Graphene-based nanocomposites have a vast potential for wide-ranging antibacterial applications due to the inherently strong biocidal activity and versatile compatibility of such nanocomposites. Therefore, graphene-based functional nanomaterials can introduce enhanced antibiofouling and antimicrobial properties to polymeric membrane surfaces. In this study, reduced graphene oxide–copper (rGOC) nanocomposites were synthesized as newly robust biocides via in situ reduction. Inspired by the emerging method of bridging ultrafiltration membrane surface cavities, loose nanofiltration (NF) membranes were designed using a rapid (2 h) bioinspired strategy in which rGOC nanocomposites were firmly codeposited with polydopamine (PDA) onto an ultrafiltration support. A series of analyses (SEM, EDS, XRD, XPS, TEM, and AFM) confirmed the successful synthesis of the rGO–Cu nanocomposites. The secure loading of rGOC composites onto the membrane surfaces was also confirmed by SEM and AFM images. Water contact angle results display a high surface hydrophilicity of the modified membranes. The PDA-rGOC functionalization layer facilitated a high water permeability (22.8 L m–2 h–1 bar–1). The PDA-rGOC modification additionally furnished the membrane with superior separation properties advantageous for various NF applications such as dye purification or desalination, as ultrahigh (99.4% for 0.5 g L–1 reactive blue 2) dye retention and high salt permeation (7.4% for 1.0 g L–1 Na2SO4, 2.5% for 1.0 g L–1 NaCl) was achieved by the PDA-rGOC-modified membranes. Furthermore, after 3 h of contact with Escherichia coli (E. coli) bacteria, the rGOC-functionalized membranes exhibited a strong antibacterial performance with a 97.9% reduction in the number of live E. coli. This study highlights the use of rGOC composites for devising loose NF membranes with strong antibacterial and separation performance.Keywords: antibacterial; fast deposition; loose nanofiltration; polydopamine; reduced graphene oxide−copper; water treatment;

Metal–organic frameworks (MOFs) are studied for the design of advanced nanocomposite membranes, primarily due to their ultrahigh surface area, regular and highly tunable pore structures, and favorable polymer affinity. However, the development of engineered MOF-based membranes for water treatment lags behind. Here, thin-film nanocomposite (TFN) membranes containing poly(sodium 4-styrenesulfonate) (PSS) modified ZIF-8 (mZIF) in a polyamide (PA) layer were constructed via a facile interfacial polymerization (IP) method. The modified hydrophilic mZIF nanoparticles were evenly dispersed into an aqueous solution comprising piperazine (PIP) monomers, followed by polymerizing with trimesoyl chloride (TMC) to form a composite PA film. FT-IR spectroscopy and XPS analyses confirm the presence of mZIF nanoparticles on the top layer of the membranes. SEM and AFM images evince a retiform morphology of the TFN-mZIF membrane surface, which is intimately linked to the hydrophilicity and adsorption capacity of mZIF nanoparticles. Furthermore, the effect of different ZIF-8 loadings on the overall membrane performance was studied. Introducing the hydrophilizing mZIF nanoparticles not only furnishes the PA layer with a better surface hydrophilicity and more negative charge but also more than doubles the original water permeability, while maintaining a high retention of Na2SO4. The ultrahigh retentions of reactive dyes (e.g., reactive black 5 and reactive blue 2, >99.0%) for mZIF-functionalized PA membranes ensure their superior nanofiltration performance. This facile, cost-effective strategy will provide a useful guideline to integrate with other modified hydrophilic MOFs to design nanofiltration for water treatment.Keywords: hydrophilizing; interfacial polymerization; MOF modification; nanofiltration; water treatment; ZIF-8;

In this work, carbonic anhydrase (CA) molecules were embedded into metal–organic frameworks (MOFs) via physical absorption and chemical bonds, which could overcome the enzymatic inactivation and the poor separation property of pristine MOF materials. And then, these nanocomposites (enzyme-embedded MOFs) as the crystal seeds were in situ grown on oriented halloysite nanotube layers to develop novel biocatalytic composite membranes. These membranes exhibited optimal separation performance with a CO2/N2 selectivity of 165.5, about 20.9 fold higher than that of the membrane without embedded CA molecules, surpassing the Robeson upper bound (2008). At the same time, the CO2 permeance increased about 3.2 fold (from 7.6 GPU to 24.16 GPU). Importantly, the biocatalytic composite membranes showed good stability and mechanical properties and were easily scalable, which could be extended to industrial applications.

•A novel and facile strategy was developed to prepared ultrathin antibacterial separation membranes.•First to directly immobilize lysozyme using interfacial polymerization.•The lysozyme membranes demonstrated a remarkable permselectivity for salts and dyes.•The lysozyme membranes exhibited sufficient antibacterial activity.This work is focused on the application of a novel antibacterial nanofiltration (NF) membrane formed by interfacial polymerization on a polyethersulfone supporting membrane for dye removal. Lysozyme, a ubiquitous and cheap enzyme with antibacterial activity was deployed as an aqueous monomer to react with 1,3,5-benzenetricarbonyl trichloride (TMC) to construct separation membranes. The formation of an ultrathin lysozyme-polymer active layer was verified by ATR-FTIR and FESEM. The degree of crosslinking, controlled by the concentration of lysozyme and TMC, had a significant influence on the physicochemical properties of the resultant membrane, as well as its separation performance. The optimum membranes show a high water flux (58.04 L m−2 h−1), distinguished rejection for low molecular weight reactive dyes (600–800 Da, >98.0%) and a high permeation of salts (>95%). Furthermore, the membranes with higher lysozyme concentration exhibited a sufficient antibacterial activity (81.9%) for E. coli bacteria. This facile strategy of enzyme immobilization not only allows for an in-situ preparation of enzyme-polymer membranes, but also maintains the native enzyme activity despite the high degree of covalent bonding between proteins. The resultant lysozyme membranes prove their potential in dyes removal, while also corroborating the value of interfacial polymerization in the field of enzyme immobilization and protein-polymer film construction.Download high-res image (205KB)Download full-size image

