Asymmetric polyamide nanofilms with highly ordered nanovoids for water purification

Distinct Efficacies of Interlayers in Tailoring Polyamide Nanofiltration Membrane Performance for Organic Micropollutant Removal: Dependent on Substrate Characteristics

Interlayered thin-film nanocomposite (TFN) membranes have shown the potential to boost nanofiltration performance for water treatment applications including the removal of organic micropollutants (OMPs). However, the effects of substrates have been overlooked when exploiting and evaluating the efficacy of certain kinds of interlayers in tailoring membrane performance. Herein, a series of TFN membranes were synthesized on different porous substrates with identical interlayers of metal-organic framework nanosheets. It was revealed that the interlayer introduction could narrow but not fully eliminate the difference in the properties among the polyamide layers formed on different substrates, and the membrane performance variation was prominent in distinct aspects. For substrates with small pore sizes exerting severe water transport hindrance, the introduced interlayer mainly enhanced membrane water permeance by affording the gutter effect, while it could be more effective in reducing membrane pore size by improving the interfacial polymerization platform and avoiding PA defects when using a large-pore-size substrate. By matching the selected substrates and interlayers well, superior TFN membranes were obtained with simultaneously higher water permeance and OMP rejections compared to three commercial membranes. This study helps us to objectively understand interlayer efficacies and attain performance breakthroughs of TFN membranes for more efficient water treatment.

Ion-Selective Transport Promotion Enabled by Angstrom-Scale Nanochannels in Dendrimer-Assembled Polyamide Nanofilm for Efficient Electrodialysis

The ion permeability and selectivity of membranes are crucial in nanofluidic behavior, impacting industries ranging from traditional to advanced manufacturing. Herein, we demonstrate the engineering of ion-conductive membranes featuring angstrom-scale ion-transport channels by introducing ionic polyamidoamine (PAMAM) dendrimers for ion separation. The exterior quaternary ammonium-rich structure contributes to significant electrostatic charge exclusion due to enhanced local charge density; the interior protoplasmic channels of PAMAM dendrimer are assembled to provide additional degrees of free volume. This facilitates the monovalent ion transfer while maintaining continuity and efficient ion screening. The dendrimer-assembled hybrid membrane achieves high monovalent ion permeance of 2.81 mol m-2 h-1 (K+), reaching excellent mono/multivalent selectivity up to 20.1 (K+/Mg2+) and surpassing the permselectivities of state-of-the-art membranes. Both experimental results and simulating calculations suggest that the impressive ion selectivity arises from the significant disparity in transport energy barrier between mono/multivalent ions, induced by the "exterior-interior" synergistic effects of bifunctional membrane channels.

Energy harvesting from acid mine drainage using a highly proton/ion-selective thin polyamide film

A huge chemical potential difference exists between the acid mine drainage (AMD) and the alkaline neutralization solution, which is wasted in the traditional AMD neutralization process. This study reports, for the first time, the harvest of this chemical potential energy through a controlled neutralization of AMD using H+-conductive films. Polyamide films with controllable thickness achieved much higher H+ conductance than a commercially available cation exchange membrane (CEM). Meanwhile, the optimal polyamide film had an excellent H+/Ca2+ selectivity of 63.7, over two orders of magnitude higher than that of the CEM (0.3). The combined advantages of fast proton transport and high proton/ion selectivity greatly enhanced the power generation of the AMD battery. The power density was 3.1 W m-2, which is over one order of magnitude higher than that of the commercial CEM (0.2 W m-2). Our study provides a new sustainable solution to address the environmental issues of AMD while simultaneously enabling clean energy production.

Empowering ultrathin polyamide membranes at the water-energy nexus: strategies, limitations, and future perspectives

