A Critical Review of Membrane Wettability in Membrane Distillation from the Perspective of Interfacial Interactions

Improving the hydrophobic nature of biopolymer based edible packaging film: A review

Biopolymer-based polymer compounds mainly protein and polysaccharide are commonly used in biodegradable packaging. Gelatin, cellulose, starch, chitosan, pectin, and alginate are widely used for producing biodegradable packaging films because of their nontoxicity and safety. However, some drawbacks need to be solved before employing them in food packaging, such as inappropriate wettability, low barrier properties, low mechanical properties and high moisture sensitivity. To overcome the hurdle, film-surface modifications with hydrophobic agents improve oxygen and carbon dioxide permeation while reducing moisture permeability. Some of the method used in tuning hydrophobicity is plasma treatment and coating, high pressure processing, electrospinning, etc. Additionally, hydrophobic coatings are applied on the inner surfaces of packaging films to minimize water losses, prevent leakages, and reduce spoilage and contamination of food. This article discusses the various methodologies in improving hydrophobicity for biodegradable packaging films by introducing hydrophobic agents and other active constituents to increase their functioning and intelligence.

Regulating gypsum scaling-induced wetting in membrane distillation by heterogeneous crystallization: Role of filter media

Mineral scaling and scaling-induced wetting are critical issues in membrane distillation (MD) during treatment of saline wastewaters. Gypsum scaling and scaling-induced wetting in MD were successfully regulated by heterogeneous crystallization with in-line granular filtration in this study. Stable water recovery increased from 32.5 % to more than 52.5 % in one-cycle operation, depending on filter media properties. Because a large mass of crystals were retained or/and adsorbed in the granular filter, the scaling mass on membrane surface was reduced by 41.2 %, 23.1 %, 54.7 % and 78.1 % by filter charged with activated carbon, sand, fiber and activated alumina, respectively. When activated carbon, sand, fiber and activated alumina were used, the final MD fluxes were 1.58, 1.04, 1.96 and 3.43 times that without filter, and permeate conductivity decreased by 43.0 %, 46.8 %, 83.2 % and 81.3 %, respectively. The multi-cycle tests showed that heterogeneous crystallization gradually occurred in the granular filter, thereby promoting seeding-induced crystallization that reduced gypsum scaling and scaling-induced wetting in MD. Excellent anti-scaling and anti-wetting performance of in-line granular filtration was also confirmed for synthetic and real industrial wastewater. The results of this study provide guidance for mineral scaling control in MD to allow resource utilization for saline wastewater.

Versatile Gas-Transfer Membrane in Water and Wastewater Treatment: Principles, Opportunities, and Challenges

Technologies using liquid-transfer membranes, such as microfiltration, ultrafiltration, and reverse osmosis, have been widely applied in water and wastewater treatment. In the last few decades, gas-transfer membranes have been introduced in various fields to facilitate mass transfer, in which gaseous compounds permeate through membrane pores driven by gradients in chemical concentration or potential. A notable knowledge gap exists among researchers working on these emerging gas-transfer membranes as they approach this subject from different angles and areas of expertise (e.g., material science versus microbiology). This review explores the versatile applications of gas-transfer membranes in water and wastewater treatment, categorizing them into three primary types according to the function of membranes: water vapor transferring, gaseous reactant supplying, and gaseous compound extraction. For each type, the principles, evolution, and potential for further development were elaborated. Moreover, this review highlights the potential knowledge transfer between different fields, as insights from one type of gas-transfer membrane could potentially benefit another. Despite their technical innovations, these processes still face challenges in practical operation, such as membrane fouling and wetting. We advocate for research focusing on more practical and sustainable membranes and careful consideration of these emerging membrane technologies in specific scenarios. The current practicality and maturity of these emerging processes in water and wastewater treatment are described by the Technology Readiness Level (TRL) framework. Particularly, ongoing fundamental progress in membranes and engineering is expected to continue fueling the future development of these technologies.

Advances in membrane distillation for wastewater treatment: Innovations, challenges, and sustainable opportunities

Water pollution and the growing demand for zero liquid discharge solutions have driven the development of advanced wastewater treatment technologies. Membrane distillation (MD) is a promising thermal-based process capable of treating high-salinity brines and wastewater. This review provides an in-depth analysis of MD configurations, operating principles, and membrane characteristics while addressing key challenges such as fouling and pore wetting which hinder large-scale implementation. To overcome these limitations, various membrane fabrication and modification strategies, including physical and chemical approaches, have been explored. The integration of MD with other processes (hybrid MD) for wastewater treatment is also examined. A comprehensive discussion on the mechanisms of organic, inorganic, and biological fouling and their impact on MD performance is presented. Additionally, recent advancements in antifouling strategies, including surface modifications, novel materials, and operational optimizations, are reviewed. Furthermore, the review critically analyzes membrane wetting, its governing mechanisms, and mitigation techniques. By summarizing the current challenges and future prospects, this work provides valuable insights into improving MD performance for practical applications. The findings serve as a foundation for further research and technological advancements in the field of wastewater treatment using MD.