•Montmorillonite (MMT) and Mg-Al hydrotalcite (HT) nanosheets were successfully prepared.•Thin-film composite membranes were developed using MMT and HT nanosheets as nanofillers.•The combination of nanosheets with organic membrane has a great potential in gas separation application.Montmorillonite (MMT) and Mg-Al hydrotalcite (HT) nanosheets were prepared via vigorous agitation and ultrasonic, respectively. In order to increase the permeability of CO2, poly (PEA-MMT-TMC)/PS and poly (PEA-HT-TMC)/PS composite membranes were prepared via interfacial polymerization by adding the dimensional (2D) inorganic nanosheets (MMT and HT) into the aqueous phase of PEA. And in consequence, the poly (PEA-MMT-TMC)/PS composite membrane showed CO2 permeability of 15.87 barrer and the CO2/N2 selectivity of 37 at 1.0 bar when the MMT concentration was 0.068 wt%. The poly (PEA-HT-TMC)/PS composite membrane also showed CO2 permeability of 15.3 barrer and the CO2/N2 selectivity of 40 at 1.0 bar when the HT concentration was 0.25 wt%. Compared with the controlled membrane (CO2 permeability: 6.9 barrer, CO2/N2 selectivity: 103), the CO2 permeability increased after incorporating the inorganic nanosheets into the membranes and maintained the pretty CO2/N2 selectivity. The addition of exfoliated MMT and HT could facilitate the gas permeation to improve the gas separation performance of the composite membranes. Also, the combination of inorganic nanosheets with organic membrane has a great potential application in the gas separation.Download high-res image (120KB)Download full-size image

•TiO2@GO nanocomposites with dilated nanochannels were synthesized using in-situ growth method.•TiO2@GO nanocomposites were incorporated to fabricate polyamide nanofiltration membranes for the first time.•The effects of the embedded TiO2@GO nanocomposites were systematically investigated.•TiO2@GO membranes showed elevated nanofiltration performance and antifouling property.Two-dimensional (2D) graphene-based nanomaterials of atomic thickness have opened a new era for fabricating membranes with outstanding performance. In this work, a novel graphene oxide (GO) based thin film nanocomposite membrane for nanofiltration (NF) was constructed. Taking advantage of the nanochannels between graphene oxide, TiO2 nanoparticles were introduced between these GO nanosheets to form a TiO2@GO nanocomposite with dilated and stable nanochannels. The TiO2@GO incorporated membranes was prepared by interfacial polymerization of piperazine (PIP) and trimesoyl chloride (TMC) monomers and embedding TiO2@GO nanocomposite in its polyamide layer. The effects of the embedded nanoparticles on the physicochemical properties of the prepared membranes, and on the NF membrane performance were investigated. The superior performance of the TiO2@GO incorporated membranes was observed in the case of 0.2 wt% TiO2@GO with water flux of 22.43 L m−2 h−1 at 0.4 MPa and Na2SO4 rejection of 98.8%. This represents an enhancement in permeate flux by a factor 2 compared to a pristine membrane, and 5 times higher than the GO modified membrane, only with a slight compromise in the solute rejection. In addition, the introduction of the TiO2@GO endows the modified TFN membranes with an improved antifouling effect.Download high-res image (295KB)Download full-size image

•Porous graphene as inorganic nanofiller was used to fabricate thin film nanocomposite membranes.•The permeability and selectivity of membranes have been improved after adding porous graphene nanosheets.•This work could provide a potential approach for the thin film nanocomposite membrane for gas separation.The inherent defects of porous graphene (PG) formed during reduction etching process could serve as nanopores, making PG emerge a potential application for the preparation of micrometre-sized separation membranes. Here, we introduced PG as inorganic nanofiller to fabricate thin film nanocomposite (TFN) membranes for CO2 capture via interfacial polymerization technique. The PG selective nanolayers not only possessed a good adhesion with polymers but also benefited from hydrogen bonding actions, simultaneously, thus ensuring the formation of high-efficiency molecular sieving passageway in the separation layer of membranes. Furthermore, the thin PG nanosheets were verified to have an significantly affect for permeability and selectivity of membranes (PG, 0.05 wt%, 1 bar), with exhibiting about 21% and 20.8% enhancement of the CO2 permeance and the CO2/N2 selectively compared to that of the membrane without PG separately. Simultaneously, the membrane also showed higher stability and the porous surface morphology of PG shortened greatly the gas transfer path. The approach offers a potential promising to exploit the ultra-thin film composite membrane for efficient gas separation.Download high-res image (245KB)Download full-size image

The precise and rapid separation of different molecules from aqueous, organic solutions and gas mixtures is critical to many technologies in the context of resource-saving and sustainable development. The strength of membrane-based technologies is well recognized and they are extensively applied as cost-effective, highly efficient separation techniques. Currently, empirical-based approaches, lacking an accurate nanoscale control, are used to prepare the most advanced membranes. In contrast, nanoscale control renders the membrane molecular specificity (sub-2 nm) necessary for efficient and rapid molecular separation. Therefore, as a growing trend in membrane technology, the field of nanoscale tailor-made membranes is highlighted in this review. An in-depth analysis of the latest advances in tailor-made membranes for precise and rapid molecule sieving is given, along with an outlook to future perspectives of such membranes. Special attention is paid to the established processing strategies, as well as the application of molecular dynamics (MD) simulation in nanoporous membrane design. This review will provide useful guidelines for future research in the development of nanoscale tailor-made membranes with a precise and rapid molecular sieve separation property.

