MXene Nanosheet Templated Nanofiltration Membranes toward Ultrahigh Water Transport

Enhancement separation selectivity of mineral ions and perfluorinated and polyfluoroalkyl substances by nanofiltration membrane through hydrogel-assisted interfacial polymerization

The presence of perfluorinated and polyfluoroalkyl substances (PFAS) in drinking water is a critical concern for water safety and public health. Nanofiltration (NF) membranes have emerged promising technology for the elimination of trace organic contaminants from drinking water, but many previous studies have sacrificed the retention of vital mineral ions in human body in pursuit of efficient removal of PFAS. In this study, hydrogel-assisted interfacial polymerization (IP) strategy was designed to enhance the selectivity of mineral ions over PFAS, optimized pore size and surface characteristics of polyamide layers were obtained by IP process assisted by hydrogel formed by chitosan and glutaraldehyde. This approach facilitated the fabrication of NF membranes with a thinner active layer, enlarged pore size, and a more negatively charged surface. The optimized modified membrane exhibited a remarkable improvement in water permeance (16 LMH/bar, over 200 % than the control membrane) and maintained high rejection rates (>90 %) for PFAS with molecular weights ranging from 214 to 514 Da, while significantly reducing the rejection of Ca2+ and Mg2+ ions (<20 %). Density functional theory calculations revealed that all membranes exhibited reduced adsorption energies for PFAS. The treatment of natural surface water indicated the superior rejection selectivity of the modified membrane for mineral ions over natural organic matter, the average gap value of inorganic ions and natural organic matter in modified membranes was 4.6, while the average gap in commercial membranes was 1.6, improved by 2.6 times in selectivity compared to existing commercial membranes. This study offers valuable insights into the targeted enhancement of mineral ions/PFAS selectivity in NF membranes, thereby paving the way of more efficient and sustainable water treatment processes.

Molecular-Level Understanding of Water Transport Mechanisms in Functionalized Ti3C2TX MXene Membrane-Combined Experimental Approaches

The hydrophilicity of two-dimensional (2D) transition-metal carbides, carbonitrides, and nitrides (MXene) nanochannels plays a critical role in water transport during filtration, yet its specific effects on MXene membranes remain inadequately understood. Herein, we systematically investigated water transport through Ti3C2TX MXene nanochannels using molecular dynamics simulations coupled with experimental validation, addressing a significant knowledge gap in MXene-based separation membranes. Our simulations demonstrated that strong interactions between water molecules and hydrophilic nanochannel MXene surfaces (Ti3C2(OH)2 MXene or Ti3C2(NH)2 MXene) facilitated the formation of ordered molecular arrangements, substantially improving water permeability. Conversely, hydrophobic nanochannels (Ti3C2O2 MXene or Ti3C2F2 MXene) exhibited disordered water molecule distributions, leading to reduced permeability. Experimental validation corroborated these simulation results, demonstrating a direct correlation between the hydrophilicity of the Ti3C2TX surface and the water flux. The highly hydrophilic Ti3C2(OH)2 MXenes exhibited water flux maximum, whereas the more hydrophobic Ti3C2F2 MXenes had the lowest water flux. By integrating molecular dynamics simulations with experimental analyses, we gained comprehensive insights into the influence of nanochannel hydrophilicity on water transport mechanisms in MXene membranes. These findings provide critical guidelines for designing high-performance MXene-based membranes for advanced water treatment and separation applications.

Innovative temperature-responsive membrane with an elastic interface for biofouling mitigation in industrial circulating cooling water treatment

To address the issues of scaling caused by heat and water evaporation in regard to circulating cooling water (CCW), TFC membrane filtration systems have been increasingly considered for terminal treatment processes because of their excellent separation performance. However, membrane biofouling phenomenon significantly hinders the widespread utilization of TFC membranes. In this study, to harness the thermal phenomenon of CCW and establish a stable and durable multifunctional antibiofouling layer, temperature-responsive Pnipam and the spectral antibacterial agent Ag were organically incorporated into commercially available TFC membranes. Biological experimental findings demonstrated that above the lower critical solution temperature (LCST), the contraction of Pnipam molecular chains facilitated the inactivation of bacteria by the antibacterial agent, resulting in an impressive sterilization efficiency of up to 99 %. XDLVO analysis revealed that below the LCST, the establishment of a hydration layer on the functional interface resulted in the creation of elevated energy barriers, effectively impeding bacterial adhesion to the membrane surface. Consequently, a high bacterial release rate of 98.4 % was achieved on the low-temperature surface. The alterations in the functional membrane surface conformation induced by temperature variations further amplified the separation between the pollutants and the membrane, creating an enhanced "elastic interface." This efficient and straightforward cleaning procedure mitigated the formation of irreversible fouling without compromising the integrity of the membrane surface. This study presents a deliberately engineered thermoresponsive antibiofouling membrane interface to address the issue of membrane fouling in membrane-based CCW treatment systems while shedding new light on the mechanisms of "inactivation" and "defense."