Membrane-based separation is one of the most energy-efficient methods to meet the growing need for a significant amount of fresh water. It is also well-known for its applications in water treatment, desalination, solvent recycling, and environmental remediation. Most typical membranes used for separation-based applications are thin-film composite membranes created using polymers, featuring a top selective layer generated by employing the interfacial polymerization technique at an aqueous-organic interface. In the last decade, various manufacturing techniques have been developed in order to create high-specification membranes. Among them, the creation of ultrathin polyamide membranes has shown enormous potential for achieving a significant increase in the water permeation rate, translating into major energy savings in various applications. However, this great potential of ultrathin membranes is greatly hindered by undesired transport phenomena such as the geometry-induced "funnel effect" arising from the substrate membrane, severely limiting the actual permeation rate. As a result, the separation capability of ultrathin membranes is still not fully unleashed or understood, and a critical assessment of their limitations and potential solutions for future studies is still lacking. Here, we provide a summary of the latest developments in the design of ultrathin polyamide membranes, which have been achieved by controlling the interfacial polymerization process and utilizing a number of novel manufacturing processes for ionic and molecular separations. Next, an overview of the in-depth assessment of their limitations resulting from the substrate membrane, along with potential solutions and future perspectives will be covered in this review.

Amphiphilic Interlayer Regulated Interfacial Polymerization for Constructing Polyamide Nanofiltration Membranes with High Perm-Selectivity of Mono-/Divalent Salts

High-quality thin-film composite (TFC) membranes with high selectivity and permeability have great significance owing to their practical applications, specifically for the accurate differentiation of monovalent and divalent ions. However, the trade-off effect between selectivity and permeability is still a big challenge due to the difficult structure adjustment of the selective layer. Herein, polydopamine (PDA) functionalized with a hydrophobic long alkane chain was first explored as a functional amphiphilic interlayer to synthesize high-quality TFC membranes via a confined interfacial polymerization (IP) reaction. The amphiphilic interlayer not only restricted the formation of the polyamide (PA) matrix in the pores of the substrate but also accelerated spatially more homogeneous polymerization and formed a PA active layer with a more uniform pore size distribution. The method may provide an effective principle for the construction of versatile polyamide-based membranes with high perm-selectivity on various supports. The NaCl/Na2SO4 separation factor of the D-8/PA membrane reached as high as 204.07, while the flux increased up to 25.71 L m-2 h-1 bar-1. This progress provides a more feasible way for the construction of high-quality TFC membranes with a devisable and creative amphiphilic interlayer for industrial application.

Polyester Nanofiltration Membranes for Efficient Cations Separation

Polyester nanofiltration membranes highlight beneficial chlorine resistance, but their loose structures and negative charge result in poor cations retention precluding advanced use in cations separation. This work designs a new monomer (TET) containing "hydroxyl-ammonium" entities that confer dense structures and positive charge to polyester nanofiltration membranes. The TET monomer undergoes efficient interfacial polymerization with the trimesoyl chloride (TMC) monomer, and the resultant TET-TMC membranes feature one of the lowest molecular weight cut-offs (389 Da) and the highest zeta potential (4 mv, pH: 7) among all polyester nanofiltration membranes. The MgCl2 rejection of the TET-TMC membrane is 95.5%, significantly higher than state-of-the-art polyester nanofiltration membranes (<50%). The Li+ /Mg2+ separation performance of TET-TMC membrane is on par with cutting-edge polyamide membranes, while additionally, the membrane is stable against NaClO though polyamide membranes readily degrade. Thus the TET-TMC is the first polyester nanofiltration membrane for efficient cations separation.

Optimized Polymeric Membranes for Water Treatment: Fabrication, Morphology, and Performance

Conventional polymers, endowed with specific functionalities, are extensively utilized for filtering and extracting a diverse set of chemicals, notably metals, from solutions. The main structure of a polymer is an integral part for designing an efficient separating system. However, its chemical functionality further contributes to the selectivity, fabrication process, and resulting product morphology. One example would be a membrane that can be employed to selectively remove a targeted metal ion or chemical from a solution, leaving behind the useful components of the solution. Such membranes or products are highly sought after for purifying polluted water contaminated with toxic and heavy metals. An efficient water-purifying membrane must fulfill several requirements, including a specific morphology attained by the material with a specific chemical functionality and facile fabrication for integration into a purifying module Therefore, the selection of an appropriate polymer and its functionalization become crucial and determining steps. This review highlights the attempts made in functionalizing various polymers (including natural ones) or copolymers with chemical groups decisive for membranes to act as water purifiers. Among these recently developed membrane systems, some of the materials incorporating other macromolecules, e.g., MOFs, COFs, and graphene, have displayed their competence for water treatment. Furthermore, it also summarizes the self-assembly and resulting morphology of the membrane materials as critical for driving the purification mechanism. This comprehensive overview aims to provide readers with a concise and conclusive understanding of these materials for water purification, as well as elucidating further perspectives and challenges.

