Effects of different surfactant properties on anti-wetting behaviours of an omniphobic membrane in membrane distillation

Charge-dominated phase separation synthesis method of Janus particles with well-defined separated lobes and patternable surface chemistries

Synthesizing Janus particles (JPs) with well-defined separated lobes and customizable surface chemistries has broad scientific and engineering application prospects but has proven extremely challenging. Here, we report a novel phase-separation-based fabrication method leveraging charge-dominated seeded emulsion polymerization, which enables the synthesis of JPs with multi-scale lobe architectures (ranging from isotropic asymmetric shapes to chemically anisotropic forms such as ellipses, dumbbells, and triblock structures) and customizable surface chemistries (including functional groups like carboxyl, sulfate, and sulfonate). Our method is based on the principles of multicomponent systems' heterogeneous nucleation and growth, where the interfacial energy is meticulously controlled by fine-tuning the surface charges/chemical properties of polystyrene (PS) seeds and methacryloxypropyl trimethoxysilane (MPS) emulsions, while the growth kinetics of polymethacryloxypropyl trimethoxysilane (PMPS) lobes are guided through a synergistic combination of radical polymerization and hydrolysis-condensation reactions. Charge-dominated repulsive forces at the interface play a crucial role in driving the phase separation, enabling the synthesis of well-defined JPs and making this strategy broadly applicable to a variety of negatively charged PS seeds or MPS emulsions for customizable two-lobe surface chemistries. Furthermore, the PMPS hemisphere can be selectively modified, enabling applications in Pickering emulsions. This work offers a scalable method for the controllable fabrication of JPs with programmable architectures and surface chemistries.

Constructing spherical-beads-on-string structure of electrospun membrane to achieve high vapor flux in membrane distillation

Hydrophobic membranes with a reentrant-like structure have shown high hydrophobicity and high anti-wetting properties in membrane distillation (MD). Here, PVDF spherical-beads-on-string (SBS) fibers were electrospun on nonwoven fabric and used in the MD process. Such a reentrant-like structure was featured with fine fibers, a low ratio of bead length to bead diameter, and high bead frequency. It was revealed that the SBS-structured membranes exhibited an exceptional capability for vapor flux, due to the formation of a network of more interconnected macropores than that of fibers and fusiform-beads-on-string structures, ensuring unimpeded vapor diffusion. In the desalination of formulated seawater (3.5 wt.% NaCl solution), a vapor flux of 61 ± 3 kg m-2 h-1 with a salt rejection of >99.98 % was achieved at a feed temperature of 60 °C. Furthermore, this SBS structured membrane showed satisfactory seawater desalination performance with a stable flux of 40 kg m-2 h-1 over a 27 h MD process. These findings suggest a viable approach for fabricating SBS-structured membranes that significantly enhance vapor flux in MD for desalination applications. Besides, the hydrophobic membranes with SBS structure can be prepared by single-step electrospinning, and it is facile to scale-up manufacture. This strategy holds promise for advancing the development of high-performance MD membranes tailored for efficient seawater desalination processes.

Effects of association colloidal structures on the oxygen solubility in oil-in-water emulsion matrix

Although association colloidal structures are believed as major oxidation places, relationship of oxygen molecules with association colloids have not been evaluated in oil-in-water (O/W) emulsion. Oxygen solubility was determined in O/W emulsion containing dispersed phases with different charges of emulsifiers, numbers of dispersed droplets, and surface areas of dispersed droplets. The rates of lipid oxidation were also examined. O/W emulsion made of positively charged emulsifier had higher oxygen solubility than negatively charged and neutral emulsifiers. As number and surface area of oil droplet in O/W emulsion increased, higher oxygen solubility was observed, implying that dispersed phases could be places for oxygen molecules. O/W emulsion made of positively charged emulsifier had higher lipid oxidation than neutral emulsifier. O/W emulsion with more interfaces had lower oxidative stability, implying interfaces of association colloids could affect rates of lipid oxidation. Dispersed phase in O/W emulsion can be places for oxygen molecules.