High-Performance Nanofiltration Membrane with Dual Resistance to Gypsum Scaling and Biofouling for Enhanced Water Purification

Nanofiltration (NF) technology is pivotal for ensuring a sustainable and reliable supply of clean water. To address the critical need for advanced thin-film composite (TFC) polyamide (PA) membranes with exceptional permselectivity and fouling resistance for emerging contaminant purification, we introduce a novel high-performance NF membrane. This membrane features a selective polypiperazine (PIP) layer functionalized with amino-containing quaternary ammonium compounds (QACs) through an in situ interfacial polycondensation reaction. Our investigation demonstrated that precise QAC functionalization enabled the construction of the selective PA layer with increased surface area, enhanced microporosity, stronger electronegativity, and reduced thickness compared to the control PIP membrane. As a result, the QAC NF membrane exhibited an approximately 51% increase in water permeance compared to the control PIP membrane, while achieving superior retention capabilities for divalent salts (>99%) and emerging organic contaminants (>90%). Furthermore, the incorporation of QACs into the PIP selective layer was proved to be effective in mitigating mineral scaling by allowing selective passage of scale-forming cations, while simultaneously exhibiting strong antimicrobial properties to combat biofouling. The in situ QAC incorporation strategy presented in this study provides valuable guidelines for the fit-for-purpose design of the selective PA layer, which is crucial for the development of high-performance NF membranes for efficient 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.

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.

Polyphenol-mediated defect patching of graphene oxide membranes for sulfonamide contaminants removal and fouling control

Graphene oxide (GO)-based laminar membranes are promising candidates for next-generation nanofiltration membranes because of their theoretically frictionless nanochannels. However, nonuniform stacking during the filtration process and the inherent swelling of GO nanosheets generate horizontal and vertical defects, leading to a low selectivity and susceptibility to pore blockage. Herein, both types of defects are simultaneously patching by utilizing tannic acid and FeⅢ. Tannic acid first partially reduced the upper GO framework, and then coordinated with FeⅢ to form a metal-polyphenol network covering horizontal defects. Due to the enhanced steric hindrance, the resulting membrane exhibited a two-fold increase in sulfonamide contaminants exclusion compared to the pristine GO membrane. A non-significant reduction in permeance was observed. In terms of fouling control, shielding defects significantly alleviated the irreversible pore blockage of the membrane. Additionally, the hydrophilic metal-polyphenol network weakened the adhesion force between the membrane and foulants, thereby improving the reversibility of fouling in the cleaning stage. This work opens up a new way to develop GO-based membranes with enhanced separation performance and antifouling ability.

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.

An economical and simple method for preparing highly permeable and chlorine-resistant reverse osmosis membranes with potential commercial applications

The improvement in the overall efficiency of thin-film composite (TFC) reverse osmosis (RO) membranes is limited by their low permeability and sensitivity to degradation by chlorine. In the present study, polypiperazine (PIP), the commonly used amine monomer in preparing commercial TFC nanofiltration (NF) membranes, was used to regulate the m-phenylenediamine (MPD) based interfacial polymerization (IP) process. The results showed that addition of PIP optimized the micro-structure and surface properties of the polyamide (PA) layer. When the MPD and PIP mass ratio was 1 : 1, the TFCW-1:1 membrane exhibited 70% flux enhancement compared to pure MPD-based TFCW-1:0 membranes. Besides, the TFCW-1:1 membrane exhibited better chlorine-resistant performance since the NaCl rejection declined to just 3.8% while it was 11.3% for TFCW-1:0 membranes after immersion in 500 ppm NaClO solution for 48 h. Such improvement can be attributed to the increased number of unreacted amine groups and the thickness of the PA layer that PIP brought, which provided a sacrificial protective layer to consume the active chlorine, and thus maintain the integrity of the inner rejection layer. In all, the novelty and purpose of the present work is to find a more simple and scalable method to fabricate high-performance TFC RO membranes by using commonly, cheaply and frequently used materials.

Nanofiltration Membranes with Salt-Responsive Ion Valves for Enhanced Separation Performance in Brackish Water Treatment: A Battle against the Limitation of Salt Concentration

Utilizing brackish water resources has imposed a high requirement on the design and construction of nanofiltration membranes. To overcome the limitation of high salt concentration on the nanofiltration separation performance resulting from the weakened Donnan effect, a nanofiltration membrane with the effect of salt-responsive ion valves was developed by incorporating zwitterionic nanospheres into the polyamide layer (PA-ZNs). The interaction between the nanospheres and membranes at high salinity was revealed through a combination analysis from the perspectives of water transport model, positron annihilation spectroscopy, and solute rejection, contributing to the formation of the valve effect. The PA-ZNs membrane presented a breakthrough in overcoming the limitation of increased salt concentrations on nanofiltration separation performance, achieving a high selectivity of 105 for mono/multivalent anions. To reveal the role of the ion valve effect in ion transport through the membrane, the membrane conductance was determined at different salt concentrations, confirming channel-controlled transport at low salinity and ion valve-controlled transport at high salinity. Moreover, the main membrane separation mechanisms were systematically studied. The concept of salt-responsive ion valves may contribute to expanding the application of nanofiltration in brackish water treatment.