Biofouling is an inevitable obstacle that impairs the overall performance of polymeric membranes, including selectivity, permeability, and long-term stability. With an increase of various biocides being utilized to inhibit biofilm formation, the enhancement of bacterial resistance against traditional bactericides is increasingly becoming an extra challenge in the development of antimicrobial membranes. Graphene-based nanomaterials are emerging as a new class of strong antibacterial agents due to their oxygen-containing functional groups, sharp edges of the one-atom-thick laminar structure, and synergistic effect with other biocides. They have been successfully employed not only to confer favorable antibacterial abilities, but also to impart superior separation properties to polymeric membranes. However, the exact bactericidal mechanism of graphene remains unclear. This review aims to examine the synthesis methods and antimicrobial behavior of graphene-based materials, offering an insight into how the nanocomposites influence their antimicrobial abilities. Most importantly, the use of graphene-based nanomaterials in the design and development of antimicrobial membranes is highlighted.

A rational manipulation of the surface structures and properties of thin-film composite membranes is important to optimize their separation performance and service durability. In this study, a facile strategy is reported for fabricating electroneutral loose nanofiltration membranes based on the rapid co-deposition of biomimetic adhesive polydopamine and poly(ethylene imine) (PEI) by using CuSO4/H2O2 as a trigger. Through this strategy, the surface properties and the filtration performance of the membranes can be easily tailored by the addition of PEI and CuSO4/H2O2, as well as tuning the deposition time. UV, XPS, SEM, AFM, zeta potential, water contact angle and nanofiltration measurements were used to investigate membrane performance. Surface characterization revealed that overall enhanced surface properties including low roughness, favourable hydrophilicity, and relatively neutral charge can be achieved after the addition of PEI. The optimum membranes, with 1 h co-deposition of PDA and PEI, show an ultrahigh water permeability (26.2 L m−2 h−1 bar−1), distinguished rejection for both negatively and positively charged dyes and a high permeation of divalent salts (>90%). Furthermore, the optimum membranes also show excellent operational stability in long-term nanofiltration operations in alkaline solution. This study provides an efficient and facile approach to tailor the membrane properties and structure for loose nanofiltration membranes.

Surface zwitterionization of graphene oxide (GO) was firstly conducted by grafting poly(sulfobetaine methacrylate) (PSBMA) onto the GO surface via reverse atom transfer radical polymerization (RATRP). Then, a novel type of GO-PSBMA/polyethersulfone (PES) loose nanofiltration membrane (NFM) was constructed by mixing with modified GO composites via phase inversion. FTIR, XRD, TEM, XPS and TGA were applied to analyze the chemical composition and morphology, confirming a favorable synthesis of GO-PSBMA composites. Besides, the effect of the embedded GO-PSBMA nanoplates on the morphology and overall performance of the hybrid membranes was systematically investigated based on the SEM images, water contact angle, zeta potential, and fouling parameters. It was found that the water flux of the hybrid membrane was greatly enhanced from 6.44 L m−2 h−1 bar−1 to 11.98 L m−2 h−1 bar−1 when the GO-PSBMA content increased from 0 to 0.22 wt%. The antifouling tests revealed that the GO-PSBMA embedded membranes had an excellent antifouling performance: a high flux recovery ratio (ca. 94.4%) and a low total flux decline ratio (ca. 0.18). Additionally, the hybrid membranes exhibited a distinct advance in the mechanical strength due to the addition of highly rigid GO. Notably, compared with unmodified membranes, the hybrid membranes had a higher retention of Reactive Black 5 (99.2%) and Reactive Red 49 (97.2%), and a lower rejection of bivalent salts (10% for Na2SO4) at an operational pressure of 0.4 MPa, rendering the membranes promising for dye/salt fractionation.

Molybdenum disulfide (MoS2) is a graphene-like two-dimensional inorganic material, which has been used for the first time as an inorganic nanofiller to prepare a composite mixed matrix membrane to separate CO2 and N2. Polysulfone (PSf) was used as a support substrate and poly(dimethylsiloxane) (PDMS) was used as the gutter layer. The selective layer was prepared by mixing a CO2-philic copolymer Pebax 1657 with MoS2 nanosheets to enhance CO2 permeance. In addition, a simple drop-coating and evaporation method was developed to prepare the selective layer. Both permeability and selectivity of the MoS2–Pebax membrane have exceeded the pristine Pebax membrane. The permeability and selectivity reached to the maximum values of 64 Barrer and 93, respectively, at 0.15 wt % MoS2 nanosheets loadings. This result has been on the Robeson’s upper bound line. The membrane also showed higher stability. The separation mechanism of the membrane is based on the well-known solution-diffusion mechanism. In addition, the stronger adsorption energy of MoS2 nanosheets to CO2 than N2 also provides the enhancement of gas selectivity.Keywords: CO2 capture; composite mixed matrix membrane; drop-coating method; MoS2 nanosheets; solution-diffusion mechanism

Inspired by the rational design concept, a novel antimicrobial agent zeolitic imidazolate framework-8 (ZIF-8)/graphene oxide (GO) was synthesized and utilized as a novel and efficient bactericidal agent to fabricate antimicrobial thin film nanocomposite (TFN) membranes via interfacial polymerization. The resultant hybrid nanosheets not only integrates the merits of both ZIF-8 and GO but also yields a uniform dispersion of ZIF-8 onto GO nanosheets simultaneously, thus effectively eliminating the agglomeration of ZIF-8 in the active layer of membranes. A ZIF-8/GO thin film nanocomposite (TFN-ZG) membrane with typical water permeability (40.63 L m–2 h–1 MPa–1) allows for efficient bivalent salt removal (rejections of Na2SO4 and MgSO4 were 100% and 77%, respectively). Furthermore, the synthesized ZIF-8/GO nanocomposites were verified to have an optimal antimicrobial activity (MIC,128 μg/mL) in comparison with ZIF-8 and GO separately, which sufficiently endowed the TFN-ZG membrane with excellent antimicrobial activity (84.3% for TFN-ZG3). Besides, the antimicrobial mechanisms of ZIF-8/GO hybrid nanosheets and TFN-ZG membranes were proposed. ZIF-8/GO functionalized membrane with high antimicrobial activity and salt retention denoted its great potential in water desalination, and we suggest that ZIF-8 based crystal may offer a new pathway for the synthesis of a multifunctional bactericide.Keywords: antimicrobial; graphene oxides; in situ growth; nanofiltration; ZIF-8

