Composite Nanofiltration Membranes Research Articles

Quantitative analysis of the influence of PVA interlayer on the permeation resistance of TFC NF membranes

The interfacial polymerization based on the interlayer is an effective method for enhancing thin-film composite (TFC) nanofiltration (NF) membranes permeability. In this study, a polyvinyl alcohol (PVA) interlayer was constructed on a polyethersulfone substrate through a physical deposition method. Then the TFC NF membranes regulated by PVA interlayer were prepared, and conducted a quantitative analysis of the impact of the PVA interlayer on the permeation resistance of TFC NF membranes. The results indicated that after regulated by the PVA interlayer, the permeation resistance of the polyamide layer decreased slightly, from 0.130 to 0.106 bar h m−1, while the permeation resistance of the substrate decreased significantly, from 0.277 to 0.087 bar h m−1. These results indicated that the dramatic decrease in permeation resistance of substrate caused by PVA interlayer was main reason for enhanced permeability, enriching our understanding of the mechanism of the interlayer.

Highly permeable polyamide nanofiltration membranes fabricated via the construction of anionic covalent organic frameworks /polydopamine composite interlayer

Thin-film composite nanofiltration (TFC NF) membranes can separate neutral/charged solutes and play an essential role in water purification. However, the permeability of traditional NF membranes is severely limited by excessive mass transfer resistance due to the thick polyamide (PA) layer. Introducing porous nanomaterials as an interlayer into PA NF membranes has been recognized as an effective approach to improve permeability. Herein, a novel composite interlayer consisting of anionic covalent organic frameworks with abundant sulfonic acid groups (COF-SO3H) and polydopamine was designed to regulate interfacial polymerization to construct the highly permeable TFC NF membranes. The surface negative charge and hydrophilicity of the TFC NF membranes can be controlled by the abundant polar groups in the composite interlayer. Moreover, mass transfer resistance can be reduced by introducing COF-SO3H to create additional water transport channels. The results showed that the optimal membrane manifested attractive permeability (44.2 L⋅m−2⋅h−1⋅bar−1) and a high Na2SO4 rejection (98.7 %), almost 3-fold higher than that of the original TFC membrane. Furthermore, the optimal membrane exhibited favorable anti-fouling performance and stability. This work provides insight into strategies for fabricating highly permeable TFC NF membranes through the introduction of an interlayer.

Antifouling and smart covalent organic framework composite nanofiltration membranes with light-gated molecular transport

Covalent organic framework (COF) holds great promise in affording precise and fast molecular separation. Inspired by the light-gated channels in cell membranes, we developed artificial light-gated molecular channel composite membranes by vacuum filtration COF with azobenzene (AZO) functional groups. The simple stimuli response could remotely adjust the membranes separation performance. The cis membranes with off state under ultraviolet (UV) light have higher dye rejection than trans membranes with on-state channels. The permeance of composite membranes increased from 19.56 to ∼ 42.30 L h−1 m−2 bar−1, dye (Rose Bengal (RB)) rejection increased from 94.40 % to 99.12 % after 10 min of 365 nm UV irradiation. In addition, the composite nanofiltration membranes exhibited good antifouling performance due to hydrophilic and highly smooth surface. By controlling the trans-to-cis AZO isomerization via UV /visible (Vis) light, the solvent permeance and dye rejection can be dynamically tuned due to the adjustable pore size.

The synergistic effects of incorporating bi-sulfonated covalent-organic frameworks nanosheets and surface modification on the durability of thin-film nanocomposite nanofiltration membranes

