Thin Polymeric Nanoparticle Membrane Coupled With Photothermal Membrane for Unprecedented Solar-Driven VOCs Removal

The removal of trace volatile organic compounds (VOCs) from water through solar-driven evaporation is crucial for ensuring access to safe drinking water. However, the faster evaporation rate of VOCs compared to water presents a significant challenge in effectively removing these hazardous compounds. To address this issue, a thin polymeric nanoparticle separation membrane is integrated with a photothermal membrane, forming a dual-layer solar evaporator. The bottom layer, a hydrogen-bonding complexed  polymer nanoparticle (HCPN) membrane, is fabricated by complexing poly(acrylic acid) and poly(vinylpyrrolidone) into well-dispersed nanoparticles through hydrogen bonding. This HCPN membrane selectively impedes VOC transport while allowing water to pass. The top layer is an in situ-grown photothermal membrane that absorbs solar energy, converting it into heat to drive the water evaporation process. Together, the two layers enable efficient separation and evaporation. This dual-layer solar evaporator demonstrates unprecedented performance, achieving a water evaporation rate of 2.18 kg m-2 h-1 under 1 sun irradiation and over 98% rejection to VOCs. The system maintains stability during 30-h continuous operation and performs effectively under real sunlight conditions. With its high efficiency, stability, and adaptability to real-world conditions, this solar evaporator offers a promising solution for providing safe, clean water in remote and underserved areas.

Toward Molecular Simulation Guided Design of Next-Generation Membranes: Challenges and Opportunities

Membranes provide energy-efficient solutions for separating ions from water, ion-ion separation, neutral or charged molecules, and mixed gases. Understanding the fundamental mechanisms and design principles for these separation challenges has significant applications in the food and agriculture, energy, pharmaceutical, and electronics industries and environmental remediation. In situ experimental probes to explore Angstrom-nanometer length-scale and pico-nanosecond time-scale phenomena remain limited. Currently, molecular simulations such as density functional theory, ab initio molecular dynamics (MD), all-atom MD, and coarse-grained MD provide physics-based predictive models to study these phenomena. The status of molecular simulations to study transport mechanisms and state-of-the-art membrane separation is discussed. Furthermore, limitations and open challenges in molecular simulations are discussed. Finally, the importance of molecular simulations in generating data sets for machine learning and exploration of membrane design space is addressed.

3D Nanoscale Structures of Hydrated Polyamide Desalination Membranes Revealed by Cryogenic Transmission Electron Microscopy Tomography

Desalination via reverse osmosis (RO) membrane technology is a preferred solution to the ongoing global challenges of freshwater scarcity. The active separation layer of RO membranes is a polyamide thin film (<200 nm), whose morphology critically influences membrane performance. However, conflicting descriptions of trends between morphology and performance abound in the literature due to the lack of a rigorous morphological description of these membranes. Notably, comprehensive three-dimensional (3D) morphological characterization of these membranes has so far been conducted exclusively under dry conditions, which contrasts with the operational, hydrated state of these membranes. Here, we present, for the first time, characterization of the hydrated 3D nanoscale morphology of polyamide films from commercial brackish water (BW) and seawater (SW) membranes using cryo-transmission electron microscopy (cryo-TEM) tomography. Our findings reveal significant morphological differences between hydrated and dry membranes, resulting in variations in key structural parameters that impact performance. Both SW and BW membranes swell and increase in total volume and thickness upon hydration, with BW membranes exhibiting more pronounced swelling (32% vs 7% in volume and 35% vs 11% in effective thickness), primarily due to the lower degree of cross-linking of BW membranes. Additionally, while the surface area decreases upon hydration for both SW and BW membranes, indicating a smoothing of surface nodules and cavities, surface roughness remains unchanged, suggesting that current roughness measurement methods such as atomic force microscopy do not capture intrinsic morphological features. Overall, this study demonstrates the feasibility of employing cryo-TEM tomography techniques to characterize RO membrane morphology under operation relevant conditions, thus enabling a better linkage between membrane morphology and performance.

Nodular networks in hydrated polyamide desalination membranes enhance water transport

For nearly half a century, thin-film composite reverse osmosis membranes have served as key separation materials for desalination. However, the precise structure of their polyamide selective layer under hydrated conditions and its relationship to membrane transport remain poorly understood. Using cryo-electron tomography, we successfully reconstructed the three-dimensional structure of six commercial polyamide membranes under hydrated conditions, revealing a fully swollen nodular network. The highly heterogeneous nodules, measuring 17.2 ± 2.8 nanometer in thickness, were directly connected to the pores of the underlying polysulfone substrate. The nodules occupied most of the surface area compared to the 75.9 ± 26.8-nanometer-thick dense layer of the polyamide film. Key structural parameters of the nodules, including surface area index and wall thickness, were correlated with the water permeance of an additional 16 polyamide membranes, validating the major role of these nodules in water transport. This study enhances our understanding of the heterogeneous structure of desalination membranes and its role in membrane transport.

