Abstract
Omniphobic membranes are attractive for membrane distillation (MD) because of their superior wetting resistance. However, a design framework for MD membrane remains incomplete, due to the complexity of omniphobic membrane fabrication and the lack of fundamental relationship between wetting resistance and water vapor permeability. Here we present a particle-free approach that enables rapid fabrication of monolithic omniphobic membranes for MD desalination. Our monolithic omniphobic membranes display excellent wetting resistance and water purification performance in MD desalination of hypersaline feedwater containing surfactants. We identify that a trade-off exists between wetting resistance and water vapor permeability of our monolithic MD membranes. Utilizing membranes with tunable wetting resistance and permeability, we elucidate the underlying mechanism of such trade-off. We envision that our fabrication method as well as the mechanistic insight into the wetting resistance-vapor permeability trade-off will pave the way for smart design of MD membranes in diverse water purification applications.
Highlights
Omniphobic membranes are attractive for membrane distillation (MD) because of their superior wetting resistance
In our approach to fabricate omniphobic polyvinylidene difluoride (PVDF) membrane, ultra-fast etching of a commercial PVDF membrane (HVHP, Durapore) by immersing it in a sodium/ naphthalene-based solution[38,39,40] for ~1 s was combined with surface chemistry modification using a fluoroalkyl silane[41,42,43,44] to impart low solid surface energy
Fluorine was stripped from the backbone of PVDF, while oxygen-containing functional groups such as hydroxyl and carboxyl groups were created to provide active sites for the subsequent grafting of fluoroalkyl silane via vapor-phase silanization[45,46]
Summary
Omniphobic membranes are attractive for membrane distillation (MD) because of their superior wetting resistance. Membrane distillation (MD) has recently attracted great attention as an emerging desalination technology for water purification, due to its superior characteristics such as moderate operational temperature, high tolerance to salinity, and unique capability of utilizing low-grade thermal energy[8,9,10]. Our monolithic omniphobic membranes display excellent wetting resistance against liquids with low surface tensions (e.g., ethanol), as well as excellent water purification performance in direct contact MD of hypersaline solutions containing the surfactant sodium dodecyl sulfate (SDS). We envision that our simple and rapid fabrication technique as well as our elucidation of the underlying mechanism of wetting resistance-vapor permeability trade-off will facilitate the practical use and smart design of omniphobic membranes in MD desalination and contribute to the mitigation of water scarcity
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