Improving the membrane durability without sacrificing perm-selectivity is an essential challenge in membrane technology. To address this challenge, a combination of incorporating bi-sulfonated covalent-organic frameworks nanosheets (TpPa-(SO3H)2 CONs) and surface modification was used to modify thin-film composite (TFC) nanofiltration membranes. The nanofiltration experiments and molecular dynamics (MD) simulations revealed that nanosheets dispersed in n-hexane could significantly modulate the piperazine diffusion. At the optimal nanosheet addition amount of 0.004 g/100 ml n-hexane, the pure water permeance (PWP) and Na2SO4 rejection of the thin-film nanocomposite (TFN) membranes increased to 9.35 L·m−2·h−1·bar−1 and 99.26 %. Meanwhile the corresponding values of the pristine TFC membranes were only 5.45 L·m−2·h−1·bar−1 and 96.99 %, respectively. Membrane surface modification was performed on the optimal TFN surface. When the surface was modified with Tp at a concentration of 0.004 g/100 ml n-hexane for 9 s, the PWP further increased to 11.42 L·m−2·h−1·bar−1, which was almost 2.10 folds higher than the original TFC membranes. In contrast, the Na2SO4 rejection slightly increased to 99.36 %. Moreover, the static immersion experiments results indicated that the surface modification enhanced the membrane durability. This study proved that the association of these two methods provides a simple and effective strategy to design nanofiltration membranes with excellent separation performance and durability simultaneously.

Preparation of N-MG-modified PVDF-CTFE substrate composite nanofiltration membrane and its selective separation of salt and dye.

Poly(vinylidene fluoride-co-chlorotrifluoroethylene) (PVDF-CTFE) is considered an ideal membrane material for the treatment of complex environmental water due to its exceptional thermal stability and chemical resistance. Thus, to expand its application in the field of nanofiltration (NF) membranes, in this study, N-methylglucamine (N-MG) was used to hydrophilically modify PVDF-CTFE, overcoming the inherent hydrophobicity of PVDF-CTFE as a porous substrate membrane, which leads to difficulties in controlling the interfacial polymerization (IP) reaction and instability of the separation layer structure. The -OH present in N-MG could replace the C-Cl bond in the CTFE chain segment, thus enabling the hydrophilic graft modification of PVDF-CTFE. The influence of the addition of N-MG on the surface and pore structure, wettability, permeability, ultrafiltration separation, and mechanical properties of the PVDF-CTFE substrate membrane was studied. According to the comparison of the comprehensive capabilities of the prepared porous membranes, the M4 membrane with the addition of 1.5 wt% N-MG exhibited the best hydrophilicity and permeability, indicating that it is a desirable modified membrane for use as an NF substrate membrane. The experiments showed that the rejection of Na2SO4 by the NF membrane was 96.5% and greater than 94.0% for various dyes. In the test using dye/salt mixed solution, this membrane exhibited a good separation selectivity (CR/NaCl = 177.8) and long-term operational stability.

Constructing high-performance nanofiltration membranes using nanofiber supports: Effects of structural stability and polyvinyl alcohol interlayers

Nanofiber supports are promising candidates for constructing high-performance nanofiltration (NF) membranes due to their unique interconnected pores and high porosity. Conventional nanofiber supports usually possess excessive surface roughness and low structural stabilities, influencing the formation of polyamide separation layers. However, research on the effects of nanofiber support properties on the NF membrane performances is relatively few. In this study, we systematically evaluated the effects of structural properties of the nanofiber supports on the performances of the fabricated thin-film nanofiber composite (TFNC) NF membranes. The polyimide (PI) support with heat-pressing post-treatments owns a smooth surface and high structural stability due to the welding and compaction of the PI nanofibers. This promoted the formation of a polyamide separation layer with high smoothness and low fouling tendencies. Consequently, the mechanical strength and long-term operation stability of the TFNC-PI NF membrane were much stronger than that of the membranes with polyacrylonitrile (PAN) and PVDF nanofiber supports. Membrane performance analyses illustrated that the TFNC NF membranes possess notable higher water permeance while slight lower salt rejection than the commercial NF membrane. Compared to TFNC-PAN NF membranes (18∼20 L m-2h−1 bar−1), the water permeance of the structurally stable TFNC-PI NF membrane was relatively low (15–16 L m-2h−1 bar−1) due to its smaller surface filtration area and membrane pore size. Further attempts illustrated that constructing interlayers with 2 wt% polyvinyl alcohol (PVA) coating solutions resulted in the formation of clear and regular Turing structures in the polyamide surface layer. It notably enhanced the water permeance of the membrane by ∼30 % without compromising the salt rejection capability of the membrane. This study provides fundamental insights into the effects of nanofiber support properties on NF performance and feasible pathways to construct high-performance TFNC NF membranes.

