High-flux Nanofiltration Research Articles

Structure design and performance optimization of PVDF-CTFE nanofibrous composite nanofiltration membrane modified by C–Cl active site grafting

The substrate membrane plays a crucial role in determining the performance of composite nanofiltration membranes prepared by interfacial polymerization (IP). In this study, persistent hydrophilic modification of polyvinylidene fluoride-co-chlorotrifluoroethylene (PVDF-CTFE) was achieved by incorporating N-methylglucamine (N-MG) without altering the membrane's morphology or pore size. Moreover, thin-film nanofibrous composite (TFNC) nanofiltration membranes were prepared using the surfactant assembly regulated interfacial polymerization (SARIP) method with the modified PVDF-CTFE@N-MG nanofibrous membrane as the substrate. The effect of N-MG addition on the substrate membrane's hydrophilicity and the structure and performance of the polyamide (PA) separation layer was investigated. The results showed that the addition of N-MG significantly improved the hydrophilicity of the PVDF-CTFE nanofibrous membrane. The TFNC-5 membrane, prepared with the modified PVDF-CTFE@N-MG nanofibrous membrane as the substrate, exhibited permeability exceeding 13.5 L m−2 h−1·bar−1 for various dye solutions at 3 bar pressure, with rejection rates above 98 %. It also demonstrated excellent selective separation performance (separation factor for NaCl/Orange G reaching 97.38) and long-term operational stability. This work provides a method for the preparation of low-pressure, high-flux nanofiltration membranes, which is applicable to seawater desalination and dye wastewater treatment.

Construction of skyloft-like polyamide membrane on tubular ceramic support for high-flux nanofiltration

Polyamide (PA) thin-film composite membrane has been widely used in water/wastewater treatment, but construction of such membrane on a three-dimensional inorganic macroporous support remains a challenge, thus limiting the further application under harsh conditions. Herein, we propose a skyloft-like PA membrane on a commercial tubular ceramic macrofiltration support. This membrane built on stilts comprises an ultrathin PA canopy of ∼ 33 nm and pillars (∼1 μm-thick) of vertically aligned CoAl-layered double hydroxide (LDH) nanosheets and covalent organic framework (COF) nanospheres. The prepared PA/OH-COF/F-COF/LDH membrane shows a high water permeance of up to 736.6 L m-2h−1 MPa−1 together with a satisfactory rejection rate of > 90 % for various small molecules including dyes and antibiotics (> ∼1 nm). The resultant membrane also demonstrates a good dye desalination performance (with a selectivity of up to 197), illustrating an outstanding permselectivty. A long-term and continuous cross-flow nanofiltration indicates a robust stability of the membrane.

Swift fabrication of thin-film composite membranes with azine-linked covalent organic framework for high-temperature organic solvent nanofiltration

Covalent organic frameworks (COFs) belong to a class of crystalline and porous solids that are generated by the self-assembly of reactive monomers via covalent linkage. Most existing methods for fabricating COF membranes are tedious, and they focus on quality control challenges associated with thickness uniformity, pore size, and mechanical properties. In this study, TraHz and TpaHz COF-based thin-film composite (TFC) membranes were prepared via the swift interfacial polymerization (IP) of hydrazine and aldehyde monomers on a polyacrylonitrile (PAN) support. Hydrazine played a dual role in one-pot synthesis, where PAN was crosslinked and IP was initiated. Varying the polymerization time allowed for fine-tuning the membrane thickness and performance. The membranes displayed robust high-temperature separation performance in polar aprotic solvents at temperatures of up to 80 °C. The toluene permeance of the membranes was remarkable. The permeance was highly dependent on both the applied pressure and temperature; however, the rejection performance was less dependent on these parameters. Based on the results of the nanofiltration performance evaluated using different molecular-weight dyes, a molecular-weight cut-off value of ∼650 g mol−1 and a rejection higher than 98 % for the Congo red and Chicago blue dyes in water, THF, and ethanol were observed. The dye rejection in toluene was higher than 98 % for Rhodanile blue, while maintaining high toluene permeances of 47 and 33 L m−2 h−1 bar−1 using TraHz-30 and TpaHz-30, respectively. In addition, both membranes exhibited stable separation performance. Our TFC membranes demonstrate a new approach for designing robust and high-flux nanofiltration membranes for precise molecular sieving even at high temperatures.

