Preparation Of Nanofiltration Membranes Research Articles

Ultra-fast interlayer construction strategy for the preparation of NF membranes with high Li+/Mg2+ separation performance

Nanofiltration (NF) membranes have demonstrated significant potential in lithium extraction from salt-lake brines due to their superior selectivity in separating monovalent cations from divalent cations. In this paper, NF membranes with high permeate flux and high lithium-magnesium separation factor (SLi,Mg) were prepared by using a polyamide (PA) layer as the separation layer, RA-PEI as the interlayer, and a polysulfone (PSf) membrane as the substrate membrane. Notably, the RA-PEI co-deposition modification method is an ultra-fast substrate modification method, requiring only 1s for modifying PSf substrates. At the same time, RA-PEI firmly adheres to the surface of the PSf substrate by the synergistic effects of intermolecular hydrogen bonding interactions and π-π interactions. This was further verified by comparing the adsorption energies of different kinds of organic interlayers and PSf substrates obtained from molecular dynamics (MD) simulations. Correlation characterization showed that the RA-PEI interlayer endowed the NF membranes with reduced average pore size as well as attenuated electronegativity, thus realizing the efficient separation of Mg2+ and Li+. The performance assessment showed that the membrane containing RA-PEI interlayer exhibited excellent separation performance and permeate flux in simulated salt-lake brine (2000 ppm, WMg/Li = 20), where SLi,Mg (92.8) and permeate flux (16.7 L·m−2·h−1·bar−1) were improved by 7.8 and 2.2 times, respectively, in comparison to the pristine NF membrane. Therefore, this work provides a useful contribution to the practical use of NF membranes for lithium extraction from salt lakes.

Incorporation of Graphene Oxide Quantum Dots in Gradient Layers of Polyethersulphone Nanofiltration Membranes for Nitrate Rejection from Aqueous Solution.

Graphene oxide quantum dots (GOQDs) have been widely used to prepare nanofiltration membranes due to the merits of excellent dispersity, ultrasmall size, and unique properties related to graphene. In this study, we first prepared the polyethersulphone-based nanofiltration (PES-NF) membrane via an interfacial polymerization process using a piperazine and m-phenylenediamine mixed solution as the aqueous phase. Then GOQDs were incorporated into the top-down gradient structured layers (i.e., ultrathin layer, interlayer, and substrate membrane layer) of the nanofiltration membrane, and subsequently the effect of GOQD addition on the nitrate rejection was evaluated. Compared with the pristine PES-NF membrane without the incorporation of GOQDs, the fabricated NF membrane (GOQD/PES-NF-2) incorporating GOQDs at both the ultrathin layer and interlayer exhibits more remarkable performances (an acceptable permeation flux of 52.2 L m-1 h-1 and excellent nitrate rejection of 96.3% at 0.6 MPa), the permeation flux of this membrane increases by nearly 2.4 times, and its nitrate rejection also shows a slight enhancement (∼7.6%) compared with those of PES-NF. Remarkably, at the operating pressure much lower than that required by reverse osmosis membranes, the GOQD/PES-NF-2 membrane possesses an equivalent monovalent ion rejection to reverse osmosis membranes but a higher permeation flux. Furthermore, the result of a 7 day continuous stability test validates the excellent durability of the GOQD/PES-NF-2 membrane, and its antifouling and chlorine resistance performances also outperform those of the PES-NF membrane.

Performance regulation of the TFC NF membrane via the introduction of oleic acid into different organic solvents

AbstractBACKGROUNDTailored design of high‐performance nanofiltration membranes is desirable. The physicochemical properties of the organic phaseare essential for the preparation of nanofiltration membranes. Therefore, byfine‐tuning the organic phase, it is possible to achieve nanofiltrationmembranes with enhanced performance.RESULTSIn this work, oleic acid, a naturally extracted fatty acid, wasintroduced into three different organic solvents (n‐heptane, n‐octane andisopar G) to construct special oil phases for interfacial polymerization. Testingresults showed that proper dosage of oleic acid addition can effectivelydecrease the nanofiltration membrane pore size and enhance the salt rejectionregardless of the solvent type. Taking n‐octane as example, 5% oleic acidaddition in it can decrease the mean pore size from 0.500 nm to 0.341 nm and improve the MgSO4 rejection from 52.4% to 95.5%. Further characterizationindicated that the introduction of oleic acid into organic solvent cansignificantly reduce the interfacial tension and promote the diffusion of piperazinefrom aqueous phase to the oil phase. In addition, oleic acid could react with piperazineand produce some white particles that could be embed into the polyamide layer, thus altering the surface morphology.CONCLUSIONIn this work, a novel modified thin film composite nanofiltration membrane was prepared by a simple and controllable oleic acid assisted interfacial polymerization strategy, which exhibits good water permeability, solute selectivity and antifouling property. Without changing the existing process, our strategy opens up a simple, environmentally friendly and operable route for the synthesis of thin film composite nanofiltration membranes. © 2024 Society of Chemical Industry (SCI).