Self-assembled dendrimer polyamide nanofilms with enhanced effective pore area for ion separation

Membrane technology using well-defined pore structure can achieve high ion purity and recovery. However, fine-tuning the inner pore structure of the separation nanofilm to be uniform and enhance the effective pore area is still challenging. Here, we report dendrimers with different peripheral groups that preferentially self-assemble in aqueous-phase amine solution to facilitate the formation of polyamide nanofilms with a well-defined effective pore range and uniform pore structure. The high permeabilities are maintained by forming asymmetric hollow nanostripe nanofilms, and their well-designed ion effective separation pore ranges show an enhancement, rationalized by molecular simulation. The self-assembled dendrimer polyamide membrane provides Cl-/SO42- selectivity more than 17 times that of its pristine polyamide counterparts, increasing from 167.9 to 2883.0. Furthermore, the designed membranes achieve higher Li purity and Li recovery compared to current state-of-the-art membranes. Such an approach provides a scalable strategy to fine-tune subnanometre structures in ion separation nanofilms.

A review on artificial water channels incorporated polyamide membranes for water purification: Transport mechanisms and performance

While thin-film composite (TFC) polyamide (PA) membranes are advanced for removing salts and trace organic contaminants (TrOCs) from water, TFC PA membranes encounter a water permeance-selectivity trade-off due to PA layer structural characteristics. Drawing inspiration from the excellent water permeance and solute rejection of natural biological channels, the development of analogous artificial water channels (AWCs) in TFC PA membranes (abbreviated as AWCM) promises to achieve superior mass transfer efficiency, enabling breaking the upper bound of water permeance and selectivity. Herein, we first discussed the types and structural characteristics of AWCs, followed by summarizing the methods for constructing AWCM. We discussed whether the AWCs acted as the primary mass transfer channels in AWCM and emphasized the important role of the AWCs in water transport and ion/TrOCs rejection. We thoroughly summarized the molecular-level mechanisms and structure-performance relationship of water molecules, ions, and TrOCs transport in the confined nanospace of AWCs, which laid the foundation for illustrating the enhanced water permeance and salt/TrOCs selectivity of AWCM. Finally, we discussed the challenges encountered in the field of AWCM and proposed future perspectives for practical applications. This review is expected to offer guidance for understanding the transport mechanisms of AWCM and developing next-generation membrane for effective water treatment.

Anhydrous interfacial polymerization of sub-1 Å sieving polyamide membrane

Highly permeable polyamide (PA) membrane capable of precise ionic sieving can be utilized for many energy-efficient chemical separations. To fulfill this target, it is crucial to innovate membrane-forming process to induce a narrow pore-size distribution. Herein, we report an anhydrous interfacial polymerization (AIP) at a solid-liquid interface where the amine layer sublimated is in direct contact with the alkane containing acyl chlorides. In such a heterophase interface, water-caused side reactions are eliminated, and the amines in compact arrangement enable an intensive and orderly IP reaction, leading to a unique PA layer with an ionic sieving accuracy of 0.5 Å. The AIP-PA membrane demonstrates excellent separation selectivities of monovalent and divalent cations such as Mg2+/Li+ (78.3) and anions such as Cl-/SO42- (29.2) together with a high water flux up to 13.6 L m-2 h-1 bar-1. Our AIP strategy may provide inspirations for engineering high-precision PA membranes available in various advanced separations.

Function-Led Design of Covalent-Organic-Framework Membranes for Precise Ion Separation

Insufficient access to clean water and resources has emerged as one of the most pressing issues affecting people globally. Membrane-based ion separation has become a focal point of research for the generation of fresh water and the extraction of energy elements. This Review encapsulates recent advancements in the selective ion transport of covalent organic framework (COF) membranes, accomplished by strategically pairing diverse monomers to create membranes with various pore sizes and environments for specific purposes. We first discuss the merits of using COF materials as a basis for fabricating membranes for ion separation. We then explore the development of COF membranes in areas such as desalination, acid recovery, and energy element extraction, with a particular emphasis on the fundamental principles of membrane design. Lastly, we address both theoretical and practical challenges, as well as potential opportunities in the targeted design of ion-selective membranes. The goal of this Review is to stimulate future investigative efforts in this field, which is of significant scientific and strategic importance.

