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Microporous membranes fabricated from hydrophobic polymers JL such as polyvinylidene fluoride (PVDF) have been widely used for membrane distillation (MD). However, hydrophobic MD membranes are prone to wetting by low surface tension substances, thereby limiting their use in treating challenging industrial wastewaters, such as shale gas produced water. In this study, we present a facile and scalable approach for the fabrication of omniphobic polyvinylidene fluoride (PVDF) membranes that repel both water and oil. Positive surface charge was imparted to an alkaline-treated PVDF membrane by aminosilane functionalization, which enabled irreversible binding of negatively charged silica nanoparticles (SiNPs) to the membrane through electrostatic attraction. The membrane with grafted SiNPs was then coated with fluoroalkylsilane (perfluorodecyltrichlorosilane) to lower the membrane surface energy. Results from contact angle measurements with mineral oil and surfactant solution demonstrated that overlaying SiNPs with ultralow surface energy significantly enhanced the wetting resistance of the membrane against low surface tension liquids. We also evaluated desalination performance of the modified membrane in direct contact membrane distillation with a synthetic wastewater containing surfactant (sodium dodecyl sulfate) and mineral oil, as well as with shale gas produced water. The omniphobic membrane exhibited a stable MD performance, demonstrating its potential application for desalination of challenging industrial wastewaters containing diverse low surface tension contaminants. [ABSTRACT FROM AUTHOR]

We investigated the factors that determine surface omniphobicity of microporous membranes and evaluated the potential application of these membranes in desalination of low surface tension wastewaters by membrane distillation (MD). Specifically, the effects of surface morphology and surface energy on membrane surface omniphobicity were systematically investigated by evaluating wetting resistance to low surface tension liquids. Single and multilevel re-entrant structures were achieved by using cylindrical glass fibers as a membrane substrate and grafting silica nanoparticles (SiNPs) on the fibers. Surface energy of the membrane was tuned by functionalizing the fiber substrate with fluoroalkylsilane (FAS) having two different lengths of fluoroalkyl chains. Results show that surface omniphobicity of the modified fibrous membrane increased with higher level of re-entrant structure and widi lower surface energy. The secondary re-entrant structure achieved by SiNP coating on the cylindrical fibers was found to play a critical role in enhancing the surface omniphobicity. Membranes coated with SiNPs and chemically modified by the FAS with a longer fluoroalkyl chain (or lower surface energy) exhibited excellent surface omniphobicity and showed wetting resistance to low surface tension liquids such as ethanol (22.1 mN m). We further evaluated performance of the membranes in desalination of saline feed solutions with varying surface tensions by membrane distillation (MD). The engineered membranes exhibited stable MD performance with low surface tension feed waters, demonstrating the potential application omniphobic membranes in desalinating complex, high salinity industrial wastewaters. [ABSTRACT FROM AUTHOR]

We evaluated the performance of trimethylamine–carbon dioxide (TMA–CO 2 ) as a potential thermolytic draw solution for engineered osmosis. Water flux and reverse solute flux with TMA–CO 2 draw solution were measured in forward osmosis (FO) and pressure retarded osmosis (PRO) modes using thin-film composite (TFC) and cellulose triacetate (CTA) FO membranes. Water flux with the TMA–CO 2 draw solution was comparable to that obtained with the more common ammonia–carbon dioxide (NH 3 –CO 2 ) thermolytic draw solution at similar (1 M) concentration. Using a TFC–FO membrane, the water fluxes produced by 1 M TMA–CO 2 and NH 3 –CO 2 draw solutions with a DI water feed were, respectively, 33.4 and 35.6 L m −2 h −1 in PRO mode and 14.5 and 15.2 L m −2 h −1 in FO mode. Reverse draw permeation of TMA–CO 2 was relatively low compared to NH 3 –CO 2 , ranging from 0.1 to 0.2 mol m −2 h −1 in all experiments, due to the larger molecular size of TMA. Thermal separation and recovery efficiency for TMA–CO 2 was compared to NH 3 –CO 2 by modeling low-temperature vacuum distillation utilizing low-grade heat sources. We also discuss possible challenges in the use TMA–CO 2 , including potential adverse impact on human health and environments. [ABSTRACT FROM AUTHOR]