Dual-functional TFNC polyester membranes utilizing maltitol for dye/salt separation and desalination

To achieve a promising application prospect in nanofiltration (NF) membrane separation, a strategic design tailored to particular functions is crucial. Here, tailor-made thin film nanofibrous composite (TFNC) NF membranes for specific applications were fabricated via interfacial polymerization (IP) between maltitol and trimesoyl chloride (TMC) on PAN nanofibrous substrates. Owing to the moderate reactivity of maltitol, a range of sugar alcohol based polyester membranes with different morphologies and cross-linking degrees were obtained by varying the monomer concentration as well as the reaction time. In this case, the finely tuned loose Mal1-TMC0.2-2 membrane exhibited a prominent permeability (110.34 L m−2 h−1 bar−1) and an extraordinary selectivity for dye/salt mixed solution, with the rejection of Congo Red up to 99.11 % and NaCl retention of only 7.41 %, indicating its suitability for textile wastewater treatment. Further, the dense Mal2-TMC0.4-10 membrane showed an excellent Na2SO4 rejection of 97.40 % with a good permeability of 18.65 L m−2 h−1 bar−1, making it a candidate for seawater and brackish water desalination. In addition, both membranes demonstrated reliable chlorine and pressure resistance along with long-term stability. This study provided new insights and inspiration to the design and preparation of NF membranes for specialized applications.

Optimization of ultrathin polyamide nanofiltration membranes with sulfonated polyethersulfone interlayers for enhanced performance

The “trade-off” effect of nanofiltration membranes may lead to problems of lower flux and higher energy cost, which limit their applications. In this work, an ultra-thin polyamide selective layer (34 nm in thickness, much lower than the conventional polyamide selective layer) was prepared by constructing a sulfonated polyethersulfone interlayer on the surface of polysulfone ultrafiltration membranes by liquid-phase deposition, taking advantage of the inhibitory effect of sulfonated polyethersulfone nanoparticles on the diffusion of piperazine. The prepared nanofiltration membrane exhibits a water flux per unit pressure of 33.2 L·m−2·h−1·bar−1, which is more than 2.5 times higher than that of the nanofiltration membrane without the introduction of the interlayer (12.2 L·m−2·h−1·bar−1). Additionally, it has a high rejection of 98.8 % for Na2SO4, which surpasses the upper limit of the equilibrium of trade-off. The study also delves into the modulation mechanism of the interfacial polymerization process by the presence of a sulfonated polyethersulfone interlayer and explores the potential reasons for the water flux enhancement. This research will likely provide theoretical support and practical experience for further optimization of nanofiltration membrane performance.

Reviewing Recent Advances in Nanofiltration Membranes for Efficient Lithium and Magnesium Separation from Salt Lake Brines

In recent years, with the rapid development of the new energy industry, electric vehicles and electronic wearable devices, the consumption of elemental lithium is increasing rapidly, and the demand for lithium resources is challenging to fulfil. Salt lake brine is rich in lithium. However, the high ratio of magnesium to lithium in salt lake brine makes the separation of lithium and magnesium from salt lake brine very challenging. Compared with extraction, adsorption, electrochemistry, reaction coupling and other methods, nanofiltration has attracted much attention as an environmentally friendly, energy-efficient and low-cost process which can effectively separate lithium and magnesium and enrich lithium to facilitate its further purification. This review analyzes nanofiltration membrane preparation methods and evaluates the effects of temperature, pressure, pH and salinity on the performance of the membranes, which prefer low pH and salinity environments. In addition, the ability of nanofiltration to reduce the Mg/Li ratio, as well as its stability, was analyzed in terms of monomer optimiza­tion, post-modification and process improvement.