Enhanced properties of a positive-charged nanofiltration membrane containing quaternarized chitosan through second interfacial polymerization for the removal of salts and pharmaceuticals.

Nanofiltration (NF) membrane technology has been widely used in the removal of salts and trace organic pollutants, such as pharmaceuticals and personal care products (PPCPs), due to its superiority. A positive-charged composite NF membrane with an active skin layer was prepared by polyethyleneimine (PEI), trimethyl benzene chloride, and quaternate chitosan (HTCC) through second interfacial polymerization on the polyethersulfone ultrafiltration membrane. The physicochemical properties of the nanocomposite membrane were investigated using surface morphology, hydrophilicity, surface charge, and molecular weight cut-off (MWCO). The influence of the concentration and reaction time of PEI and HTCC was documented. The optimized membrane had a MWCO of about 481 Da and possessed a pure water permeability of 25.37 L·m-2·h-1·MPa-1. The results also exhibited salt rejection ability as MgCl2 > CaCl2 > MgSO4 > Na2SO4 > NaCl > KCl, showing a positive charge on the fabricated membrane. In addition, the membrane had higher rejection to atenolol, carbamazepine, amlodipine, and ibuprofen at 89.46, 86.02, 90.12, and 77.21%, respectively. Moreover, the anti-fouling performance and stability of the NF membrane were also improved.

Membrane fouling control and contaminant removal during direct nanofiltration of surface water

Direct nanofiltration (NF) is an attractive separation process that produces high-quality drinking water without pretreatment. However, the propensity for membrane fouling is a considerable challenge. This study evaluated the potential of direct NF treatment of surface water using submerged flat-sheet (polyamide composite) NF membranes to achieve stable operation and high-quality water production. Direct NF treatment at a constant permeate flux of 5.0 L/m2h (NF270) and 2.5 L/m2h (NF90) was successfully continued for over 100 d with a monthly one-wipe sponge cleaning. Sponge cleaning eliminated the foulants on the NF270 membrane surface, thereby recovering the membrane permeability. Contrastingly, tight NF90 membranes did not exhibit significant changes in membrane permeability. The rejection of odorous compounds and other dissolved organic matter by the NF90 membrane was significant (>95 %), whereas those by the NF270 membrane considerably varied with the feed temperature. Further investigation suggested that the visible foulant on the membrane surface could be a porous cake layer mainly composed of constituents with a molecular weight of >300 Da, resulting in low hydraulic resistance. One-wipe sponge cleaning can readily detach fluffy cake layers. The results of this study suggest the viability of direct NF treatment as a low-energy advanced drinking water treatment system.

Scalable, Interfacially Synthesized, Covalent-Organic Framework (COF)-Based Thin-Film Composite (TFC) Hollow Fiber Membranes for Organic Solvent Nanofiltration (OSN).

Covalent organic frameworks have great potential for energy-efficient molecular sieving-based separation. However, it remains challenging to implement COFs as an alternative membrane material due to the lack of a scalable and cost-effective fabrication mechanism. This work depicts a new method for fabricating a scalable in situ COF hollow fiber (HF) membrane by an interfacial polymerization (IP) approach at room temperature. The 2D COF film was constructed on a polyacrylonitrile HF substrate using aldehyde (1,3,5-trimethylphloroglucinol, Tp) and amine (4,4'-azodianiline (Azo) and 4,4',4″-(1,3,5-triazine- 2,4,6-triyl) trianiline (Tta)) as precursors. The COF membrane on the PAN substrate showed 99% rejection of Direct red-80 with remarkable solvent permeance. The rejection analysis revealed that the structural aspects of the solute molecule play a major role in rejection rather than the molecular weight. We further optimized the precursor concentrations to improve the permeation performance of the resulting membrane. The durability study reveals excellent stability of the membrane toward organic solvents. This study also demonstrated the easy scalability of the membrane fabrication approach. The approach was further extrapolated to fabricate a cation-based COF membrane. These charged membranes exhibited an enhanced rejection performance. Finally, this approach can facilitate industrially challenging molecular sieving applications using COF-based membranes.

