# Suppressing Salt Transport through Composite Pervaporation Membranes for Brine Desalination
## 3 Results
### Anti-Fouling Performance  
GO nanosheets are known to improve the anti-fouling property for the mixed matrix filtration membranes [18; 20; 21; 22; 23]. This is ascribed to the improved membrane's hydrophilicity due to the hydrophilic groups on GO. However, whether GO can promote the anti-fouling performance of pervaporation membranes has not been explored yet. The fouling study on pure PVA/PVDF composite membranes (Figure 10) showed that the presence of 10 mg/L humic acid in 100 g/L NaCl aqueous solution had a negligible effect on the flux, but increased the permeate conductivity significantly (from 370 uS/cm to 2.8 mS/cm at the end of 72 h operation). This may indicate that the attachment of humic acid on the surface of PVA layer and the binding between humic acid and salt ions accelerated the salt transport through the membrane [55]. With GO incorporated in the PVA coating layer, increased permeate conductivity was also observed (from 140 to 355 uS/cm at the end of 120 h operation), which well aligned with the membrane without GO. However, the increase was less significant. This indicated that the incorporation of GO in PVA matrix stabilized the composite membrane's performance when processing brine containing organic foulant (humic acid).  
We further investigated the effect of CaCl\({}_{2}\) on the membrane performance by adding 1.26 g/L CaCl\({}_{2}\) in the mixture of 100 g/L NaCl and 10 mg/L humic acid: the presence of the inorganic salt ions had a negligible effect on the permeation flux. It is interesting that the presence of calcium slightly reduced the permeation conductivity for both pure PVA/PVDF and PVA-GO/PVDF composite membranes, which can be attributed to the formation of the bulkier calcium-humic complex [55]. The results showed that a much lower permeate conductivity was obtained for the composite membrane containing GO than the pure PVA/PVDF membrane, which clearly demonstrated the improved anti-fouling property for the composite membrane after the addition of GO. This observation also suggested better operational stability for the pervaporation desalination process compared with the MD process: the presence of CaCl\({}_{2}\) can lead to severe fouling and flux decline for virgin hydrophobic PVDF (flux dropping to 0 with permeate conductivity rising to 1 mS/cm at the end of 20 h' operation) and modified superhydrophobic membrane (flux dropping to 0 with permeate conductivity rising to 3 mS/cm at the end of 40 h' operation) during the vacuum MD process, as suggested by our previous work [56].  
The permeate solution composition at the different operating time was analyzed by ICP when using feed solution containing 100 g/L NaCl, 10 mg/L humic acid and 1.26 g/L CaCl\({}_{2}\) with the composite membrane containing 0.1 wt. % GO (Table 4). With the progress of the pervaporation operation, the permeate conductivity increased due to the gradual diffusion of the sodium ions and  
Figure 9: (a) Flux and (b) permeate conductivity profiles of the composite membrane using a feed solution of 100 g/L NaCl at 65 \({}^{\circ}\)C.  
calcium ions through the membrane, and the diffusion rate of the smaller Na ions was higher than the bulkier Ca ions. In all, the incorporation of small amount of GO in PVA matrix efficiently slowed down the transport of salt ions through the membrane during the pervaporation process, although this was not completely prevented.