3D Monolayer Silanation of Porous Structure Facilitating Multi-Phase Pollutants Removal

Activated carbon (AC) is widely used to removing hazardous pollutants from air and water, owing to its exceptional adsorption properties. However, the high affinity of water molecules with the surface oxygen-containing functional groups can adversely affect the adsorption performance of AC. In this study, a facile and efficient method is presented for fabrication of hydrophobic AC through surface monolayer silanation. Compared to initial AC, the hydrophobic AC improves the water contact angle from 29.7° to 123.5° while maintaining high specific surface area and enhances the removal capacity of multi-phase pollutants (emulsified oil and toluene). Additionally, the hydrophobic AC exhibits excellent adsorption capability to harmful algal bloom species (Chlorella) (97.56%) and algal organic matter (AOM) (96.23%) owing to electrostatic interactions and surface hydrophobicity. The study demonstrates that this method of surface monolayer silanation can effectively weaken the effect of water molecules on AC adsorption capacity, which has significant potential for practical use in air and water purification, as well as in the control of harmful algal blooms.

Enhancing the removal of organic micropollutants by nanofiltration membrane with Fe (III)-tannic acid interlayer: Mechanisms and environmental implications

Nanofiltration technology has been applied in a variety of water treatment scenarios. However, conventional thin-film composite (TFC) membranes fail to remove emerging organic micropollutants (OMPs) efficiently. Here we applied thin-film nanocomposite membrane with an interlayer (TFNi) of Fe (III)-tannic acid to remove various types of OMPs, such as endocrine disrupting chemicals (EDCs), pharmaceutically active compounds (PhACs), and perfluoroalkyl substances (PFASs). Compared to the pristine TFC membrane, TFNi membrane exhibited crumpled morphology and its rejection layer was denser, better cross-linked and possessed smaller average pore size with narrower distribution. Significant enhancement in water-OMPs selectivity of PhACs and PFASs was observed. The mechanism lies in the effects of interlayer in improving the membrane permeance to water and meanwhile reducing the permeance to some OMPs by enhancing size exclusion effects. This work confirms the effectiveness of using TFNi membrane to simultaneously enhance the OMPs rejection and water permeance. The unraveled mechanism might inspire the future development of high-performance nanofiltration membranes targeting OMPs removal.

Quaternization-spiro design of chlorine-resistant and high-permeance lithium separation membranes

Current polyamide lithium extraction nanofiltration membranes are susceptible to chlorine degradation and/or low permeance, two problems that are hard to reconcile. Here we simultaneously circumvented these problems by designing a quaternized-spiro piperazine monomer and translating its beneficial properties into large-area membranes (1 × 2 m2) via interfacial polymerization with trimesoyl chloride. The quaternary ammonium and spiral conformation of the monomer confer more positive charge and free volume to the membrane, leading to one of the highest permeance (~22 L m-2 h-1 bar-1) compared to the state-of-the-art Mg2+/Li+ nanofiltration membranes. Meanwhile, membrane structures are chlorine resistant as the amine-acyl bonding contains no sensitive N-H group. Thus the high performance of membrane is stable versus 400-h immersion in sodium hypochlorite, while control membranes degraded readily. Molecular simulations show that the high permeance and chlorine resistance, which were reproducible at the membrane module level, arise from the spiral conformation and secondary amine structures of the monomer.

Enhancing Nanofiltration Selectivity of Metal-Organic Framework Membranes via a Confined Interfacial Polymerization Strategy

Development of well-constructed metal-organic framework (MOF) membranes can bring about breakthroughs in nanofiltration (NF) performance for water treatment applications, while the relatively loose structures and inevitable defects usually cause low rejection capacity of MOF membranes. Herein, a confined interfacial polymerization (CIP) method is showcased to synthesize polyamide (PA)-modified NF membranes with MOF nanosheets as the building blocks, yielding a stepwise transition from two-dimensional (2D) MOF membranes to polyamide NF membranes. The CIP process was regulated by adjusting the loading amount of piperazine (PIP)-grafted MOF nanosheets on substrates and the additional content of free PIP monomers distributed among the nanosheets, followed by the reaction with trimesoyl chloride in the organic phase. The prepared optimal membrane exhibited a high Na2SO4 rejection of 98.4% with a satisfactory water permeance of 37.4 L·m-2·h-1·bar-1, which could be achieved by neither the pristine 2D MOF membranes nor the PA membranes containing the MOF nanosheets as the conventional interlayer. The PA-modified MOF membrane also displayed superior stability and enhanced antifouling ability. This CIP strategy provides a novel avenue to develop efficient MOF-based NF membranes with high ion-sieving separation performance for water treatment.