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.

Can MXene be the Effective Nanomaterial Family for the Membrane and Adsorption Technologies to Reach a Sustainable Green World?

Environmental pollution has intensified and accelerated due to a steady increase in the number of industries, and exploring methods to remove hazardous contaminants, which can be typically divided into inorganic and organic compounds, have become inevitable. Therefore, the development of efficacious technology for the separation processes is of paramount importance to ensure the environmental remediation. Membrane and adsorption technologies garnered attention, especially with the use of novel and high performing nanomaterials, which provide a target-specific solution. Specifically, widespread use of MXene nanomaterials in membrane and adsorption technologies has emerged due to their intriguing characteristics, combined with outstanding separation performance. In this review, we demonstrated the intrinsic properties of the MXene family for several separation applications, namely, gas separation, solvent dehydration, dye removal, separation of oil-in-water emulsions, heavy metal ion removal, removal of radionuclides, desalination, and other prominent separation applications. We highlighted the recent advancements used to tune separation potential of the MXene family such as the manipulation of surface chemistry, delamination or intercalation methods, and fabrication of composite or nanocomposite materials. Moreover, we focused on the aspects of stability, fouling, regenerability, and swelling, which deserve special attention when the MXene family is implemented in membrane and adsorption-based separation applications.

In situ oxidized TiO2/MXene ultrafiltration membrane with photocatalytic self-cleaning and antibacterial properties

Self-cleaning, antimicrobial ultrafiltration membranes are urgently needed to alleviate the low flux problems caused by membrane fouling in water treatment processes. In this study, in situ generated nano-TiO₂ MXene lamellar materials were synthesized and then 2D membranes were fabricated using vacuum filtration. The presence of nano TiO₂ particles as an interlayer support layer widened the interlayer channels, and also improved the membrane permeability. The TiO₂/MXene composite on the surface also showed an excellent photocatalytic property, resulting in enhanced self-cleaning properties and improved long-term membrane operational stability. The best overall performance of the TiO₂/MXene membrane at 0.24 mg cm^-2 loading was optimal, with 87.9% retention and 211.5 L m^-2 h^-1 bar^-1 flux at a filtration of 1.0 g L^-1 bovine serum albumin solution. Noticeably, the TiO₂/MXene membranes showed a very high flux recovery under UV irradiation with a flux recovery ratio (FRR) of 80% as compared to the non-photocatalytic MXene membranes. Moreover, the TiO₂/MXene membranes demonstrated over 95% resistance against E. coli. And the XDLVO theory also showed that the loading of TiO₂/MXene slowed down the fouling of the membrane surface by protein-based contaminants.

Effect of Interlayer Construction on TFC Nanofiltration Membrane Performance: A Review from Materials Perspective

Polyamide (PA) thin-film composite (TFC) nanofiltration (NF) membranes, which are extensively utilized in seawater desalination and water purification, are limited by the upper bounds of permeability-selectivity. Recently, constructing an interlayer between the porous substrate and the PA layer has been considered a promising approach, as it may resolve the trade-off between permeability and selectivity, which is ubiquitous in NF membranes. The progress in interlayer technology has enabled the precise control of the interfacial polymerization (IP) process, which regulates the structure and performance of TFC NF membranes, resulting in a thin, dense, and defect-free PA selective layer. This review presents a summary of the latest developments in TFC NF membranes based on various interlayer materials. By drawing from existing literature, the structure and performance of new TFC NF membranes using different interlayer materials, such as organic interlayers (polyphenols, ion polymers, polymer organic acids, and other organic materials) and nanomaterial interlayers (nanoparticles, one-dimensional nanomaterials, and two-dimensional nanomaterials), are systematically reviewed and compared. Additionally, this paper proposes the perspectives of interlayer-based TFC NF membranes and the efforts required in the future. This review provides a comprehensive understanding and valuable guidance for the rational design of advanced NF membranes mediated by interlayers for seawater desalination and water purification.