Basically, commercialized nanofiltration membranes exhibit a salt (NaCl) rejection of >30%, which are difficult to accomplish the separation of low-molecular-weight organics from their salts-containing wastewater. To solve this problem, in this study, a facile and novel loose nanofiltration membrane was developed by the embedment of modified hydrotalcite (mHT) in poly(ether sulfone) (PES) membrane matrix upon a phase inversion method. Membrane performance was characterized by scanning electron microscopy (SEM), water contact angle, transmission electron microscopy (TEM), atomic force microscopy (AFM), water uptake, tensile strength and percentage elongation, and thermal stability. Nanofiltration tests were performed using a series of salts (MgCl2, MgSO4, NaCl, and Na2SO4, 0.5 g/L) and dyes (reactive black 5 and reactive red 49, 1 g/L) aqueous solutions to evaluate membrane permeation properties. The resulted membrane showed higher surface hydrophilicity, enhanced mechanical and thermal stability, as well as higher dyes retention (above 95% for reactive black 5 and around 90% for reactive red 49) and near-zero salts rejection properties. Moreover, the short-term operation test demonstrated the stability of flux and rejection of mHT mixed PES membrane for dyes desalination. Therefore, this loose nanofiltration membrane may have potential applications in separation of dyes from salts-containing wastewater.Keywords: Dyes desalination; Hydrotalcite nanosheets; Loose nanofiltration membrane; Poly(ionic liquid) brushes; Positively charged

A novel fixed carrier composite membrane was prepared via interfacial polymerization by using graphene oxide nanosheets (GO), hyperbranched polyethylenimine (HPEI), and trimesoyl chloride (TMC) coating on a polysulfone membrane. The interfacial polymerization was confirmed with SEM, TEM, ATR-FTIR, XPS, DSC, and water contact angle. Further gas separation tests with CO2/N2 (10:90 v:v) mixed gas confirmed that the addition of GO could significantly improve the CO2 permeance and CO2/N2 selectivity. The highest CO2 permeance in this work was 9.7 GPU, while the selectivity was over 80. A further gas separation test under different feed gas humidity confirmed the facilitated transport was the main mechanism of gas separation through the membrane, while the addition of GO into the membrane exhibited a synergistic effect with the gas carriers: the surface defects acted as molecule sieves, and the interlayer fixed flow channels ensured a high water content microenvironment to improve the reactivity between CO2 and amino based carriers. Besides, superior stability of the composite membrane was also testified.

Halloysite nanotubes (HNTs) are naturally occurring clay mineral with nanotubular structures and have found increasing potential applications in industrial fields. Here, after a brief introduction of the general structure, main properties and newly emerging applications of HNTs, particular attention is paid to HNT-derived applications in water treatment. We mainly review the recent progress in applications of HNT-derived nanocomposites in heavy metal ion, dye or organic pollutant removal from wastewater and HNT-containing membranes for water filtration. The HNT-derived composites exhibit superior properties for water treatment in various ways and are promising to be used in practical applications. Finally, we summarize the predominant mechanisms acting in the applications of water treatment and future prospects are discussed.

Organized arrays of halloysite clay nanotubes have great potential in molecular separation, absorption, and biomedical applications. A highly oriented layer of halloysite on polyacrylonitrile porous membrane was prepared via a facile evaporation-induced method. Scanning electronic microscopy, surface attenuated total reflection Fourier transform infrared spectroscopy, and energy dispersive X-ray spectroscopy mapping indicated formation of the nanoarchitecture-controlled membrane. The well-ordered nanotube coating allowed for the excellent dye rejection (97.7% for reactive black 5) with high salt permeation (86.5% for aqueous NaCl), and thus these membranes were suitable for dye purification or concentration. These well-aligned nanotubes’ composite membranes also showed very good fouling resistance against dye accumulation and bovine serum albumin adsorption as compared to the pristine polyacrylonitrile or membrane coated with disordered halloysite layer.Keywords: antifouling; composite membranes; controlled thickness; oriented halloysite nanotubes; self-assembly;

Membrane fouling by microbial and organic components is considered as the “Achilles heel” of membrane processes as it not only reduces the membrane performance but also leads to membrane biodegradation. In this work, a novel high flux, antibacterial and antifouling ultrafiltration membrane was fabricated by blending the silver nanoparticles (AgNPs)–halloysite nanotubes (HNTs)–reduced graphene oxide (rGO) nanocomposite (AgNPs–HNTs–rGO) into a polyethersulfone (PES) membrane matrix. HNTs were applied to expand the interlayer space between neighboring rGO sheets and eliminate the leaching on AgNPs. The hybrid membranes had higher hydrophilicity, surface smoothness and higher water permeation flux when compared with the pure PES membrane. Both dynamic and static BSA adsorption tests revealed improved antifouling behavior of the hybrid membrane. In addition, the incorporated AgNPs were evenly attached onto the rGO support with an average size of 10 nm, which ensured its good antibacterial performance: the hybrid membrane had an ideal bacteriostasis rate against Escherichia coli (E. coli) even after six months of storage.