Advances in nanomedicines for lymphatic imaging and therapy

Lymph nodes play a pivotal role in tumor progression as key components of the lymphatic system. However, the unique physiological structure of lymph nodes has traditionally constrained the drug delivery efficiency. Excitingly, nanomedicines have shown tremendous advantages in lymph node-specific delivery, enabling distinct recognition and diagnosis of lymph nodes, and hence laying the foundation for efficient tumor therapies. In this review, we comprehensively discuss the key factors affecting the specific enrichment of nanomedicines in lymph nodes, and systematically summarize nanomedicines for precise lymph node drug delivery and therapeutic application, including the lymphatic diagnosis and treatment nanodrugs and lymph node specific imaging and identification system. Notably, we delve into the critical challenges and considerations currently facing lymphatic nanomedicines, and futher propose effective strategies to address these issues. This review encapsulates recent findings, clinical applications, and future prospects for designing effective nanocarriers for lymphatic system targeting, with potential implications for improving cancer treatment strategies.

Asymmetric gradient orbital interaction of hetero-diatomic active sites for promoting C - C coupling

Diatomic-site catalysts (DACs) garner tremendous attention for selective CO₂ photoreduction, especially in the thermodynamical and kinetical mechanism of CO₂ to C₂₊ products. Herein, we first engineer a novel Zn-porphyrin/RuCu-pincer complex DAC (ZnPor-RuCuDAC). The heteronuclear ZnPor-RuCuDAC exhibits the best acetate selectivity (95.1%), while the homoatomic counterparts (ZnPor-Ru₂DAC and ZnPor-Cu₂DAC) present the best CO selectivity. In-situ spectroscopic measurements reveal that the heteronuclear Ru-Cu sites easily appear C₁ intermediate coupling. The in-depth analyses confirm that due to the strong gradient orbital coupling of Ru4d-Cu3d resonance, two formed ^*CO intermediates of Ru-Cu heteroatom show a significantly weaker electrostatic repulsion for an asymmetric charge distribution, which result from a side-to-side absorption and narrow dihedral angle distortion. Moreover, the strongly overlapped Ru/Cu-d and CO molecular orbitals split into bonding and antibonding orbitals easily, resulting in decreasing energy splitting levels of C₁ intermediates. These results collectively augment the collision probability of the two ^*CO intermediates on heteronuclear DACs. This work first provides a crucial perspective on the symmetry-forbidden coupling mechanism of C₁ intermediates on diatomic sites.

Polycage membranes for precise molecular separation and catalysis

The evolution of the chemical and pharmaceutical industry requires effective and less energy-intensive separation technologies. Engineering smart materials at a large scale with tunable properties for molecular separation is a challenging step to materialize this goal. Herein, we report thin film composite membranes prepared by the interfacial polymerization of porous organic cages (POCs) (RCC3 and tren cages). Ultrathin crosslinked polycage selective layers (thickness as low as 9.5 nm) are obtained with high permeance and strict molecular sieving for nanofiltration. A dual function is achieved by combining molecular separation and catalysis. This is demonstrated by impregnating the cages with highly catalytically active Pd nanoclusters ( ~ 0.7 nm). While the membrane promotes a precise molecular separation, its catalytic activity enables surface self-cleaning, by reacting with any potentially adsorbed dye and recovering the original performance. This strategy opens opportunities for the development of other smart membranes combining different functions and well-tailored abilities.

