Ultrathin Polyamide Membrane with Decreased Porosity Designed for Outstanding Water-Softening Performance and Superior Antifouling Properties

Construction of PDA-PEI/ZIF-L@PE tight ultra-filtration (TUF) membranes on porous polyethylene (PE) substrates for efficient dye/salt separation

Tight ultra-filtration (TUF) membranes were constructed by in situ growing zinc imidazole frameworks micro-crystalline leaves (ZIF-L) in polyethylene imine (PEI) and polydopamine (PDA) deposit layers on porous polyethylene (PE) substrates. The effects of preparation conditions on the surface physical and chemical structures as well as on the dye/salt separation performance of the formed TUF membranes were systematically investigated. By inserting selective water permeation channels and increasing contacting surface areas, in situ-grown ZIF-L arrays tightly cross-linked in the coating matrix greatly increased water permeation without trading off dye/salt retention selectivity. The morphology of the included ZIF-L particles could be varied by adjusting the ligand/Zn molar ratio (α) in the preparation processes. Optimized PDA-PEI/ZIF-L@PE TUF membranes containing ZIF-L of cross-cross block morphology showed very high pure water permeability of 180 ± 20 L·m-2·h-1·bar-1 (LMHB) and retention selectivity (SCR/Na2SO4 and SMB/Na2SO4) of 267 and 43, respectively, as well as excellent stability and anti-fouling properties.

Ice-confined synthesis of highly ionized 3D-quasilayered polyamide nanofiltration membranes

Existing polyamide (PA) membrane synthesis protocols are underpinned by controlling diffusion-dominant liquid-phase reactions that yield subpar spatial architectures and ionization behavior. We report an ice-confined interfacial polymerization strategy to enable the effective kinetic control of the interfacial reaction and thermodynamic manipulation of the hexagonal polytype (Ih) ice phase containing monomers to rationally synthesize a three-dimensional quasilayered PA membrane for nanofiltration. Experiments and molecular simulations confirmed the underlying membrane formation mechanism. Our ice-confined PA nanofiltration membrane features high-density ionized structure and exceptional transport channels, realizing superior water permeance and excellent ion selectivity.

Title Preparation and Desalination of Semi-Aromatic Polyamide Reverse Osmosis Membranes (ROMs)

Reverse osmosis membrane (ROM) technology has a series of advantages, such as a simple process, no secondary pollution, high efficiency, energy saving, environmental protection, and good separation and purification effects. High-performance semi-aromatic polyamide reverse osmosis membranes (ROMs) were prepared by interfacial polymerization (IP) of novel cyclopentanecarbonyl chloride (CPTC) and m-phenylenediamine (MPD) monomers. The surface morphology, hydrophilicity and charge of the ROMs were characterized by field-emission scanning electron microscopy (SEM), a contact angle tester and a solid-surface zeta potential analyzer. The effects of CPTC concentration, MPD concentration, oil-phase solvent type, IP reaction time and additive concentration on the performance of semi-aromatic polyamide ROMs were studied. SEM morphology characterization showed that the surface of the prepared polyamide ROMs presented a multinodal structure. The performance test showed that when the concentration of MPD in the aqueous phase was 2.5 wt.%, the concentration of sodium dodecylbenzene sulfonate (SDBS) was 0.2%, the residence time in the aqueous phase was 2 min, the concentration of CPTC/cyclohexane in the oil phase was 0.13 wt.%, the IP reaction was 20 s, the NaCl rejection rate of the semi-aromatic polyamide ROM was 98.28% and the flux was 65.38 L/m2·h, showing good desalination performance. Compared with an NF 90 commercial membrane, it has a good anti-BSA pollution ability.