Comparison of Different Polymeric Membranes in Direct Contact Membrane Distillation and Air Gap Membrane Distillation Configurations

Membrane distillation (MD) is an evolving thermal separation technique most frequently aimed at water desalination, compatible with low-grade heat sources such as waste heat from thermal engines, solar collectors, and high-concentration photovoltaic panels. This study presents a comprehensive theoretical-experimental evaluation of three commercial membranes of different materials (PE, PVDF, and PTFE), tested for two distinct MD modules-a Direct Contact Membrane Distillation (DCMD) module and an Air Gap Membrane Distillation (AGMD) module-analyzing the impact of key operational parameters on the performance of the individual membranes in each configuration. The results showed that increasing the feed saline concentration from 7 g/L to 70 g/L led to distillate flux reductions of 12.2% in the DCMD module and 42.9% in the AGMD one, averaged over the whole set of experiments. The increase in feed temperature from 65 °C to 85 °C resulted in distillate fluxes up to 2.36 times higher in the DCMD module and 2.70 times higher in the AGMD one. The PE-made membrane demonstrated the highest distillate fluxes, while the PVDF and PTFE membranes exhibited superior performance under high-salinity conditions in the AGMD module. Membranes with high contact angles, such as PTFE with 143.4°, performed better under high salinity conditions. Variations in operational parameters, such as flow rate and temperature, markedly affect the temperature and concentration polarization effects. The analyses underscored the necessity of a careful selection of membrane type for each distillation configuration by the specific characteristics of the process and its operational conditions. In addition to experimental findings, the proposed heat and mass transfer-reduced model showed good agreement with experimental data, with deviations within ±15%, effectively capturing the influence of operational parameters. Theoretical predictions showed good agreement with experimental data, confirming the model's validity, which can be applied to optimization methodologies to improve the membrane distillation process.

Gravity-driven membrane integrated with membrane distillation for efficient shale gas produced water treatment

Substantial volumes of hazardous shale gas produced water (SGPW) generated in unconventional natural gas exploration. Membrane distillation (MD) is a promising approach for SGPW desalination, while membrane fouling, wetting, and permeate deterioration restrict MD application. The integration of gravity-driven membrane (GDM) with MD process was proposed to improve MD performance, and different pretreatment methods (i.e., oxidation, coagulation, and granular filtration) were systematically investigated. Results showed that pretreatment released GDM fouling and improved permeate quality by enrich certain microbes' community (e.g., Proteobacteria and Nitrosomonadaceae), greatly ensured the efficient desalination of MD. Pretreatment greatly influences GDM fouling layer morphology, leading to different flux performance. Thick/rough/hydrophilic fouling layer formed after coagulation, and thin/loose fouling layer formed after silica sand filtration improved GDM flux by 2.92 and 1.9 times, respectively. Moreover, the beneficial utilization of adsorption-biodegradation effects significantly enhanced GDM permeate quality. 100 % of ammonia and 53.99 % of UV254 were efficiently removed after zeolite filtration-GDM and granular activated carbon filtration-GDM, respectively. Compared to the surged conductivity (41.29 μS/cm) and severe flux decline (>82 %) under water recovery rate of 75 % observed in single MD for SGPW treatment, GDM economically controlled permeate conductivity (1.39-19.9 μS/cm) and MD fouling (flux decline=8.3 %-27.5 %). Exploring the mechanisms, the GDM-MD process has similarity with Janus MD membrane in SGPW treatment, significantly reduced MD fouling and wetting.

Selective O2-to-H2O2 Electrosynthesis by a High-Performance, Single-Pass Electrofiltration System Using Ibuprofen-Laden CNT Membranes

Producing H2O2 through a selective, two-electron (2e) oxygen reduction reaction (ORR) is challenging, especially when it serves as an advanced oxidation process (AOP) for cost-effective water decontamination. Herein, we attain a 2e-selectivity H2O2 production using a carbon nanotube electrified membrane with ibuprofen (IBU) molecules laden (IBU@CNT-EM) in an ultrafast, single-pass electrofiltration process. The IBU@CNT-EM can generate H2O2 at a rate of 25.62 mol gCNT-1 h-1 L-1 in the permeate with a residence time of 1.81 s. We demonstrated that an interwoven, hydrophilic-hydrophobic membrane nanostructure offers an excellent air-to-water transport platform for ORR acceleration. The electron transfer number of the ORR for IBU@CNT at neutral pH was confirmed as 2.71, elucidating a near-2e selectivity to H2O2. Density functional theory (DFT) studies validated an exceptional charge distribution of the IBU@CNT for the O2 adsorption. The adsorption energies of the O2 and *OOH intermediates are proportional to the H2O2 selectivity (64.39%), higher than that of the CNT (37.81%). With the simple and durable production of H2O2 by IBU@CNT-EM electrofiltration, the permeate can actuate Fenton oxidation to efficiently decompose emerging pollutants and inactivate bacteria. Our study introduces a new paradigm for developing high-performance H2O2-production membranes for water treatment by reusing environmental functional materials.

Improving membrane distillation performance by Fe(II) activated sodium percarbonate oxidation during the treatment of shale gas produced water

Membrane distillation (MD) offers promise for recycling shale gas produced water (SGPW), while membrane fouling is still a major obstacle in standalone MD. Herein, sodium percarbonate (SPC) oxidation was proposed as MD pretreatment, and the performance of the single MD, SPC-MD hybrid process and Fe(II)/SPC-MD hybrid process for SGPW treatment were systematically evaluated. Results showed that compared to raw SGPW, the application of SPC and Fe(II)/SPC led to the decrease of the fluorescent organics by 28.54 % and 54.52 %, respectively. The hydrophobic fraction decreased from 52.75 % in raw SGPW to 37.70 % and 27.20 % for SPC and Fe(II)/SPC, respectively, and the MD normalized flux increased from 0.19 in treating raw SGPW to 0.65 and 0.81, respectively. The superiority of SPC oxidation in reducing the deposited membrane foulants and restoring membrane properties was further confirmed through scanning electron microscopy observation, attenuated total reflection fourier transform infrared, water contact angle and surface tension analyses of fouled membranes. Correlation analysis revealed that hydrophobic/hydrophilic matters and fluorescent organics in SGPW took a crucial role in MD fouling. The mechanism of MD fouling mitigation by Fe(II)/SPC oxidation was attributed to the decrease in concentrations and hydrophobicity of organic by synergistic oxidation, coagulation and adsorption.

