Omniphobic Nanofibrous Membrane with Pine-Needle-Like Hierarchical Nanostructures: Toward Enhanced Performance for Membrane Distillation.

Wetting and fouling phenomena are the main concerns for membrane distillation (MD) in treating high-salinity industrial wastewater. This work developed an omniphobic membrane by growing titanium dioxide (TiO2) nanorods on polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP) nanofibers using a hydrothermal technique. The TiO2 nanorods form a uniform pine-needle-like hierarchical nanostructure on PVDF-HFP fibers. A further fluorination treatment provides the membrane with a low-surface-energy omniphobic surface, displaying contact angles of 168° and 153° for water and mineral oil, respectively. Direct contact MD experiments demonstrated that the resulting membrane shows a high and stable salt rejection of >99.9%, while the pristine PVDF-HFP nanofibrous membrane suffers a rejection decline caused by intense pore wetting and oil fouling in the desalination process in the presence of surfactant and mineral oil. The superior antiwetting and antifouling behaviors were ascribed to a nonwetting Cassie-Baxter state established by the accumulation of a great deal of air in the hydrophobized hierarchical re-entrant structures. The development of omniphobic membranes with pine-needle-like hierarchical nanostructures provides an approach to mitigate membrane wetting and fouling in the MD process for the water reclamation from industrial wastewater.