Breaking the trade-off between lithium purity and lithium recovery: A comprehensive mathematical modeling based on membrane structure-property-performance relationships

The application of nanofiltration (NF) membranes for resource recovery, particularly lithium (Li) extraction from high magnesium (Mg) brines, is a rapidly growing research area. However, the trade-off between high Li+ purity and recovery remains challenging. In our study, we extend the widely adopted Donnan Steric Pore Model with Dielectric Exclusion (DSPM-DE) to analyze membrane structure-property-performance relationships at the process scale. For the first time, we quantify how membrane intrinsic parameters (e.g., pore size, effective thickness, and charge density) affect Li+ purity and recovery under module-scale processes. Under this framework, we demonstrate that electrically neutral and positively charged membranes outperform negatively charged membranes, albeit at the cost of slightly higher required hydraulic pressure. Notably, positively charged membranes with smaller pore size yet high water permeance (40-80 L m-2 h-1 bar-1) are preferred, which could simultaneously achieve excellent Li+ purity (∼98 %) and high Li+ recovery (∼93 %) in the single-pass process, effectively overcoming the purity-recovery trade-off correlation. We further demonstrate that negative Li+ rejection plays a crucial role in overcoming the trade-off correlation by significantly increasing Li+ recovery. Nevertheless, poor system flux distribution is inadvertently observed in the regions where strong negative rejection occurs, highlighting the need for careful consideration of the balance between system stability and lithium extraction performances. Our study identifies critical membrane parameters for achieving optimal lithium extraction performance at the process scale, offering fundamental insights for designing high-performance membranes for resource recovery.

Bimodal Ionic Conduction through Polymer Films due to Nano Confinement

Kinetic and selectivity of ionic conduction through polymer films is a key factor in the function of many systems including membrane technology, energy devices, sensors, and corrosion. Typically, the ionic conduction is accelerated by increasing the ionic strength of the electrolyte. However, counterintuitively, polymer films with very high electrochemical impedance demonstrate the opposite effect, i. e. ionic conduction slows down by increasing the ionic strength of the electrolyte. This bimodal nature of ionic conduction was discovered five decades ago and yet the mechanism has remained poorly understood. This minireview explains the long-standing anomaly through the lens of recent advances in nano-confinement electrochemistry. Emerging evidence shows that changes in solvation shells occurs due to the size of the channel through which ionic diffusion takes place. These solvation shells can significantly affect the mobility of ions, their interactions with functional groups of the polymer, and the overall conductivity of the electrolyte. At dimensions only a few nanometers, partially hydrated ions form stronger bonds with polymer slowing down the ionic conduction. Increasing ionic strength will add ion-pairing interactions further reducing ions mobility. The interplay of solvated ions, polymer chemistry, and ionic content offer an explanation for the anomalous bimodal ionic conduction.

Solution-processable polymer membranes with hydrophilic subnanometre pores for sustainable lithium extraction

Membrane-based separation processes hold great promise for sustainable extraction of lithium from brines for the rapidly expanding electric vehicle industry and renewable energy storage. However, it remains challenging to develop high-selectivity membranes that can be upscaled for industrial processes. Here we report solution-processable polymer membranes with subnanometre pores with excellent ion separation selectivity in electrodialysis processes for lithium extraction. Polymers of intrinsic microporosity incorporated with hydrophilic functional groups enable fast transport of monovalent alkali cations (Li+, Na+ and K+) while rejecting relatively larger divalent ions such as Mg2+. The polymer of intrinsic microporosity membranes surpasses the performance of most existing membrane materials. Furthermore, the membranes were scaled up and integrated into an electrodialysis stack, demonstrating excellent selectivity in simulated salt-lake brines. This work will inspire the development of selective membranes for a wide range of sustainable separation processes critical for resource recovery and a global circular economy.

