Omniphobic membrane with nest-like re-entrant structure via electrospraying strategy for robust membrane distillation

Abstract

Omniphobic membranes have acquired some recognitions for alleviating the membrane wetting in the robust membrane distillation because of the super-repulsion towards different surface tension liquids. In this study, the Ti-CNFs/PVDF composite membrane with omniphobicity and high permeate flux was fabricated via electrospraying multilevel heat-conducting nanofibers on the PVDF membrane and fluorosilane treatment simultaneously with one-step strategy. In detail, carbon nanofibers decorated by TiO₂ with hydrothermal treatment were immobilized on the surface by the adhesive attraction of PDMS to establish the re-entrant structure. Meanwhile, 1H,1H,2H,2H-perfluorodecyltriethoxysilane was presented in the spraying liquid to gain the low surface energy for the surface. Additionally, the composite nanofibers played a role in mitigating the temperature polarization. A series of characterizations on membrane morphologies, wetting behaviors, structural peculiarities and stable properties were investigated systematically. The results showed that the re-entrant morphology consisted of the intersecting nanofibers was constructed skillfully, looking like the nest's hierarchical structure. The composite membrane displayed outstanding contact angles of 172.0 ± 1.8°, 170.5 ± 3.5°, 160.1 ± 2.6°, 165.0 ± 3.0°, 162.5 ± 4.5°, 151.0 ± 1.5°, 153.5 ± 3.5° and 118.0 ± 3.0° towards DI water, 3.5 wt% NaCl, 0.4 mM SDS, 0.4 mM CTAB, 0.4 mM Tween-20, ethanol, soybean milk and dodecane, respectively. The modified membrane presented a 167% LEP measurement of the pristine membrane and satisfactory chemical and thermal stabilities along with silver mirror phenomena. Furthermore, the omniphobic membrane exhibited anti-fouling and anti-wetting performances with high and stable permeate fluxes (36.3 kg/m²·h) as well as a nearly 100% salt rejection rate in the consecutive DCMD tests, where HA and diverse surfactants were added to the brine feed. This suggests the omniphobic membrane as a promising alternative for wastewater treatment in membrane distillation.