Interface-Confined Channels Facilitating Water Transport through an IL-Enriched Nanocomposite Membrane

Improving the permeance of the polyamide (PA) membrane while maintaining the rejection is crucial for promoting the development of membrane separation technology in the practical water-treatment industry. Herein, a novel metal-ionic liquid (Zn-IL) coordination compound was synthesized by in situ growth to improve the water permeance of PA nanofiltration membranes, using an amine-functionalized IL (1-aminopropyl-3-methylimidazolium chloride, [AEMIm][Cl]) as a ligand to react with Zn(NO₃)₂·6H₂O. Piperazine (PIP) and trimesoyl chloride (TMC) were adopted to prepare the PA layer covering the Zn-IL complex. Due to the unique property of the Zn-IL complex, the Zn-IL/PIP-TMC absorbing force to water was increased, enabling the fast transport of water molecules through the membrane pore channels in the form of free water. The resulting Zn-IL/PIP-TMC nanocomposite membrane exhibited a high permeance of up to 26.5 L m^-2 h^-1 bar^-1, which is 3 times that of the PIP-TMC membrane (8.8 L m^-2 h^-1 bar^-1), combined with rejection above 99% for dyes such as methyl blue.

Development and Investigation of Hierarchically Structured Thin-Film Nanocomposite Membranes from Polyamide/Chitosan Succinate Embedded with a Metal-Organic Framework (Fe-BTC) for Pervaporation

Thin-film composite membranes (TFC) obtained by the formation of a selective layer on a porous membrane-substrate via interfacial polymerization (IP) are indispensable for separation procedures in reverse osmosis, nanofiltration, pervaporation, and gas separation. Achieving high selectivity and permeability for TFC membranes is still one of the main challenges in membrane science and technology. This study focuses on the development of thin film nanocomposite (TFN) membranes with a hierarchically structured polyamide (PA)/chitosan succinate (ChS) selective layer embedded with a metal-organic framework of iron 1,3,5-benzenetricarboxylate (Fe-BTC) for the enhanced pervaporation dehydration of isopropanol. The aim of this work was to study the effect of Fe-BTC incorporation into the ChS interlayer and PA selective layer, obtained via IP, on the structure, properties, and performance of pervaporation TFN membranes. The structure and hydrophilicity of the developed TFN membranes were investigated using scanning electron microscopy (SEM) and atomic force microscopy (AFM), along with water contact angle measurements. The developed TFN membranes were studied in the pervaporation dehydration of isopropanol (12-30 wt % water). It was found that incorporation of Fe-BTC into the ChS interlayer yielded the formation of a smoother, more uniform, and defect-free PA ultrathin selective layer via IP, due to the amorpho-crystalline structure of particles serving as the amine storage reservoir and led to an increase in membrane selectivity toward water, and a slight decrease in permeation flux compared to the ChS interlayered TFC membranes. The best pervaporation performance was demonstrated by the TFN membrane with a ChS-Fe-BTC interlayer and the addition of 0.03 wt % Fe-BTC in the PA layer, yielding a permeation flux of 197-826 g·m^-2·h^-1 and 98.50-99.99 wt % water in the permeate, in the pervaporation separation of isopropanol/water mixtures (12-30 wt % water).

Ultrathin Membranes for Separations: A New Era Driven by Advanced Nanotechnology

Ultrathin membranes are at the forefront of membrane research, offering great opportunities in revolutionizing separations with ultrafast transport. Driven by advanced nanomaterials and manufacturing technology, tremendous progresses are made over the last 15 years in the fabrications and applications of sub-50 nm membranes. Here, an overview of state-of-the-art ultrathin membranes is first introduced, followed by a summary of the fabrication techniques with an emphasis on how to realize such extremely low thickness. Then, different types of ultrathin membranes, categorized based on their structures, that is, network, laminar, or framework structures, are discussed with a focus on the interplays among structure, fabrication methods, and separation performances. Recent research and development trends are highlighted. Meanwhile, the performances and applications of current ultrathin membranes for representative separations (gas separation and liquid separation) are thoroughly analyzed and compared. Last, the challenges in material design, structure construction, and coordination are given, in order to fully realize the potential of ultrathin membranes and facilitate the translation from scientific achievements to industrial productions.