Enhanced high-salinity brines treatment using polyamide nanofiltration membrane with tunable interlayered MXene channel

The introduce of a nanomaterial interlayer between the substrate and polyamide is identified as a promising strategy to construct highly performed membranes. Two-dimensional (2D) materials are potential candidates as interlayer for advanced thin-film nanocomposite interlayer (TFNi) membranes. Nevertheless, low permeability, selectivity and long-term stability are still critical issues in TFNi membrane manufacture. Herein, a scalable approach for constructing TFNi membranes was implemented using stacked MXene nanosheets as interlayer, wherein the Fe₃O₄ nanoparticles worked as the sacrificial template to regulate the interlayer spacing of the 2D channels. SEM, XPS, water contact angle, and zeta potential were used to characterize the physical and chemical properties of prepared TFNi membranes, and the results shows that the presence of MXene interlayer increased the hydrophilicity, thinness and roughness of polyamide layer compared to that of pure TFC membranes. Besides, the enlarged interlayer channel after the sacrifice of the Fe₃O₄ nanoparticles greatly boosted the transport of the water molecules. The resultant membranes exhibited nearly double fold of water flux (66.4 ± 3.45 L·m^-2·h^-1) and higher selective separation factor (48.4) compared with those prepared without interlayer, while the outstanding salt rejection (>97 %) was maintained. This work achieves an innovative strategy for multifunctional polyamide nanofiltration membrane construction.

Enhancing Stability of Tannic Acid-FeIII Nanofiltration Membrane for Water Treatment: Intercoordination by Metal-Organic Framework

Tannic acid (TA)-Fe^III nanofiltration (NF) membrane has been demonstrated to possess more favorable removal of trace organic contaminants (TrOCs) over the conventional polyamide NF membrane. However, the drawback of acid instability severely hinders the practical application of TA-Fe^III NF membrane in the treatment of (weak) acidic wastewater containing TrOCs (e.g., pharmaceutical wastewater, surface water, and drinking water). Herein, we introduced the MIL-101(Cr) nanoparticle, a kind of metal-organic framework (MOF), into the TA-Fe^III selective layer to enhance the membrane acid stability. The acid-tolerance parameter of MIL-101(Cr)-stabilized TA-Fe^III membrane (TA-Fe^III-MOF membrane, 12,000 ppm/s^-1) was two orders of magnitude larger than that of the TA-Fe^III membrane (50 ppm/s^-1), and the TA-Fe^III-MOF membrane can withstand acid treatment at pH = 4 for more than 30 days. Meanwhile, the TA-Fe^III-MOF membrane displayed increased water permeance from 9.5 to 12.7 L/(m²·h·bar) after the MOF addition, without compromising the selectivity. The enhanced acid stability for the TA-Fe^III-MOF membrane was ascribed to an intercoordination mechanism, where Fe^III centers (from TA-Fe^III complex) coordinated with -COOH groups (from terephthalic acid of MOF) and Cr^III centers (from MOF) coordinated with -OH groups (from TA of TA-Fe^III complex), which was verified by the density functional theory calculation. This study highlights a new approach for the development of a TA-Fe^III-based NF membrane with markedly enhanced acid stability, which is important for its real application in wastewater treatment and water reuse.

Efficient capture of endocrine-disrupting compounds by a high-performance nanofiltration membrane for wastewater treatment

Conventional polyamide (PA) nanofiltration (NF) membranes can readily adsorb aromatic compounds, such as endocrine disrupting compounds (EDCs). Therefore, these substances can easily be transported across the membrane by solution-diffusion, resulting in a poor EDC-rejection. In this work, a novel thin film nanocomposite (TFN) membrane was fabricated by incorporating covalent organic frameworks (COFs) into the PA layer via an interfacial polymerization reaction. COFs with functional groups can provide abundant active binding sites for highly efficient EDC-capture. The rejection of the optimal TFN-COF membrane for bisphenol A, bisphenol AF, and sodium 2-biphenylate was 98.3%, 99.1%, and 99.3%, respectively, which was much higher than of the rejection of the pristine NF-membrane (82.4%, 95.5%, and 96.4%, respectively). Additionally, the TFN-COF membrane could be regenerated fast and efficiently by washing with ethanol for some minutes. COF nanofillers with porous structures provide additional water channels, making it possible to overcome the permeability-selectivity trade-off of NF membranes. The water permeance (17.1 L m^-2 h^-1 bar^-1) of the optimal membrane was about two times higher than for the pristine NF-membrane (8.7 L m^-2 h^-1 bar^-1). In addition, the TFN-COF membrane with a COF-loading of 0.05% w/v had an excellent Na₂SO₄ rejection (95.2%) due to size exclusion and strong Donnan effect. This work combines traditional NF membranes and adsorption materials to achieve efficient capture and rapid release of EDCs without sacrificing salt rejections, which opens the door to develop fit-for-purpose adsorptive NF membranes.

