# Suppressing Salt Transport through Composite Pervaporation Membranes for Brine Desalination
## 3 Results  
### Investigation on Cross-linking of PVA by Glutaraldehyde  
#### 3.1.1 Pervaporation Performance Using Single Salt Brine  
PVA/PVDF membranes fabricated with different molar ratio of glutaraldehyde to PVA repeat unit (MR values of 0.025, 0.1, 0.2) were investigated regarding their pervaporation desalination performance using 100 g/L NaCl as the feed (Figure 2). In this work, each pervaporation test was carried out with at least two membrane samples, and the difference in permeation flux and salt rejection was less than 10% throughout the whole testing process.  
As shown in Figure 2a, all membrane samples had comparable water flux under the same operating condition although a slightly lower flux was observed for membranes with higher MR value. Besides, all the membranes experienced continuous flux declines during the whole testing process due to the gradual increase of the feed concentration. In terms of the permeate conductivity, all membrane samples showed an increased conductivity with a longer operation time, and the membrane with the highest MR value (0.2) showed the most stable salt rejection after 96 h operation. This trend suggested that the cross-linking of PVA can benefit the operational stability of the membrane, which can be attributed to more acetal/ether linkages with higher MR value, leading to the improved structural stability with reduced free volume and swelling tendency [32; 33]. Comparatively, the virgin supporting membrane, hydrophobic PVDF membrane with nominal pore size of 0.22 \(\upmu\)m, showed a permeate conductivity of 2 mS/cm after 10 min of operation under the same operating condition. This indicated that the salt rejection efficiency of the pervaporation process was not attributed to the supporting membrane used in the composite membrane.  
The water flux results in this work were comparable to the values reported in previous literatures of pervaporation membranes, when taking the operating temperature and feed solution properties into consideration. For example, composite membranes containing 100-1000 nm PVA surface coating on polysulfone membrane yielded fluxes of 4.6-7.4 L/m2h with 30 g/L NaCl solution at 70 \({}^{\circ}\)C [34]. In another line of research, a slightly higher water flux (5.57 L/m2h) was obtained with a PVA/polyacrylamideonitrile (PAN)/polyethylenimine (PEI) triple-layered membrane when using 50 g/L NaCl solution as feed under room temperature [35]. In terms of the salt rejection, the PVA membrane in this work exhibited higher salt rejections even with more concentrated feed solution compared with the literatures [34; 35].We also explored the membrane stability by recycling the membrane and testing the membrane using different feed concentrations and salt types, as shown in Figures S2 and S3.  
Figure 2: (**a**) Flux and (**b**) permeate conductivity profile of composite PVA/PVDF membrane with different molar ratio of glutaraldehyde to PVA repeat unit (MR 0.025, 0.1, 0.2) using a feed solution of 100 g/L NaCl at 65 \({}^{\circ}\)C.  
However, considering the permeate flux was relatively invariant, the gradual increase of permeate conductivity for all the PVA/PVDF membranes indicated a more rapid transport of salt ions as the pervaporation test proceeded. The following characterizations and discussion were aimed to better understand the transport mechanism of salt through the composite PVA/PVDF membrane, to provide guidance for the further modification of such membranes.