Novel Poly(ester amide) Membranes with Tunable Crosslinked Structures for Nanofiltration

Tuning the crosslinking density of interfacial-polymerized nanofiltration (NF) membranes varying from loose to dense structures can make them meet the demand of various applications. The properties (e.g., pore size and porosity) of NF membranes can be tuned by choosing monomers with different structures and reactivities. Herein, tris(hydroxymethyl)aminomethane (THAM), a low-cost and green monomer, is first employed for the preparation of poly(ester amide) (PEA) thin-film composite membranes via interfacial polymerization. The moderate reactivity of THAM enables rational regulation of the crosslinking density of PEA membranes from loose to dense structures by varying the THAM concentration, which can hardly be achieved for traditional polyamide or polyester membranes. The developed PEA membranes with a wide tunability range of crosslinking densities broaden their potential utility in NF. PEA membranes with dense structures show exceptional desalination performance with a water permeance of 11.1 L m^-2 h^-1 bar^-1 and a Na₂SO₄ rejection of 97.1%. However, loose PEA membranes exhibit good dye/salt separation performance with a dye removal rate over 95.0% and negligible NaCl rejection (<7.5%), as well as high water permeance (>45 L m^-2 h^-1 bar^-1). This work implies that PEA membranes with tunable crosslinked structures provide new possibilities for the development of task-specific separation membranes.

Tailored design of nanofiltration membranes for water treatment based on synthesis-property-performance relationships

Tailored design of high-performance nanofiltration (NF) membranes is desirable because the requirements for membrane performance, particularly ion/salt rejection and selectivity, differ among the various applications of NF technology ranging from drinking water production to resource mining. However, this customization greatly relies on a comprehensive understanding of the influence of membrane fabrication methods and conditions on membrane properties and the relationships between the membrane structural and physicochemical properties and membrane performance. Since the inception of NF, much progress has been made in forming the foundation of tailored design of NF membranes and the underlying governing principles. This progress includes theories regarding NF mass transfer and solute rejection, further exploitation of the classical interfacial polymerization technique, and development of novel materials and membrane fabrication methods. In this critical review, we first summarize the progress made in controllable design of NF membrane properties in recent years from the perspective of optimizing interfacial polymerization techniques and adopting new manufacturing processes and materials. We then discuss the property-performance relationships based on solvent/solute mass transfer theories and mathematical models, and draw conclusions on membrane structural and physicochemical parameter regulation by modifying the fabrication process to improve membrane separation performance. Next, existing and potential applications of these NF membranes in water treatment processes are systematically discussed according to the different separation requirements. Finally, we point out the prospects and challenges of tailored design of NF membranes for water treatment applications. This review bridges the long-existing gaps between the pressing demand for suitable NF membranes from the industrial community and the surge of publications by the scientific community in recent years.

Inner Surface Hydrophilic Modification of PVDF Membrane with Tea Polyphenols/Silica Composite Coating

The use of Polyvinylidene fluoride (PVDF) membranes is constrained in wastewater treatment because of their hydrophobic nature. Therefore, a large number of researchers have been working on the hydrophilic modification of their surfaces. In this work, a superhydrophilic tea polyphenols/silica composite coating was developed by a one-step process. The composite coating can achieve not only superhydrophilic modification of the surface, but also the inner surface of the porous PVDF membrane, which endows the modified membrane with excellent water permeability. The modified membrane possesses ultrahigh water flux (15,353 L·m⁻²·h⁻¹). Besides this, the modified membrane can realize a highly efficient separation of oil/water emulsions (above 96%).

Multirole Regulations of Interfacial Polymerization Using Poly(acrylic acid) for Nanofiltration Membrane Development

Effective control of monomer diffusion and reaction rate is the key to achieving a controlled interfacial polymerization (IP) and a high-performance nanofiltration (NF) membrane. Herein, an integration of multirole regulations was synchronously realized using poly(acrylic acid) (PAA) as an active additive in a piperazine (PIP) aqueous phase. Thanks to synergistic interactions, including hydrogen bonding, electrostatic interaction, and covalent bonding between PAA and PIP molecules, together with the increased viscosity of the solution, PIP diffusion was rationally controlled. Moreover, interfacial polycondensation was also restrained via the modestly reduced pH of the aqueous solution. These contribute to the formation of a thinner, looser, more hydrophilic, and higher negatively charged PAA-decorated polyamide selective layer with a unique nanostrand-nodule morphology. The harvested NF-PAA/PIP membrane showed an ∼70% rise in water permeability (up to 23.5 L·m-2·h-1·bar-1) while retaining high Na2SO4 and dye rejections. Furthermore, the optimized NF-PAA/PIP membrane presented a superior fouling resistance capability for typical pollutants, as well as long-term stability during successive filtration. Thus, this work offers a straightforward and impactful approach to regulating IP and promoting NF membrane properties.

