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Recent years, membrane separation technology has attracted significant research attention because of the efficient and environmentally friendly operation. The selection of suitable materials to improve the membrane selectivity, permeability and other properties has become a topic of vital research relevance. Two-dimensional (2D) materials, a novel family of multifunctional materials, are widely used in membrane separation due to their unique structure and properties. In this respect, as a novel 2D material, graphitic carbon nitride (g-C3N4) have found specific attention in membrane separation. This study reviews the application of carbon nitride in gas separation membranes, pervaporation membranes, nanofiltration membranes, reverse osmosis membranes, ion exchange membranes and catalytic membranes, along with describing the separation mechanisms. © 2020 The Chemical Industry and Engineering Society of China, and Chemical Industry Press Co., Ltd.. All rights reserved.

Membranes with asymmetric wettability have attracted significant interest by virtue of their unique transport characteristics and functionalities arising from different wetting behaviors of each membrane surface. The cross-sectional wettability distinction enables a membrane to realize directional liquid transport or multifunction integration, resulting in rapid advance in applications, such as moisture management, fog collection, oil-water separation, and membrane distillation. Compared with traditional homogeneous membranes, these membranes possess enhanced transport performance and higher separation efficiency owing to the synergistic or individual effects of asymmetric wettability. This Review covers the recent progress in fabrication, transport mechanisms, and applications of electrospun membranes with asymmetric wettability and provides a perspective on future development in this important area.

Two-dimensional (2D) graphitic carbon nitride (g-C3N4) nanosheets are widely used in photocatalysis, energy storage and membrane separation owing to their high specific surface area, rapid photogenic carrier migration rate and numerous active sites. In this study, high performance g-C3N4 nanosheets with a particle size of about 290 nm were prepared by gas exfoliation. It was observed that the structure and properties of g-C3N4 nanosheets prepared at 500 °C were superior as compared to 300 °C and 400 °C. The state of g-C3N4 layers at different temperatures was also studied by molecular dynamics simulation. It was revealed that the liquid nitrogen (L-N2) molecules could only penetrate the edge space between g-C3N4 layers at 500 °C, however, no penetration took place in the layers. The traditional gas exfoliation mechanisms were observed to be incapable in explaining this phenomenon. Combining the experimental studies and molecular dynamics simulations, a novel gas exfoliation mechanism was proposed in this study so as to prepare few-layered g-C3N4 nanosheets. Based on this, the L-N2 molecules entered the edge space in g-C3N4 layers and subsequently pushed deeper to exfoliate the nanosheets.

The fabrication of porous silicon carbide (SiC) ceramics prepared by existing reaction bonding technology requires high-temperature sintering beyond 1400 °C. We used low-cost industrial grade water glass and zirconia as sintering aids to prepare porous SiC ceramic filters to reduce the sintering temperature while improving the commerciality. Additionally, we investigated the effects of different sintering temperatures and the addition of water glass on the samples. Loading water glass and zirconia reduced the sintering temperature to 600 °C. The additive-loaded ceramic filter sintered at 600 °C exhibited an average pore size of 0.58 μm, a porosity of 47.4%, a bending strength reaching 56 MPa, a pure water permeance exceeding 4800 L m−2h−1bar−1, and good acid corrosion resistance. In addition, the SiC filter prepared at 1000 °C showed good chemical resistance and an average pore size of 0.75 μm as well as mechanical strength and pure water permeance of over 80 MPa and 6294 L m−2h−1bar−1, respectively. The low-cost, high-performance porous SiC ceramic filter exhibits broad commercial application prospects in water treatment.

Carbon nanotubes (CNTs) with high degrees of uniformity, orientation and controlled dimensions on porous supports are highly desirable for various applications such as separation of O/W emulsions and air purification. In this work, CNTs were fabricated on silicon carbide (SiC) porous supports with different porosities and pore sizes by chemical vapor deposition (CVD). The growth processes of CNTs on the surface and in the pore channels of the SiC support were studied in detail. Based on microstructural characterization by SEM, Raman spectroscopy and TEM, it was found that these CNTs grown in the pore channels of SiC supports had a higher degree of orientation and purity than those grown on the surface due to the spatially confined effect. The growth processes of various types of CNTs on the microporous supports were proposed, which were further verified by CNTs with different steric configurations (S-CNTs and VACNTs) and on Al

