# Suppressing Salt Transport through Composite Pervaporation Membranes for Brine Desalination
## 3 Results
### Effect of Permeate Pressure on Pervaporation Performance  
As the pervaporation process is a thermal-driven process, the vapor pressure difference across the membrane plays a major role in the membrane water vapor permeability. While the main goal of this study was to suppress the salt transport through the composite membrane by modifications on PVA layer, there is still a need to suggest how the permeate pressure influenced on the membranes pervaporation performance, especially the membrane productivity. Conducting a thorough study could be an independent endeavor, however, in this study a limited number of tests were conducted to explore the effect of permeate pressure on membrane performance. A secondary condensation unit was applied next to the membrane cell in the permeate side, in order to maintain the permeate pressure at 3 kPa (instead of the initial pressure of 24 kPa). The membrane PVA/PVDF and PVA0.1 GO/PVDF (both with 0.2 MR value and effective membrane area of 14.7 cm\({}^{2}\)) were further tested, with both Milli-Q water and 100 g/L NaCl as the feeds. For the Milli-Q water feed, PVA/PVDF membrane and PVA0.1 GO/PVDF membrane had very comparable flux (average flux 39.7 and 39.2 L/m\({}^{2}\)h for 4 h operation after stabilization, results not shown). The pervaporation performance using highly concentrated NaCl as the feed was shown in Figure 11 an initial volume of 2 L feed solution with a concentration of 100 g/L was used without top-up during a 24 h operation. Both membranes showed an initial flux of 27-28 L/m\({}^{2}\)h, which was followed by obvious flux decline (around 25%) as a result of continuously increased feed concentration. After the 24 h pervaporation test using salt water, a flux of  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Operation Time (h)** & **Conductivity (\(\mu\)S/cm)** & **Na\({}^{+}\) (mg/L)** & **Ca\({}^{2+}\) (mg/L)** \\ \hline
19.2 & 3.7 & 0.55 & 0.00 \\
66.5 & 78.8 & 13.6 & 0.03 \\
116.4 & 205 & 37.1 & 0.18 \\
164.25 & 493 & 127 & 1.07 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Permeate property of membrane PVA0.1 GO/PVDF at different operation time by using feed solution of 100 g/L NaCl, 10 mg/L humic acid and 1.26 g/L CaCl\({}_{2}\), where initial feed conductivity is around 155 mS/cm, with Na\({}^{+}\) 39.4 g/L and Ca\({}^{2+}\) 0.164 g/L.  
Figure 10: (**a**) Flux and (**b**) permeate conductivity profiles of the composite membrane PVA/PVDF, and PVA0.1 GO/PVDF prepared by cast coating method using different feed solution.  
around 39 L/m2h was restored using Milli-Q water as the feed for both membranes (results not shown). In terms of permeate conductivity, the membrane PVA0.1 GO/PVDF obtained a constant value of 1-1.2 uS/cm, while the membrane PVA/PVDF showed a value of 15 uS/cm at the end of 24 h testing. This also confirmed that the addition of 0.1 wt. % GO effectively suppressed salt diffusion through the membrane. More importantly, the flux value using the same feed concentration was 9 times to that of using permeate pressure of 24 kPa. This suggested the further direction for optimization for such pervaporation wastewater treatment processes.