A facile and novel method for the fabrication of mixed matrix membranes (MMMs) has been developed, i.e., in situ synthesis of quaternized polyethylenimine (QPEI) soft nanoparticles (SNPs) followed by quaternization with bromoethane in poly(ether sulfone) (PES) casting solution. The resulting composite membranes were constructed via phase inversion method. The influences of SNPs on the morphology and performance of the hybrid membranes were systematically investigated by scanning electron microscopy, dynamic water contact angle, antifouling measurement, etc. The composite membranes exhibited a thin top layer and porous finger-like structure, which were greatly affected by in situ synthesized SNPs. Contact angle and water uptake measurements indicated that the hydrophilicity of hybrid membranes markedly improved in contrast with that of unfilled membrane. Meanwhile, the water flux of the membranes significantly enhanced due to the incorporation of SNPs. The ion-exchange capacity (IEC) value could achieve as high as 0.72 mmol g–1 with an initial PEI content of 1.5 wt %. The salts rejection of MMMs followed the order: MgCl2 > MgSO4 > Na2SO4 > NaCl, confirming that the hybrid membranes were positively charged. Meanwhile, the fouling parameters demonstrated that the composite membranes exhibited a preferable antifouling property. The newly developed membranes demonstrated an impressive prospect for the dye purification due to the high rejection of reactive dyes with a high permeation flux, as well as low multivalent ions retention. The possible separation mechanism of dyes and salts for composite membranes influenced by synthesized SNPs was also proposed in this study.Keywords: antifouling property; dyes purification; in situ synthesis; mixed matrix membranes; Polyethylenimine; soft nanoparticles;

Carrier-based immobilization has been developed to enhance enzymatic stability and activity, which permits the employment of enzymes in different solvents, at wide ranges of pH and temperature as well as high substrate concentrations. In this study, a novel carrier was prepared with halloysite nanotubes (HNTs) and layered double hydroxide (LDH) via a layer-by-layer (LbL) deposition process followed by an in situ growth technique. The in situ growth of LDH nanoplatelets on a HNTs support was demonstrated producing a well-defined three-dimensional architecture (HNTs@LDH). These flowerlike structural materials possess a high lysozyme immobilized amount (237.6 mg/g support) compared with individual HNTs and LDH. And such lysozyme immobilized composites (HNT@LDH–Ly) exhibit a superior antibacterial property against Escherichia coli (E. coli).Keywords: Enzyme immobilization; Halloysite nanotubes; In situ growth; Layered double hydroxide;

In this study, a facilitated transport mixed matrix membrane was fabricated by a surface coating method. Polyvinyl amine (PVAm) and chitosan (Cs) were used as the polymer matrix materials and coated onto a porous polysulfone (PS) support. Graphene oxide (GO) grafted with hyperbranched polyethylenimine (HPEI-GO) was added as the nanofiller. The gas separation tests with CO2/N2 (10:90 v:v) mixed gas suggest that the addition of GO could improve CO2/N2 selectivity. The highest CO2 permeance was 36 GPU in the membrane with 2.0 wt % HPEI-GO, and the optimal selectivity was 107 in the membrane with 3.0 wt % HPEI-GO. Herein, GO could provide a transport channel for CO2 and enhance the long-term stability of the membranes. Further gas separation tests under various relative humidities confirmed that facilitated transport was the main mechanism of gas separation through the membrane. The stability test suggests that the membrane has long-term stability. CO2 transports through the membrane mainly by the facilitated transport mechanism with assistance from the solution-diffusion mechanism, while N2 transports only by the solution-diffusion mechanism.Keywords: Chitosan; CO2 facilitated transport; Graphene oxide; Mixed matrix membrane; Polyvinyl amine

•SiO2-PSS with various molecular weights was prepared via SI-ATRP.•Negatively charged NF membranes were fabricated by blending with SiO2-PSS.•The membranes showed a potential application in dye purification and desalination.Silica spheres in nanoscale were prepared via sol–gel method and then sodium 4-styrene sulfonate was grafted onto the surfaces of SiO2 (PSS-SiO2) by surface-initiated atom transfer radical polymerization (SI-ATRP). Then, a negatively charged loose SiO2-PSS/polyethersulfone (PES) nanofiltration membrane with high flux was fabricated via phase inversion method. FT-IR and TEM results showed that SiO2 nanoparticles were synthesized and modified successfully. GPC results further proved the “living”/controlled behavior of SI-ATRP. The morphology, hydrophilicity of the membranes were investigated by SEM, static water contact angle and water ratio. The results revealed that the surface hydrophilicity and water permeability of hybrid membranes were greatly improved after adding SiO2-PSS and thus may enhance fouling resistance to a certain extent. The salt permeation and separation of dye/salt mixture of the hybrid membranes were significantly superior to the pure PES membrane, and the order of permeation for different salt solutions was NaCl > MgCl2 > MgSO4 > Na2SO4. When the content of SiO2-PSS was 3.0 wt%, the hybrid membrane showed optimal performance with IEC value of 0.07 mmol/g and pure water flux of 269.5 L m−2 h−1 and the rejections for all types of salts declined to under 11%. The above results indicated that SiO2-PSS incorporated into PES matrix played an important role in enhancing the performance of NF membranes, which may possess a significant impact on the application in dye purification and desalination.Graphical abstract

In this study, halloysite nanotubes (HNTs) were used to immobilize lysozyme via a covalent binding reaction. Immobilized lysozyme (HNTs–Ly) was then added to a polyethersulfone (PES) polymer solution to prepare hybrid antibacterial ultrafiltration membranes via classic phase inversion. The results showed that the surface hydrophilicity and the water flux of the hybrid membranes were significantly improved after adding HNTs–Ly. When the content of HNTs–Ly was 3.0 wt%, the water flux of the resultant membranes could achieve values as high as 400 L m−2 h−1 and maintain higher rejections for PEG 20000 (69%) and PVA 30000–70000 (99.6%). The tensile strength and the elongation at the break of the hybrid membranes were increased after adding HNTs–Ly, which revealed that the mechanical strength of the membranes was also enhanced. Moreover, the hybrid membrane showed a good antibacterial activity against Gram-negative bacteria (E. coli) with a high bacteriostasis rate of 63%.

N-Halamine grafted halloysite nanotubes (N-halamine@HNTs) were used as an antibacterial agents to fabricate polyethersulfone (PES) ultrafiltration (UF) hybrid membranes. N-Halamine@HNTs were characterized by Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectra (XPS). The chemical compositions, storage modulus, tanδ, morphology and performance of the membranes were characterized by attenuated total reflection-Fourier transform infrared spectra (ATR-FTIR), dynamic mechanical analysis (DMA), water contact angle, scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), overall porosity and pore size measurements. The results showed that the hydrophilicity of the membranes was improved greatly after adding N-halamine@HNTs. Water flux of the hybrid membrane could reach as high as 248.3 L m−2 h−1 when the content of N-halamine@HNTs was 1.0 wt%. In addition, the antibacterial test indicated that the hybrid membranes showed good antibacterial activity against E. coli.

