# Suppressing Salt Transport through Composite Pervaporation Membranes for Brine Desalination
## 1 Introduction  
Desalination has been widely utilized to relieve the shortage of fresh water in many parts of the world. However, a large amount of brine wastewater is also produced as a by-product of desalination. For example, seawater reverse osmosis (RO) reentate may contain concentrated salt, humic acid and other dissolved solids. The disposal of brine wastewater into the ocean or inland could lead to environmental and ecological problems [1]. Due to high osmotic pressures, RO cannot be utilized to treat such wastewater. On the other hand, the treatment efficiency of thermally driven processes like membrane distillation (MD) is less dependent on the feed solution concentration, and can be regarded as a promising candidate in brine wastewater treatment [2; 3]. However, the major problems for the industrial application of MD are membrane fouling/scaling and pore wetting. More recently, to address the pore wetting, a dense hydrophilic pervaporation membrane has been explored for brine treatment. Pervaporation is also a thermally driven process, and the main water transport mechanism is solution-diffusion [4]. It has been extensively studied for organic dehydration [5; 6]. The purpose of both desalination and organic solvents dehydration is to separate water from the bulk feed solution; however, feed properties vary significantly. For organic solvent dehydration, water usually only accounts for less than 10% of the bulk solution [7], while water is the major component in feed for desalination (more than 90%). Thus, the major problem of applying many conventional pervaporation membranes for desalination is that the membranes are not stable in the solution with both high water fractions and elevated temperatures. Polyvinyl alcohol (PVA) pervaporation membranes have high water vapor permeability, satisfactory membrane-forming capability and excellent anti-fouling property. Recently, PVA membranes have been applied for pervaporation desalination, but a certain amount of salt can still diffuse through the polymeric layers in the form of hydrated salt ions [8; 9]. This unfavorable effect can be partially mitigated by polymer cross-linking. However, a higher salt rejection would be always compromised by a reduced permeability [10; 11]. Aside from the organic cross-linker, the addition of inorganic nanofillers can also enhance the stability of the composite pervaporation membranes. For example, the PVA/maleic acid/silica freestanding membranes exhibited a reduced swelling degree with higher silica concentration [12; 13]. Furthermore, graphene oxide (GO), a unique 2D inorganic material, has good interfacial compatibility with PVA due to the formation of hydrogen bonds [14], showing good potential to enhance PVA stability. GO has been extensively investigated for gas and liquid separation membranes [15; 16]. For porous filtration membranes, the incorporation of GO can improve the permeation flux, salt rejection and anti-fouling properties due to improved hydrophilicity [17; 18; 19; 20; 21; 22; 23]. For a porous hydrophobic membrane for MD desalination applications, the immobilization of GO by PVDF as the binder material on the surface of polytetrafluoroethylene (PTFE) membranes showed enhanced flux due to selective sorption, nanocapillary effect, reduced temperature polarization and polar functional groups in GO [24]. It can also improve the thermal, mechanical and electrical properties of the composite membranes [25; 26]. Considering its 2D structure, the incorporation of GO can potentially suppress the passage of hydrated salt ions within the PVA polymer. In addition, stacked GO laminates can also form highly efficient molecular sieving channels within polymeric matrix [27; 28]. However, these aspects have yet to be explored for pervaporation membranes. Furthermore, most studies on pervaporation desalination so far only use single monovalent salt solutions as feed, and the systems are only evaluated for a relatively short period.