Applications of electrically conductive membranes in water treatment via membrane distillation: Joule heating, membrane fouling/scaling/wetting mitigation and monitoring

Membrane distillation (MD) is a thermally driven separation process that is driven by phase change. The core of this technology is the hydrophobic microporous membrane that prevents mass transfer of the liquid while allowing the vapor phase to pass through the membrane's pores. Currently, MD is challenged by its high energy consumption and membrane degradation due to fouling, scaling and wetting. The use of electrically conductive membranes (ECMs) is a promising alternative method to overcome these challenges by inducing localized Joule heating, as well as mitigating and monitoring membrane fouling/scaling/wetting. The objective of this review is to consolidate recent advances in ECMs from the standpoint of conductive materials, membrane fabrication methodologies, and applications in MD processes. First, the mechanisms of ECMs-based MD processes are reviewed. Then the current trends in conductive materials and membrane fabrication methods are discussed. Thereafter, a comprehensive review of ECMs in MD applications is presented in terms of the different processes using Joule heating and various works related to membrane fouling, scaling, and wetting control and monitoring. Key insights in terms of energy consumption, economic viability and scalability are furnished to provide readers with a holistic perspective of the ECMs potential to achieve better performances and higher efficiencies in MD. Finally, we illustrate our perspectives on the innovative methods to address current challenges and provide insights for advancing new ECMs designs. Overall, this review sums up the current status of ECMs, looking at the wide range of conductive materials and array of fabrication methods used thus far, and putting into perspective strategies to deliver a more competitive ECMs-based MD process in water treatment.

Progress on Improved Fouling Resistance-Nanofibrous Membrane for Membrane Distillation: A Mini-Review

Nanofibrous membranes for membrane distillation (MD) have demonstrated promising results in treating various water and wastewater streams. Significant progress has been made in recent decades because of the development of sophisticated membrane materials, such as superhydrophobic, omniphobic and Janus membranes. However, fouling and wetting remain crucial issues for long-term operation. This mini-review summarizes ideas as well as their limitations in understanding the fouling in membrane distillation, comprising organic, inorganic and biofouling. This review also provides progress in developing antifouling nanofibrous membranes for membrane distillation and ongoing modifications on nanofiber membranes for improved membrane distillation performance. Lastly, challenges and future ways to develop antifouling nanofiber membranes for MD application have been systematically elaborated. The present mini-review will interest scientists and engineers searching for the progress in MD development and its solutions to the MD fouling issues.

Enhanced Anti-Wetting Methods of Hydrophobic Membrane for Membrane Distillation

Increasing issues of hydrophobic membrane wetting occur in the membrane distillation (MD) process, stimulating the research on enhanced anti-wetting methods for membrane materials. In recent years, surface structural construction (i.e., constructing reentrant-like structures), surface chemical modification (i.e., coating organofluorides), and their combination have significantly improved the anti-wetting properties of the hydrophobic membranes. Besides, these methods change the MD performance (i.e., increased/decreased vapor flux and increased salt rejection). This review first introduces the characterization parameters of wettability and the fundamental principles of membrane surface wetting. Then it summarizes the enhanced anti-wetting methods, the related principles, and most importantly, the anti-wetting properties of the resultant membranes. Next, the MD performance of hydrophobic membranes prepared by different enhanced anti-wetting methods is discussed in desalinating different feeds. Finally, facile and reproducible strategies are aspired for the robust MD membrane in the future.