Poly(4-vinylpyridine) (P4VP) with various molecular weights was grafted onto halloysite nanotubes (HNTs) via reverse atom transfer radical polymerization (RATRP). Then copper nanoparticles (Cu NPs) were loaded onto the surface of HNTs by complexation and reduction (Cu NPs@HNTs). Finally, a novel mixed matrix membrane, polyethersulfone (PES) ultrafiltration membrane, containing Cu NPs@HNTs was fabricated via the classical phase inversion method. The results showed that the pure water flux of the hybrid membrane was greatly enhanced, and the maximum could reach as high as 212 L m−2 h−1. The contact angle and atomic force microscopy (AFM) results indicated that the hydrophilicity of the prepared membranes was improved and the surface became smoother, compared with the virgin membranes. Importantly, the antibacterial test indicated that the hybrid membranes showed good antibacterial activity against E. coli with a high bacteriostasis rate of 94.5%.

Copper nanoparticle supported halloysite nanotubes with a 15 nm lumen and 30 nm external diameter via surface initiation reverse atom transfer radical polymerization were fabricated and showed good antibacterial activity against Escherichia coli (E. coli).

Zeolite–reduced graphene oxide (zeolite–rGO) composites were first fabricated from halloysite nanotubes (HNTs) by blending with GO via a hydrothermal reaction and then were used as an effective adsorbent for the removal of cationic dyes (methylene blue and malachite green). Zeolite–rGO was presented as having spherical structure with the diameter of 2–2.5 μm. The adsorption behavior of the two cationic dyes was affected by equilibrium time, pH, etc. Additionally, the most appropriate adsorption process of dyes was well-fitted to Langmuir models. It was found that adsorption of dyes was favorable and the maximum adsorption capacity of methylene blue reached 53.3 mg g–1 (48.6 mg g–1 for malachite green). The superior behavior of zeolite–rGO for cationic dye removal from an aqueous solution should be amenable to potential water treatment applications in consideration of zeolite–rGO’s low-cost, abundance in clay minerals, and favorable adsorption processes.

To investigate the synergistic effect of multi-nanofillers in a superabsorbent nanocomposite, a poly (sodium acrylate–acrylamide) superabsorbent nanocomposite incorporating graphene oxide and halloysite nanotubes (PAA-AAm–HNT–GO) was synthesized via the inverse suspension polymerization method. Graphene oxide (GO) was prepared by an improved method and halloysite nanotubes (HNTs) were modified by grafting carboxyl groups. Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were carried out to examine the structure and morphology of the resulting superabsorbent nanocomposite. It was found that HNTs, GO and poly(sodium acrylate-acrylamide) (PAA-AAm) copolymers combine well with each other during the polymerization process. Meanwhile, the particle sizes of the resulting superabsorbent nanocomposite reduced to about one-tenth of the original size after the introduction of HNTs and GO. The PAA-AAm–HNT–GO superabsorbent nanocomposite exhibited a significant improvement in its water absorption and water retention abilities, due to the synergistic effect of the HNTs and GO, compared with controls, which may make it suitable for use in some special applications that demand a higher water absorption and retention capacity.

Halloysite nanotubes (HNTs), natural nanotube, have been developed as a support for loading of antibacterial agents. Firstly, HNTs were modified by silane coupling agent (KH-792). And then, modified HNTs were immersed in silver nitrate solution and a complex reaction between the two amino groups of KH-792 and silver ions formed, leading to large clusters on the surface of HNTs. Finally, these silver containing clusters were converted into silver nanoparticles (Ag NPs) with about 5 nm diameter by reduction process. A new antibacterial agent, Ag NPs/HNTs, was characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and scanning transmission electron microscopy-energy dispersive X-ray analysis (STEM-EDX). The antibacterial test indicated that Ag NPs/HNTs showed good antibacterial performance against Gram-negative bacteria (Escherichia coli) and Gram-positive bacteria (Staphylococcus aureus).Silver nanoparticles (Ag NPs) with about 5 nm diameter were homogeneously distributed and bound to the surface of HNTs. Ag NP/HNT nanocomposite powders showed good antibacterial activities against Gram-negative bacteria (Escherichia coli) and Gram-positive bacteria (Staphylococcus aureus).Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► A novel silver nanoparticle-halloysite nanotubes nanocomposite powders was prepared. ► Ag NPs with about 5 nm diameters uniformly distributed across the surface of HNTs. ► Ag NPs/HNTs nanocomposite powders showed good antibacterial activities.

In this study, poly(4-vinylpyridine) (P4VP) was first grafted onto the surface of halloysite nanotubes (HNTs) via in situ polymerization, and then, silver ions were immobilized on P4VP via complex reaction. Finally, silver ions were reduced to silver nanoparticles (Ag NPs). Polyethersulfone (PES) ultrafiltration membranes bending with modified HNTs loaded with Ag NPs were prepared via phase inversion. FT-IR spectra and TGA results showed that HNTs were modified successfully. The contact angle data indicated that the hydrophilicity of the membranes was enhanced by the addition of modified HNTs. The permeation properties of the hybrid membranes were significantly superior to the pure PES membrane, especially when the modified HNTs content was 3%; the pure water flux of the membrane reached the maximum at 396.5 L·m–2·h–1, which was about 251.5% higher than that of the pure PES membrane, and the rejection was slightly affected by the addition of the modified HNTs. The microstructure of the membranes was characterized using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The results showed that the structure of membrane was not obviously affected by addition of the modified HNTs. Antibacterial activity of the hybrid membrane was evaluated with the viable cell count method using antibacterial rate against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). The antibacterial rates of the hybrid membranes against E. coli and S. aureus were about 99.9% and 99.8%, respectively.