Effect of Tetramethylurea (TMU) on Polysulfone Membrane Performance for Atrazine-containing Wastewater Treatment

Tetramethylurea (TMU) is a good solvent for organic substances and has received little attention as compared to other solvents. The TMU is a polar solution and is one of the molecules with an amphiphilic character. In the present work, an attempt has been made to use TMU as an additive in the preparation of nanofiltration membranes to improve the hydrophilicity of the membrane. The polysulfone membrane has been modified by incorporating different concentrations of TMU (0, 0.5, and 1 wt.%) in order to check the rejection of atrazine in water. This study aim is to optimize the conditions to enhance the flux and the rejection of atrazine. It was observed that the rejection of atrazine was enhanced when feed pH changed to acidic and with increasing the evaporation time. The prepared membranes were subjected to different analyses, such as contact angle measurement, FTIR, porosity, and mean pore size. The effect of the coagulation bath, evaporation time, and pH on the atrazine rejection was also studied. Membrane with 0.5 wt.% TMU shows maximum rejection of atrazine at the operating pressure of 15 kgf/cm2.

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.

Preparation of polysulfone/sulfonated polysulfone-SiO2 nanofiltration membrane and its performance characterization

Purpose This study aims to optimize the preparation conditions and modify the nanofiltration (NF) membranes to prepare high-performance polysulfone/sulfonated polysulfone composite nanofiltration (PSF/SPSF-NF) membranes through interfacial polymerization. Design/methodology/approach Investigating the impacts of anhydrous piperazine (PIP) concentration, trimesoyl chloride (TMC) concentration and basement membrane type on NF membrane performance, the optimal membrane was prepared. In addition, nano-SiO2 was added to the active separation layer to modify the NF membranes. Findings The comprehensive performance of PSF/SPSF-NF membranes was optimized when the concentration of PIP was 0.75 Wt.% and the concentration of TMC was 0.15 Wt.%, at which time the water flux was 66.1 L·m−2·h−1 and the retention rate of Na2SO4 was 98.1%. The comprehensive performance of polysulfone/sulfonated polysulfone-SiO2 nanofiltration (PSF/SPSF-SiO2-NF) membranes was optimized when the blending ratio of nano-SiO2 to PIP was 2:3, with a pure water flux of 81.9 L·m−2·h−1 and a Na2SO4 retention rate of 95.9%. Compared to polysulfone nanofiltration (PSF-NF) membranes and PSF/SPSF-NF membranes, NF membranes with nano-SiO2 increased the flux recovery rate by 22.9% and 8.7%. Practical implications PSF/SPSF-SiO2-NF membrane exhibits excellent antifouling properties. Originality/value There is currently no literature available on the preparation of NF membranes using polysulfone/sulfonated polysulfone (PSF/SPFS) as a substrate. This provides a method for modifying NF membranes, starting with the modification of the basement membrane and then modifying the active separation layer.

Acetone extraction induced piperazine diffusion reaction for regulating thin film composite nanofiltration membrane

The regulation of monomer diffusion in interfacial polymerization played a vital role in manipulating structure and enhancing separation performance of nanofiltration (NF) membrane. The co-solvent strategy has been recognized to be valid and applicable for improving membrane performance. However, the underlying mechanism for its effect on the diffusion progress of amine monomers was still unclear and contradictory. In this study, we found that the added co-solvent (acetone) induced piperazine diffusion flood through interfacial co-solvent extraction effect due to the interfacial enrichment of acetone and innovatively proposed a concept of “acetone extraction effect”. The mechanism and molecular dynamics simulation (MD) were used to systematically explain the effect of diffusion on the morphology and properties of the membranes. The prepared NF membranes exhibited improved separation performance for Calcium (CaCl2, 71.5 ± 0.6 %) and Magnesium (MgCl2, 83.1 ± 0.8 %) ions, which were better than many of the recorded membranes. This study provided a new idea for the co-solvent effect in interfacial polymerization and promising guidance for the preparation of superior NF membranes.