Interfacial polymerization process based on diffusion control: Role of chemical composition and morphology on fouling resistance

Nanofiltration, as a technology with low energy consumption, no secondary pollution, and continuous and compact operation, is widely used in the fields of seawater desalination, resource recovery and reuse, and drinking water purification, etc. However, “trade-off” effect and membrane fouling are the main problems limiting the application of nanofiltration membranes. Herein, glycerol was introduced into the aqueous reactive solution in interfacial polymerization (IP) for regulating chemical composition and membrane structure simultaneously. As glycerol raised the viscosity of the reactive solution which slowing down the diffusion rate of amine monomer, crumpled structure generated by the diffusion-driven instability. At the same time, the addition of glycerol varied the oxygen content in the skin layer of the membrane. At 60% glycerol content, the as-prepared membrane achieved a pure water permeance of 21.14 L·m−2·h−1·bar−1 with sodium sulfate rejection of 95.6%. However, the oxygen content in the chemical composition of the membrane showed a minimum value with the increase of glycerol concentration indicating a competitive reaction between the amine monomer and glycerol. Through the anti-fouling analysis of membranes with different chemical compositions and structures, it was found that the increase in roughness of nanofiltration membranes with similar chemical composition resulted in an increase of irreversible pollution. The nanofiltration membrane with similar membrane structure and higher oxygen content had stronger anti-fouling ability to humic acid. It is concluded that the rational design of the crumpled membrane with high oxygen-chemical composition is expected to obtain a high flux nanofiltration membrane with anti-fouling performance.

Removal of pharmaceutically active compounds from water sources using nanofiltration and reverse osmosis membranes: Comparison of removal efficiencies and in-depth analysis of rejection mechanisms

Trace organic compounds from effluent streams are not completely removed by conventional purification techniques and hence, contaminating groundwater sources. Herein, we report the removal efficiency and rejection mechanisms of three common pharmaceutically active compounds (PhACs); caffeine (CFN), omeprazole (OMZ), and sulfamethoxazole (SMX), using commercial nanofiltration (NF) and reverse osmosis (RO) membranes with different surface characteristics. The RO membranes showed near-complete removal of all PhACs with rejection rates >99%. On the other hand, retention capabilities for the NF membranes varied and were influenced by the characteristics of the PhACs, membranes, and the feed solution. In general, during long-term testing, the rejection did not show much variation and followed a trend compatible with the size exclusion (steric hindrance) mechanism. When a real matrix was used, the rejection of CFN by the more tight NF membranes, HL TFC and NFW decreased by ∼10%, whereas the removal of SMX by the loose NF membrane, XN45, increased by the same ratio. In short-term testing, the rejection of negatively charged SMX increased significantly (∼20–40%) at a higher pH of ∼8 and in the presence of salts. Fouling by the PhACs was more severe on the high-flux NF membranes, HL TFC and XN45, as witnessed by the significant change in Contact angle (CA) values (∼25–50°) as well as the flux decline (∼15%) during long-term testing. To summarize, the removal of PhACs by membranes is a complex phenomenon and depends upon a combination of several factors.

Restricted piperazine diffusion by polyacrylic acid for high flux nanofiltration membrane

Restricted piperazine diffusion by polyacrylic acid for high flux nanofiltration membrane

Covalent organic framework-immobilized wood membrane for efficient, durable, and high-flux nanofiltration

Covalent organic frameworks (COFs) are promising for the construction of nanofiltration membranes owing to their uniform, ordered channel structures and high chemical stability. However, developing a low-cost strategy for fabricating COF-based nanofiltration membranes and increasing the long-term stability of the membranes are challenging. Here, we developed a TpPa-wood membrane through the in-situ formation of massive imine COFs on the top surface of a wood block for the rapid and efficient nanofiltration-based separation of organic pollutants. The numerous hydroxyl groups of cellulose in wood provided nucleation sites for the growth of dense, defect-free COFs with a diameter of 1.8 nm. The nano porous COFs enabled efficient separation, while the long and low-tortuosity channels of wood with a diameter of 10–30 μm facilitated rapid water transport and augmented the mechanical robustness of the membrane. Therefore, the prepared TpPa-wood membrane showed an outstanding separation efficiency of ∼97.0% toward organic pollutants such as dyes and pharmaceutical molecules at a flux rate of 600 L m−2 h−1. In addition, the TpPa-wood membrane exhibited excellent stability at a wide pH range (3–11) and over continuous filtration for 48 h. The TpPa-wood membrane opens up a new avenue for rapid and efficient nanofiltration by combining the merits of nano porous COFs and biomass with numerous micro-channels and high hydrophilicity.

High-flux nanofiltration membrane with modified highly dispersed MOF particles as nano filler.