Controlling Superhydrophobicity and Oleophobicity of Polydimethylsiloxane-Coated Silica Hybrid Particles

Hierarchical functional organic-inorganic hybrid particles for versatile control of surface wettability have attracted much attention in a wide range of applications from makeup cosmetics to anti-smudging optoelectronic devices. In this study, superhydrophobic and oleophobic organic-inorganic hybrid particles were prepared by a simple and systematic fabrication strategy using the synergistic combination of commonly available silica particles and polydimethylsiloxanes (PDMSs) with hydrophobic chain ends. Various types of PDMSs with different chain lengths and chemical structures were surface-grafted to silica microparticles through facile physical dispersion and subsequent thermal treatment to form hydrogen bonds or covalent bonds between the inorganic silica and organic PDMS polymers and thus induce a core-shell structure for the hybrid particles, which imparts superhydrophobicity and oleophobicity to the surface of silica particles. The prepared PDMS-coated silica hybrid particles with long PDMS chains exhibited a water contact angle of 151.2° and an oil contact angle of 15.2° due to the rough surface morphology and hydrophobic long-chain effects. Furthermore, the resulting organic-inorganic hybrid particles were thermally stable up to 420 °C. This controlled approach endowed the organic-inorganic hybrid particles with both superhydrophobic and oleophobic surfaces and, therefore, these particles were proven to be suitable for waterproof applications.

Coaxial Electrospun Nanofibrous Membranes for Enhanced Water Recovery by Direct Contact Membrane Distillation

Membrane distillation (MD) is an emerging technology for water recovery from hypersaline wastewater. Membrane scaling and wetting are the drawbacks that prevent the widespread implementation of the MD process. In this study, coaxially electrospun polyvinylidene fluoride-co-hexafluoropropylene (PVDF-co-HFP) nanofibrous membranes were fabricated with re-entrant architecture and enhanced hydrophobicity/omniphobicity. The multiscale roughness was constructed by incorporating Al2O3 nanoparticles and 1H, 1H, 2H, 2H Perfluorodecyltriethoxysilane in the sheath solution. High resolution transmission electron microscopy (HR-TEM) could confirm the formation of the core-sheath nanofibrous membranes, which exhibited a water contact angle of ~142.5° and enhanced surface roughness. The membrane displayed a stable vapor flux of 12 L.m−2.h−1 (LMH) for a 7.0 wt.% NaCl feed solution and no loss in permeate quality or quantity. Long-term water recovery from 10.5 wt.% NaCl feed solution was determined to be 8−10 LMH with >99.9% NaCl rejection for up to 5 cycles of operation (60 h). The membranes exhibited excellent resistance to wetting even above the critical micelle concentration (CMC) for surfactants in the order sodium dodecyl sulphate (SDS) (16 mM) > cetyltrimethylammonium bromide (CTAB) (1.5 mM) > Tween 80 (0.10 mM). The presence of salts further deteriorated membrane performance for SDS (12 mM) and Tween-80 (0.05 mM). These coaxial electrospun nanofibrous membranes are robust and can be explored for long-term applications.

Study on Spacing Regulation and Separation Performance of Nanofiltration Membranes of GO

Graphene oxide (GO) membranes have attracted significant attention in the field of water processing in recent years due to their unique characteristics. However, few reports focus on both membrane stability and the “trade-off” effect. In this study, a series of aliphatic diamines (1, 2-ethylenediamine, 1, 4-butanediamine, and 1, 6-hexamethylenediamine) of covalent crosslinked GO were used to prepare diamine-modified nanofiltration membranes, BPPO/AX-GO, with adjustable layer spacing using the vacuum extraction−filtration method. Moreover, Ax-GO-modified nanofiltration membranes modified with adipose diamine had higher layer spacing, lower mass-transfer resistance, and better stability. When the number of carbon atoms was 5, the best layer spacing was reached, and when the number of carbon atoms was greater than 4, the modified membrane nanosheets more easily accumulated. With the increase in layer spacing, the water flux of the composite film increased to 26.27 L/m2·h·bar. Meanwhile, adipose diamine crosslinking significantly improved the stability of GO films. The interception sequence of different valence salts in the composite membrane was NaCl > Na2SO4 > MgSO4, and the rejection rate of bivalent salts was higher than that of monovalent salts. The results can provide some experimental basis and research ideas for overcoming the “trade-off” effect of a lamellar GO membrane.