Incorporating etched ZnO nanoparticles to fabricate positively charged composite membrane for heavy metal removal

The removal of heavy metal ions by nanofiltration (NF) membranes is attractive, but it is limited by a trade-off effect between permeation and solute rejection. Generally, the permeability of the NF membrane is closely related to its hydrophilicity, while the solute rejection is closely related to size exclusion and the Donnan effect. In this study, porous etched zinc oxide (ZnO) nanoparticles were prepared through alkali etching, and a thin-film nanocomposite (TFN) NF membrane was fabricated by interfacial polymerization (IP) method between PEI and TMC on a polysulfone (PSF) substrate. The incorporation of 0.01 wt% etched ZnO nanoparticles increased the hydrophilicity and positive charge of the NF membrane, resulting in an increase in the permeability (from 6.0 L m−2 h−1 bar−1 to 8.1 L m−2 h−1 bar−1) and rejection of heavy metal ions (Cu2+, Zn2+, and Ni2+ increased from 85 % to over 96 %). Long-term filtration experiments using the simulated wastewater demonstrated that the prepared NF membrane exhibited good stability and had the potential for industrial applications. It opens a new pathway for studying TFN NF membranes, indicating that alkaline etching can transform smooth nanoparticles into nanoparticles with higher specific surface area and porosity, thereby improving the performance of membranes.

Evaluation of nanofiltration and reverse osmosis membranes for efficient rejection of organic micropollutants

Nanofiltration (NF) and reverse osmosis (RO) membranes have revolutionized water treatment processes. However, emerging separations, such as the removal of organic micropollutants (OMPs), pose a challenge for traditional RO and NF membranes due to either low water permeance (RO) or inadequate OMP rejection (NF). In this work, we investigated the rejection of thirteen frequently detected OMPs at environmentally relevant low feed concentration of 12 μg/L by using a wide range of commercial RO and NF polyamide thin-film composite (TFC) membranes and an in-house prepared NF membrane. The membranes were characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), contact angle, and surface charge measurements. We investigated removal capabilities of OMPs at standard brackish water RO operating conditions (i.e., 15.5 bar transmembrane pressure, pH 8) and also examined the effect of feed solution transmembrane pressure on membrane performance. The rejection of ionic OMPs was strongly influenced by electrostatic repulsion effects, whereas rejection of non-ionic solutes was primarily driven by size exclusion and hydrophobic/hydrophilic interactions. The best performing TFC membranes — a lab-made semi-aromatic piperazine-based NF polyamide membrane and a fully aromatic commercial polyamide NF membrane (NF1) — displayed excellent rejection of OMPs comparable to a commercial RO seawater membrane (RO4), but exhibited ∼2-fold and ∼6-fold higher water permeance, respectively.

Modulation of charge properties inside thin-film composite polyamide membranes towards enhanced nanofiltration performance

Nanofiltration (NF) is receiving increasing attention worldwide as a high-efficiency technology for water purification. One viable approach to enhance the performance of NF membranes involves the meticulous optimization of their physicochemical and structural attributes through the introduction of additives. Nonetheless, the predominant NF membrane additives currently accessible primarily exert their influence at the membrane's surface. In this context, we introduce a novel strategy for fabricating polyamide (PA) NF membranes endowed with augmented negative charge density, capable of permeating the innermost stratum of the separation layer. This innovation hinges on the straightforward inclusion of 2-Methylimidazole (Hmim) into the aqueous phase. The ramifications of Hmim's incorporation of the structural configuration, morphological composition, physicochemical characteristics, and overall performance of the thin-film composite (TFC) membrane were meticulously scrutinized through an array of comprehensive characterizations. The integration of Hmim affords PA nanofiltration membranes the remarkable capability to achieve a rejection rate as formidable as 99.5% for Na2SO4 and 42.8% for NaCl, all the while maintaining an unswerving flux. As a result, a high SO42-/Cl- separation factor of up to 116.1 was achieved. Our work provides insights and directions for the development of additives in the preparation of NF membranes.

Porous organic polymer interlayers modulated nanofiltration membranes for ultra-permselective Li+ /Mg2+ separation

The ion-ion precise separation is of paramount importance but presents significant technical challenges. Achieving such an accurate separation utilizing nanofiltration (NF) membranes demands ion-selective channels with appropriate pore geometry structure and charge property. In this work, thinner polyamide (PA) NF membranes with narrower pore size distribution than conventional NF membranes were fabricated via the porous organic polymer (POP) interlayer-mediated interfacial polymerization. The dual-rigid-distorted POP interlayers were incorporated in situ onto the substrates through azo-coupling reactions. The POP interlayers exhibited a high-density adsorption capacity of amine monomers and acted as a space manipulator that regulated their diffusion behavior into the oil phase, enabling their reaction with the acyl chloride. The synergistic effects engendered a uniform PA active layer and impeded further growth of the membrane, compared with the typical PA membranes, facilitating the generation of a thin and a narrow pore size distribution PA active layer with a nanostripe surface. The prepared NF membranes showed an outstanding ion-ion separation factor of Li+/Mg2+ (up to 78.56), almost 18 times that of the pristine NF membrane (4.32), along with a two-fold increase in water permeance. Furthermore, the newly developed NF membranes demonstrate long-term stability under operational pressure, thus highlighting the promising potential of these NF membranes for extracting Li+ from salt-lake brines.