The synthesis of optimized thin film nanocomposite (TFN) membrane with no or few defects is an efficacious method which can improve nanofiltration performance. However, poor dispersion of fillers in the organic phase and wrong compatibility between fillers and polymerizate are still a serious problem. In this study, the particle size of metal organic framework (MOF), aluminum-based metal-organic frameworks (CAU-1) was modulated and for the first time, dodecyl aldehyde was used to modify the surface hydrophobicity of CAU-1, which improved the dispersibility and inhibited the aggregation in the trimesoyl chloride (TMC)/n-hexane solution; later CAU-1 and modified CAU-1 were incorporated into the polyamide (PA) selective layer to synthesize TFN membrane by interfacial polymerization (IP). The particle size modulation and modification of the CAU-1 were demonstrated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) characterization. The characterization showed that PA selective layer was synthesized on the top layer of polysulfone (PSF) substrate. The pure water flux of the TFN membrane was increased to 79.89 ± 1.24 L·m-2·h-1·bar-1 compared to the original thin film composite (TFC) membrane, which was due to the polymerization of 100 nm modified CAU-1 on the PA layer to form a new water molecular channel, thus increasing the water flux by about 70%.

High Flux Nanofiltration Membranes with Double-Walled Carbon Nanotube (DWCNT) as the Interlayer.

Nanofiltration (NF) membranes with a high permeability and rejection are of great interest in desalination, separation and purification. However, how to improve the permeation and separation performance still poses a great challenge in the preparation of NF membranes. Herein, the novel composite NF membrane was prepared through the interfacial polymerization of M-phenylenediamine (MPD) and trimesoyl chloride (TMC) on a double-walled carbon nanotube (DWCNT) interlayer supported by PES substrate. The DWCNT interlayer had a great impact on the polyamide layer formation. With the increase of the DWCNT dosage, the XPS results revealed an increase in the number of carboxyl groups, which decreased the crosslinking degree of the polyamide layer. Additionally, the AFM results showed that the surface roughness and specific surface area increased gradually. The water flux of the prepared membrane increased from 25.4 L/(m2·h) and 26.6 L/(m2·h) to 109 L/(m2·h) and 104.3 L/(m2·h) with 2000 ppm Na2SO4 and NaCl solution, respectively, under 0.5 MPa. Meanwhile, the rejection of Na2SO4 and NaCl decreased from 99.88% and 99.38% to 96.48% and 60.47%. The proposed method provides a novel insight into the rational design of the multifunctional interlayer, which shows great potential in the preparation of high-performance membranes.

Polyvinyl alcohol and sodium alginate hydrogel coating with different crosslinking procedures on a PSf support for fabricating high-flux NF membranes

Polyvinyl alcohol (PVA) and sodium alginate (SA) hydrogel-coated nanofiltration (NF) membranes with high-flux and permselectivity were prepared. The coating of PVA and SA hydrogel selective layer on a porous polysulfone (PSf)/non-woven fabric ultrafiltration substrate membrane was conducted through different three procedures including pre-crosslinking, in-situ crosslinking, and immersing crosslinking and the use of glutaraldehyde as a crosslinking agent. The properties and performances of all types of the prepared membranes were evaluated through ATR-FTIR spectroscopy, AFM, SEM, zeta potential, contact angle, and cross-flow permeation tests. The immersing technique resulted in the formation of TFC membranes with higher hydrophilicity, smoother surface layer, higher negative charge, higher permeation flux, higher salt rejection and better anti-fouling performance. Also, the higher negative surface charge of the immersing coated TFC membranes due to dissociation of hydrophilic functional groups of the PVA and SA hydrogel selective layer resulted in higher As(III) rejection. SA coated NF membrane through immersing method exhibited a higher pure water permeability of 11.2 L m−2 h−1 bar−1, NaCl, MgSO4, and Na2SO4 rejection of 38.2%, 55.1%, and 70.4%, respectively with As(III) rejection of 60.6%. All types of the PVA and SA hydrogel-coated PSf membranes possessed improved fouling resistance to BSA aqueous solution, superior anti-fouling performance was obtained with SA hydrogel coating through immersing method. Such optimum membranes indicated high stability in the long-term experiments. This study showed that the coating of the SA hydrogel layer on a PSf support through immersing method could be a promising candidate for fabricating high-flux NF membranes.