Addressing respiratory infectious diseases remains one of the main priorities due to the increased risk of exposure caused by population growth, increasing international travel and commerce, and most recently, the COVID-19 outbreak. In the war against respiratory diseases, facemasks are powerful tools to obstruct the penetration of microorganisms, thereby protecting the wearer from infections. Nonetheless, the intercepted microorganisms on the surface of facemasks may proliferate and lead to secondary infection. To solve this problem, atomic layer deposition is introduced to deposit uniform and mechanically robust ZnO layers on polypropylene (PP) nonwoven fabrics, a widely used raw material in fabricating facemasks. The loading of ZnO demonstrates no adverse effects on the separation performance of facemasks, and the filtration efficiency of the facemasks towards different types of nanoparticles remains higher than 98.9%. Moreover, the modified PP nonwoven fabrics are granted with excellent antibacterial activity and photocatalytic sterilization ability, which can inactivate both germ-negative and germ-positive bacteria (E. coli and S. aureus) effectively with and without light illumination. Therefore, the modified PP nonwoven fabrics are potential candidates to be used as the outer layer on facemasks and endow them with photocatalytic antibacterial activity.

Severe fouling to poly(vinylidene fluoride) (PVDF) membrane is usually caused as filtrating the papermaking wastewater in the ultrafiltration (UF) process. In the paper, fouling behavior and mechanism were investigated, and the low-concentration polyvinyl alcohol (PVA) contained in the sedimentation tank wastewater was found as the main foulant. Consequently, the corresponding cleaning approach was proposed. The experiment and modeling results elaborated that the fouling mode developed from pore blockage to cake layer along with filtration time. Chemical cleaning conditions including the composition and concentration of reagents, cleaning duration and trans-membrane pressure were investigated for their effect on cleaning efficiency. Pure water flux was recovered by over 95% after cleaning the PVDF membrane using the optimal conditions 0.5 wt% NaClO (as oxidant) and 0.1 wt% sodium dodecyl benzene sulfonate (SDBS, as surfactant) at 0.04 MPa for 100 min. In the chemical cleaning method, hypochlorite (ClO−) could first chain-scissor PVA macromolecules to small molecules and SDBS could wrap the fragments in micelles, so that the foulants were removed from the pores and surface of membrane. After eight cycling tests, pure water flux recovery maintained above 95% and the reused membrane was found intact without defects.

Airborne fine particles have severe effects on abating the duration and efficiency of filters due to their high penetration probability. One-dimensional (1D) nanomaterials-constructed filters show ...

Membrane Fenton-like process with cobalt–copper spinel catalyst is proposed to degrade nitrogenous organic hazardous in wastewater. Cu-auxiliary cobalt oxide (CuxCo3–xO4) was synthesized with the dispersion co-precipitation method. As-prepared CuCo2O4 performed as the best catalyst for Fenton degradation on nitrogen-containing organic compound as methylene blue (C16H18N3ClS) in water and N,N-dimethylacetamide [CH3CON(CH3)2] in industrial wastewater. Lattice substation of Cu2+ to Co2+ ion increased the oxygen vacancies and catalytic activity. The catalyst stability was improved owing to hybrid polarization confinement (total metal elution less than 0.8 mg/L). Methylene blue in water (C0 = 100 mg/L) was completely decolorized in 50 min using 0.6 g/L CuCo2O4 and 0.09 M H2O2 at intrinsic pH 6.7. Degradation reaction rate constant was increased by two folds as k = 0.076 min−1 catalyzed by CuCo2O4 over k = 0.039 min−1 by Co3O4. DMAc-contaminant wastewater of high CODs 19,080 mg/L was treated using 2.4 g/L CuCo2O4 and 2.4 M H2O2 at intrinsic pH = 7.6. Removal rate of DMAc was enhanced from 81 to 99% in the ceramic membrane reactor when H2O2 was activated, and self-sweeping effect was suppressed. As a result, Total Organic Nitrogen (2900 mg/L) in the wastewater was mineralized by 99.9% with a fast transformation to inorganic NO3− ion. Biodegradability of the resistant organic wastewater was remarkably improved, and BOD/COD was increased by 18 folds.

A multifunctional air filtration membrane is urgently required for high efficiency removal of particulate matter (PM) and capture of airborne toxic gases (SO2, NOx, VOCs, etc.). Zeolite imidazole f...

