Development of Durable Anti-Wetting and Anti-Fouling Membranes for Water Production via Membrane Distillation (MD)

Membrane distillation (MD) has gained a growing interest in seawater desalination, wastewater treatment, and separation of volatile compounds as of late due to its low operating pressure, moderate temperature requirements, and high salt rejection. However, membrane fouling and pore wetting are two major problems that hinder maintaining long-term stable flux and salt rejection of MD when treating various feed solutions. Electrospun nanofiber membranes (ENMs) are promising for MD applications due to the discernable advantages of their high hydrophobicity and porosity. Utilization of micro/nano-particles and surface fluoro-silanization are effective modification methods to enhance anti-fouling and anti-wetting properties of MD membranes. The objective of this doctoral research is to develop effective membranes with durable fouling/wetting resistance for water production by MD. To achieve this goal, a highly hydrophobic poly (vinylidene fluoride­co­hexafluoropropylene)/ reduced graphene oxide (PVDF­HFP/ rGO) flat­sheet membrane was prepared via a facile electrospinning technique. The rGO incorporated membranes exhibited excellent stability and durability with a salt rejection of over 99.97% and an average flux of 20.37 kg/m2h using 60 g/L sodium chloride as the feed solution. To expand the application of conventional MD membranes to treat challenging wastewaters that contain low surface tension contaminants, one simple modification method was applied by utilizing fluoroalkylsilane of low surface energy on the pristine PVDF-HFP/rGO substrate to achieve surface superamphiphobicity (both superhydrophobicity and near-superoleophobicity). The resulting membranes demonstrated superamphiphobicity, confirmed by their wetting resistance evaluated with water and low surface tension liquids. The antiwetting performance of the membrane was tested through desalinating 35 g/L sodium chloride solution in the presence of a surfactant (sodium dodecyl sulfate, SDS) in the direct contact membrane distillation (DCMD) unit. Finally, a highly permeable membrane for membrane distillation applications by employing porous MCM-48 silica as the nanofiller was developed. The addition of hydrophobic MCM-48 nanoparticles was found to significantly increase water vapor transport, with a flux enhancement 87.16% greater than that observed in the pristine PVDF-HFP membrane