•SiO2 spheres were modified by poly (ionic liquid) brushes via RATRP.•Positively charged NF membranes were fabricated by incorporation of SiO2-PIL.•The membranes exhibited higher rejection for dyes and superior penetration for salts.Silica spheres modified by poly (ionic liquid) brushes, a novel positively charged nanomaterial is prepared by atom transfer radical polymerization (ATRP). A high flux positively charged loose nanofiltration membrane is fabricated via “blending-phase inversion” method. The morphology structures, hydrophilicity, thermal and mechanical properties, permeation performance of these membranes are investigated in detail. The results reveal that the hybrid membranes have enhanced surface hydrophilicity, water permeability, thermal stability, and mechanical properties. Characterization of membrane separation properties shows that the hybrid membranes possess higher salt permeability and relatively higher rejection for reactive dyes, which may open opportunities for the recycling of reactive dyes wastewater. Moreover, such hybrid membranes have an outstanding operational stability and salts concentration showed little effect on the separation properties.Download full-size image

•HNTs were firstly modified by poly (ionic liquid) brushes via RATRP.•Molecular separation membranes were fabricated by incorporation of HNTs-PIL.•These membranes possess high salt passage and organic matter removal.A positively charged nanomaterial was prepared adopting the graft polymerization of ionic liquid monomers on halloysite nanotubes (HNTs) via reverse atom transfer radical polymerization (RATRP). A novel and facile organic–inorganic hybrid molecular separation membrane was then fabricated by the incorporation of modified HNTs via phase inversion method. This hybrid membrane was investigated in terms of morphology structure, hydrophilicity, thermal, mechanical and electrical properties, and separation performances. The results revealed that the hybrid membranes represented thickened and loosened skin layer, enhanced surface hydrophilicity and water flux, as well as good thermal and mechanical properties. Most importantly, the hybrid membranes showed stabilized rejection for Reactive Black 5 (above 90%) and Reactive Red 49 (80%–90%), whereas the rejection for sorts of salts declined to below 10% indicating a potential molecular separation characteristic for dye desalination.Download full-size image

•Mg/Al hydrotalcite was in-situ exfoliated in dimethylacetamide (DMAc).•Exfoliated Mg/Al hydrotalcite/PES hybrid membrane was firstly fabricated.•The membranes showed higher rejections for organic matter, but lower for MgSO4.Layered double hydroxides (LDHs) recently have attracted intense research for their electric property. In particular, the exfoliation of LDHs provides a new type ultimate two-dimensional nanosheet with higher positive charge density. In this work, Mg/Al hydrotalcite was prepared and exfoliated in dimethylacetamide (DMAc) and then the resulting colloidal solution was used to fabricate antisymmetric membranes using polyethersulfone as membrane material via phase inversion method. Both ultrafiltration and nanofiltration membranes were prepared to investigate the effect of exfoliated Mg/Al hydrotalcite content on the membrane properties. The morphology of membranes was investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Hydrophilicity, water flux, and rejections of all prepared membranes were tested and the results indicated that both ultrafiltration and nanofiltration membranes showed good separation property. Ultrafiltration membranes exhibited higher rejection (above 85%) for PEG 20000 and lower protein adsorption amount for positively charged proteins, which may be used in some separation fields of proteins. Nanofiltration membranes showed higher rejection for PEG 400 but a much lower value for MgSO4, and therefore such membrane may have a potential application in desalination for organic matter.

•N-Halamine was used as a novel antibacterial agent to fabricate membranes.•The membranes blending with SiO2@N-Halamine showed good hydrophilicity.•The membranes showed good antifouling and antibacterial properties.SiO2@N-Halamine/polyethersulfone (PES) ultrafiltration membranes were prepared by phase inversion method. The morphology, hydrophilicity, permeation performance, porosity, antifouling and antibacterial properties of the membrane were investigated. FT-IR spectra, TEM and XPS spectra results showed that SiO2 nanoparticles were prepared and modified successfully. SEM images indicated that the cross-section morphology of membrane was influenced by the introduction of SiO2@N-Halamine. The surface hydrophilicity of membranes was significantly improved after adding SiO2@N-Halamine. The filtration results indicated that the permeation properties of the hybrid membranes were significantly superior to the pure PES membrane. The water flux of the hybrid membranes increased with the additional amount of SiO2@N-Halamine increased, when the SiO2@N-Halamine content was 5%, the water flux of the membranes reached the maximum at 384.4 L·m− 2·h− 1. Moreover, the hybrid membranes showed good antifouling and antibacterial properties, which might expand the usage of PES in water treatment and also could make some potential contributions to membrane antifouling.

•HNTs-MPC were synthesized by chemical modification of HNTs with MPC via RATRP.•The hybrid membranes containing HNTs-MPC possessed higher water flux.•The hybrid membranes showed good antifouling performance and stability.Polyethersulfone ultrafiltration hybrid membrane containing halloysite nanotubes grafted with 2-methacryloyloxyethyl phosphorylcholine (HNTs-MPC) was prepared via phase inversion method for the purpose of enhancing the antifouling property of the membrane. HNTs-MPC were synthesized by chemical modification of HNTs with MPC via reverse atom transfer radical polymerization (RATRP). The performance and morphology of the membranes were characterized by water contact angle and SEM. The hybrid membrane was shown to be more hydrophilic with a higher pure water flux. The thickness of the thin separating layer on the top tended to decrease with the addition of HNTs-MPC. The BSA adsorption experiment indicated that the adsorption amounts of bovine serum albumin (BSA) on the membrane were dramatically decreased. BSA ultrafiltration experiment also showed that the antifouling ability of the membrane with the addition of HNTs-MPC was better than the pure PES membrane. Meanwhile, the long term ultrafiltration experiment showed that the hybrid membrane had an ideal stability.Download full-size image

