# Suppressing Salt Transport through Composite Pervaporation Membranes for Brine Desalination
## 3 Results
### Incorporation of GO into PVA Matrix
#### 3.2.2. Pervaporation Performance Using Single Salt Brine  
The composite membranes with different GO loadings in PVA matrix (0.1, 0.2, and 0.3 wt. %) were tested in the pervaporation setup, which was then compared with the pure PVA/PVDF membrane (MR value of 0.2). For each membrane, Milli-Q water permeation tests were firstly conducted for at least 4 h. Compared with pure PVA/PVDF membrane, the incorporation of 0.1 wt. % GO showed the negligible effect on the Milli-Q water flux (4.38 L/m\({}^{2}\)h). However, further increase of the GO loading can lead to a marginal loss of the flux: 94.4 and 85.5% of the original PVA/PVDF flux can be obtained for composite membranes containing 0.2 and 0.3 wt. % GO (results not shown). In highly cross-linked PVA polymer chains, the free volume was organized in a tortuous manner, which led to the random and non-directional water passage pathway [49]. After the addition of GO, the presence of a large amount of hydrophobic aromatic rings on its surface can facilitate the rapid \"sliding\" of water molecules along its surface [50]. This aspect would facilitate the water transport through the membrane. On the other hand, the incorporation of impermeable GO sheets also increased the transport pathway length of water vapor. Meanwhile, lower free volume, measured as the EWC value, was obtained as a result of hydrogen bonding between GO and PVA chains. As a result, the initial addition of GO (0.1 wt. %) had a negligible effect on the Milli-Q water flux and a further increase of the GO loading (0.2 and 0.3 wt. %) slightly reduced the permeation flux. In terms of the brine desalination tests with 100 g/L NaCl solution, similar flux profiles (Figure 9) were observed as to the Milli-Q results: the initial addition of 0.1 wt. % GO had a negligible effect on the brine solution flux, and a further increase of the GO loading can lead to more obvious loss of the permeation flux. In terms of permeate conductivity, it constantly increased to over 650 uS/cm (salt rejection of over 99.6%) after 96 h operation for the pure PVA/PVDF membrane, while the composite membrane containing 0.1 wt. % GO had a much more stable performance: the permeate conductivity gradually increased to less than 150 uS/cm (salt rejection of over 99.9%) for 120 h operation while maintaining lower than 50 uS/cm in the first 50 h. As discussed above, the presence of GO nanosheets within the PVA matrix can facilitate the selective water molecule transport along with its \"slippery\" surface, which suppressed the transport of salt ions through the composite membrane. The reduced free volume of PVA matrix had a more significant blocking effect on the larger hydrated salt ions compared with smaller water molecules. As a result, the addition of GO can improve the salt rejection for the composite membrane. When 0.2 wt. % GO was added into the PVA matrix, the permeate conductivity was more stable (less than 30 uS/cm in 82 h operation). However, in this case, the permeate flux was much lower than the PVA0.1 GO/PVDF membrane, and a significant flux decline was observed after 10 h operation. Due to the reduced free volume after GO addition, the blockage effect by the hydrated salt ions to the smaller water molecules can be more significant. This can also explain the more significant flux loss for membrane containing 0.3 wt. % GO. In addition, for the membrane containing 0.3 wt. % GO, the permeate conductivity increased more rapidly compared with a membrane containing 0.1 and 0.2 wt. % GO. The possible reason was that some defects or cracks may exist due to strong filler interactions and agglomerations. Similar results were also observed with a composite membrane containing silica nanoparticles and ordered mesoporous carbon (OMC), less water uptake and permeable flux were detected with higher loading of nanoparticles in the thin-film layer, and the overdose of nanofillers can also lead to the loss of membrane selectivity [51; 52; 53].