Recent advances in thin film nanocomposite membranes containing an interlayer (TFNi): fabrication, applications, characterization and perspectives

Polyamide (PA) reverse osmosis and nanofiltration membranes have been applied widely for desalination and wastewater reuse in the last 5-10 years. A novel thin-film nanocomposite (TFN) membrane featuring a nanomaterial interlayer (TFNi) has emerged in recent years and attracted the attention of researchers. The novel TFNi membranes are prepared from different nanomaterials and with different loading methods. The choices of intercalated nanomaterials, substrate layers and loading methods are based on the object to be treated. The introduction of nanostructured interlayers improves the formation of the PA separation layer and provides ultrafast water molecule transport channels. In this manner, the TFNi membrane mitigates the trade-off between permeability and selectivity reported for polyamide composite membranes. In addition, TFNi membranes enhance the removal of metal ions and organics and the recovery of organic solvents during nanofiltration and reverse osmosis, which is critical for environmental ecology and industrial applications. This review provides statistics and analyzes the developments in TFNi membranes over the last 5-10 years. The latest research results are reviewed, including the selection of the substrate and interlayer materials, preparation methods, specific application areas and more advanced characterization methods. Mechanistic aspects are analyzed to encourage future research, and potential mechanisms for industrialization are discussed.

Two-Dimensional Membranes with Highly Charged Nanochannels for Osmotic Energy Conversion

Inadequate mass transportation of semipermeable membranes causes poor osmotic energy conversion from salinity-gradient. Here, the lamellar graphene oxide membranes (GOMs) constructed with numerous fusiform-like nanochannels, that are pre-filled with negatively charged polyanion electrolytes, to both enhance the ion permeability and ion selectivity of the membrane for energy harvest from the salinty gradient, were developed. The as-prepared membrane achieved the maximum output power density of ∼4.94 W m^-2 under a 50 fold salinity gradient, which is 3.5 fold higher than that of pristine GOM. The enhancement could be ascribed to the synergistic impact of the expanded nanochannels and the enhanced space charge density. Via feeding with the artificial salinity water and monovalent cation electrolytes, the system could realise the power output up to 14.7 W m^-2 and 34.1 W m^-2 , respectively. Overall, this material design strategy could provide an alternative concept to effectively enhance ion transport of other two-dimensional (2D) membranes for specific purposes.

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.

Enhancing the Permselectivity of Thin-Film Composite Membranes Interlayered with MoS2 Nanosheets via Precise Thickness Control

The demand for highly permeable and selective thin-film composite (TFC) nanofiltration membranes, which are essential for seawater and brackish water softening and resource recovery, is growing rapidly. However, improving and tuning membrane permeability and selectivity simultaneously remain highly challenging owing to the lack of thickness control in polyamide films. In this study, we fabricated a high-performance interlayered TFC membrane through classical interfacial polymerization on a MoS₂-coated polyethersulfone substrate. Due to the enhanced confinement effect on the interface degassing and the improved adsorption of the amine monomer by the MoS₂ interlayer, the MoS₂-interlayered TFC membrane exhibited enhanced roughness and crosslinking. Compared to the control TFC membrane, MoS₂-interlayered TFC membranes have a thinner polyamide layer, with thickness ranging from 60 to 85 nm, which can be tuned by altering the MoS₂ interlayer thickness. A multilayer permeation model was developed to delineate and analyze the transport resistance and permeability of the MoS₂ interlayer and polyamide film through the regression of experimental data. The optimized MoS₂-interlayered TFC membrane (0.3-inter) had a 96.8% Na₂SO₄ rejection combined with an excellent permeability of 15.9 L m^-2 h^-1 bar^-1 (LMH/bar), approximately 2.4 times that of the control membrane (6.6 LMH/bar). This research provides a feasible strategy for the rational design of tunable, high-performance NF membranes for environmental applications.

Functionalized-MXene Thin-Film Nanocomposite Hollow Fiber Membranes for Enhanced PFAS Removal from Water

Due to adverse health effects and the broad sources of per- and polyfluoroakyl substances (PFAS), PFAS removal is a critical research area in water purification. We demonstrate the functionalization of thin-film composite (TFC) hollow fiber nanofiltration (HFN) membranes by MXene nanosheets during the interfacial polymerization (IP) process for enhanced removal of perfluorooctane sulfonic acid (PFOS) from water. A MXene-polyamide (PA) selective layer was fabricated on top of a polysulfone (PSF) hollow fiber support via IP of trimesoyl chloride (TMC) and a mixture of piperazine (PIP) and MXene nanosheets to form MXene-PA thin-film nanocomposite (TFN) membranes. Incorporating MXene nanosheets during the IP process tuned the morphology and negative surface charge of the selective layer, resulting in enhanced PFOS rejection from 72% (bare TFC) to more than 96% (0.025 wt % MXene TFN), while the water permeability was also increased from 13.19 (bare TFC) to 29.26 LMH/bar (0.025 wt % MXene TFN). Our results demonstrate that both electrostatic interaction and size exclusion are the main factors governing the PFOS rejection, and both are determined by PA selective layer structural and chemical properties. The lamella structure and interlayer of MXene nanosheets inside the PA layer provided different transport mechanisms for water, ions, and PFAS molecules, resulting in enhanced water permeability and PFAS rejection due to traveling through the membrane by both diffusions through the PA layer and the MXene intralayer channels. MXene nanosheets showed very promising capability as a 2D additive for tuning the structural and chemical properties of the PA layer at the permeability-rejection tradeoff.