Preparation and Modification of PVDF Membrane and Study on Its Anti-Fouling and Anti-Wetting Properties

Membrane distillation (MD) has unique advantages in the treatment of high-salt wastewater because it can make full use of low-grade heat sources. The high salinity mine water in western mining areas of China is rich in Ca2+, Mg2+, SO42− and HCO3−. In the MD process, the inorganic substances in the feed will cause membrane fouling. At the same time, low surface tension organic substances which could be introduced in the mining process will cause irreversible membrane wetting. To improve the anti-fouling and anti-wetting properties of the membrane, the PVDF omniphobic membrane in this paper was prepared by electrospinning. The water contact angle (WCA) can reach 153°. Direct contact membrane distillation (DCMD) was then used for treating high-salinity mine water. The results show that, compared with the unmodified membranes, the flux reduction rate of the omniphobic membrane was reduced by 34% in 20 h, showing good anti-fouling property. More importantly, the omniphobic membrane cannot be wetted easily by the feed containing 0.3 mmol/L SDS. The extended Derjaguin–Landau–Verwey–Overbeek (XDLVO) theory was used to analyze the free energy of the interface interaction between the membrane and pollutants, aiming to show that the omniphobic membrane was more difficult to pollute. The result was consistent with the flux variation in the DCMD process, providing an effective basis for explaining the mechanism of membrane fouling and membrane wetting.

Long-Chain PFASs-Free Omniphobic Membranes for Sustained Membrane Distillation

Omniphobic membranes possessing high wetting resistance have been created for the treatment of challenging hypersaline feedwaters with low surface tension through membrane distillation (MD). However, virtually all such membranes are fabricated with long-chain per- and polyfluoroalkyl substances (PFASs, ≥8 fluorinated carbons). The environmental risks and high bioaccumulation potential of long-chain PFASs have raised increasing concerns. Developing highly wetting-resistant MD membranes while avoiding the use of long-chain PFASs is essential to improve the viability of MD for resilient and sustainable water purification. We demonstrate that MD membranes with exceptional wetting resistance can be designed through the combination of hierarchically structured membranes consisting of re-entrant texture at different length scales and (ultra)short-chain fluorocarbons, which have lower acute toxicity and bioaccumulation potentials than long-chain PFASs. Our hierarchically structured membrane with three-tier micro/nanostructure fabricated with short-chain fluorocarbon possesses superior wetting resistance, which is comparable to or higher than the long-chain PFASs-based omniphobic membranes reported in the literature. Furthermore, the hierarchically structured membranes fabricated with ultrashort-chain fluorocarbons display improved wetting resistance against feedwaters with low surface tension. Our findings indicate that long-chain PFASs are not required when designing wetting-resistant membranes and that the balance between sustainability and wetting resistance should be tailored to the wetting potential of the feedwater.

Mitigation of membrane biofouling via immobilizing Ag-MOFs on composite membrane surface for extractive membrane bioreactor