Reverse osmosis and nanofiltration membranes for highly efficient PFASs removal: overview, challenges and future perspectives

PFASs are called “forever chemicals” because they do not fully degrade. They have become so ubiquitous in the environment that it is difficult to prevent exposure. This review aims to provide a set of improved technologies to remove PFASs from water.

Asymmetric polyamide nanofilms with highly ordered nanovoids for water purification

Tailor-made structure and morphology are critical to the highly permeable and selective polyamide membranes used for water purification. Here we report an asymmetric polyamide nanofilm having a two-layer structure, in which the lower is a spherical polyamide dendrimer porous layer, and the upper is a polyamide dense layer with highly ordered nanovoids structure. The dendrimer porous layer was covalently assembled in situ on the surface of the polysulfone (PSF) support by a diazotization-coupling reaction, and then the asymmetric polyamide nanofilm with highly ordered hollow nanostrips structure was formed by interfacial polymerization (IP) thereon. Tuning the number of the spherical dendrimer porous layers and IP time enabled control of the nanostrips morphology in the polyamide nanofilm. The asymmetric polyamide membrane exhibits a water flux of 3.7−4.3 times that of the traditional monolayer polyamide membrane, showing an improved divalent salt rejection rate (more than 99%), which thus surpasses the upper bound line of the permeability−selectivity performance of the existing various structural polyamide membranes. We estimate that this work might inspire the preparation of highly permeable and selective reverse osmosis (RO), organic solvent nanofiltration (OSNF) and pervaporation (PV) membranes.

Structure and morphology are critical to the performance of permeable and selective polyamide membranes in water purification. Here, the authors report a two layer asymmetric polyamide nanofilm in which a spherical polyamide dendrimer porous lower and a polyamide dense upper layer form hierarchical nanovoids.

PIP/TMC Interfacial Polymerization with Electrospray: Novel Loose Nanofiltration Membrane for Dye Wastewater Treatment

A loose nanofiltration (NF) membrane with excellent dye rejection and high permeation of inorganic salt is required to fractionate dye/salt mixture in dye wastewater treatment. In this study, we fabricated the loose NF membrane by using the electrospray interfacial polymerization (EIP) method. It is a novel and facile interfacial polymerization method, which controls the thickness of the poly(piperazine-amide) (PPA) layer in nanometers (1 nm/min) and changes cross-linking degree of PPA layer and pore size by varying the electrospray time; consequently, water permeance and dye/salt rejection ratio can be handled. The fabricated EIP membrane with an optimized fabrication condition (M30, electrospray time was 30 min) possessed excellent pure water permeance (20.2 LMH/bar), high dye rejection (e.g., 99.6% for congo red (CR)), and low salt rejection (e.g., 6.3% for NaCl). Moreover, the EIP membrane exhibited enhanced antifouling property than commercial NF membrane (NF90) with a high flux recovery rate (FRR) of 87.1% and low irreversible fouling (R_ir) of 12.9% after fouled by bovine serum albumin (BSA) due to its great smooth surface (average roughness (R_a) is 12.2 nm), hydrophilicity property, enhanced zeta potential, and low protein adsorption. The results indicate that the EIP loose NF membrane had a high potential for dye wastewater treatment.

Flexible and Hydrophilic Copolyamide Thin-Film Composites on Hollow Fiber Membranes for Enhanced Nanofiltration Performance

Flexible and hydrophilic copolyamide (Co-PA) thin-film composite (TFC) membranes were fabricated as a selective layer on the outer surface of the polyvinylidene fluoride hollow fiber membrane substrate. The fabrication process was carried out by the dip-coating method to create three TFC membranes. The first layer is tannic acid and the second layer is (3-aminopropyl)triethoxysilane, which is followed by Co-PA as a final selective layer. The Co-PA TFC membrane was prepared through interfacial polymerization via the combination of various short-chain aliphatic diamines and conventional aromatic diamines with trimesoyl chloride. The influence of coating layers and total diamine concentration on the Co-PA TFC membrane was investigated in terms of the membrane's physicochemical and mechanical properties, morphology, surface thickness and roughness, water contact angle, surface charge, and nanofiltration (NF) performance. The obtained Co-PA TFC membrane system was operated under low pressure (2 bar) with pure water flux in the range of 23.8-83.9 L m^-2 h^-1 and exhibited better hydrophilicity, flexibility, molecular weight cutoff, and NF performance compared to the conventional PA TFC membrane. The superior properties of Co-PA are due to the increased chain mobilities provided by short-chain aliphatic diamines in its structure. The best Co-PA TFC membranes, which were synthesized using diamines containing four carbon atoms, achieved a significant improvement in NF membrane performance and selectivity (pure water flux = 56.9 L m^-2 h^-1 and salt and dye rejection in the range of 46.2-99.2%). This Co-PA TFC membrane is a promising membrane for its high flexibility, hydrophilicity, and selectivity of the NF membrane.