Ultrathin polyamide nanofiltration membrane prepared by triazine-based porous organic polymer as interlayer for dye removal

Separation membrane with high flux is generally encouraged in industrial application, because of the tremendous needs for decreasing membrane areas, usage costs and space requirements. The most effective and direct method for obtaining the high flux is to decrease membrane thickness. Polyamide (PA) nanofiltration membrane is conventionally prepared by the direct interfacial polymerization (IP) on substrate surface, and results in a thick PA layer. In this work, we proposed a strategy that constructing triazine-based porous organic polymer (TRZ-POP) as the interlayer to prepare the ultrathin PA nanofiltration membranes. TRZ-POP is firstly deposited on the polyethersulfone substrate, and then the formed TRZ-POP provides more adhesion sites towards PA based on its high specific surface areas. The chemical bonding between terminal amine group of TRZ-POP and the amide group of PA further improves the binding force, and strengthens the stability of PA layer. More importantly, the high porosity of TRZ-POP layer causes the higher polymerization of initial PA owning to the stored sufficient amino monomer; and H-bonding interaction between amine groups of TRZ-POP and piperazine (PIP) can astrict the release of PIP. Thus, IP process is controlled, and the thinnest thickness of prepared PA layer is only < 15 nm. As expected, PA/TRZ-POP membrane shows a more excellent water flux of 1414 L·m−2·h−1·MPa−1 than that of the state-of-the-art nanofiltration membranes, and without sacrificing dye rejection. The build of TRZ-POP interlayer develops a new method for obtaining a high-flux nanofiltration membrane.

Ultrathin polyamide membranes enabled by spin-coating assisted interfacial polymerization for high-flux nanofiltration

While nanofiltration (NF) membranes have much potential for precise molecule/ion separations, the thick nanofilms fabricated via rapid and irreversible interfacial polymerization (IP) result in low water permeance. The distribution and location of aqueous monomers play a pivotal role in the IP reaction and thus in the polyamide films, yet are rarely reported to improve the performance. In this study, a one-step spin-coating procedure was utilized to deposit diamines atop porous polysulfone substrates followed by IP reaction for construction of ultrathin polyamide membranes. Direct spin-coating of aqueous diamines onto a porous substrate allows for a uniform diamine deposition, demonstrated by confocal laser scanning microscope analysis. In addition to the uniform distribution, this procedure is able to enrich the diamines beyond the surface pores, contributing to the formation of ultrathin polyamide layers based on Freger’s theory. Importantly, rational regulation of spinning speed, aqueous phase volume, and IP reaction parameters allows to optimize the membrane microstructure and separation performance. The optimal TFC membranes with 9.7-nm-thick polyamide nanofilm evince a remarkably high water permeance of 36.1 L m-2 h−1 bar−1, and an acceptable divalent Na2SO4 rejection of 96.2%. An extended use of spin-coating assisted IP onto other substrates also yields high-flux polyamide NF membranes (TFC-Kevlar: 37.9 L m-2 h−1 bar−1, 97.5%, TFC-polyacrylonitrile: 41.7 L m-2 h−1 bar−1, 96.2%), confirming the applicability and high efficiency of this approach. In contrast with established methods, spin-coating assisted IP (SCIP) features distinct superiorities in economizing diamine dosage, ease of control, and attaining an excellent sieving performance, which paves the way of constructing high-performance NF membranes.

Cation Modulated Graphene Oxide Membranes for High-Flux Nanofiltration

Cation Modulated Graphene Oxide Membranes for High-Flux Nanofiltration

Engineering novel thin-film composite membranes with crater-like surface morphology using rigidly-contorted monomer for high flux nanofiltration

Developing thin-film composite (TFC) nanofiltration (NF) membranes with high water permeability and salt rejection is highly desirable in water desalination. In this study, a new TFC membrane with high NF performances was developed to achieve such a goal by constructing active separation layer with a special crater-like surface morphology using a rigidly-contorted spirocyclic monomer, 5,5,6′,6′-tetrahydroxy-3,3,3′,3′-tetramethyl spirobisindane (TTSBI), as the co-monomer with piperazine (PIP) in aqueous phase during interfacial polymerization (IP) process. This design endowed the separation layer with the characteristics of low thickness, high microporosity, and high permeation surface area. The pure water permeability was therefore increased to 9.9 L m−2 h −1 bar−1, approximately two times higher than that of the typical TFC NF membrane prepared with PIP and trimesoyl chloride, without compromise in the rejection (the rejection of Na2SO4 up to 98%). The introduction of TTSBI tuned the diffusion and reaction behaviors of monomers at the aqueous-organic interface and controlled the special microporous structure and surface morphology, which are advantageous to the improvement of water permeability. This study provides new insights for synthesizing highly permeable TFC NF membranes with remarkable rejection characteristics to address the crisis of the current potable water scarcity.

