Development of Janus membranes with asymmetric wettability for desalination of impaired seawater by direct contact membrane distillation

Membrane distillation (MD) is an emerging technology for direct seawater desalination. The widely employed hydrophobic membranes are susceptible to pore wetting/ fouling in the presence of organic contaminants in seawater. Here, we demonstrate a facile method of preparing Janus membranes with hydrophilic/hydrophobic faces having excellent anti-fouling and anti-wetting properties. The hydrophobic substrate is a PVDF-co-HFP membrane incorporating fluoroalkylsilane (FAS) grafted hydrophobic MIL-53(Al)- a form of metal organic framework (h-MIL-53(Al)), and the hydrophilic coating is prepared by the direct deposition of polydopamine (PDA) (top layer). The optimum membrane exhibited a stable flux of 11 L/m2h and permeate conductivity ∼10 µS/cm for up to 60 h of seawater desalination. The anti-wetting property of the Janus membrane was demonstrated from the stable performance when challenging the membrane with impaired seawater containing low surface tension compounds such as sodium dodecyl sulphate (SDS) and propylene glycol (PG), and oil (hexadecane). The prepared membrane exhibited wetting resistance of 45 h and 18 h when feed was seawater containing 0.3 mM SDS and 3 wt% PG, respectively. The membrane displayed excellent resistance to fouling for desalination from SDS-stabilized hexadecane (500 ppm)/seawater feeds for up to 50 h (5 cycles). The membrane fabricated in this study holds the potential for seawater desalination.