# Suppressing Salt Transport through Composite Pervaporation Membranes for Brine Desalination
## 3 Results
### Investigation on Cross-linking of PVA by Glutaraldehyde
#### 3.1.2 Transport Mechanism of Salt  
The salt transport through the membrane was further confirmed by the EDX results (Figure 3): the presence of salt crystals was observed on the membrane feed side as well as within the porous substrate after 96 h pervaporation test. The transport of salt ions through the PVA layer could be originated via the free volume between PVA polymeric chains. In this work, the gradual increase of the conductivity suggested a change of PVA chain structure overtime, possibly due to the swelling effect by hot water feed during the pervaporation process. To further explore the PVA membrane pore structure, a porometer test was carried out with the composite membrane (both fresh membranes and membranes after 96 h permeation test) using Galwick as a wetting agent at room temperature. However, no direct liquid penetration through the membrane was observed for all the membranes with different MR values under maximum feed of 10 bar. To further explore the pore size (free volume) range of the composite membrane, we conducted the dead-end filtration test with 15 bar feed pressure using fresh composite membranes at room temperature. As suggested in Table 1, the resistance for MR 0.2 membrane was in the range of RO membranes, with a pure water flux of 1.4 \(\pm\) 0.4 L/m\({}^{2}\)h. The rejection of dextran (1-2 nm) can remain \(\sim\)83.4% over 10 h operation. In comparison, the rejection of NaCl (diameter of hydrated Na\({}^{+}\) 0.716 nm, hydrated Cl\({}^{-}\) 0.664 nm) was much lower at around 33%, which was lower than the reported PVA composite nanofiltration (NF) membrane [36; 37]. Thus, it was believed that the pore size (free volume) of the composite membrane (MR of 0.2) was in the range of NF membrane (below 1 nm). [PERSON] et al. also reported that the diameter of casted/cross-linked PVA membrane's free volume was around 0.426 nm tested by bulk positron annihilation lifetime spectroscopy (PALS) [38]. Figure 3: SEM and EDX autopsy of the composite membrane after 96 h pervaporation test (membrane with MR of 0.2). (**a**) Feed side, (**b**) permeate side and (**c**) cross-section of the composite membrane; and (**d**) EDX mapping the membrane cross-section beneath the PVA coating layer. It also indicated that the change of MR value had a significant effect on the PVA layer pore (free volume) structure. With the increase of MR value, the membrane resistance gradually increased, accompanied with the increase of rejections for salt ions and dextran. In this work, the lowest resistance was observed for the membrane with MR 0.025: it fell into the range of UF or even MF membranes. Figure 4 showed the SEM and atomic force microscope (AFM) analysis of membrane with different MRs. With the increase of MR value, the PVA layer thickness became more homogeneous, accompanied with the loss of surface roughness. The surface roughness of PVDF membrane after KOH pre-treatment was \(\sim\)200 nm, which was comparable to the composite membrane with MR of 0.025. This indicated that the coating of PVA with low MR value (0.025) did not significantly change the membrane's surface roughness. Furthermore, the change of MR value did not only alter the membrane pore (free volume) structure; it can also affect the chemical component within the selective layer. Theoretically, the cross-linking of PVA with glutaraldehyde can consume the hydroxyl groups for the polymer. It can also increase the polymer chain structure rigidity. Both aspects can lead to the reduced EWC value for the membrane as shown in Table 2, which well aligns other studies [32; 33]. This could well explain the observation of slightly lower permeate flux but improved salt rejection for the membrane with higher MR values (Figure 2): the transport rate of bulkier penetrant (hydrated salt ions) was more sensitive to the changes in free volume than those of smaller penetrant (water vapor) [39; 40]. Therefore, the decreased diffusivity of salt through the membrane with higher MR (Table 2) can be attributed to its lower free volume.