The emerging vapor pressure-driven osmosis (VPDO) membrane technology enables direct conversion of abundant low-temperature (<100 °C) heat resources to useful work. In this study, a theoretical model is established to understand mass and heat transfer of VPDO, and two hydrophobic nanoporous membranes, polypropylene (PP) and polytetrafluoroethylene (PTFE), of different chemistry and structural properties were evaluated. Although the PP membrane has a less effective transport pathway, the considerably larger pore size yields a much higher Knudsen diffusivity that results in consistently higher vapor fluxes across different temperature-pressure conditions. This finding provides strong evidence that mass transfer in VPDO is dominated by Knudsen diffusion. Additionally, we find that operation at higher pressurizations caused vapor flux decline that is attributed to the membrane morphological deformation. However, the PP membrane is less sensitive to the effects of compaction, underlining the importance of membrane mechanical robustness for VPDO. Lastly, the study shows that evaporative heat transfer is significantly greater than conductive losses and the PP membrane, with higher water fluxes, has better evaporation thermal efficiencies. This study provides fundamental understanding on the impacts of membrane properties on mass and heat transfer in VPDO, and highlights the centrality of vapor permeability and mechanical robustness in developing high-performance membranes. Image 1 • Presentation of theoretical framework for mass and heat transfer in VPDO. • Study on impacts of different membrane chemical/structural properties on mass and heat transfer. • Knudsen diffusion mechanism dominates mass transfer in VPDO hydrophobic nanoporous membranes. • Vapor flux decline at high pressurizations is due to membrane morphological deformations. • Evaporative heat transfer is significantly greater than conductive losses in VPDO operation. [ABSTRACT FROM AUTHOR]

Desalination by membrane distillation (MD) using low‐grade or waste heat provides a potential route for sustainable water supply. Nonwetting, porous membranes that provide a selective pathway for water vapor over nonvolatile salt are at the core of MD desalination. Conventional water‐repelling MD membranes (i.e., hydrophobic and superhydrophobic membranes) fail to ensure long‐term desalination performance due to pore wetting and surface fouling. To address these challenges, a defect‐free carbon nanotube forest (CNTF) is engineered in situ on a porous electrospun silica fiber substrate. The engineered CNTF forms an ultrarough and porous interface structure, allowing outstanding wetting resistance against water in air and oil underwater. As a result of this antiwetting property, the composite CNTF membrane displays a stable water vapor flux and a near complete salt rejection (>99.9%) in the desalination of highly saline water containing low surface tension contaminants. The antimicrobial property of the composite CNTF membrane imparted by the unique forest‐like architecture and the oxidative effect of carbon nanotubes (CNTs) are further demonstrated. The results exemplify an effective strategy for engineering CNT architecture to elucidate the structure–property–performance relationship of the nanocomposite membranes and to guide the design of robust thermal desalination membranes. [ABSTRACT FROM AUTHOR]

Carbon Nanotube Forests: Engineering Carbon Nanotube Forest Superstructure for Robust Thermal Desalination Membranes (Adv. Funct. In article number 1903125, Menachem Elimelech and co-workers develop a novel thermal desalination membrane via in situ engineering of a carbon nanotube forest (CNTF) superstructure on a porous nanofiber substrate. Precise design of the CNTF architecture endows the composite membrane with outstanding wetting resistance against both water and oil, stable water vapor flux, and near complete salt rejection (>99.9%). [Extracted from the article]

Abstract: A numerical model for predicting the flux decline due to colloidal fouling was developed for a forward osmosis (FO) membrane system. We derived the kinetic equation of the cake layer growth based on a first-order reaction and control volume approach. Based on the model simulation, it was found that the deposited particles on a membrane surface are proportional to the feed concentration and the permeate flux. Moreover, the simulation result reveals that the cake-enhanced osmotic pressure (CEOP) is a key factor diminishing the permeate flux for large colloidal foulants. For small colloidal foulants, the hydraulic resistance of the cake layer is dominant in flux decline at the beginning of the fouling and CEOP increasingly become significant as fouling progresses. The effects of the reverse draw solute permeation on the flux decline were also simulated. Interestingly, the increased reverse draw solute permeation obtained by increasing the solute permeability showed little effect on the flux decline. Contrarily, variation of the diffusivity significantly influenced the flux decline. Consequently, the numerical model developed in this paper suggests that the selection of draw solute for an FO membrane process should be carefully regarded, along with the fouling mechanism. [Copyright &y& Elsevier]