Performance enhancement and mechanism of electroenhanced peroxymonosulfate activation by single-atom Fe catalyst modified electrodes

Peroxymonosulfate-based electrochemical advanced oxidation processes (PMS-EAOPs) have great potential for sustainable water purification, so an in-depth understanding of its catalytic mechanism is imperative to facilitate its practical application. Herein, the performance enhancement and mechanism of electroenhanced PMS activation by single-atom Fe catalyst modified carbon felt was investigated. Compared with the anode, the cathode exhibited faster bisphenol A degradation (kcathode = 0.073 vs. kanode = 0.015 min-1), increased PMS consumption (98.8 vs. 10.3%), and an order of magnitude reduction of Fe dissolution (0.068 vs. 0.787 mg L-1). Mass transfer is a key factor limiting PMS activation, while the electrostriction of water in the hydrophobic region caused by cathode electric field (CEF) significantly increased mass transfer coefficient (km, cathode = 1.49 × 10-4 vs. km, anode = 2.68 × 10-5 m s-1). The enhanced activation of PMS is a synergistic result between electroactivation and catalyst-activation, which is controlled by the applied current density. 1O2 and direct electron transfer are the main active species and activation pathway, which achieve high degradation efficiency over pH 3 to 10. Density functional theory calculations prove CEF increases the adsorption energy, lengthens the O-O bond in PMS, and promotes charge transfer. A flow-through convection unit achieves sustainable operation with high removal efficiency (99.5% to 97.5%), low electrical energy consumption (0.15 kWh log-1 m-3), and low Fe leaching (0.81% of the total single atom Fe). This work reveals the critical role of electric fields in modulating Fenton-like catalytic activity, which may advance the development of advanced oxidation processes and other electrocatalytic applications.

An in-depth analysis of membrane distillation research (1990-2023): Exploring trends and future directions through bibliometric approach

This bibliometric analysis offers a comprehensive investigation into membrane distillation (MD) research from 1990 to 2023. Covering 4389 publications, the analysis sheds light on the evolution, trends, and future directions of the field. It delves into authorship patterns, publication trends, prominent journals, and global contributions to reveal collaborative networks, research hotspots, and emerging themes within MD research. The findings demonstrate extensive global participation, with esteemed journals such as Desalination and the Journal of Membrane Science serving as key platforms for disseminating cutting-edge research. The analysis further identifies crucial themes and concepts driving MD research, ranging from membrane properties to strategies for mitigating membrane fouling. Co-occurrence analysis further highlights the interconnectedness of research themes, showcasing advancements in materials, sustainable heating strategies, contaminant treatment, and resource management. Overlay co-occurrence analysis provides temporal perspective on emerging research trends, delineating six key topics that will likely shape the future of MD. These include innovations in materials and surface engineering, sustainable heating strategies, emerging contaminants treatment, sustainable water management, data-driven approaches, and sustainability assessments. Finally, the study serves as a roadmap for researchers and engineers navigating the dynamic landscape of MD research, offering insights into current trends and future trajectories, ultimately aiming to propel MD technology towards enhanced performance, sustainability, and global relevance.

Ultra-antiwetting Membrane for Hypersaline Water Crystallization in Membrane Distillation

Membrane distillation (MD) has great potential in the management of hypersaline water for zero liquid discharge (ZLD) due to its high salinity tolerance. However, the membrane wetting issue significantly restricts its practical application. In this study, a composite membrane tailored for extreme concentrations and even crystallization of hypersaline water is synthesized by coating a commercial hydrophobic porous membrane with a composite film containing a dense polyamide layer, a cation exchange layer (CEL), and an anion exchange layer (AEL). When used in direct contact MD for treating a 100 g L-1 NaCl hypersaline solution, the membrane achieves supersaturation of feed solution and a salt crystal yield of 38.0%, with the permeate concentration at <5 mg L-1. The composite membrane also demonstrates ultrahigh antiwetting stability in 360 h of long-term operation. Moreover, ion diffusion analysis reveals that the ultrahigh wetting resistance of the composite membrane is attributed to the bipolar AEL and CEL that eliminate ion crossover. The literature review elucidates that the composite membrane is superior to state-of-the-art membranes. This study demonstrates the great potential of the composite membrane for direct crystallization of hypersaline water, offering a promising approach to filling the gap between reverse osmosis and conventional thermal desalination processes for ZLD application.

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.

Crosslinking and fluorination reinforced PTFE nanofibrous membrane with excellent amphiphobic performance for low-scaling membrane distillations

Membrane distillation (MD) has emerged as a promising technology for desalination and concentration of hypersaline brine. However, the efficient preparation of a structurally stable and salinity-resistant membrane remains a significant challenge. In this study, an amphiphobic polytetrafluoroethylene nanofibrous membrane (PTFE NFM) with exceptional resistance to scaling has been developed, using an energy-efficient method. This innovative approach avoids the high-temperature sintering treatment, only involving electrospinning with PTFE/PVA emulsion and subsequent low-temperature crosslinking and fluorination. The impact of the PVA and PTFE contents, as well as the crosslinking and subsequent fluorination on the morphology and MD performance of the NFM, were systematically investigated. The optimized PTFE NFM displayed robust amphiphobicity, boasting a water contact angle of 155.2º and an oil contact angle of 132.7º. Moreover, the PTFE NFM exhibited stable steam flux of 52.1 L·m-2·h-1 and 26.7 L·m-2·h-1 when fed with 3.5 wt % and 25.0 wt % NaCl solutions, respectively, and an excellent salt rejection performance (99.99 %, ΔT = 60 °C) in a continuous operation for 24 h, showing exceptional anti-scaling performance. It also exhibited stable anti-wetting and anti-fouling properties against surfactants (sodium dodecyl sulfate) and hydrophobic contaminants (diesel oil). These results underscore the significant potential of the PTFE nanofibrous membrane for practical applications in desalination, especially in hypersaline or polluted aqueous environments.

