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Antiwetting and antifouling Janus membrane for desalination of saline oily wastewater by membrane distillation

University of British Columbia

Membrane distillation (MD) is an emerging desalination technology employing a hydrophobic (water-repelling) microporous membrane that is promising for water reclamation from highly saline streams that conventional reverse osmosis (RO) cannot treat. However, conventional hydrophobic membranes are prone to wetting and fouling when treating complex waste streams, such as oil- and gas-produced waters, which limits the applications of MD. Typically, two conflicting surface properties (i.e., hydrophobic and hydrophilic) are required to mitigate pore wetting and membrane fouling, respectively. In this thesis, 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 re-entrant 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 low-surface-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 MD (DCMD) 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 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.

Text

2019-08-26

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/71468

Civil Engineering

University of British Columbia

UBCV

http://creativecommons.org/licenses/by-nc-nd/4.0/