Compact SiOC ceramic composite nanofiltration membranes by slow dip coating for water purification

In this work, novel silicon oxycarbide (SiOC) ceramic composite nanofiltration membranes with high performance were successfully synthesized at low sintering temperatures by gradual dip-coating approach for the first time. The slow dip-coating process facilitated improved wetting and even spreading of the precursor solution across the substrate. This controlled method ensured the formation of uniform and continuous SiOC ceramic membranes without the introduction of substantial defects or irregularities. Consequently, the systematic approach minimized the occurrence of pinholes, cracks and other structural imperfections in the SiOC ceramic membranes. The synthesis of the SiOC separation layer involved the sintering of polymeric polysilicon as precursor, which caused a lower decomposition temperature in contrast to the conventional method employing silicon carbide powder for sintering. The specific feature enabled a sharp decrease in the sintering temperature which was required for the formation of the separation layer. The prepared SiOC ceramic composite nanofiltration membranes exhibited good permeability with high rejection of dye molecules (>95%) and maintained stable water flux for at least 24 h, showing good long-term stability during cross-flow filtration. This progress provides new approach for the fabrication of compact ceramic nanofiltration membranes with low sintering temperatures, high separation efficiency and enhanced stability.

A sub-10 nm polyamide nanofiltration membrane from polyvinylpyrrolidone-mediated interfacial polymerization

In conventional interfacial polymerization (IP) for thin-film composite nanofiltration membranes, the separation layer is typically thick (∼100 nm), which limits its water permeance. In this study, we successfully synthesized an ultrathin polyamide layer (∼8.6 nm) from polyvinylpyrrolidone (PVP)-mediated IP. With the surfactant concentration over its critical micelle concentration, PVP not only formed a monolayer at the water-heptane interface, but also stayed in the aqueous solution. The CO groups of PVP formed the N–H⋯O hydrogen bond interactions with N–H bonds of piperazine (PIP) monomers in the aqueous phase to slow down the diffusion of PIP monomers. Besides, PVP with amphiphilic groups minimized the interfacial tension at the water-heptane interface to regulate IP processes. By adjusting the concentration of PVP, we obtained a sub-10 nm polyamide nanofiltration membrane. Moreover, hydrophilic PVP macromolecules improved the membrane surface hydrophilicity. Due to the ultrathin thickness and improved hydrophilicity, the optimized polyamide membrane demonstrated a threefold increase in permeance compared to that of the pristine polyamide membrane without PVP addition, and maintained 98.4% rejection for 1000 ppm Na2SO4 solution. This study provides a new insight into fabricating sub-10 nm polyamide membranes for nanofiltration processes.

Effects of mixed amines on the microstructure and performance of PVDF LNF membranes by Co-deposition via IP

Selective recovery of recyclables is a major challenge when treating textile wastewater. For this reason, there is an urgent need to develop nanofiltration membranes with high permeability and selective filtration. In response, a novel composite PVDF loose nanofiltration membrane was developed by combining two different mixed amines system/dopamine (DA) followed by interfacial polymerization with benzoyl chloride (TMC). The two different amines systems were a mixture of GAZ and PEI and a mixture of PIP and PEI, respectively. The effects of GAZ/PEI and PIP/PEI on the microstructure was characterized by SEM, AFM, FTIR and XPS and performance of the membrane was also investigated. The effects of GAZ/PEI and PIP/PEI on the microstructure and the selective layer of membranes were different. The results stated that with the increase of GAZ/PEI the thickness of the selective layer, surface roughness decreased, while the fluxes of pure water and dye wastewater increased. However, with increasing PIP/PEI, surface nodules and roughness both increased. Contrarily, the thickness of the selective layer and PEG800 rejection first decreased and then increased, while the flux of pure water and the dye wastewater first increased and then decreased. The optimal GAZ/PEI and PIP/PEI were 9/91 and 50/50, respectively. When the GAZ/PEI was 9/91, the pure water flux and the RB5 wastewater flux were 20.7 L∙(m2∙h)−1 and 11.5 L∙(m2∙h)−1. The dye (RB5) retention rate was 94.3% and salt retention rate was 2.1%. However, when the PIP/PEI was 50/50, the pure water flux and RB5 wastewater flux were 26.6 L∙(m2∙h)−1 and 17.1 L∙(m2∙h)−1 increased by 87.3% and 134.2% respectively. The retention rates of RB5 and NaCl were 96.3% and 1.7%. The above results indicate that the composite PVDF LNF membranes prepared with mixed amine monomers instead of single amine monomers have good selective filtration performance.