Three-Dimensional Reconstruction-Characterization of Polymeric Membranes: A Review

Polymeric membranes serve as vital separation materials in diverse energy and environmental applications. A comprehensive understanding of three-dimensional (3D) structures of membranes is critical to performance evaluation and future design. Such quantitative 3D structural information is beyond the limit of most employed conventional two-dimentional characterization techniques such as scanning electron microscopy. In this review, we summarize eight types of 3D reconstruction-characterization techniques for membrane materials. Originated from life and materials science, these techniques have been optimized to reveal the 3D structures of membrane materials in the separation field. We systematically introduce the theories of each technique, summarize the sample preparation procedures developed for membrane materials, and demonstrate step-by-step data processing, including 3D model reconstruction and subsequent characterization. Representative case studies are introduced to show the progress of this field and how technical challenges have been overcome over the years. In the end, we share our perspectives and believe that this review can serve as a useful reference for 3D reconstruction-characterization techniques developed for membrane materials.

Highly Permselective Contorted Polyamide Desalination Membranes with Enhanced Free Volume Fabricated by mLbL Assembly

The permeability-selectivity trade-off in polymeric desalination membranes limits the efficiency and increases the costs of reverse osmosis and nanofiltration systems. Ultrathin contorted polyamide films with enhanced free volume demonstrate an impressive 8-fold increase in water permeance while maintaining equivalent salt rejection compared to conventional polyamide membranes made with m-phenylenediamine and trimesoyl chloride monomers. The solution-based molecular layer-by-layer (mLbL) deposition technique employed for membrane fabrication sequentially reacts a shape-persistent contorted diamine monomer with a trimesoyl chloride monomer, forming highly cross-linked, dense polyamide networks while avoiding the kinetic and mass transfer limitations of traditional interfacial polymerization. The mLbL process allows precise nanoscale control over polyamide selective layer thickness, network structure, and surface roughness. The resulting controlled film thicknesses enable direct measurements of water and NaCl permeabilities. The permselectivities of contorted polyamide membranes surpass those of commercial desalination membranes and approach the reported polyamide upper bound. Solution-diffusion transport modeling indicates that this high permselectivity may be attributed to enhanced water transport pathways in the contorted polyamides that increase water diffusivity-permeability while maintaining high solute rejection through solubility-selectivity.

Supramolecular Interfacial Assembly: Integrating Supramolecular Hosts into Polymeric Membranes through an Aqueous Interface

Efficient incorporation of macrocycles in polymeric membranes can impart the overall matrix with new properties for a range of cutting-edge applications. Here, we introduce a Supramolecular Interfacial Assembly (SIA) method for the fabrication of polymeric membranes featuring embedded macrocycles. Through harnessing the quasi-liquid nature of the concentrated polymer solution, SIA orchestrates the homogeneous spreading of macrocycles in an aqueous layer on its surface, leading to the creation of an interface between "water/water" phases, subsequently forming a cross-linked membrane driven by supramolecular electrostatic interactions. Remarkably, compared to the traditional interfacial polymerization, SIA adheres to a "green" paradigm without the need for organic solvents. The resultant composite membrane exhibits superior performance in organic solvent nanofiltration (OSN), owing to the precise molecular sieving property provided by the macrocycles with well-defined permanent cavities. This fabrication method holds great promise for the innovative design and production of composite membranes that seamlessly integrates macrocycles with conventional polymers, which can greatly impact the design and preparation of advanced membrane materials in the future.

Approaching Ideal Selectivity with Bioinspired and Biomimetic Membranes

The applications of polymeric membranes have grown rapidly compared to traditional separation technologies due to their energy efficiency and smaller footprint. However, their potential is not fully realized due, in part, to their heterogeneity, which results in a "permeability-selectivity" trade-off for most membrane applications. Inspired by the intricate architecture and excellent homogeneity of biological membranes, bioinspired and biomimetic membranes (BBMs) aim to emulate biological membranes for practical applications. This Review highlights the potential of BBMs to overcome the limitations of polymeric membranes by utilizing the "division of labor" between well-defined permeable pores and impermeable matrix molecules seen in biological membranes. We explore the exceptional performance of membranes in biological organisms, focusing on their two major components: membrane proteins (biological channels) and lipid matrix molecules. We then discuss how these natural materials can be replaced with artificial mimics for enhanced properties and how macro-scale BBMs are developed. We highlight key demonstrations in the field of BBMs that draw upon the factors responsible for transport through biological membranes. Additionally, current state-of-the-art methods for fabrication of BBMs are reviewed with potential challenges and prospects for future applications. Finally, we provide considerations for future research that could enable BBMs to progress toward scale-up and enhanced applicability.