Atomic-scale understanding of the Na and Cl trapping on the Mo1.33C(OH)2-MXene

Drinking water scarcity in arid and semi-arid regions is a reality that may turn into a global healthcare problem in the next few years. The scientific community is always looking for new materials to achieve effective sea and brackish water desalination to reduce water scarcity. Commonly, theoretical, and experimental methods make a synergy to better understand and explain the chemical and physical processes in water desalination electrodes. In this way, experimental evidence pointed Mo1.33CTx MXene as an efficient ion intercalation material, in which both Na+ and Cl- are removed. However, the atomic scale understanding of the physicochemical processes due to the Na and Cl interaction with the MXene is still unknown. We report the Na0 and Cl0 interaction with an OH functionalized Mo1.33C monolayer through a comprehensive first-principles density functional theory assessment. Results demonstrate that Na atoms attach to Oxygen, whereas Cl atoms bond through hydrogen bonds to the functional groups in the MXene, these bonds have two energy contributions: electrostatic and charge transfer, which increases its adsorption energy. Electrostatic potential isosurfaces, Bader charge analysis, and non-covalent interactions index help clarifying the way Na0 and Cl0 attach to the MXene layer. Oxygen atoms have an affinity for the electropositive Na0 atoms, which after interaction oxidizes to Na+, whereas hydrogen atoms-of the hydroxyl groups-interact with the electronegative Cl0 atoms, which upon adsorption reduce to Cl-. Our findings explain why OH-functionalized Mo1.33C can efficiently remove both Na and Cl atoms based on their affinities with the functional groups present in the MXene layer.

A novel ceramic-based thin-film composite nanofiltration membrane with enhanced performance and regeneration potential

The rational design of a ceramic-based nanofiltration membrane remains a significant challenge due to its performance and fabrication cost. Herein, we report a high-performance ceramic-based thin-film composite (TFC) membrane fabricated via a typical interfacial polymerization on an interwoven net substrate assembled by titanium dioxide (TiO₂) nanowires. The chemical properties and morphologies were systematically investigated for ceramic substrates and their corresponding TFC membranes. Due to the significantly improved hydrophilicity of the TiO₂ framework, more reactive amine monomers were uniformly adsorbed on the modified surface of the ceramic substrate, yielding an ultrathin polyamide layer with less resistance. In addition, the smooth surface and decreased pore size of the TiO₂ framework contributed to forming a defect-free polyamide layer. As a result, the obtained ceramic-based TFC membrane evinced high permeance of 26.4 L m^-2 h^-1 bar^-1 and excellent salt rejection efficiency, leading to simultaneous improvements compared with the control TFC membrane without the TiO₂ framework. Notably, the potential regeneration ability of the ceramic-based TFC membrane could be achieved via facile low-temperature calcination and re-polymerization process due to the varied thermostability between the polyamide layer and the robust ceramic substrate. The operation of regeneration helped to prolong the lifetime and decrease the cost for the ceramic-based TFC membrane. This research provides a feasible protocol to fabricate sustainable ceramic-based nanofiltration membranes with enhanced performance for water treatment.

Mechanistic Insights of a Thermoresponsive Interface for Fouling Control of Thin-Film Composite Nanofiltration Membranes

In spite of extensive research, fouling is still the main challenge for nanofiltration membranes, generating an extra transport resistance and requiring a larger operational pressure in practical applications. We fabricated a highly antifouling nanofiltration membrane by grafting poly(N-isopropylacrylamide) (PNIPAM) chains on a bromine-containing polyamide layer. The resulting membrane was found to have a double permeance compared to the pristine membrane, while the rejection of multivalent ions remained the same. In addition, PNIPAM chains yielded a better deposition resistance and adhesion resistance, thereby mitigating the increase of fouling and promoting the recovery of flux during the filtration and traditional cleaning stages, respectively. Moreover, PNIPAM chains shrank when the water temperature was above the lower critical solution temperature (LCST), indicating the formation of a buffer layer between the membrane and pollutants. The buffer layer would eliminate the membrane-foulant interaction energy, thus further enhancing the detachment of pollutants. This simple and efficient cleaning method could act as an enhanced cleaning procedure to remove irreversible fouling. This provides new insights into the fabrication of enhanced antifouling membranes using smart responsive polymer chains.