From environmental issue to purification aid: Novel positively charged functionalized algal biochar as robust modifier of composite nanofiltration membranes

Robust membrane modifiers were achieved for the first time by functionalizing the algal biochar of unique porous structure. The biochar was prepared through the pyrolysis of Cladophora glomerata, the most widespread freshwater macroalga, functionalized by diethylenetriamine and dendrimer poly(amidoamine), and employed to fabricate positively charged composite nanofiltration membranes. The presence of hydrophilic functionalizers of positive charge on the membrane was verified through Fourier transform infrared and energy dispersive X-ray analyses and atomic force microscopy and zeta potential measurements were performed to determine surface roughness and confirm positive charge of the modified membranes. Dispersion of modifiers on the surface and morphology of the were also revealed through field-emission scanning electron microscopy images. It has shown that, compared to the pristine membrane, pure water fluxes were increased by 214% and 185%, and water contact angles were reduced from 66.1° to 39.5° and 43.3° in those modified by biochar functionalized with dendrimer poly(amidoamine) and diethylenetriamine, respectively. More than 90% dye rejections and salt and heavy metals removals were recorded for the membranes possessed 0.6 wt% of modifiers. Finally, a comparative study conducted between the novel modifier introduced in this study and those reported in the literature, indicated that C. glomerata biochar decorated with amine functional groups could be considered as a robust and practical alternative to the common modifiers used to manipulate nanocomposite membranes characteristics.

Application of a functionalized thin-film composite nanofiltration membrane in water desalination

To prepare a thin-film nanocomposite (TFN) nanofiltration (NF) membrane, characterized with high desalination rate and antifouling performance, carboxylated multi-walled magnetic 1,2,3,4-butanetetracarboxylic acid carbon nanotubes (hereafter, the C-MWCNT-MBTCA) with a choice of hydrophilic groups were initially synthesized in this study, and then incorporated into the polyamide (PA) layer via interfacial polymerization (IP). After that, the surface morphology and hydrophilicity of the fabricated NF membrane was examined by the field emission scanning electron microscopy (FESEM) and water contact angle measurements, respectively. To assess the membrane application, the solutions (2000 mg/L) of sodium sulfate (Na2SO4), magnesium sulfate (MgSO4), calcium chloride (CaCl2), and sodium chloride (NaCl) were consistently tested. On account of the ample carboxylic acid (R-COOH) groups in the modified TFN membrane, the study outcomes revealed that the given membrane could enhance the flux along with the desalination rate upon the addition of the C-MWCNT-MBTCA (0.04 wt%). Likewise, the pure water flux significantly grew from 58.8 to 110.5 Lm-2h−1 after the utilization of the C-MWCNT-MBTCA (0.04 wt%), and the Na2SO4 removal rate promoted from 87 to 98 %. Ultimately, it was established that the C-MWCNT-MBTCA could be used as an economical modifying agent to boost the performance of NF membranes for water desalination purposes.

Scalable nanofiltration membranes with sharpened pore distribution and enhanced negativity for mono/divalent anion separation