A Janus membrane with electro-induced multi-affinity interfaces for high-efficiency water purification

Drinking water with micropollutants is a notable environmental concern worldwide. Membrane separation is one of the few methods capable of removing micropollutants from water. However, existing membranes face challenges in the simultaneous and efficient treatment of small-molecular and ionic contaminants because of their limited permselectivity. Here, we propose a high-efficiency water purification method using a low-pressure Janus membrane with electro-induced multi-affinity. By virtue of hydrophobic and electrostatic interactions between the functional interfaces and contaminants, the Janus membrane achieves simultaneous separation of diverse types of organics and heavy metals from water via single-pass filtration, with an approximately 100% removal efficiency, high water flux (>680 liters m-2 hour-1), and 98% lower energy consumption compared with commercial nanofiltration membranes. The electro-induced switching of interfacial affinity enables 100% regeneration of membrane performance; thus, our work paves a sustainable avenue for drinking water purification by regulating the interfacial affinity of membranes.

High Breakthrough Pressure in Hydrogels Enabled Ultrastable Treatment of Hypersaline Wastewaters

Surface effects of low-surface-tension contaminants accumulating at the evaporation surface easily induce wetting in membrane distillation, especially in hypersaline scenarios. Herein, we propose a novel strategy to eliminate the surface effect and redistribute contaminants at the evaporation interface simply by incorporating a layer of hydrogel. The as-fabricated composite membrane exhibits remarkable stability, even when exposed to solution with salt concentration of 5 M and surfactant concentration of 8 mM. Breakthrough pressure of the membrane reaches 20 bar in the presence of surfactants, surpassing commercial hydrophobic membranes by one to two magnitudes. Density functional theory and molecular dynamics simulations reveal the important role of the hydrogel-surfactant interaction in suppressing the surface effect. As a proof of concept, we demonstrate the membrane in stably processing synthetic wastewater containing 144 mg L-1 surfactants, 1 g L-1 mineral oils, and 192 g L-1 NaCl, showing its potential in addressing challenges of hypersaline water treatment.

Molecular insights into the adsorption and penetration of oil droplets on hydrophobic membrane in membrane distillation

Membrane fouling induced by oily substances significantly constrains membrane distillation performance in treating hypersaline oily wastewater. Overcoming this challenge necessitates a heightened fundamental understanding of the oil fouling phenomenon. Herein, the adsorption and penetration mechanism of oil droplets on hydrophobic membranes in membrane distillation process was investigated at the molecular level. Our results demonstrated that the adsorption and penetration of oil droplets were divided into four stages, including the free stage, contact stage, spreading stage, and equilibrium stage. Due to the extensive non-polar surface distribution of the polytetrafluoroethylene (PTFE) membrane (comprising 95.41 %), the interaction between oil molecules and PTFE was primarily governed by van der Waals interaction. Continuous oil droplet membrane fouling model revealed that the new oil droplet molecules preferred to penetrate into membrane pores where oil droplets already existed. The penetration of resin (a component of medium-quality oil droplets) onto PTFE membrane pores required the "pre-paving" of light crude oil. Finally, the ΔE quantitative structure-activity relationships (QSAR) models were developed to evaluate the penetration mechanism of pollutant molecules on the PTFE membrane. This research provides new insights for improving sustainable membrane distillation technologies in treating saline oily wastewater.

Ultra-fast green hydrogen production from municipal wastewater by an integrated forward osmosis-alkaline water electrolysis system

Recent advancements in membrane-assisted seawater electrolysis powered by renewable energy offer a sustainable path to green hydrogen production. However, its large-scale implementation faces challenges due to slow power-to-hydrogen (P2H) conversion rates. Here we report a modular forward osmosis-water splitting (FOWS) system that integrates a thin-film composite FO membrane for water extraction with alkaline water electrolysis (AWE), denoted as FOWSAWE. This system generates high-purity hydrogen directly from wastewater at a rate of 448 Nm3 day-1 m-2 of membrane area, over 14 times faster than the state-of-the-art practice, with specific energy consumption as low as 3.96 kWh Nm-3. The rapid hydrogen production rate results from the utilisation of 1 M potassium hydroxide as a draw solution to extract water from wastewater, and as the electrolyte of AWE to split water and produce hydrogen. The current system enables this through the use of a potassium hydroxide-tolerant and hydrophilic FO membrane. The established water-hydrogen balance model can be applied to design modular FO and AWE units to meet demands at various scales, from households to cities, and from different water sources. The FOWSAWE system is a sustainable and an economical approach for producing hydrogen at a record-high rate directly from wastewater, marking a significant leap in P2H practice.

Pioneering the preparation of porous PIM-1 membranes for enhanced water vapor flow

