Elucidating the Trade-off between Membrane Wetting Resistance and Water Vapor Flux in Membrane Distillation

Abstract

Membrane distillation (MD) has been receiving considerable attention as a promising technology for desalinating industrial wastewaters. While hydrophobic membranes are essential for the process, increasing membrane surface hydrophobicity generally leads to the reduction of water vapor flux. In this study, we investigate the mechanisms responsible for this trade-off relation in MD. We prepared hydrophobic membranes with different degrees of wetting resistance through coating quartz fiber membranes with a series of alkylsilane molecules while preserving the fiber structures. A trade-off between wetting resistance and water vapor flux was observed in direct-contact MD experiments, with the least-wetting-resistant membrane exhibiting twice as high vapor flux as the most wetting-resistant membrane. Electrochemical impedance analysis, combined with fluorescence microscopy, elucidated that a lower wetting resistance (still water-repelling) allows deeper penetration of the liquid–air interfaces into the membrane, resulting in an increased interfacial area and therefore a larger evaporative vapor flux. Finally, we performed osmotic distillation experiments employing anodized alumina membranes that possess straight nanopores with different degrees of wetting resistance, observed no trade-off, and substantiated this proposed mechanism. Our study provides a guideline to tailor the membrane surface wettability to ensure stable MD operations while maximizing the water recovery rate.

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

“…The initial flux for MP-PVDF and CF4-MP-PVDF was very similar at 31.2 kg m −2 h −1 and 32.1 kg m −2 h −1 , respectively but the flux of FAS-MP-PVDF was 21.6 kg m −2 h −1 . Chemical surface fluorination by FAS leading to reduced initial flux has been demonstrated previously, 47 showing that the fluorinated coating can increase wetting resistance, and impede deeper penetration of the liquid–air interfaces into the membrane, resulting in a decreased interfacial area and therefore a decreased flux in comparison to MP-PVDF. 47…”

“…Chemical surface fluorination by FAS leading to reduced initial flux has been demonstrated previously, 47 showing that the fluorinated coating can increase wetting resistance, and impede deeper penetration of the liquid–air interfaces into the membrane, resulting in a decreased interfacial area and therefore a decreased flux in comparison to MP-PVDF. 47…”

Synthesis and Water Treatment Applications of Nanofibers by Electrospinning

“…The MD process depends on the temperature difference of the feed and permeates solution. Thus, the water vapor pressure is the driving force [48,49] across the hydrophobic membrane. The heat and mass transfer processes in MD affect the membrane hydrophobicity, wettability, porosity, etc.…”

Non-solvent Flux Augmentation of an LDPE-Coated Polytetrafluoroethylene Hollow Fiber Membrane for Direct Contact Membrane Distillation

“…The importance and potential benefits of exploring the MD for seawater remediation and as an effective technique for wastewater treatment have necessitated numerous research efforts to improve the MD concept . The central idea here is to make the fabricated membrane more thermally active to enhance its productivity and energy efficiency. ,, The use of dense polymeric membranes has been hypothesized to have the ability to increase the flux rate of MD by suppressing the limiting factors associated with the conventional porous membranes. , The significant limitations of using hydrophobic membranes in the MD process are wetting after prolonged usage and fouling due to continuous accumulation of biofilm, organic, inorganic, and colloidal substances on either the surface of the membrane or in its internal pore structure. , …”

“…In general, any parameter that improves flux production results in the suppression of delayed wetting and reduced wetting rate in a membrane . Unfortunately, the preferred hydrophobic types of membranes utilized in MD also reduce the membrane’s water vapor flux . Therefore, the primary task is to circumvent the mechanisms responsible for the trade-off relationship between water vapor flux and resistance to resistance in MD.…”