Locking Patterned Carbon Nanotube Cages by Nanofibrous Mats to Construct Cucurbituril[n]-Based Ultrapermselective Dye/Salt Separation Membranes

Surface patterning is a promising strategy to overcome the trade-off effect of separation membranes. Herein, a bottom-up patterning strategy of locking micron-sized carbon nanotube cages (CNCs) onto a nanofibrous substrate is developed. The strongly enhanced capillary force triggered by the abundant narrow channels in CNCs endows the precisely patterned substrate with excellent wettability and antigravity water transport. Both are crucial for the preloading of cucurbit[n]uril (CB6)-embeded amine solution to form an ultrathin (∼20 nm) polyamide selective layer clinging to CNCs-patterned substrate. The CNCs-patterning and CB6 modification result in a 40.2% increased transmission area, a reduced thickness, and a lowered cross-linking degree of selective layer, leading to a high water permeability of 124.9 L·m^-2 h^-1 bar^-1 and a rejection of 99.9% for Janus Green B (511.07 Da), an order of magnitude higher than that of commercial membranes. The new patterning strategy provides technical and theoretical guidance for designing next-generation dye/salt separation membranes.

Polyamide membranes with nanoscale ordered structures for fast permeation and highly selective ion-ion separation

Fast permeation and effective solute-solute separation provide the opportunities for sustainable water treatment, but they are hindered by ineffective membranes. We present here the construction of a nanofiltration membrane with fast permeation, high rejection, and precise Cl^-/SO₄^2- separation by spatial and temporal control of interfacial polymerization via graphitic carbon nitride (g-C₃N₄). The g-C₃N₄ nanosheet binds preferentially with piperazine and tiles the water-hexane interface as revealed by molecular dynamics studies, thus lowering the diffusion rate of PIP by one order of magnitude and restricting its diffusion pathways towards the hexane phase. As a result, membranes with nanoscale ordered hollow structure are created. Transport mechanism across the structure is clarified using computational fluid dynamics simulation. Increased surface area, lower thickness, and a hollow ordered structure are identified as the key contributors to the water permeance of 105 L m²·h^-1·bar^-1 with a Na₂SO₄ rejection of 99.4% and a Cl^-/SO₄^2- selectivity of 130, which is superior to state-of-the-art NF membranes. Our approach for tuning the membrane microstructure enables the development of ultra-permeability and excellent selectivity for ion-ion separation, water purification, desalination, and organics removal.

A review on polyester and polyester-amide thin film composite nanofiltration membranes: Synthesis, characteristics and applications

Nanofiltration (NF) membranes have been widely used in various fields including water treatment and other separation processes, while conventional thin film composite (TFC) membranes with polyamide (PA) selective layers suffer the problems of fouling and chlorine intolerance. Due to the abundant hydrophilic hydroxyl groups and ester bonds free from chlorine attack, the TFC membranes composed of polyester (PE) or polyester-amide (PEA) selective layers have been proven to possess enhanced anti-fouling properties and superior chlorine resistance. In this review, the research progress of PE and PEA nanofiltration membranes is systematically summarized according to the variety of hydroxyl-containing monomers for membrane fabrication by the interfacial polymerization (IP) reaction. The synthesis strategies as well as the mechanisms for tailoring properties and performance of PE and PEA membranes are analyzed, and the membrane application advantages are demonstrated. Moreover, current challenges and future perspectives of the development of PE and PEA nanofiltration membranes are proposed. This review can offer guidance for designing high-performance PE and PEA membranes, thereby further promoting the efficacy of nanofiltration.

N-Oxide Zwitterion Functionalized Positively Charged Polyamide Composite Membranes for Nanofiltration

N-Oxide zwitterionic polyethyleneimine (ZPEI), a new kind of aqueous phase monomer synthesized by commercially branched polyethyleneimine (PEI) via oxidation reaction, was prepared for fabrication of thin-film composite (TFC) polyamide membranes via interfacial polymerization. The main factors, including the monomer concentration and immersion time of the aqueous phase and organic phase, were investigated. Compared with PEI-TFC membranes, the obtained optimal defect-free ZPEI-TFC membranes exhibited a lower roughness (3.3 ± 0.3 nm), a better surface hydrophilicity, and a smaller pore size (238 Da of MWCO). The positively charged ZPEI-TFC membranes (isoelectric point at pH 8.05) showed higher rejections toward both divalent cationic (MgCl₂, 93.0%) and anionic (Na₂SO₄, 96.1%) salts with a water permeation flux of up to 81.0 L·m^-2·h^-1 at 6 bar, which surpassed currently reported membranes. More importantly, mainly owing to N-oxide zwitterion with strong hydration capability, ZPEI-TFC membranes displayed a high flux recovery ratio (97.0%) toward a model protein contaminant (bovine serum albumin), indicating good anti-fouling properties. Therefore, the novel N-oxide zwitterion functionalized positively charged nanofiltration membranes provide an alternative for water desalination and sewage reclamation.

