Suppressing Salt Transport through Composite Pervaporation Membranes for Brine Desalination

Pervaporation membranes have gained renewed interest in challenging feedwaters desalination, such as reverse osmosis (RO) concentrated brine wastewater. In this study, composite polyvinyl alcohol (PVA)/polyvinylidene fluoride (PVDF) pervaporation membranes were prepared for brine treatment. The composite membrane was firstly studied by adjusting the cross-linking density of PVA by glutaraldehyde: the membrane with higher cross-linking density exhibited much higher salt rejection efficiency for long-term operation. A trace of salt on the permeate side was found to diffuse through the membrane in the form of hydrated ions, following solution-diffusion mechanism. To further suppress the salt transport and achieve long-term stable operation, graphene oxide (GO) was incorporated into the PVA layer: the addition of GO had minor effects on water permeation but significantly suppressed the salt passage, compared to the pure PVA/PVDF membranes. In terms of brine wastewater containing organic/inorganic foulant, improved anti-fouling performance was also observed with GO-containing membranes. Furthermore, the highest flux of 28 L/m2h was obtained for the membrane with 0.1 wt. % of GO using 100 g/L NaCl as the feed at 65 °C by optimising the pervaporation rig, with permeate conductivity below 1.2 µS/cm over 24 h (equivalent to a salt rejection of >99.99%).