Unraveling the Atomistic Mechanisms Underlying Effective Reverse Osmosis Filtration by Graphene Oxide Membranes

The graphene oxide (GO) membrane displays promising potential in efficiently filtering ions from water. However, the precise mechanism behind its effectiveness remains elusive, particularly due to the lack of direct experimental evidence at the atomic scale. To shed light on this matter, state-of-the-art techniques are employed such as integrated differential phase contrast-scanning transmission electron microscopy and electron energy loss spectroscopy, combined with reverse osmosis (RO) filtration experiments using GO membranes. The atomic-scale observations after the RO experiments directly reveal the binding of various ions including Na+, K+, Ca2+, and Fe3+ to the defects, edges, and functional groups of GO. The remarkable ion-sieving capabilities of GO membranes are confirmed, which can be attributed to a synergistic interplay of size exclusion, electrostatic interactions, cation-π, and other non-covalent interactions. Moreover, GO membranes modified by external pressure and cation also demonstrated further enhanced filtration performance for filtration. This study significantly contributes by uncovering the atomic-scale mechanism responsible for ion sieving in GO membranes. These findings not only enhance the fundamental understanding but also hold substantial potential for the advancement of GO membranes in reverse osmosis (RO) filtration.

Water transport mechanisms during pressure-driven transport through polyamide nanogaps

Molecular-scale simulations of pressure-driven transport through polyamide nanogaps (5-100 Å) were performed to investigate fundamental transport mechanisms. Results show that transport in nanogaps ≤  10 Å is always subdiffusive, but superdiffusive transport was observed in nanogaps ≥  20 Å. Near typical operating pressures for applications ( Δ p  = 100 atm), only the 100 Å nanogap exhibited superdiffusive behavior. Since openings in common membrane materials are typically <20 Å, results indicate that subdiffusive to diffusive transport dominates for typical applications, such as reverse osmosis.

Turing covalent organic framework membranes via heterogeneous nucleation synthesis for organic solvent nanofiltration

Although covalent organic frameworks (COFs) demonstrate notable potential for developing advanced separation membranes, contemporary COF membranes still lack controlled stacking and highly efficient sieving. Here, Turing-architecture COF membranes were constructed by engineering a reaction-diffusion assembly process via heterogeneous nucleation synthesis with tannic acid (TA). TA covalently binds with amine monomers to form a composite precursor with increased reactivity and decreased diffusivity. This altered the pathway of Schiff base reactions with aldehyde monomers, fulfilling suitable reaction-diffusion conditions, and ultimately formed the labyrinthine stripe or spot-patterned Turing COF film with controlled stacking and uniform pore structure. This endows our COF membrane with highly efficient molecule sieving ability for organic solvent nanofiltration while exhibiting a flux that is 621% greater than that of commercial membranes. Thus, this study provides a paradigm for the in situ synthesis of highly efficient COF membranes for diversely sustainable separations.

Nanofiltration Membrane with Enhanced Ion Selectivity Based on a Precision-Engineered Ultrathin Polyethylene Supporting Layer

Nanofiltration (NF) technology is increasingly used in the water treatment and separation fields. However, most research has focused on refining the selective layer while overlooking the potential role of the supporting layer. With expertise in ultrathin polymer films, particularly in the production of polyethylene (PE) membranes, we explore the possibility of improving NF membrane performance by precisely controlling the structure and surface properties of the ultrathin supporting layer in this work. Here, we introduced an innovative NF membrane that used a submicrometer ultrathin PE membrane produced through a biaxial stretching process, which is significantly thinner than commercial PE membranes available on the market. The core innovations are as follows: first, we focused on precise control of the supporting layer rather than just the selective layer, achieving significant enhancements in overall NF membrane performance; second, the ultrathin PE supporting layer served as a tunable interface for interfacial polymerization, offering possibilities for structural control of the selective layer and advancing membrane performance innovations. The resulting NF membrane boasts an overall thickness of ∼630 nm, which represents the thinnest NF membrane documented to date. This ultrathin NF membrane showed an ultrahigh Cl-/SO42- selectivity of 338.03, placing it at the forefront of existing literature. This study sheds light on the important role of the supporting layer in the preparation of selective layers. We believe that this approach has the potential to contribute to the development of ultrathin, high-performance NF membranes.

Nanofoamed Polyamide Membranes: Mechanisms, Developments, and Environmental Implications

Thin film composite (TFC) polyamide membranes have been widely applied for environmental applications, such as desalination and water reuse. The separation performance of TFC polyamide membranes strongly depends on their nanovoid-containing roughness morphology. These nanovoids not only influence the effective filtration area of the polyamide film but also regulate the water transport pathways through the film. Although there have been ongoing debates on the formation mechanisms of nanovoids, a nanofoaming theory─stipulating the shaping of polyamide roughness morphology by nanobubbles of degassed CO2 and the vapor of volatile solvents─has gained much attention in recent years. In this review, we provide a comprehensive summary of the nanofoaming mechanism, including related fundamental principles and strategies to tailor nanovoid formation for improved membrane separation performance. The effects of nanovoids on the fouling behaviors of TFC membranes are also discussed. In addition, numerical models on the role of nanovoids in regulating the water transport pathways toward improved water permeance and antifouling ability are highlighted. The comprehensive summary on the nanofoaming mechanism in this review provides insightful guidelines for the future design and optimization of TFC polyamide membranes toward various environmental applications.

