modification. The rejection rate is an important parameter for characterizing the filtration performance of an ultrafiltration membrane [30]. In this paper, BSA was used to simulate the filtration performance of the ultrafiltration membrane for protein pollutants. It can be seen from Figure 5b that the rejection rate of BSA by the pure PVDF ultrafiltration membrane is 60.4%. With the addition of GO or CGO, the rejection rate decreases and both show a trend of first decreasing and then increasing. This is due to the increased porosity and average pore size of ultrafiltration membrane with the addition of GO or CGO. When the dosage of GO or CGO was 0.75 wt%, the rejection rate of the ultrafiltration membrane reached the minimum, which was 56.9 and 57.6%, respectively. However, with increased GO content, the GO agglomerates and porosity decreased, so the rejection rate increased. The data show that the rejection rate of the ultrafiltration membrane with CGO was higher than that with GO. This is because there are more active groups on the surface of the membrane with CGO added and some BSA molecules are intercepted by non-covalent bonds. The flux recovery rate (FRR) is an important parameter to characterize antifouling performance [31]. Figure 5c shows the FRR of a PVDF ultrafiltration membrane with different GO or CGO additions. It can be seen that the FRR of a pure PVDF ultrafiltration membrane is about 42% and with the addition of GO or CGO, it first increases and then decreases. This result can be explained by hydrophilicity. The better the hydrophilicity of the composite membrane surface, the more easily water molecules can be adsorbed on the surface during the filtration process.