The petrochemical industry is an important producer of high salinity wastewater, necessitating the efficient separation of salts in zero discharge processes. This study reports the development of an industrially scalable thin film composite nanofiltration membranes that facilitate the effective separation of NaCl/Na2SO4. Employing a one-step functionalization process, the membrane pore distribution is sharpened, and the surface negativity is intensified. Branched polyethyleneimine with abundant reactive amine groups is employed as an aqueous co-monomer, leading to the generation of high density grafting active sites. Nucleophilic substitution reaction is employed to functionalize the membrane surface with negatively charged sulfonic acid groups. Simultaneously, large voids within the free volume are filled and the pore size distribution is effectively narrowed. The resultant membranes demonstrate an impressive rejection of 99.5 % for Na2SO4, a selectivity of 163.4 for NaCl/Na2SO4 and a selectivity of 488 for Cl−/SO42−, with a water flux of 45 LMH under 0.5 MPa. The corresponding 8″ × 40″ sized spiral wound membrane modules exhibit a NaCl/Na2SO4 selectivity of 100.4 and a Cl−/SO42− selectivity of 258.3, demonstrating high practical applicability. A pilot trial is planned to integrate these novel membrane products into zero discharge processes within the internal chemical enterprises of Sinopec.

A gelatin-zirconium phosphate nanoparticles composite interlayer for enhancing compaction resistance and antifouling performance of TFC NF membrane

From a practical point of view, the thin-film composite (TFC) nanofiltration (NF) membrane should not only have superior separation performance but also excellent compaction resistance and antifouling properties. Generally, polyamide (PA) separation layers of the TFC NF membrane prepared on the support with hydrophilic interlayer are crumpled, which may be detrimental to the compaction resistance and antifouling properties of TFC NF membranes. A flexible polymer-rigid nanoparticles composite interlayer was constructed on the pristine polysulfone (PSF) support in this work to design a TFC NF membrane with compaction resistance, antifouling, and excellent separation performance. Morphology and crosslinking degree of PA separation layers were investigated by scanning electron microscopy (SEM), atomic force microscope (AFM), and X-ray photoelectron spectroscopy (XPS). The results demonstrated that the composite interlayer can not only regulate the distribution and diffusion of piperazine (PIP) but also act as a template to induce the generation and growth of PA during the interfacial polymerization (IP) process, leading to the formation of the PA separation layer with a small nodular structure. The compaction resistance, antifouling properties, and separation performance of the TFC membrane were also investigated by nanoindentation, fouling experiment, and filtration test, respectively. The TFC NF membrane with the composite interlayer showed greatly improved mechanical properties with its hardness and elastic modulus approximately 1.5 and 1.6 times that of the reference, respectively. Furthermore, the flux recovery of the TFC NF membrane with the composite interlayer (93.8 %) was higher than that of the control (87.2 %), indicating that the TFC NF membrane with the composite interlayer had better antifouling properties. In addition, the obtained TFC NF membrane with the composite interlayer showed a water flux approximately twice as high as that of the reference with satisfactory rejection. This advancement will contribute to the development of more efficient and stable TFC NF membranes for industrial wastewater treatment.

Construction of a high-performance hollow fiber composite NF membrane via the simultaneous modification of substrate and selective layer

Hollow fiber composite (HFC) nanofiltration (NF) membrane demonstrates significant commercial potential profiting from its self-supporting characteristic, high packing density and strong fouling resistance. In this work, a high-performance HFC NF membrane was successfully developed through simultaneous modification of the substrate via alkaline hydrolysis and incorporation of iopamidol (IPD) into the selective layer. Alkaline hydrolysis enhanced the hydrophilicity of the polyacrylonitrile (PAN) substrate by converting –CN groups into –COOH, thereby sharply improving the success rate of HFC NF membrane fabrication. The addition of IPD into piperazine (PIP) solution effectively increased the water permeability of the HFC NF membrane without compromising its salt rejection. Specifically, the optimized membrane sample denoted as NF-0.15P + 0.5I with 0.5 w/v% IPD added demonstrated a 44 % increase in PWP from 21.8 L·m−2·h−1·bar−1 to 31.5 L·m−2·h−1·bar−1, while maintaining a Na2SO4 rejection over 96.8 %. This improvement can be attributed to the decreased selective layer thickness and reduced MWCO. Furthermore, the membrane exhibited exceptional dye/salt separation performance. This work represents a novel approach leading to the development of a HFC NF membrane suitable for dye/salt separation.