Ultrafast Water Transport of Reverse Osmosis Membrane Based on Quasi-Vertically Oriented 2D Interlayer

Interlayered thin-film composite (i-TFC) membranes based on 2D materials have been widely studied due to their high efficiency in mass transfer. However, the randomly stacked 2D nanosheets usually increase the fluid path length to some extent. Herein, in situ-grown quasi-vertically oriented 2D ZIF-L was introduced as an interlayer for preparing high-performance reverse osmosis (RO) membranes. Through the optimization of the crystal growth based on the inert polyethylene substrate, the novel i-TFC RO membrane via interfacial polymerization shows an outstanding water permeance (5.50 L m-2 h-1 bar-1) and good NaCl rejection (96.3%). The membrane also shows promising potential in domestic water purification and organic solvent separation applications. Compared with the randomly stacked ZIF-L interlayer, the advantages of the vertically oriented one were ascribed to the excellent storage capacity of the amine monomers and the intensified gutter effect. This work will encourage more exploration on the interlayer architectures for high-performance i-TFC membranes.

Emerging Advances around Nanofluidic Transport and Mass Separation under Confinement in Atomically Thin Nanoporous Graphene

Membrane separation stands as an environmentally friendly, high permeance and selectivity, low energy demand process that deserves scientific investigation and industrialization. To address intensive demand, seeking appropriate membrane materials to surpass trade-off between permeability and selectivity and improve stability is on the schedule. 2D materials offer transformational opportunities and a revolutionary platform for researching membrane separation process. Especially, the atomically thin graphene with controllable porosity and structure, as well as unique properties, is widely considered as a candidate for membrane materials aiming to provide extreme stability, exponentially large selectivity combined with high permeability. Currently, it has shown promising opportunities to develop separation membranes to tackle bottlenecks of traditional membranes, and it has been of great interest for tremendously versatile applications such as separation, energy harvesting, and sensing. In this review, starting from transport mechanisms of separation, the material selection bank is narrowed down to nanoporous graphene. The study presents an enlightening overview of very recent developments in the preparation of atomically thin nanoporous graphene and correlates surface properties of such 2D nanoporous materials to their performance in critical separation applications. Finally, challenges related to modulation and manufacturing as well as potential avenues for performance improvements are also pointed out.

Smart membranes for separation and sensing

Self-assembled membranes are extensively applied across various fields due to their non-thermal and low-carbon footprint characteristics. Recently, smart membranes with stimuli responsiveness have garnered significant attention for their ability to alter physical and chemical properties in response to different stimuli, leading to enhanced performance and a wider range of applications compared to traditional membranes. This review highlights the recent advancements in self-assembled smart membranes, beginning with widely used membrane preparation strategies such as interfacial polymerization and blending. Then it delves into the primary types of stimuli-responses, including light, pH, and temperature, illustrated in detail with relevant examples. Additionally, the review explores the latest progress in the use of smart membranes for separation and sensing, addressing the challenges and opportunities in both fields. This review offers new insights into the design of novel smart membrane platforms for sustainable development and provides a broader perspective on their commercial potential.

Unraveling Anomalies in Preferential Liquid Transport through the Intrinsic Pores of Cyclodextrin in Polyester Nanofilms

The precise manipulation of the porous structure of the nanofiltration membrane is critical for unlocking enhanced separation efficiencies across various liquids and solutes. Ultrathin films of crosslinked macrocycles, specifically cyclodextrins (CDs), have drawn considerable attention in this area owing to their ability to facilitate precise molecular separation with high liquid permeance for both polar and non-polar liquids, resembling Janus membranes. However, the functional role of the intrinsic cavity of CD in liquid transport remains inadequately understood, demanding immediate attention in designing nanofiltration membranes. Here, the synthesis of polyester nanofilms derived from crosslinked β-CD, demonstrating remarkable Na2SO4 rejection (≈92 - 99.5%), high water permeance (≈4.4 - 37.4 Lm-2h-1bar-1), extremely low hexane permeance (<1 Lm-2h-1bar-1), and extremely high ratio (α > 500) of permeances for polar and non-polar liquids, is reported. Molecular simulations support the findings, indicating that neither the polar nor the non-polar liquids flow through the β-CD cavity in the nanofilm. Instead, liquid transport predominantly occurs through the 2.2 nm hydrophilic aggregate pores. This challenges the presumed functional role of macrocyclic cavities in liquid transport and raises questions about the existence of the Janus structure in nanofiltration membranes produced from the macrocyclic monomers.