In this study, porous polymers of intrinsic microporosity (PIM-1) membranes were prepared by non-solvent induced phase inversion (NIPS) and investigated for water vapor transport in view of their application in membrane distillation (MD). Due to the lack of high boiling point solvents for PIM-1 that are also water miscible, the mixture of tetrahydrofuran (THF) and N-methyl-2-pyrrolidone (NMP) was found to be optimal for the formation of a membrane with a developed porous system both on the membrane surface and in the bulk. PIM-1 was synthesized by using low and high temperature methods to observe how molecular weight effects the membrane structure. Low molecular weight PIM-1 was produced at low temperatures, while high molecular weight PIM-1 was obtained at high temperatures. Several membranes were prepared, including PM-6, PM-9, and PM-11 from low molecular weight PIM-1, and PM-13 from high molecular weight PIM-1. Scanning electron microscopy (SEM) was used to image the surface and cross-section of different porous PIM-1 membranes. Among all the PIM-1 membranes (PM) obtained, PM-6, PM-9, PM-11 and PM-13 showed the most developed porous structure, while PM-13 showed large voids in the bulk of the membrane. Contact angle measurements showed that all PIM-1 porous membranes are highly hydrophobic. Liquid water flux measurements showed that PM-6, PM-9 and PM-11 showed minimal water fluxes due to small surface pore size, while PM-13 showed a high water flux due to a large surface pore size. Water vapor transport measurements showed high permeance values for all membranes, demonstrating the applicability of the developed membranes for MD. In addition, a thin film composite (TFC) membrane with PIM-1 selective layer was prepared and investigated for water vapor transport to compare with porous PIM-1 membranes. The TFC membrane showed an approximately 4-fold lower vapor permeance than porous membranes. Based on these results, we postulated that the use of porous PIM-1 membranes could be promising for MD due to their hydrophobic nature and the fact that the porous membranes allow vapor permeability through the membrane but not liquid water. The TFC membrane can be used in cases where the transfer of water-soluble contaminants must be absolutely avoided.

Development of multifunctional membranes via plasma-assisted nonsolvent induced phase separation

Demands on superhydrophobic, self-cleaning and piezoelectric membranes have gained significantly due to their potential to overcome global shortages in clean water and energy. In this study, we have discovered a novel plasma-assisted nonsolvent induced phase separation (PANIPS) method to prepare superhydrophobic, self-cleaning and piezoelectric poly(vinylidene difluoride) (PVDF) membranes without additional chemical modifications or post-treatments. The PANIPS membranes exhibit water contact angles ranging from 151.2° to 166.4° and sliding angles between 6.7° and 29.7°. They also show a high piezoelectric coefficient (d33) of 10.5 pC N-1 and can generate a high output voltage of 10 Vpp. The PANIPS membranes can effectively recover pure water from various waste solutions containing Rose Bengal dye, humic acid, or sodium dodecyl sulfate via direct contact membrane distillation (DCMD). This study may provide valuable insights to fabricate PANIPS membranes and open up new avenues to molecularly design advanced superhydrophobic, self-cleaning, and piezoelectric membranes in the fields of clean water production, motion sensor, and piezoelectric nanogenerator.

Making waves: Magneto-responsive membranes with special and switchable wettability: new opportunities for membrane distillation

Membrane distillation (MD) has promising potential in the water purification and wastewater treatment industries; however, fouling and wetting are the main obstacles to its commercialization, and higher fluxes and energy efficiencies are essential. Magneto-responsive membranes (MagMem) with integrated magnetic nanoparticles (MNPs) enable in situ fouling mitigation and switchable separation by nano-mixing or nano-heating, triggered by external magnetic fields, in a range of membrane processes, but not yet been demonstrated in MD. This perspective discussed the potential paths of MagMem utilization in MD based on the research status and dilemmas of MD. It can be envisioned that MagMem will lead to a paradigm shift in MD, especially by in situ fouling/wetting mitigation and enhancing energy efficiency via in-place actuation and localized heating by MNPs. Moreover, remotely controllable pore tuning and specific or switchable wettability can also be anticipated. Overall, MagMem provides attractive opportunities for advanced robust and efficient MD.

Ambient-mediated wetting on smooth surfaces

A consensus was built in the first half of the 20th century, which was further debated more than 3 decades ago, that the wettability and condensation mechanisms on smooth solid surfaces are modified by the adsorption of organic contaminants present in the environment. Recently, disagreement has formed about this topic once again, as many researchers have overlooked contamination due to its difficulty to eliminate. For example, the intrinsic wettability of rare earth oxides has been reported to be hydrophobic and non-wetting to water. These materials were subsequently shown to display dropwise condensation with steam. Nonetheless, follow on research has demonstrated that the intrinsic wettability of rare earth oxides is hydrophilic and wetting to water, and that a transition to hydrophobicity occurs in a matter of hours-to-days as a consequence of the adsorption of volatile organic compounds from the ambient environment. The adsorption mechanisms, kinetics, and selectivity, of these volatile organic compounds are empirically known to be functions of the substrate material and structure. However, these mechanisms, which govern the surface wettability, remain poorly understood. In this contribution, we introduce current research demonstrating the different intrinsic wettability of metals, rare earth oxides, and other smooth materials, showing that they are intrinsically hydrophilic. Then we provide details on research focusing on the transition from wetting (hydrophilicity) to non-wetting (hydrophobicity) on somooth surfaces due to adsorption of volatile organic compounds. A state-of-the-art figure of merit mapping the wettability of different smooth solid surfaces to ambient exposure as a function of the surface carbon content has also been developed. In addition, we analyse recent works that address these wetting transitions so to shed light on how such processes affect droplet pinning and lateral adhesion. We then conclude with objective perspectives about research on wetting to non-wetting transitions on smooth solid surfaces in an attempt to raise awareness regarding this surface contamination phenomenon within the engineering, interfacial science, and physical chemistry domains.

Application of coal fly ash based ceramic membranes in wastewater treatment: A sustainable alternative to commercial materials

The continued increase in the global population has resulted in increased water demand for domestic, agricultural, and industrial purposes. These activities have led to the generation of high volumes of wastewater, which has an impact on water quality. Consequently, more practical solutions are needed to improve the current wastewater treatment systems. The use of improved ceramic membranes for wastewater treatment holds significant prospects for advancement in water treatment and sanitation. Hence, different studies have employed ceramic membranes in wastewater treatment and the search for low-cost and environmentally friendly starting materials has continued to engender research interests. This review focuses on the application of coal fly ash in membrane technology for wastewater treatment. The processes of membrane fabrication and the various limitations of the material. Several factors that influence the properties and performance of coal fly ash ceramic membranes in wastewater treatment are also presented. Some possible solutions to the limitations are also proposed, while cost analysis of coal fly ash-based membranes is explored to evaluate its potential for large-scale applications.