Treatment to crystallization mother liquor containing high concentration of organic and inorganic substances is a challenge in zero liquid discharge of industrial wastewater. Acid precipitation coupled membrane-dispersion advanced oxidation process (MAOP) was proposed for organics degradation before salt crystallization by evaporation. With acid-MAOP treatment CODCr in mother liquor of pulping wastewater was eliminated by 55.2% from ultrahigh initial concentration up to 12,500 mg·L−1. The decolorization rate was 96.5%. Recovered salt was mainly NaCl (83.3 wt%) having whiteness 50 brighter than industrial baysalt of whiteness 45. The oxidation conditions were optimized as CO3 = 0.11 g·L−1 and CH2O2 = 2.0 g·L−1 with dispersing rate 0.53 mL·min−1 for 100 min reaction toward acidified liquor of pH = 2. Acidification has notably improved evaporation efficiency during crystallization. Addition of H2O2 made through membrane dispersion has eliminated hydroxyl radical “quench effect” and enhanced the degradation capacity, in particular, the breakage of carbon-chloride bonds (of both aliphatic and aromatic). As a result, the proposed coupling method has improved organic pollutants reduction so as the purity of salt from the wastewater mixture which can facilitate water and salt recycling in industry.

Low-temperature selective catalytic reduction of NOx combined with dust removal technique due to its energy conservation characteristic has been attracted much attention for fume purification. In this work, the MnOx wrapped PTFE membrane with efficient dust removal and low-temperature NH3-SCR has been prepared with a facile route. MnOx with different crystal structures was uniformly grown around the PTFE fibrils through water bath. The flower-sphere-like MnOx@PTFE(O-MnOx@PTFE) and lamellar-interlaced ripple-like MnOx@PTFE(W-MnOx@PTFE) have large specific surface area which is favorable for enhancing catalytic performance. Also, the uniformly wrapped W-MnOx around the PTFE fibrils optimized the pore structure for ultrafine dust capture. The membrane can almost 100% reject particles that are smaller than 1.0 μm with a low filtration resistance. Meanwhile, W-MnOx@PTFE with more surface chemisorbed oxygen has the best NO conversion efficiency of 100% at a comparatively low and wide activity temperature window of 160–210 °C, which is far to the thermal limitation of the PTFE. Therefore, this efficient and energy conserving membrane has a bright application prospects for tail gas treatment compared to the original treatment process.

The collaborative removal of dust-containing toxic gas caused by emission of industrial fumes is an intractable problem, and catalytic membranes that combine catalysis and separation are an effecti...

Inefficient separation of inorganic salts and organic matters in crystallization mother liquor is still a problem to industrial wastewater treatment since the high salinity significantly impedes organic pollutant degradation by oxidation or incineration. In the study, acidification combined electrodialysis (ED) was attempted to effectively separate Cl− ions from organics in concentrate pulping wastewater. Membrane's rejection rate to total organic carbon (TOC) was 85% at wastewater intrinsic pH = 9.8 and enhanced to 93% by acidifying it to pH = 2 in ED process. Negative-charged alkaline organic compounds (mainly lignin) could be liberated from their sodium salt forms and coagulated in acidification pretreatment. Neutralization of the organic substances also made their electro-migration less effective under electric driving force and in particular improved separation efficiency of chloride and organics. After acid-ED coupled treatment (pH = 2 and J = 40 mA·cm−2) [TOC] remarkably reduced from 1.315 g·L−1 to 0.048 g·L−1 and [Cl−] accumulated to 130 g·L−1 in concentrate solution. Recovery rate of NaCl was 89% and the power consumption was 0.38 kW·h·kg−1 NaCl. Irreversible fouling was not caused as electric resistance of membrane pile maintained stably. In conclusion, acidic-ED is a practical option to treat salinity organic wastewater when current techniques including thermal evaporation and pressure-driven membrane separation present limitations.

Air pollution is harmful to human health. Membrane technology has gradually become the mainstream method for air purification because it is efficient, environmentally friendly, and energy-saving. Membranes can not only efficiently remove particulate matter (PM) through surface filtration but can also realize the synchronous removal of other gaseous pollutants such as volatile organic compounds (VOCs), NOx, and SO2. In this chapter, first, the source, types, and hazards of air pollutants are briefly introduced. Second, membranes suitable for air purification, such as silicon carbide (SiC) ceramic membranes, biaxial stretching polytetrafluoroethylene (PTFE) membranes, and electrospun nanofiber membranes, are introduced in detail. Finally, the application and filtration mechanism of membranes for PM purification are described, and the membranes for the integration of the separation of PM with the removal of VOCs, desulfurization, denitration, and antibiosis are summarized. The integrated purification of multi-component air pollutants is the development direction of functional membrane materials for air purification.