Fluid energy theory of membrane

Membrane science is the key strategy to solve water shortage in the future, and its essence is energy and mass transfer. Due to the complexity and variety of the internal structure of membrane, the energy transfer theory of membrane is still a black box theory. Herein, a new fluid mechanics principle is introduced to establish the energy fluid theory of membrane, which is translated into the energy formula: such as the initial total pressure difference (ΔP), the flow rate of fluid exiting the membrane (v1 and v2), fluid density (ρ), and energy consumption by salt resistance (NSR): { [Formula: see text] +12ρv23}. The theoretical framework is not only helpful for the data analysis of the energy transfer process of membranes, but also helps to allow for more in-depth and specific theoretical research. For instance, the relationship between NSR and the concentration difference (C) of salt can be expressed as NSR = aCb (a-product constant, b-exponential constant, R2>0.99). Hence, the basic theory can not only be widely applied to a variety of membranes with complex internal structure, but also have a profound impact on the application and research of membrane science.

Nanoarchitectonics of ultrafine molybdenum carbide nanocrystals into three-dimensional nitrogen-doped carbon framework for capacitive deionization

Molybdenum carbide (MoC) has emerged as a promising material for capacitive deionization (CDI), but the poor electrochemical kinetics in conventional MoC owing to the bulk structure and low electric conductivity limit its CDI performance. To address this challenge, herein, we develop a novel strategy to synthesize ultrafine MoC nanocrystals that are embedded within a three-dimensional nitrogen-doped carbon framework (NC/MoC). This synthesis method involves the space-confined pyrolysis of molybdate precursors within metal-organic frameworks (MOFs). In this process, molybdates are confined into the MOF crystalline structure, where MOFs provide a confined reactor and carbon source. The resulting NC/MoC with the uniformly distributed MoC nanocrystals provides sufficient active sites for the electrosorption of salt ions, while the MOF-derived NC matrix facilitates charge transfer and provides the space-confined effect for preventing the possible aggregations of MoC nanocrystals during the CDI process. The NC/MoC exhibits an impressive salt adsorption capacity (SAC, 84.2 mg g-1, 1.2 V), rapid desalination rate, and high cycling stability (91.4% SAC retention after 200 cycles), better than those of most previously reported carbon-based CDI materials. Besides, the possible mechanisms are systematically investigated by ex situ characterization and density functional theory calculations. This study opens up new avenues for the construction of metal carbide-based nanocrystals for CDI and other electrochemical applications.

Advancing Ion Separation: Covalent-Organic-Framework Membranes for Sustainable Energy and Water Applications

ConspectusMembranes are pivotal in a myriad of energy production processes and modern separation techniques. They are essential in devices for energy generation, facilities for extracting energy elements, and plants for wastewater treatment, each of which hinges on effective ion separation. While biological ion channels show exceptional permeability and selectivity, designing synthetic membranes with defined pore architecture and chemistry on the (sub)nanometer scale has been challenging. Consequently, a typical trade-off emerges: highly permeable membranes often sacrifice selectivity and vice versa. To tackle this dilemma, a comprehensive understanding and modeling of synthetic membranes across various scales is imperative. This lays the foundation for establishing design criteria for advanced membrane materials. Key attributes for such materials encompass appropriately sized pores, a narrow pore size distribution, and finely tuned interactions between desired permeants and the membrane. The advent of covalent-organic-framework (COF) membranes offers promising solutions to the challenges faced by conventional membranes in selective ion separation within the water-energy nexus. COFs are molecular Legos, facilitating the precise integration of small organic structs into extended, porous, crystalline architectures through covalent linkage. This unique molecular architecture allows for precise control over pore sizes, shapes, and distributions within the membrane. Additionally, COFs offer the flexibility to modify their pore spaces with distinct functionalities. This adaptability not only enhances their permeability but also facilitates tailored interactions with specific ions. As a result, COF membranes are positioned as prime candidates to achieve both superior permeability and selectivity in ion separation processes.In this Account, we delineate our endeavors aimed at leveraging the distinctive attributes of COFs to augment ion separation processes, tackling fundamental inquiries while identifying avenues for further exploration. Our strategies for fabricating COF membranes with enhanced ion selectivity encompass the following: (1) crafting (sub)nanoscale ion channels to enhance permselectivity, thereby amplifying energy production; (2) implementing a multivariate (MTV) synthesis method to control charge density within nanochannels, optimizing ion transport efficiency; (3) modifying the pore environment within confined mass transfer channels to establish distinct pathways for ion transport. For each strategy, we expound on its chemical foundations and offer illustrative examples that underscore fundamental principles. Our efforts have culminated in the creation of groundbreaking membrane materials that surpass traditional counterparts, propelling advancements in sustainable energy conversion, waste heat utilization, energy element extraction, and pollutant removal. These innovations are poised to redefine energy systems and industrial wastewater management practices. In conclusion, we outline future research directions and highlight key challenges that need addressing to enhance the ion/molecular recognition capabilities and practical applications of COF membranes. Looking forward, we anticipate ongoing advancements in functionalization and fabrication techniques, leading to enhanced selectivity and permeability, ultimately rivaling the capabilities of biological membranes.