Water and salt recovery from shale gas produced water by vacuum membrane distillation followed by crystallization

A vacuum membrane distillation (VMD) followed by crystallization (VMD-C) was developed for the recovery of water and salts from shale gas produced water (SGPW). Before VMD, the pretreatment of SGPW with Fenton oxidation-flocculation is applied, with the chemical oxygen demand (COD) concentration reduction of 75% and the total removal of the total suspended solids (TSS), Ca2+, and Mg2+ in SGPW. The pretreatment of SGPW mitigated the membrane fouling in the VMD and effectively prevented the reduction of membrane flux over time. The average flux of the PTFE membrane reached 12.1 kg m-2 h-1 during the separation of the pretreated SGPW at a feed flux of 40 L h-1 and a feed temperature of 40 °C. The rejection rate of the membrane to TDS in SGPW was over 99%. Fresh water with a conductivity of below 20 μs cm-1 was produced by VMD-C. The salts concentrated upstream of the membrane were recovered by a stirring crystallization process. The VMD-C system resulted in a 61% cost savings compared to conventional SGPW treatment.

Mineral scaling induced membrane wetting in membrane distillation for water treatment: Fundamental mechanism and mitigation strategies

The scaling-induced wetting phenomenon seriously affects the application of membrane distillation (MD) technology in hypersaline wastewater treatment. Unlike the large amount of researches on membrane scaling and membrane wetting, scaling-induced wetting is not sufficiently studied. In this work, the current research evolvement of scaling-induced wetting in MD was systematically summarized. Firstly, the theories involving scaling-induced wetting were discussed, including evaluation of scaling potential of specific solutions, classical and non-classical crystal nucleation and growth theories, observation and evolution of scaling-induced processes. Secondly, the primary pretreatment methods for alleviating scaling-induced wetting were discussed in detail, focusing on adding agents composed of coagulation, precipitation, oxidation, adsorption and scale inhibitors, filtration including granular filtration, membrane filtration and mesh filtration and application of external fields including sound, light, heat, electromagnetism, magnetism and aeration. Then, the roles of operation conditions and cleaning conditions in alleviating scaling-induced wetting were evaluated. The main operation parameters included temperature, flow rate, pressure, ultrasound, vibration and aeration, while different types of cleaning reagents, cleaning frequency and a series of assisted cleaning measures were summarized. Finally, the challenges and future needs in the application of nucleation theory to scaling-induced wetting, the speculation, monitoring and mitigation of scaling-induced wetting were proposed.

Membrane Distillation-Crystallization for Sustainable Carbon Utilization and Storage

Anthropogenic greenhouse gas emissions from power plants can be limited using postcombustion carbon dioxide capture by amine-based solvents. However, sustainable strategies for the simultaneous utilization and storage of carbon dioxide are limited. In this study, membrane distillation-crystallization is used to facilitate the controllable production of carbonate minerals directly from carbon dioxide-loaded amine solutions and waste materials such as fly ash residues and waste brines from desalination. To identify the most suitable conditions for carbon mineralization, we vary the membrane type, operating conditions, and system configuration. Feed solutions with 30 wt % monoethanolamine are loaded with 5-15% CO2 and heated to 40-50 °C before being dosed with 0.18 M Ca2+ and Mg2+. Membranes with lower surface energy and greater roughness are found to more rapidly promote mineralization due to up to 20% greater vapor flux. Lower operating temperature improves membrane wetting tolerance by 96.2% but simultaneously reduces crystal growth rate by 48.3%. Sweeping gas membrane distillation demonstrates a 71.6% reduction in the mineralization rate and a marginal improvement (37.5%) on membrane wetting tolerance. Mineral identity and growth characteristics are presented, and the analysis is extended to explore the potential improvements for carbon mineralization as well as the feasibility of future implementation.

Lotus Effect Inspired Hydrophobic Strategy for Stable Zn Metal Anodes

Zn-ion batteries (ZIBs) have long suffered from the unstable Zn metal anode, which faces numerous challenges concerning dendrite growth, corrosion, and hydrogen evolution reaction. The absence of H2 O adsorption control techniques has become a bottleneck for the further development of ZIBs. Using the stearic acid (SA)-modified Cu@Zn (SA-Cu@Zn) anode as an example, this work illustrates how the lotus effect controls the H2 O adsorption energy on the Zn metal anode. In situ integrated Cu nanorods arrays and hydrophobic long-chain alkyl groups are constructed, which provide zincophilic ordered channels and hydrophobic property. Consequently, the SA-Cu@Zn anode exhibits long-term cycling stability over 2000 h and high average Coulombic efficiency (CE) of 99.83% at 1 mA cm-2 for 1 mAh cm-2 , which improves the electrochemical performance of the Zn||V2 O5 full cell. Density functional theory (DFT) calculations combined with water contact angle (CA) measurements demonstrate that the SA-Cu@Zn exhibits larger water CA and weaker H2 O adsorption than Zn. Moreover, the presence of Cu ensures the selective adsorption of Zn on the SA-Cu@Zn anode, well explaining how the excellent reversibility is achieved. This work demonstrates the effectiveness of the lotus effect on controllable H2 O adsorption and Zn deposition mechanism, offering a universal strategy for achieving stable ZIB anodes.

