Antiwetting and Antifouling Janus Membrane for Desalination of Saline Oily Wastewater by Membrane Distillation

Abstract

In this study, we develop Janus membranes comprising a hydrophilic zwitterionic polymer layer and an omniphobic (all-liquid-repelling) porous substrate that simultaneously possess fouling and wetting resistances. An omniphobic membrane was first fabricated by attaching silica nanoparticles (SiNPs) to the fibers of a quartz fiber mat, creating multilevel reentrant structures, followed by surface fluorination to reduce the surface energy. The Janus membrane was then fabricated by grafting a zwitterionic polymer brush layer via surface-initiated atom-transfer radical-polymerization (ATRP) on the omniphobic substrate. Membrane characterizations, including Fourier-transform infrared spectroscopy, fluorescence microscopy, and contact angle measurements, confirm that the surface hydrophilicity can be finely tuned by adjusting the duration of the ATRP reaction. Also, the zwitterionic polymer brush layer was confined on the top surface of the Janus membrane, rendering the surface hydrophilic, while the remaining part of the Janus membrane remained omniphobic, resisting the wicking of lowsurface-tension liquids including ethanol and hexane. A static oil-fouling test showed that crude oil droplets irreversibly fouled an omniphobic membrane (without a hydrophilic top layer) in water. In contrast, oil droplets placed on the Janus membrane in air were immediately desorbed upon its immersion in water. Finally, we performed direct-contact membrane distillation (MD) experiments using a crude-oil-in-saline (NaCl) water emulsion as a feed solution, simulating highly saline oily wastewater. The Janus membrane exhibited superior wetting and fouling resistances, with a stable water flux and nearly perfect salt rejection, while an omniphobic membrane failed in the desalination process. Our work highlights the great potential of antiwetting and antifouling Janus membranes for water reclamation from challenging industrial wastewaters through MD.

Asymmetric Surface Engineering for Janus Membranes

“…Single‐side irradiation provides an energy gradient because the irradiation intensity is attenuated across an opaque membrane. As a result, the photochemical reactions [ 82–99 ] or plasma activation [ 100–102 ] can be carried out nonuniformly along the direction of membrane thickness. The Janus structure is established through photoetching, [ 82–92 ] photodegradation, [ 93–97 ] or photoinitiated grafting [ 98,99 ] under a directional irradiation of laser or ultraviolet.…”

“…Likewise, a plasma of nitrogen and oxygen is able to directly activate the surface of hydrophobic membranes, in situ producing hydrophilic amino and hydroxyl groups on one side to obtain a Janus structure. [ 100–102 ] However, the inevitable structural damage arisen from the high‐energy light or plasma irradiation should be carefully considered with respect to the stability of the membranes.…”

“…The introduction of hydrophilic layer effectively depresses the fouling phenomenon for both vacuum membrane distillation [ 109 ] and direct contact membrane distillation. [ 26–28,45,52,55,63,101,110 ] Besides, a hydrophilic layer with a certain thickness shortens the pathway for mass transfer from the hot feed fluid to the cold permeate for achieving a high steam flux. Further increasing the thickness of hydrophilic layer would compromise the salt rejection and even cause a breakthrough by water for the membranes.…”

Atomic Layer Deposition for Polypropylene Film Engineering—A Review

“…[ 63 ] Most recent scientific reports about applications of Janus membranes focus on the recovery of oil–water emulsions. [ 59b,63a,64 ] Two approaches to the Janus membrane are selective hydrophobization to one side of the initially hydrophilic membrane or selective hydrophilization to one side of the initially hydrophobic membrane. Janus membranes are mostly prepared by selective modification via vapor deposition or plasma treatment methods.…”

Fabricating Janus membranes via physicochemical selective chemical vapor deposition

“…With the advent of novel surface functionalization methods, the fabrication of materials with asymmetric surface properties has gained increasing attention 1‐7 . To this end, membranes with asymmetric wettability, referred to as Janus membranes, have found widespread applications 8‐14 . To date, extensive research has been devoted to fabricating Janus membranes 15‐24 ; however, the complexities involved with the fabrication schemes have hindered the development of these membranes.…”