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.

Beyond nothingness in the formation and functional relevance of voids in polymer films

Voids-the nothingness-broadly exist within nanomaterials and impact properties ranging from catalysis to mechanical response. However, understanding nanovoids is challenging due to lack of imaging methods with the needed penetration depth and spatial resolution. Here, we integrate electron tomography, morphometry, graph theory and coarse-grained molecular dynamics simulation to study the formation of interconnected nanovoids in polymer films and their impacts on permeance and nanomechanical behaviour. Using polyamide membranes for molecular separation as a representative system, three-dimensional electron tomography at nanometre resolution reveals nanovoid formation from coalescence of oligomers, supported by coarse-grained molecular dynamics simulations. Void analysis provides otherwise inaccessible inputs for accurate fittings of methanol permeance for polyamide membranes. Three-dimensional structural graphs accounting for the tortuous nanovoids within, measure higher apparent moduli with polyamide membranes of higher graph rigidity. Our study elucidates the significance of nanovoids beyond the nothingness, impacting the synthesis‒morphology‒function relationships of complex nanomaterials.

An all-in-one FeOx-rGO sponge fabricated by solid-phase microwave thermal shock for water evaporation and purification

Developing high-efficiency photothermal seawater desalination devices is of significant importance in addressing the shortage of freshwater. Despite much effort made into photothermal materials, there is an urgent need to design a rapidly synthesized photothermal evaporator for the comprehensive purification of complex seawater. Therefore, we report on all-in-one FeOx-rGO photothermal sponges synthesized via solid-phase microwave thermal shock. The narrow band gap of the semiconductor material Fe3O4 greatly reduces the recombination of electron-hole pairs, enhancing non-radiative relaxation light absorption. The abundant π orbitals in rGO promote electron excitation and thermal vibration between the lattices. Control of the surface hydrophilicity and hydrophobicity promotes salt resistance while simultaneously achieving the purification of various complex polluted waters. The optimized GFM-3 sponge exhibitedan enhanced photothermal conversion rate of 97.3% and a water evaporation rate of 2.04 kg/(m2·hr), showing promising synergistic water purification properties. These findings provide a highly efficient photothermal sponge for practical applicationsof seawater desalination and purification,as well as develop a super-rapid processing methodology for evaporation devices.

Double charge flips of polyamide membrane by ionic liquid-decoupled bulk and interfacial diffusion for on-demand nanofiltration

Fine design of surface charge properties of polyamide membranes is crucial for selective ionic and molecular sieving. Traditional membranes face limitations due to their inherent negative charge and limited charge modification range. Herein, we report a facile ionic liquid-decoupled bulk/interfacial diffusion strategy to elaborate the double charge flips of polyamide membranes, enabling on-demand transformation from inherently negative to highly positive and near-neutral charges. The key to these flips lies in the meticulous utilization of ionic liquid that decouples intertwined bulk/interfacial diffusion, enhancing interfacial while inhibiting bulk diffusion. These charge-tunable polyamide membranes can be customized for impressive separation performance, for example, profound Cl-/SO42- selectivity above 470 in sulfate recovery, ultrahigh Li+/Mg2+ selectivity up to 68 in lithium extraction, and effective divalent ion removal in pharmaceutical purification, surpassing many reported polyamide nanofiltration membranes. This advancement adds a new dimension to in the design of advanced polymer membranes via interfacial polymerization.