Robust ceramic-based graphene membrane for challenging water treatment with enhanced fouling and scaling resistance

Membrane fouling and scaling are two challenges for efficient treatment of hypersaline wastewater, greatly hindering separation performance and operation stability of desalination membranes. In this work, we report a smooth ceramic-based graphene desalination membrane, exhibiting enhanced anti-fouling and anti-scaling ability and operational performance for efficient treatment of both synthetic and real industrial wastewaters, outperforming polypropylene (PP) membrane. For treatment of hypersaline waters containing organic or inorganic substance, we demonstrate that the graphene membrane exhibits more stable water flux and almost complete salt rejection (>99.9%) during constant operation. Enhanced anti-fouling and desalination performance of graphene membrane could be attributed to the lower attractive interaction force with foulant (-4.65 mJ m-2), lower surface roughness (Ra = 2.2 ± 0.1 nm) and higher affinity with water than PP membrane. Furthermore, an anti-scaling mechanism enabled by graphene membrane is evidenced, with a highlight on the roles of smooth graphene surface with lower roughness, less nucleation sites and lower binding force with scaling crystals. Importantly, even for industrial petrochemical wastewater, such a graphene membrane also exhibits relatively more stable water flux and promising oil and ions rejection during long-term operation, outperforming PP membrane. This study further confirms a promising practical application potential of robust ceramic-based graphene membrane for efficient treatment of more challenging hypersaline wastewater with complicated compositions, which is not feasible by conventional desalination membranes.

Patterned dense Janus membranes with simultaneously robust fouling, wetting and scaling resistance for membrane distillation

Membrane fouling, wetting and scaling are three prominent challenges that severely hinder the practical applications of membrane distillation (MD). Herein, polyamide/polyvinylidene fluoride (PA/PVDF) Janus membrane comprising a hydrophobic PVDF substrate and a patterned dense PA layer by reverse interfacial polymerization (R-IP) was developed. Direct contact MD experiments demonstrated that PA/PVDF Janus membrane could exhibit simultaneously superior resistance towards surfactant-induced wetting, oil-induced fouling and gypsum-induced scaling without compromising flux. Importantly, the size-sieving effect, rather than the breakthrough pressure of the membrane, was revealed as the critical factor that probably endowed its resistance to wetting. Furthermore, a unique possible anti-scaling mechanism was unveiled. The superhydrophilic patterned dense PA layer with strong salt rejection capability not only prevented scale-precursor ions from intruding the substrate but also resulted in the high surface interfacial energy that inhibited the adhesion and growth of gypsum on the membrane surface, while its relatively low surface -COOH density benefited from R-IP process further ensured the membrane with a low scaling propensity. This study shall provide new insights and novel strategies in designing high-performance MD membranes and enable robust applications of MD facing the challenges of membrane fouling, wetting and scaling.

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.

CO2/CH4 separation performance of SiO2/PES composite membrane prepared by gas phase hydrolysis and grafting coating in gas-liquid membrane contactor: A comparative study

The gas-liquid membrane contactor (GLMC) is a new and promising kind of gas separation technique, but still exhibits limitations, especially in membrane performance. In order to solve the above problems, we fabricated and characterized novel OH/SiO2/PES composite membranes using gas phase hydrolysis and graft coating methods, respectively. In the preparation process, whether to use alkali to pretreat the membrane was used as an evaluation index. The CO2/CH4 separation performance was tested using the modified OH/SiO2/PES hollow fiber membrane as the membrane contactor in GLMC. In the experiment, we conducted a single factor experiment with diethanolamine (DEA) as the adsorbent to analyze the effect of the flow rate and concentration of DEA on the separation of CO2/CH4. The collected gas had a CH4 content of 99.92% and a CO2 flux of 10.1059 × 10-3 mol m-2 s-1 while DEA at a concentration of 1 mol/L was flowing at a rate of 16 L/h. The highest separation factor occurred at this moment, which was 833.67. Overall, the CO2/CH4 separation performance in GLMC was enhanced with the use of the fluorinated OH/SiO2/PES composite membrane.

Early monitoring of pore wetting in membrane distillation using ultrasonic time-domain reflectometry (UTDR)

Pore wetting induced by surfactants and salt scaling is a major obstacle to the industrial application of membrane distillation (MD). Identifying the transition of wetting stages and achieving early monitoring of pore wetting are crucial for wetting control. Herein, we made a pioneering attempt to use ultrasonic time-domain reflectometry (UTDR) technique to non-invasively detect the pore wetting in a direct contact MD, and explain the UTDR waveform with the help of optical coherence tomography (OCT) imaging. The results showed that the water-vapor interface had a strong reflection to ultrasound (reflection coefficient = 0.9995), while the water-membrane and water-scaling layer interfaces showed relatively weak reflection. Therefore, UTDR could effectively detect the movement of water-vapor interface with the low interference from the signals generated by the membrane and scaling layer. For the surfactant-induced wetting, the occurrence of wetting could be successfully detected by the right-shift in phase and the reduction in amplitude of the UTDR waveform. Moreover, the wetting depth could be accurately calculated by the time of flight (ToF) and ultrasonic velocity. For scaling-induced wetting, the waveform slightly shifted to the left at the beginning due to the growth of scaling layer, then to the right because the left-shift was surpassed by the right-shift of the waveform caused by pore wetting. Both for the surfactant- and scaling-induced wetting, the variation of the UTDR waveform was sensitive to wetting dynamics, and the right-shift of phase and the reduction in amplitude of the waveform could act as early monitoring signals to the occurrence of wetting.

