# 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
During the pervaporation process, salt ions could diffuse through the PVA layer in the form of hydrated ions and condensed together with water due to the temperature drop in the permeate side. The increase of cross-linking density can retard but not completely block the passage of salt ions. At the same time, the gradual up-take of water can lead to the swelling of PVA due to the preferable adsorption of water molecules by the hydroxyl groups on PVA chains. In the swollen PVA especially the one with large free volume, more water was adsorbed by the hydroxyl groups of PVA chains, where a small fraction of hydrated salt ions could fit into such enlarged free volume, allowing their gradual passage. As a result, the increased free volume allows fast dissolution and diffusion of bulkier hydrated salt ions through the membrane [34; 41; 42], which explained the gradual \begin{table}
\begin{tabular}{c c c c c c} \hline
**Pressurized Filtration Test Results** & **Unit** &
\begin{tabular}{c} **Commercial** \\ **PVDF** \\ \end{tabular} & **MR0.025** & **MR0.1** & **MR0.2** \\ \hline
**Pure water flux** & L/m\({}^{2}\)h & \(>\)15,000 & 240 \(\pm\) 56 & 5.5 \(\pm\) 0.27 & 1.4 \(\pm\) 0.4 \\
**Permability** & L/m\({}^{2}\)h-bar & \(>\)1000 & 16 \(\pm\) 3.5 & 0.37 \(\pm\) 0.018 & 0.093 \(\pm\) 0.027 \\
**Average membrane resistance** & m\({}^{-1}\) & \(<\)3.9 \(\times\) 10\({}^{11}\) & \(-\)2.5 \(\times\) 10\({}^{13}\) & -1.1 \(\times\) 10\({}^{15}\) & 4.3 \(\times\) 10\({}^{15}\) \\
**Flux using 30 g/L NaCl as the feed** & L/m\({}^{2}\)h & / & 129 \(\pm\) 16.5 & 3.74 \(\pm\) 0.23 & 0.54 \(\pm\) 0.1 \\
**Salt rejection using 30 g/L NaCl as the feed** & \% & / & \(\sim\)0 & \(\sim\)9 & \(\sim\)33 \\
**Flux using 100 mg/L Dextran as the feed** & L/m\({}^{2}\)h & / & 142 \(\pm\) 21 & 4.69 \(\pm\) 0.42 & 0.99 \(\pm\) 0.18 \\
**Rejection using 100 mg/L Dextran as the feed** & \% & / & \(\sim\)0 & 53.9\% & 83.4 \\ \hline \end{tabular}
\end{table} 
Table 1: Pressurized filtration test results of composite PVA/PVDF membrane using feed solutions of Milli-Q water, 30 g/L NaCl or 100 mg/L dextran, under TMP of 15 bar at room temperature. The flux was recorded after the stabilization of 30 min. The membrane resistance (R\({}_{\rm m}\)) was calculated based on \(R_{m}=\frac{\Delta P}{\mu J}\), where \(\Delta\)P, \(\mu\) and J indicated the trans-membrane pressure, viscosity of Milli-Q water and corresponded pure water flux, respectively. \begin{table}
\begin{tabular}{c c c c c} \hline
**Composite** & **Surface** & **EWC (\%) in Water** & **EWC (\%) in 100** & **NaCl Diffusivity** \\
**Membrane** & **Roughness (nm)** & **g/L NaCl** & **(10\({}^{-6}\) cm\({}^{2}\)/s)** \\ \hline
**MR0.025** & 281.5 \(\pm\) 7.78 & 184.5 \(\pm\) 35.3 & 129.7 \(\pm\) 16.8 & 2.02 \(\pm\) 0.97 \\
**MR0.1** & 140 \(\pm\) 31.1 & 60.4 \(\pm\) 5.2 & 56.6 \(\pm\) 2.5 & 1.42 \(\pm\) 0.36 \\
**MR0.2** & 106.4 \(\pm\) 13.58 & 49.6 \(\pm\) 7.0 & 38.1 \(\pm\) 6.3 & 0.64 \(\pm\) 0.13 \\ \hline \end{tabular}
\end{table} 
Table 2: Characterization results of composite PVA/PVDF membrane (Diffusivity of NaCl in pure water is 14.7 \(\times\) 10\({}^{-6}\) cm\({}^{2}\)/s). MR: Molar Ratio. EWC: Equilibrium Water Content. loss of the salt rejection during the extended pervaporation test (Figure 5). Thus, in the following session, GO nanosheets, impermeable hydrophilic nanosheets, were incorporated into the PVA matrix to limit the free volume increase with time in PVA layer and further suppress salt transport through the membrane. MR value of 0.2 was applied to fabricate the membranes with GO because of its highest salt rejection efficiency compared to membranes with lower MR values.