Construction of covalent organic frameworks membranes in situ through nonsolvent-induce phase separation for fast and accurate nanofiltration

With the development of covalent organic frameworks (COFs), their superior separation performance in membrane separation has gradually becoming apparent. The method of combining COFs with membrane materials is crucial for their applications prospect. In this study, inspired by the traditional Chinese mortise and tenon structure, we selected 1,3,5-triacetaldehyde mesityl phenol (TFP) and 1,3,5-tris(4-aminophenyl)benzene (TAPB) as the reactive building monomers. P-Phenylenediamine (PDA) was introduced into the casting solution to construct Polyether-sulfone (PES) bottom membranes with amino ligand sites, utilizing the phase transition effect generated by the non-invasive solvent-induced method (NIPS). Subsequently, COFs composite nanofiltration membranes with two-dimensional layered structures were prepared through mild interfacial polymerization (IP) under optimized reaction conditions. The synthesis of COFs composite nanofiltration membranes was demonstrated through a series of characterizations, including scanning transmission electron microscopy and infrared spectroscopy. The COFs composite nanofiltration membranes synthesized by this method exhibited better alignment, greater chemical stability, and higher separation efficiency compared to traditional interracially polymerized COFs composite nanofiltration membranes. As the method involves the addition of additives, it can be more widely applied to a broad range of organic polymer membranes. Furthermore, we demonstrated the effectiveness of the COF composite nanofiltration membranes produced using this method in the separation of molecular dyes. They exhibited rejection exceeding 99% for 0.1 g L−1 methyl blue (MB), Congo red (CR) and chrome black-T (EBT). Additionally, they achieved separation fluxes of up to 300 L m−2 h−1 bar−1 for mixed solutions (dyes/salts) and proved to be effective under extreme conditions, such as 1 M HCl/NaOH. Moreover, COFs-NH2PES composite nanofiltration membranes exhibited the ability to efficiently separate dye molecules of different sizes. This work introduces new ideas for constructing stable and durable COF functional composite membrane materials for practical applications.

Preparation and performance of magnetic carbon nanotubes modified PVC substrate composite nanofiltration membranes

Polyamide (PA)/ferroferric oxide/oxidized multi-walled carbon nanotubes (Fe3O4/o-MWCNTs)/polyvinyl chloride (PVC) composite nanofiltration (NF) membranes were prepared by the interfacial polymerization (IP) process with the Fe3O4/o-MWCNTs/PVC porous membrane as the substrate membrane, which was prepared by the magnetic field assisted non-solvent induced phase separation method (NIPS) with the synthesized superparamagnetic Fe3O4/o-MWCNTs nanoparticles (NPs) as the additives of the functional intermediate layer integrated with the substrate. The effects of the magnetic carbon nanotubes modified PVC porous substrate membrane on the surface chargeability, PA layer thickness, mechanical properties, pressure resistance and permeability of the composite NF membranes were investigated, and the comprehensive performance of composite NF membranes was enhanced. The results indicated that the porosity and hydrophilicity of the Fe3O4/o-MWCNTs/PVC porous substrate membrane were improved by the migration of magnetic Fe3O4/o-MWCNTs NPs to the upper edge of the substrate membrane and uniform enrichment under magnetic field induction, which further affected the structure and properties of the PA/Fe3O4/o-MWCNTs/PVC composite NF membrane. The prepared NF membrane presented smaller roughness, thinner PA layer, larger pore size, with excellent mechanical properties and better pressure resistance. Meanwhile, the PA/Fe3O4/o-MWCNTs/PVC composite NF membrane presented excellent divalent salt rejection, with the rejection of MgSO4 and Na2SO4 maintaining more than 95.2% and 93.4%, respectively, while the permeation flux could reach 35.4 L·m−2·h−1. In summary, the modification of the substrate integrated with the functional intermediate layer had certain enlightening significance for the preparation of the NF membrane with excellent comprehensive performance.

Dually Charged Acid Stable Tight Polyamine Composite Nanofiltration Membranes with Low Molecular Weight Amine Molecules for MgCl2 Rejection

Dually Charged Acid Stable Tight Polyamine Composite Nanofiltration Membranes with Low Molecular Weight Amine Molecules for MgCl₂ Rejection