Virus rejection and removal in pilot-scale air-gap membrane distillation

Membrane distillation (MD) is a thermally-driven process that can treat high concentration streams and provide a dual barrier for rejection and reduction of pathogens. Thus, MD has potential applications in treating concentrated wastewater brines for enhancing water recovery and potable water reuse. In bench-scale studies, it was demonstrated that MD can provide high rejection of MS2 and PhiX174 bacteriophage viruses, and when operating at temperatures greater than 55 °C, can reduce virus levels in the concentrate. However, bench-scale MD results cannot directly be used to predict pilot-scale contaminant rejection and removal of viruses because of the lower water flux and higher transmembrane hydraulic pressure difference in pilot-scale systems. Thus far, virus rejection and removal have not been quantified in pilot-scale MD systems. In this work, the rejection of MS2 and PhiX174 at low (40 °C) and high (70 °C) inlet temperatures is quantified in a pilot-scale air-gap MD system using tertiary treated wastewater. Both viruses were detected in the distillate which suggests the presence of pore flow; the virus rejection at a hot inlet temperature of 40 °C for MS2 and PhiX174 were 1.6-log₁₀ and 3.1-log₁₀, respectively. At 70 °C, virus concentrations in the brine decreased and were below the detection limit (1 PFU per 100 mL) after 4.5 h, however, viruses were also detected in the distillate in that duration. Results demonstrate that virus rejection is lower in pilot-scale experiments because of increased pore flow that is not captured in bench-scale experiments.

Mapping the research on desulfurization wastewater: Insights from a bibliometric review (1991-2021)

Desulfurization wastewater in coal-fired power plants (CFPPs) is a great environmental challenge. This study aimed at the current status and future research trends of desulfurization wastewater by bibliometric analysis. The desulfurization wastewater featured with high sulfate (8000 mg/L), chlorite (8505 mg/L), magnesium (2882 mg/L) and calcium (969 mg/L) but low sodium (801.82 mg/L), and the concentrations of the main contaminants were critically summarized. There was an increasing trend in the annual publications of desulfurization wastewater in the period from 1991 to 2021, with an average growth rate of 15%. Water Science and Technology, Desalination and Water Treatment, Energy & Fuels, Chemosphere, and Journal of Hazardous Materials are the top 5 journals in this field. China was the most productive country (58.3% of global output) and the core country in the international cooperation network. Wordcloud analysis and keyword topic trend demonstrated that removal/treatment of pollutants dominated the global research in the field of desulfurization wastewater. The primary technologies for desulfurization wastewater treatment were systematically evaluated. The physicochemical treatment technologies occupied half of the total treatment methods, while membrane-based integrated processes showed potential applications for beneficial reuse. The challenges and outlook on desulfurization wastewater treatment for achieving zero liquid discharge are summarized.

Insights into the Superior Bioavailability of Biogenic Sulfur from the View of Its Unique Properties: The Key Role of Trace Organic Substances

Elemental sulfur (S0) is widely utilized in environmental pollution control, while its low bioavailability has become a bottleneck for S0-based biotechnologies. Biogenic sulfur (bio-S0) has been demonstrated to have superior bioavailability, while little is known about its mechanisms thus far. This study investigated the bioavailability and relevant properties of bio-S0 based on the denitrifying activity of Thiobacillus denitrificans with chemical sulfur (chem-S0) as the control. It was found that the conversion rate and removal efficiency of nitrate in the bio-S0 system were 2.23 and 2.04 times those of the chem-S0 system. Bio-S0 was not pure orthorhombic sulfur [S: 96.88 ± 0.25% (w/w)]. Trace organic substances detected on the bio-S0 surface were revealed to contribute to its hydrophilicity, resulting in better dispersibility in the aqueous liquid. In addition, the adhesion force of T. denitrificans on bio-S0 was 1.54 times that of chem-S0, endowing a higher bacterial adhesion efficiency on the sulfur particle. The weaker intermolecular binding force due to the low crystallinity of bio-S0 led to enhanced cellular uptake by attached bacteria. The mechanisms for the superior bioavailability of bio-S0 were further proposed. This study provides a comprehensive view of the superior bioavailability of bio-S0 and is beneficial to developing high-quality sulfur resources.

Electrospun Silica-Polyacrylonitrile Nanohybrids for Water Treatments

In this work, the removal of NOM (natural organic matter) as represented by humic acid by means of electrospun nanofiber adsorptive membranes (ENAMs) is described. Polyacrylonitrile (PAN) was used for the preparation of ENAMs incorporating silica nanoparticles as adsorbents. The addition of silica to the polymer left visible changes on the structural morphology and fibers' properties of the membrane. The membrane samples were characterized by pure water permeability, contact angle measurement, SEM, XPS, and XRD. This study assesses the preliminary performance of PAN-Si membranes for the removal of natural organic matter (NOM). The membrane rejected the humic acid, a surrogate of NOM, from 69.57% to 87.5%.

Fouling and chemically enhanced backwashing performance of low-pressure membranes during the treatment of shale gas produced water

The treatment of shale gas produced water (SGPW) for beneficial reuse is currently the most dominant and economical option. Membrane filtration is one preferred method to deal with SGPW, but membrane fouling is an unavoidable problem. In this study, two types of ultrafiltration (UF) membranes and one type of microfiltration (MF) membrane were investigated to treat SGPW from Sichuan basin. Results showed that increased total dissolved solid (31-40 g/L) and UV₂₅₄ (10-42.9 m^-1) were observed for the same shale gas plays, and the primary fluorescent organic substances were humic acid-like components. Compared to UF membranes with the flux decline by 2% to 60%, MF membranes with larger pore size were more likely to be fouled with the flux decline by 43% to 95%. Cake layer filtration was verified to be the primary membrane fouling mechanism. Statistical analysis showed that UV₂₅₄ played the most significant role in membrane fouling which had the highest correlation (0.76 to 0.93). Compared to permeate backwashing (13%), deionized water backwashing and chemically enhanced backwashing (CEB) using NaClO, H₂O₂ and citric acid improved the cleaning efficiencies (31%-95%). CEB using NaOH prepared by deionized water aggravated membrane fouling, while excellent cleaning efficiencies (39%-79%) were observed for CEB using NaOH prepared by permeate. The difference in cleaning behaviors for fouled membranes by SGPW was verified by morphology observation and element composition analysis.