The extractive membrane bioreactor (EMBR) combines an extractive membrane process and bioreactor to treat highly saline recalcitrant organic wastewater, in which the organic contaminations diffuse through a semi-permeable polydimethysiloxane (PDMS) composite membrane from the feed wastewater to the receiving biomedium. During the long-term EMBR operation, membrane biofouling is an inevitable phenomenon, which is one of the main obstacles impeding its wide applications. The excessive biofilm deposited on membrane surface could significantly reduce the organic mass transfer coefficient of composite membranes by more than 40%. Therefore, in this work, the silver (Ag)-metal organic frameworks (MOFs) were synthesized and immobilized on the PDMS surface of nanofibrous composite membranes to mitigate the membrane biofouling. The robustness of Ag-MOFs coating on membrane surface was well demonstrated by ultrasonic treatment. In addition, the silver nanoparticles (AgNPs) were coated on the PDMS surface of composite membranes for comparison. In contrast with the unmodified composite membrane #M0, the AgNPs-coated (#M1) and Ag-MOFs modified (#M2) composite membranes possessed less hydrophobic and negatively charged surfaces due to the coating layers. Although the modified membranes exhibited lower phenol mass transfer coefficients (k0's) in the aqueous-aqueous extractive membrane process due to these additional modification layers, both #M1 and #M2 displayed better long-term performance in the 12-days continuous EMBR operations due to their excellent anti-biofouling properties. Moreover, #M2 exhibited the most stable EMBR performance among the composite membranes developed in this work and other reported membranes with a finally stabilized k0 of 33.0 × 10-7 m/s (89% of initial k0). The least amounts of proteins, polysaccharides and total suspended solids (TSS) on the surface of tested #M2 also demonstrated its outstanding biofouling resistance. This excellent anti-biofouling performance should be attributed to the stable, controlled and long-lasting Ag+release from Ag-MOFs, as well as its less hydrophobic and negative charged surface properties, which made #M2 undergo the k0's increasing and gradual stabilization stages in the long-term EMBR operations.

Membrane Distillation of Saline Water Contaminated with Oil and Surfactants

Application of the membrane distillation (MD) process for the treatment of high-salinity solutions contaminated with oil and surfactants represents an interesting area of research. Therefore, the aim of this study is to investigate the effect of low-concentration surfactants in oil-contaminated high-salinity solutions on the MD process efficiency. For this purpose, hydrophobic capillary polypropylene (PP) membranes were tested during the long-term MD studies. Baltic Sea water and concentrated NaCl solutions were used as a feed. The feed water was contaminated with oil collected from bilge water and sodium dodecyl sulphate (SDS). It has been demonstrated that PP membranes were non-wetted during the separation of pure NaCl solutions over 960 h of the module exploitation. The presence of oil (100–150 mg/L) in concentrated NaCl solutions caused the adsorption of oil on the membranes surface and a decrease in the permeate flux of 30%. In turn, the presence of SDS (1.5–2.5 mg/L) in the oil-contaminated high-salinity solutions slightly accelerated the phenomenon of membrane wetting. The partial pores’ wetting accelerated the internal scaling and affected degradation of the membrane’s structure. Undoubtedly, the results obtained in the present study may have important implications for understanding the effect of low-concentration SDS on MD process efficiency.

Bio-inspired super liquid-repellent membranes for membrane distillation: Mechanisms, fabrications and applications

With the aggravation of the global water crisis, membrane distillation (MD) for seawater desalination and hypersaline wastewater treatment is highlighted due to its low operating temperature, low hydrostatic pressure, and theoretically 100% rejection. However, some issues still impede the large-scale applications of MD technology, such as membrane fouling, scaling and unsatisfactory wetting resistance. Bio-inspired super liquid-repellent membranes have progressed rapidly in the past decades and been considered as one of the most promising approaches to overcome the above problems. This review for the first time systematically summarizes and analyzes the mechanisms of different super liquid-repellent surfaces, their preparation and modification methods, and anti-wetting/fouling/scaling performances in the MD process. Firstly, the topology theories of in-air superhydrophobic, in-air omniphobic and underwater superoleophobic surfaces are illustrated using different models. Secondly, the fabrication methods of various super liquid-repellent membranes are classified. The merits and demerits of each method are illustrated. Thirdly, the anti-wetting/fouling/scaling mechanisms of super liquid-repellent membranes are summarized. Finally, the conclusions and perspectives of the bio-inspired super liquid-repellent membranes are elaborated. It is anticipated that the systematic review herein can provide readers with foundational knowledge and current progress of super liquid-repellent membranes, and inspire researchers to overcome the challenges up ahead.