Nanofiltration membrane based on a dual-reinforcement strategy of support and selective layers for efficient Mg2+/Li+ separation

With the consumption of lithium resources increasing year by year, the investigation of high-performance salt lake lithium extraction technology is extremely important. In order to effectively separate Mg2+ and Li+ from brine, a novel charge-positive nanofiltration (NF) membrane with high permeability and excellent Mg2+/Li+ separation factor was designed by interfacial polymerization (IP) in this work. A B15C5-MX-NF membrane was fabricated by blending hydrophilic two-dimensional (2D) MXene nanosheets into a PES membrane substrate to enhance the permeability, followed by the modification of polyamide (PA) layer with a crown ether molecule (Benzo-15-crown-5) to optimize the surface properties of membrane. The PA layer thickness was effectively reduced (58 nm) due to the enhancement in the interaction between the substrate membrane and PEI by the incorporation of MXene, while the addition of B15C5 provided additional Li+ permeation channels. The B15C5-MX-NF membrane exhibited excellent permeability (16.1 L·m−2·h−1·bar−1) and superior selectivity for Mg2+/Li+ (SLi, Mg of 15.8) under the dual optimization of membrane substrate and PA layer. In addition, it was demonstrated by molecular dynamics (MD) simulations that the B15C5 molecules in the B15C5-MX-NF membranes had a specific recognition ability of Li+ to allow its preferential permeation, thereby enhancing the Mg2+/Li+ selectivity of the membranes. This study provides a new idea for the preparation of NF membranes for efficient Mg2+/Li+ separation.

Tailoring the performance of composite PEI nanofiltration membranes via incorporating activated cyclodextrins

Cyclodextrins (CDs) with unique cavity structures have been used as materials for nanofiltration membrane fabrications. In the present work, the activated CD (O-CD), oxidated by NaIO4, and polyethyleneimine (PEI) were co-deposited on a hydrolyzed polyacrylonitrile support, post-treated by glycerol protection and heating treatment, to prepare nanofiltration membranes with low molecular weight cut-off (MWCO). As the cavities in CD present and the aldehyde groups introduced after oxidation, the O-CDs were expected to crosslink the PEI layer and provide extra permeating channels. The filtration experiments showed that the incorporation of O-CDs improved the permeances of the O-CD-PEI/HPAN nanofiltration membranes. The performance can be tailored by the control of the loading or the oxidation degree of the O-CD. At optimal conditions, the permeance increment was nearly double (from 9.2 to 21.1 Lm-2·h−1·bar−1). While the selectivity was without significant sacrifice, the rejection of PEG 200 remained around 90%. Meanwhile, the membrane stability was demonstrated by pro-longed filtratiing a PEG 200 aqueous solution. The constant permeance and rejection confirmed the O-CD-PEI/HPAN membranes were stable. The incorporation of activated CD in PEI offers a facile strategy to promote the permeance of PEI-based membranes.

Hybrid ceramic nanofiltration membranes prepared by impregnation and solid-state grafting of organo-phosphonic acids

Grafting of oligomers into the mesopores of ceramic membranes (e.g. γ-alumina) represents an attractive strategy for deploying durable, resistant, and efficient hybrid membranes for the industrial treatment of wastewater containing organic solvents and divers solutes. In general, to control the membrane/solute/solvent interactions, homogeneous surface modification and high grafting yields are required. However, reaching this goal is rather challenging with the current grafting approaches where the active diffusion of the oligomers to the membrane pore surface may be hampered. To mitigate such diffusion limitation, we present here a novel synthesis method consisting of an infiltration of concentrated PEG organophosphonic acid oligomer solution in ceramic ultrafiltration membranes before initiating the solid-state grafting reaction to form a PEG-brush-grafted nanofiltration ceramic membrane. The infiltration step was found to be decisive for the formation of weak bonds between the oligomer linking functions and the ceramic membrane ensuring the final synthesis of robust PEG-based membranes by a grafting reaction. The influence of the solvent polarity (water vs. cyclohexane) on the conformation of the grafted PEG brush inside the γ-alumina mesopores was observed as a key parameter in the analysis of the pore size and permeability results. In addition to this, it was studied here for the first time by molecular dynamic simulations. The as-prepared PEG-brush/ceramic membranes were found very efficient in the separation of small organic dyes such as Rhodamine B (479 g mol−1) up to 85%. As a result, the present synthesis method represents a sustainable and simple alternative way for the preparation of hybrid nanofiltration membranes with controlled surface properties and attractive separation performance. Moreover, the vacuum impregnation and solid-state grafting approach requires a minimal amount of functional oligomers and solvent (0.5 mmol for 2–3 mL of solvent) and can be applied to other porous materials used in separation technologies.