CNTs Intercalated LDH Composite Membrane for Water Purification with High Permeance

The pursuit of improved water purification technology has motivated extensive research on novel membrane materials to be carried out. In this paper, one-dimensional carboxylated carbon nanotubes (CNTs) were intercalated into the interlayer space of layered double hydroxide (LDH) to form a composite membrane for water purification. The CNTs/LDH laminates were deposited on the surface of the hydrolyzed polyacrylonitrile (PAN) ultrafiltration membrane through a vacuum-assisted assembly strategy. Based on the characterization of the morphology and structure of the CNTs/LDH composite membrane, it was found that the intercalation of CNT created more mass transfer channels for water molecules. Moreover, the permeance of the CNTs/LDH membrane was improved by more than 50% due to the low friction and rapid flow of water molecules in the CNT tubes. Additionally, the influence of preparation conditions on the separation performance was investigated using Evans blue (EB). Optimized fabrication conditions were given (the concentration of CoAl-LDH was 0.1 g/L and the weight ratio of CNTs was 2 wt.%). Next, the separation performances of the prepared CNTs/LDH composite membrane were evaluated using both single and mixed dye solutions. The results showed that the composite membrane obtained possessed a retention of 98% with a permeance of 2600 kg/(m²·h·MPa) for EB, which was improved by 36% compared with the pristine LDH composite membrane. Moreover, the stability of the CNTs/LDH composite membrane was investigated in 100 h with no obvious permeance drop (less than 13%), which exhibited its great potential in water purification.

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.

Reduced graphene oxide/TiO2(B) immobilized on nylon membrane with enhanced photocatalytic performance

Taking advantage of the unique properties of reduced graphene oxide (rGO) and monoclinic crystalline titanium dioxide (TiO₂(B)) nanomaterials, a novel rGO-TiO₂(B) composite membrane (MrGO-TiO₂(B)) was constructed by UV-light-assisted self-assembly of rGO and TiO₂ on a nylon membrane. The structure of MrGO-TiO₂(B) was characterized by scanning electron microscopy, transmission electron microscopy, UV-visible diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction analysis. Through 2D/2D self-assembly, rGO and TiO₂(B) were more tightly combined, and then MrGO-TiO₂(B) exhibited outstanding photocatalytic activity and an excellent methylene blue (MB) removal rate. MB was completely removed in 60 min at a constant rate of 0.042 min^-1 by the MrGO-TiO₂(B)/H₂O₂/MB system upon solar simulating Xe lamp irradiation. The synergistic effect of rGO and TiO₂(B) facilitated the photocatalytic degradation of MB. TiO₂(B) was excited and generated electrons and holes upon irradiation. Some electrons migrated to the surface of TiO₂(B) to react with H₂O₂ to produce hydroxyl radicals (OH), while the other electrons migrated to the surface of rGO to react with H₂O₂, producing OH. In addition, a number of superoxide radicals (O₂^-) was detected. The holes in the valence band of TiO₂(B) directly oxidized MB. The catalytic activity of MrGO-TiO₂(B) toward MB degradation remained stable after four rounds of reuse. Therefore, the surface modification of a nylon membrane with TiO₂(B) and rGO can serve as a promising route to fabricate photocatalytic membranes for use in the water treatment industry.

Toward Enhancing Desalination and Heavy Metal Removal of TFC Nanofiltration Membranes: A Cost-Effective Interface Temperature-Regulated Interfacial Polymerization

Polyamide (PA) chemistry-based nanofiltration (NF) membranes have an important role in the field of seawater desalination and wastewater reclamation. Achieving an ultrathin and defect-free active layer via precisely controlled interfacial polymerization (IP) is an effective routine to improve the separation efficiencies of NF membranes. Herein, the morphologies and chemical structures of the thin-film composite (TFC) NF membranes were accurately regulated by tailoring the interfacial reaction temperature during the IP process. This strategy was achieved by controlling the temperature (-15, 5, 20, 35, and 50°) of the oil-phase solutions. The structural compositions, morphological variations, and separation features of the fabricated NF membranes were studied in detail. In addition, the formation mechanisms of the NF membranes featuring different PAs were also proposed and discussed. The temperature-assisted IP (TAIP) method greatly changed the compositions of the resultant PA membranes. A very smooth and thin PA film was obtained for the NF membranes fabricated at a low interfacial temperature; thus, a high 19.2 L m^-2 h^-1 bar^-1 of water permeance and 97.7% of Na₂SO₄ rejection were observed. With regard to the NF membranes obtained at a high interfacial temperature, a lower water permeance and higher salt rejection with fewer membrane defects were achieved. Impressively, the high interfacial temperature-assisted NF membranes exhibited uniform coffee-ring-like surface morphologies. The special surface-featured NF membrane showed superior separation for selected heavy metals. Rejections of 93.9%, 97.9%, and 87.7% for Cu²⁺, Mn²⁺, and Cd²⁺ were observed with the optimized membrane. Three cycles of fouling tests indicated that NF membranes fabricated at low temperatures exhibited excellent antifouling behavior, whereas a high interface temperature contributed to the formation of NF membranes with high fouling tendency. This study provides an economical, facile, and universal TAIP strategy for tailoring the performances of TFC PA membranes for environmental water treatment.