Polyamide nanofilms synthesized via controlled interfacial polymerization on a "jelly" surface

A thermal-sensitive "jelly" was used to control the diffusion of a diamine monomer for synthesizing polyamide free-standing nanofilms with an adjustable thickness of 5-35 nm. The reduced reaction rate of the interfacial polymerization at the hexane-"jelly" interface made the synthesized nanofilms show high water permeation flux and suitable salt rejection, and they also have highly negative surface charges and fairly smooth surfaces.

Catalytic Membrane Microreactors with an Ultrathin Freestanding Membrane for Nitrobenzene Hydrogenation

A catalytic membrane microreactor is a potential tool for the application in multiphase catalytic reactions. Currently, the installation of a permeable ultrathin membrane to separate the reactants with different phases and the grafting of a catalyst layer on the membrane in a microreactor remain challenging. In this work, a catalytic ultrathin membrane microreactor with an ultrathin freestanding supporting membrane was developed for nitrobenzene hydrogenation, which was formed by the interfacial polycondensation reaction with parallel laminar flow in a microchannel, followed by catalyst layer immobilization on one side of the ultrathin membrane surface using the layer-by-layer self-assembly technique and in situ reduction. The ultrathin freestanding membrane ensured a stable gas/liquid contact interface and good hydrogen permeability. The developed membrane microreactor could exhibit high catalytic performance under mild conditions, demonstrating its superior mass transport. In addition, the effects of the thickness of the ultrathin freestanding membrane, flow rates, and inlet nitrobenzene concentration were investigated to obtain the optimum operating conditions. The experimental results demonstrate that this newly developed membrane microreactor is a promising candidate for multiphase catalytic reactions.

Fabrication of a Novel Nanofiltration Membrane with Enhanced Performance via Interfacial Polymerization through the Incorporation of a New Zwitterionic Diamine Monomer

It is known that the polyamide (PA) barrier layer's inherent microstructure and surface physicochemical properties of thin film composite nanofiltration membrane are crucial for its separation performance. Herein, we designed and synthesized a new zwitterionic aromatic diamine monomer 3-(4-(2-((4-aminophenyl)amino)ethyl)morpholino-4-ium)propane-1-sulfonate (PPD-MEPS) through a three steps reaction, and this hydrophilic molecule was incorporated into the active layer to tailor the poly(piperazine-amide)-based nanofiltration membranes with significantly improved water permeability and antifouling properties. As a p-phenylenediamine (PPD) derivative, PPD-MEPS possesses two active amine units, which can react with trimesoyl chloride in the organic phase during the interfacial polymerization reaction process. Thus, the super-hydrophilic zwitterions were not only on the membrane surface but also across the whole PA layer to facilitate water molecule transportation. The successful augmentation of zwitterions into the PA layer was well illustrated by attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) results and X-ray photoelectron spectroscopy analysis. With increasing loading content of PPD-MEPS in PIP aqueous solution, the as-fabricated nanofiltration membranes (NFMs) exhibited higher hydrophilicity, increased active layer thickness, and molecular weight cut off. When the zwitterionic monomer reached 60% to PIP for NFM-4, the water permeability went up to 9.82 L m^-2 h^-1 bar^-1, increasing by 45%; meanwhile, the Na₂SO₄/NaCl selectivity increased from 2.54 to 4.03. In addition, the fouling experiments illustrated that the fouling resistance of the zwitterion-modified NFMs to bovine serum albumin was significantly improved.