Roles of Anion-Cation Coupling Transport and Dehydration-Induced Ion-Membrane Interaction in Precise Separation of Ions by Nanofiltration Membranes

Nanofiltration (NF) membranes are playing increasingly crucial roles in addressing emerging environmental challenges by precise separation, yet understanding of the selective transport mechanism is still limited. In this work, the underlying mechanisms governing precise selectivity of the polyamide NF membrane were elucidated using a series of monovalent cations with minor hydrated radius difference. The observed selectivity of a single cation was neither correlated with the hydrated radius nor hydration energy, which could not be explained by the widely accepted NF model or ion dehydration theory. Herein, we employed an Arrhenius approach combined with Monte Carlo simulation to unravel that the transmembrane process of the cation would be dominated by its pairing anion, if the anion has a greater transmembrane energy barrier, due to the constraint of anion-cation coupling transport. Molecular dynamics simulations further revealed that the distinct hydration structure was the primary origin of the energy barrier difference of cations. The cation having a larger incompressible structure after partial dehydration through subnanopores would induce a more significant ion-membrane interaction and consequently a higher energy barrier. Moreover, to validate our proposed mechanisms, a membrane grafting modification toward enlarging the energy barrier difference of dominant ions achieved a 3-fold enhancement in ion separation efficiency. Our work provides insights into the precise separation of ionic species by NF membranes.

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).

Nanofiltration Membranes with Crumpled Polyamide Films: A Critical Review on Mechanisms, Performances, and Environmental Applications

Nanofiltration (NF) membranes have been widely applied in many important environmental applications, including water softening, surface/groundwater purification, wastewater treatment, and water reuse. In recent years, a new class of piperazine (PIP)-based NF membranes featuring a crumpled polyamide layer has received considerable attention because of their great potential for achieving dramatic improvements in membrane separation performance. Since the report of novel crumpled Turing structures that exhibited an order of magnitude enhancement in water permeance ( Science 2018, 360 (6388), 518-521), the number of published research papers on this emerging topic has grown exponentially to approximately 200. In this critical review, we provide a systematic framework to classify the crumpled NF morphologies. The fundamental mechanisms and fabrication methods involved in the formation of these crumpled morphologies are summarized. We then discuss the transport of water and solutes in crumpled NF membranes and how these transport phenomena could simultaneously improve membrane water permeance, selectivity, and antifouling performance. The environmental applications of these emerging NF membranes are highlighted, and future research opportunities/needs are identified. The fundamental insights in this review provide critical guidance on the further development of high-performance NF membranes tailored for a wide range of environmental applications.

Unveiling the Growth of Polyamide Nanofilms at Water/Organic Free Interfaces: Toward Enhanced Water/Salt Selectivity

The permeance and selectivity of a reverse osmosis (RO) membrane are governed by its ultrathin polyamide film, yet the growth of this critical film during interfacial polymerization (IP) has not been fully understood. This study investigates the evolution of a polyamide nanofilm at the aqueous/organic interface over time. Despite its thickness remaining largely constant (∼15 nm) for the IP reaction time ranging from 0.5 to 60 min, the density of the polyamide nanofilm increased from 1.25 to 1.36 g cm^-3 due to the continued reaction between diffused m-phenylenediamine and dangling acyl chloride groups within the formed polyamide film. This continued growth of the polyamide nanofilm led to a simultaneous increase in its crosslinking degree (from 50.1 to 94.3%) and the healing of nanosized defects, resulting in a greatly enhanced rejection of 99.2% for NaCl without sacrificing water permeance. Using humic acid as a molecular probe for sealing membrane defects, the relative contributions of the increased crosslinking and reduced defects toward better membrane selectivity were resolved, which supports our conceptual model involving both enhanced size exclusion and healed defects. The fundamental insights into the growth mechanisms and the structure-property relationship of the polyamide nanofilm provide crucial guidance for the further development and optimization of high-performance RO membranes.