A rational manipulation of the surface structures and properties of thin-film composite membranes is important to optimize their separation performance and service durability. In this study, a facile strategy is reported for fabricating electroneutral loose nanofiltration membranes based on the rapid co-deposition of biomimetic adhesive polydopamine and poly(ethylene imine) (PEI) by using CuSO4/H2O2 as a trigger. Through this strategy, the surface properties and the filtration performance of the membranes can be easily tailored by the addition of PEI and CuSO4/H2O2, as well as tuning the deposition time. UV, XPS, SEM, AFM, zeta potential, water contact angle and nanofiltration measurements were used to investigate membrane performance. Surface characterization revealed that overall enhanced surface properties including low roughness, favourable hydrophilicity, and relatively neutral charge can be achieved after the addition of PEI. The optimum membranes, with 1 h co-deposition of PDA and PEI, show an ultrahigh water permeability (26.2 L m−2 h−1 bar−1), distinguished rejection for both negatively and positively charged dyes and a high permeation of divalent salts (>90%). Furthermore, the optimum membranes also show excellent operational stability in long-term nanofiltration operations in alkaline solution. This study provides an efficient and facile approach to tailor the membrane properties and structure for loose nanofiltration membranes.

Biofouling is an inevitable obstacle that impairs the overall performance of polymeric membranes, including selectivity, permeability, and long-term stability. With an increase of various biocides being utilized to inhibit biofilm formation, the enhancement of bacterial resistance against traditional bactericides is increasingly becoming an extra challenge in the development of antimicrobial membranes. Graphene-based nanomaterials are emerging as a new class of strong antibacterial agents due to their oxygen-containing functional groups, sharp edges of the one-atom-thick laminar structure, and synergistic effect with other biocides. They have been successfully employed not only to confer favorable antibacterial abilities, but also to impart superior separation properties to polymeric membranes. However, the exact bactericidal mechanism of graphene remains unclear. This review aims to examine the synthesis methods and antimicrobial behavior of graphene-based materials, offering an insight into how the nanocomposites influence their antimicrobial abilities. Most importantly, the use of graphene-based nanomaterials in the design and development of antimicrobial membranes is highlighted.

Membrane fouling by microbial and organic components is considered as the “Achilles heel” of membrane processes as it not only reduces the membrane performance but also leads to membrane biodegradation. In this work, a novel high flux, antibacterial and antifouling ultrafiltration membrane was fabricated by blending the silver nanoparticles (AgNPs)–halloysite nanotubes (HNTs)–reduced graphene oxide (rGO) nanocomposite (AgNPs–HNTs–rGO) into a polyethersulfone (PES) membrane matrix. HNTs were applied to expand the interlayer space between neighboring rGO sheets and eliminate the leaching on AgNPs. The hybrid membranes had higher hydrophilicity, surface smoothness and higher water permeation flux when compared with the pure PES membrane. Both dynamic and static BSA adsorption tests revealed improved antifouling behavior of the hybrid membrane. In addition, the incorporated AgNPs were evenly attached onto the rGO support with an average size of 10 nm, which ensured its good antibacterial performance: the hybrid membrane had an ideal bacteriostasis rate against Escherichia coli (E. coli) even after six months of storage.

Surface zwitterionization of graphene oxide (GO) was firstly conducted by grafting poly(sulfobetaine methacrylate) (PSBMA) onto the GO surface via reverse atom transfer radical polymerization (RATRP). Then, a novel type of GO-PSBMA/polyethersulfone (PES) loose nanofiltration membrane (NFM) was constructed by mixing with modified GO composites via phase inversion. FTIR, XRD, TEM, XPS and TGA were applied to analyze the chemical composition and morphology, confirming a favorable synthesis of GO-PSBMA composites. Besides, the effect of the embedded GO-PSBMA nanoplates on the morphology and overall performance of the hybrid membranes was systematically investigated based on the SEM images, water contact angle, zeta potential, and fouling parameters. It was found that the water flux of the hybrid membrane was greatly enhanced from 6.44 L m−2 h−1 bar−1 to 11.98 L m−2 h−1 bar−1 when the GO-PSBMA content increased from 0 to 0.22 wt%. The antifouling tests revealed that the GO-PSBMA embedded membranes had an excellent antifouling performance: a high flux recovery ratio (ca. 94.4%) and a low total flux decline ratio (ca. 0.18). Additionally, the hybrid membranes exhibited a distinct advance in the mechanical strength due to the addition of highly rigid GO. Notably, compared with unmodified membranes, the hybrid membranes had a higher retention of Reactive Black 5 (99.2%) and Reactive Red 49 (97.2%), and a lower rejection of bivalent salts (10% for Na2SO4) at an operational pressure of 0.4 MPa, rendering the membranes promising for dye/salt fractionation.

Poly(4-vinylpyridine) (P4VP) with various molecular weights was grafted onto halloysite nanotubes (HNTs) via reverse atom transfer radical polymerization (RATRP). Then copper nanoparticles (Cu NPs) were loaded onto the surface of HNTs by complexation and reduction (Cu NPs@HNTs). Finally, a novel mixed matrix membrane, polyethersulfone (PES) ultrafiltration membrane, containing Cu NPs@HNTs was fabricated via the classical phase inversion method. The results showed that the pure water flux of the hybrid membrane was greatly enhanced, and the maximum could reach as high as 212 L m−2 h−1. The contact angle and atomic force microscopy (AFM) results indicated that the hydrophilicity of the prepared membranes was improved and the surface became smoother, compared with the virgin membranes. Importantly, the antibacterial test indicated that the hybrid membranes showed good antibacterial activity against E. coli with a high bacteriostasis rate of 94.5%.

Halloysite nanotubes (HNTs) are naturally occurring clay mineral with nanotubular structures and have found increasing potential applications in industrial fields. Here, after a brief introduction of the general structure, main properties and newly emerging applications of HNTs, particular attention is paid to HNT-derived applications in water treatment. We mainly review the recent progress in applications of HNT-derived nanocomposites in heavy metal ion, dye or organic pollutant removal from wastewater and HNT-containing membranes for water filtration. The HNT-derived composites exhibit superior properties for water treatment in various ways and are promising to be used in practical applications. Finally, we summarize the predominant mechanisms acting in the applications of water treatment and future prospects are discussed.