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

Abstract

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.

Understanding the fouling/scaling resistance of superhydrophobic/omniphobic membranes in membrane distillation

“…11 shows the recent tailor-made nanofiber-based superhydrophobic [46,74,111] and omniphobic [45,116,118] membranes with superior fouling and scaling resistance. The preparation of these membranes followed a similar concept: firstly adding nanoparticles to the nanofibers or creating surface roughness by a hydrothermal technique [118,119] or growing metal oxides [116]; then chemically modify the surface via fluorination. Fig.…”

Scaling resistance by fluoro-treatments: the importance of wetting states

“…23–31 A superhydrophobic membrane often possess lower surface energy, which can be achieved via chemical fluorination modification, 32,33 or CF4 physical treatment. 34–36 Minimizing the liquid solid contact area is also important, which can be achieved by introducing hierarchical structure 31,37–39 or increasing surface porosity. 40,41 Although most current superhydrophobic membranes show good anti-scaling properties, there are some contradictory results, where some membranes are scaling resistant, but others not.…”

Experimental and theoretical studies on the formation of pure β-phase polymorphs during fabrication of polyvinylidene fluoride membranes by cyclic carbonate solvents

“…This trend has been observed in various studies. [67][68][69][70][71][72][73][74][75][76] In addition, for the saline solution, the permeate flux was decreased by 25.5% and 16.3% at 40°C and 80°C, respectively, compared to the corresponding permeate flux obtained for distilled water. This reduction of the permeate flux upon the adding of NaCl to the aqueous feed solution is attributed to the reduced water vapor pressure of the feed solution and to the concentration polarization effect.…”