Ultrathin Polyamide Membranes Enabled by Spin-Coating Assisted Interfacial Polymerization for High-Flux Nanofiltration

Ultrathin Polyamide Membranes Enabled by Spin-Coating Assisted Interfacial Polymerization for High-Flux Nanofiltration

Ultrathin Polyamide Membranes Enabled by Spin-Coating Assisted Interfacial Polymerization for High-Flux Nanofiltration

Ultrathin Polyamide Membranes Enabled by Spin-Coating Assisted Interfacial Polymerization for High-Flux Nanofiltration

Ultrathin Polyamide Membranes Enabled by Spin-Coating Assisted Interfacial Polymerization for High-Flux Nanofiltration

Ultrathin Polyamide Membranes Enabled by Spin-Coating Assisted Interfacial Polymerization for High-Flux Nanofiltration

High-performance nanofiltration membrane intercalated by FeOOH nanorods for water nanofiltration

Nanofiltration (NF) is considered a promising candidate to improve the separation efficiency in water desalination, and the membranes with incomparable water permeance and great selectivity are urgently requried to reduce the energy consumption. Iron oxohydroxides (FeOOH) nanorods, goethite (α-FeOOH) with a diameter of 30–40 nm and length of about 340 nm, were synthesized and deposited on PES microfiltration (MF) membrane with the pore size of 0.1 μm by the assistance of vacuum filtration. Then a high flux NF membrane intercalated by well-dispersed FeOOH nanorods was fabricated using piperazine (PIP) and trimesoyl chloride (TMC) as monomers. The influence of FeOOH deposition on the surface properties and the nanofiltration performance of the fabricated membranes was studied. XPS, SEM, TEM, AFM, water contact angle (WCA) and NF measurements were utilized to investigate membrane performance. The optimum membranes showed a high water permeability (34.1 L·m−2·h−1·bar−1), high rejection for Na2SO4 (95.0%) and good ion selectivity towards Cl−/SO42− (23.9). Furthermore, the optimized NF membranes are acid-stable to some extent, and showed good stability in long-term NF operations. This work presents a facile and low-cost strategy for constructing high performance NF membranes.

High-flux robust PSf-b-PEG nanofiltration membrane for the precise separation of dyes and salts

We report a novel high-flux nanofiltration (NF) membrane for the precise separation of dyes and salts, which was fabricated using polysulfone-block-polyethylene glycol (PSf-b-PEG) block copolymer through the non-solvent induced phase separation (NIPS) method. The PSf-b-PEG membrane exhibited a very dense sponge-like porous structure with rich interconnected pores, contributing to its superior mechanical strength of 22.4 ± 0.9 MPa. The pore size of PSf-b-PEG membrane was mostly distributed around 1.3 nm in diameter with the mean effective pore size of 3.8 nm in diameter, while the membrane molecular weight cut off (MWCO) towards dyes was about 655 Da. The PSf-b-PEG membrane exhibited a very high water permeance of 49.3 ± 0.9 L m−2 h−1 bar−1 and a precise separation of salts and dyes with high above 98% Congo Red dye rejection and nearly 100% Na2SO4 passage, succeeding the separation performances of current NF membranes used for the separation of dyes and salts. Long-term NF tests prove the highly stable and outstanding separation performances of PSf-b-PEG membrane with the top dense skin layer as the effective selective layer. Our work suggests that the PSf-b-PEG membrane has a great potential to be further optimized and used for practical applications.

Pressure-modulated synthesis of self-repairing covalent organic frameworks (COFs) for high-flux nanofiltration

Covalent organic frameworks (COFs), with their inherent merits of specific pore size and uniform channels, have been extensively employed to produce nanofiltration (NF) membranes. However, COF-based NF towards precise separations, especially for ion separations, are often unsatisfying due to the intercrystalline defects in selective layers. Herein, a pressure-modulated synthesis method has been explored to prepare crystalline and defect-free COF membranes on seeded substrates for fast NF. Amine and aldehyde solutions are separately placed with a level interval to create vertical hydraulic pressure, which can regulate the mobility of the amine monomers and thus results in the self-repairing of defects in the COF planes and remediation of intercrystalline gaps. The abundant nucleation sites on seeded substrates promote the confined growth of COF crystallites in top layers, leading to an ultrathin selective layer with improved permeance. The resultant COF membrane shows tight methyl orange (~90.4%) and Na2SO4 (~63.6%) rejections with a pronounced water permeance of up to ~44.2 L m−2 h−1 bar−1, which is ~2–10 times higher than other NF membranes with similar rejections. This pressure-modulated synthetic strategy establishes not only the self-repairing synthesis of COFs but also the controllable preparation of defect-free COF-based NF membranes, thus enabling precise and fast separation of molecules and ions.