Two-Dimensional Transition Metal Carbides and Nitrides (MXenes) for Water Purification and Antibacterial Applications

Two-dimensional (2D) materials such as graphene, graphene oxide (GO), metal carbides and nitrides (MXenes), transition metal dichalcogenides (TMDS), boron nitride (BN), and layered double hydroxide (LDH) metal-organic frameworks (MOFs) have been widely investigated as potential candidates in various separation applications because of their high mechanical strength, large surface area, ideal chemical and thermal stability, simplicity, ease of functionalization, environmental comparability, and good antibacterial performance. Recently, MXene as a new member of the 2D polymer family has attracted significant attention in water purification, desalination, gas separation, antibacterial, and antifouling applications. Herein, we review the most recent progress in the fabrication, preparation, and modification methods of MXene-based lamellar membranes with the emphasis on applications for water purification and desalination. Moreover, the antibacterial properties of MXene-based membranes show a significant potential for commercial use in water purification. Thus, this review provides a directional guide for future development in this emerging technology.

In Situ Assembly of Polyamide/Fe(BTC) Nanocomposite Reverse Osmosis Membrane Assisted by Fe3+-Polyphenolic Complex for Desalination

The metal-organic framework (MOF)-based polyamide (PA) membranes applied for desalination with high permeability and selectivity are attracting more and more attention. However, the design and fabrication of high-quality and stable MOF-based PA nanocomposite reverse osmosis (RO) membrane still remain a big challenge. Herein, Fe³⁺-polyphenolic complex coating via interfacial coordination was first explored as an interlayer of an in situ assembled stable and high-quality Fe(BTC)-based PA nanocomposite RO membranes for desalination. Although depositing the Fe³⁺-polyphenolic complex on the polymer support, sufficient heterogeneous nucleation sites for the in situ synthesizing Fe(BTC) are provided. Using this strategy, we can not only facilely prepare continuous MOF-based PA nanocomposite RO membranes, ignoring the complicated and time-consuming co-blending process and the MOF-particle aggregation, but also restrict the formation of PA matrix inside the pores of the support membrane and increase the rigidity of the polyamide chain. The method also gives a proper level of generality for the fabrication of versatile stable MOF-based PA RO membranes on various supports. The prepared PA/Fe(BTC) composite membrane exhibited excellent separation performance with a large permeate flux of 2.93 L m^-2 h^-1 bar^-1 and a high NaCl rejection of 96.8%.

Recent Progress in Emerging Two-Dimensional Transition Metal Carbides

As a new member in two-dimensional materials family, transition metal carbides (TMCs) have many excellent properties, such as chemical stability, in-plane anisotropy, high conductivity and flexibility, and remarkable energy conversation efficiency, which predispose them for promising applications as transparent electrode, flexible electronics, broadband photodetectors and battery electrodes. However, up to now, their device applications are in the early stage, especially because their controllable synthesis is still a great challenge. This review systematically summarized the state-of-the-art research in this rapidly developing field with particular focus on structure, property, synthesis and applicability of TMCs. Finally, the current challenges and future perspectives are outlined for the application of 2D TMCs.

Intrinsic Pseudocapacitive Affinity in Manganese Spinel Ferrite Nanospheres for High-Performance Selective Capacitive Removal of Ca2+ and Mg2

Pseudocapacitor-type hybrid capacitive deionization (PHCDI) has been developed extensively for deionization, which enables to address the worldwide freshwater shortage. However, the exploitation of selective hardness ion removal in resourceful hard water via the intrinsic pseudocapacitive effect, rather than the ion-sieving or ion-swapping effect based on the electric double layer (EDL) of porous carbon, is basically blank and urgent. Herein, manganese spinel ferrite (MFO) nanospheres were successfully fabricated by one-step solvothermal synthesis and used as the cathode for PHCDI assembled with commercial activated carbon. The MFO electrode exhibited prominent capacities of 534.6 μmol g^-1 (CaCl₂) and 980.4 μmol g^-1 (MgCl₂), outperforming those of other materials ever reported in the literature. Fascinatingly, systematic investigation of binary and ternary ion solutions showed the high electro-affinity of hardness ions (Ca²⁺ and Mg²⁺) toward Na⁺, especially the leading affinity of Mg²⁺, in which the superhigh hardness selectivity of 34.76 was achieved in the ternary solution with a molar ratio of Na-Ca-Mg as 20:1:1. Unexpectedly, the ion-swapping trace in a multi-ion environment was also first detected in our pseudocapacitive-based electrode. The electrochemical response in unary and multiple electrolytes disclosed that the unique pseudocapacitive affinity based on the cation (de)intercalation-redox mechanism was from the synergistic effect of the relative redox potential, ionic radius, and valence, in which the redox potential was the dominant factor.