Catalyst-anchored secondary polymerization for highly permeable acid-resistant nanofiltration membrane preparation

Nanofiltration (NF) is considered a competitive technique for acidic stream treatments. However, poorly reactive acid-resistant monomers normally cause thick and uneven separation layers with ultra-low permeability. Herein, catalyst-anchored secondary polymerization is employed after the interfacial polymerization (IP) reaction between trimesoyl chloride (TMC) and 3-aminobenzenesulfonamide (ABSA) to fabricate an extremely acid-resistant NF membrane. Acylation catalysts in ethanol are grafted on the nascent layer by reacting with residual acyl chloride groups to enhance the monomer reactivity, while the ethanol-induced nascent layer structure regulation greatly improves the surface uniformity. Benefiting from the improved self-inhibition effect and optimized phase integrity of the modified IP process, the resulting acid-resistant membrane has unprecedented water permeance (up to 31.4 Lm−2h−1bar−1), desirable Na2SO4 rejection (92.8%), high dye/H+ selectivity and impressive acid stability (20 wt% of H2SO4 for 50 days), outperforming almost all advanced acid-resistant NF membranes. Our work provides a versatile approach for the synthesis of high-performance specialty membranes.

Highly permeable polyamide nanofiltration membranes with crumpled structures regulated by polydopamine-piperazine-halloysite interlayer

Highly selective and permeable nanofiltration (NF) membranes are desirable for energy-efficient desalination and water treatment. However, the preparation of NF membranes with thinner defect-free selective layers and high separation performance remains challenging. Here a new strategy is reported for designing an NF membrane with a crumpled selective layer using polydopamine-piperazine-halloysite nanotubes (PDP-HNTs) as an interlayer and fabricated through vacuum filtration assisted interfacial polymerization (VFIP) method. Specifically, the PDP-HNTs/piperazine (PIP) dispersion was loaded onto the substrate surface through vacuum filtration, achieving construction of the interlayer and loading of the aqueous phase in one step. The PDP-HNTs interlayer served as a regulator to control the selective layer morphologies by retarding the polymerization reaction, forming an NF membrane with loose and crumpled structures. The resulting membranes display a high performance with the water permeance of 45.3 L m−2 h−1 bar−1, while maintaining a Na2SO4 rejection of 97.1 %. Because of the versatility and simplicity of this strategy, our work provides a new approach for fabricating NF membranes with outstanding separation performance.

Polyethyleneimine modified polyamide composite nanofiltration membrane for separation of lithium and magnesium

The application of nanofiltration (NF) membrane with positive charge on the surface in the separation of lithium‑magnesium is a convenient, energy-saving and sustainable strategy. In this work, polyethyleneimine (PEI) was introduced into the composite NF membrane through two methods to investigate and make a comparation in surface morphology and properties, application in the separation of membrane to Li+ and Mg2+, respectively. Both PEI and PIP were involved in the preparation of membranes as aqueous monomers, which can reduce the cross-linking degree of active layer but reduce the electronegativity of membrane surface. The rejection of Mg2+ reached 98.1 % and the purity of Li+, which is the mass fraction of Li+ in cations of permeate, reached 62 %, respectively, the separation factor SLi,Mg reached 32.6. By contrast, PEI with different molecular weight and concentration was grafted on the PA layer which improved the density of membrane but reduced the electronegativity of the membrane surface. The rejection of Mg2+ reached 97.2 % and the purity of Li+ reached 63 %, respectively, the SLi,Mg reached 33.6 with an acceptable flux. Both of two modification methods had a preferable performance on separation of lithium‑magnesium with a higher SLi,Mg. Therefore, it is an efficient strategy to add PEI through different methods into the preparation of NF membrane to separate lithium‑magnesium and extract the Li+ from salt lake brine with high Mg/Li mass ratio.