Airborne transmission of pathogens is the most probable cause for the spread of respiratory diseases, which can be intercepted by personal protective equipment such as masks. In this study, an efficient antiviral personal protective filter was fabricated by coupling the biocompatible curcumin (CCM) with nanofibrous polytetrafluoroethylene (PTFE) membrane. The CCM extracted from plants was first dissolved in acidified ethanol at a certain pH and temperature to optimize its loading concentration, antiviral activation, and binding forces on the polyethylene terephthalate (PET) support to form a pre-filtration layer at the front section of the filter. Ultrathin PTFE membrane was then fabricated on the antibacterial-antiviral PET support (A-A PET) by controllable heating lamination. This functional layer of the filter exhibits good gas permeance (3423.6 m

The strategy of constructing catalytic membrane has a significant influence on its structure and performance. In this work, Co3O4-Cx@SiO2 nanofiber membranes (NFMs) were fabricated by an in-situ growth–pyrolysis–oxidation strategy. The Co3O4-Cx catalyst derived from ZIF-67 was wrapped around nanofibers, which helps to maintain a stable membrane structure, then suppressing the reduction of gas permeability. Among the Co3O4-Cx catalyst, the carbon skeleton can prevent the agglomeration of Co3O4 nanoparticles, obtaining an ultra-fine Co3O4 nanoparticles with high dispersibility, redox property and surface area. The obtained Co3O4-C300@SiO2 NFM exhibits excellent filtration efficiency and low pressure drop for PM2.5 (99.99% and 55 Pa) and outstanding catalytic performance with T90 of 245 °C for NH3-SCR, which is 40.3% higher than that of Co3O4@SiO2 NFM. This work might provide a universal strategy for the preparation of catalytic membrane with high-performance.

A surfactant-free approach is proposed to fabricate various-shaped Cu2O particles with good uniformity by ceramic membrane dispersion. Glucose reduced cupric salt and precipitated Cu2O (dimension ca. 1 μm and crystal size 18.5 nm) whose homogeneity was distinctly improved by distributing glucose solution through the ceramic mesoporous membrane. Particle shape including cube, sphere-like, octahedron and eight-pod was controlled kinetically by changing the concentration of sodium carbonate and sodium citrate with constant copper concentration. Morphologic transformation mechanism was studied with diffraction intensity ratio of (111)/(100) crystal planes. Micro-cubes formed at low pH and octahedral ones formed at high pH since crystal facet (111) is more stable than (100) with increased alkalinity. At the same pH, eight-pod grains were formed instead of cubes by adding citrate anions. Cu2O powders in octahedral shape was of the highest catalytic activity to degrade the organic pollutants in a heterogeneous Fenton reaction. CODs in an industrial pulp wastewater was eliminated from initial value 133 mg/L as low as to 26 mg/L. Reduction rate of CODs in water by the octahedral Cu2O was 80% while the commercial Cu2O was only 30%.

Both the pore size and surface properties of silicon carbide (SiC) membranes are demonstrated to significantly affect their separation efficiency when used for oily water treatment. However, the potential influences of open porosity together with the pore size of SiC membranes on their surface properties and oil-water separation performance have rarely been investigated. In this work, porous SiC ceramic membranes with tunable open porosity and pore size were purposely prepared and selected to systematically study the effect of pore structure-dependent wettability on the oil-water separation performance. The measured pure water flux of selected membranes as a function of open porosity (34-48%) and pore size (0.43-0.67 μm) was well-fitted by using a modified H-P equation. Interestingly, the hydrophilicity of SiC membranes was improved with the increase in open porosity and pore size, as evidenced by the gradually decreased dynamic water contact angle and underwater adhesion of oil droplets. Further, the open porosity of SiC membranes was found to contribute more to the improved surface wettability. As a result, the stable flux of SiC membranes in oil-in-water (O/W) emulsions was increased by 24% with the increased open porosity while the oil rejection rate remained above 90%. This work quantitatively reveals the contributions of the pore structure to the surface wettability of ceramic membranes, and thus provides an effective pathway to improve their performance in oil-water separation.