Harmonic amide bond density as a game-changer for deciphering the crosslinking puzzle of polyamide

It is particularly essential to analyze the complex crosslinked networks within polyamide membranes and their correlation with separation efficiency for the insightful tailoring of desalination membranes. However, using the degree of network crosslinking as a descriptor yields abnormal analytical outcomes and limited correlation with desalination performance due to imperfections in segmentation and calculation methods. Herein, we introduce a more rational parameter, denoted as harmonic amide bond density (HABD), to unravel the relationship between the crosslinked networks of polyamide membranes and their desalination performance. HABD quantifies the number of distinct amide bonds per unit mass of polyamide, based on a comprehensive segmentation of polyamide structure and consistent computational protocols derived from X-ray photoelectron spectroscopy data. Compared to its counterpart, HABD overcomes the limitations and offers a more accurate depiction of the crosslinked networks. Empirical data validate that HABD exhibits the expected correlation with the salt rejection and water permeance of reverse osmosis and nanofiltration polyamide membranes. Notably, HABD is applicable for analyzing complex crosslinked polyamide networks formed by highly functional monomers. By offering a powerful toolbox for systematic analysis of crosslinked polyamide networks, HABD facilitates the development of permselective membranes with enhanced performance in desalination applications.

Building Semipermeable Films One Monomer at a Time: Structural Advantages via Molecular Layer Deposition vs Interfacial Polymerization

Molecular layer deposition (MLD) provides the opportunity to perform condensation polymerization one vaporized monomer at a time for the creation of precise, selective nanofilms for desalination membranes. Here, we compare the structure, chemistry, and morphology of two types of commercial interfacial polymerzation (IP) membranes with lab-made MLD films. M-phenylenediamine (MPD) and trimesoyl chloride (TMC) produced a cross-linked, aromatic polyamide often used in reverse osmosis membranes at MLD growth rates of 2.9 Å/cycle at 115 °C. Likewise, piperazine (PIP) and TMC formed polypiperazine amide, a common selective layer in nanofiltration membranes, with MLD growth rates of 1.5 Å/cycle at 115 °C. Ellipsometry and X-ray reflectivity results suggest that the surface of the MLD films is comprised of polymer segments roughly two monomers in length, which are connected at one end to the cross-linked bulk layer. As a result of this structure as well as the triple-functionality of TMC, MPD-TMC had a temperature window of stable growth rate from 115 to 150 °C, which is unlike any non-cross-linked MLD chemistries reported in the literature. Compared to IP films, corresponding MLD films were denser and morphologically conformal, which suggests a reduction in void volumes; this explains the high degree of salt rejection and reduced flux previously observed for exceptionally thin MPD-TMC MLD membranes. Using X-ray photoelectron spectroscopy and infrared spectroscopy, MLD PIP-TMC films evidenced a completely cross-linked internal structure, which lacked amine and carboxyl groups, pointing to a hydrophobic bulk structure, ideal for optimized water flux. Grazing-incidence wide-angle X-ray scattering showed broad features in each polyamide with d-spacings of 5.0 Å in PIP-TMC compared to that of 3.8 Å in MPD-TMC. While MLD and IP films were structurally identical to PIP-TMC, MPD-TMC IP films had a structure that may have been altered by post-treatment compared to MLD films. These results provide foundational insights into the MLD process, structure-performance relationships, and membrane fabrication.

Interplay of the forces governing steroid hormone micropollutant adsorption in vertically-aligned carbon nanotube membrane nanopores

Vertically-aligned carbon nanotube (VaCNT) membranes allow water to conduct rapidly at low pressures and open up the possibility for water purification and desalination, although the ultralow viscous stress in hydrophobic and low-tortuosity nanopores prevents surface interactions with contaminants. In this experimental investigation, steroid hormone micropollutant adsorption by VaCNT membranes is quantified and explained via the interplay of the hydrodynamic drag and friction forces acting on the hormone, and the adhesive and repulsive forces between the hormone and the inner carbon nanotube wall. It is concluded that a drag force above 2.2 × 10-3 pN overcomes the friction force resulting in insignificant adsorption, whereas lowering the drag force from 2.2 × 10-3 to 4.3 × 10-4 pN increases the adsorbed mass of hormones from zero to 0.4 ng cm-2. At a low drag force of 1.6 × 10-3 pN, the adsorbed mass of four hormones is correlated with the hormone-wall adhesive (van der Waals) force. These findings explain micropollutant adsorption in nanopores via the forces acting on the micropollutant along and perpendicular to the flow, which can be exploited for selectivity.

Desalination Performance in Janus Graphene Oxide Channels: Geometric Asymmetry vs Charge Polarity

Improving the desalination performance of membranes is always in the spotlight of scientific research; however, Janus channels with polarized surface charge as nanofiltration membranes are still unexplored. In this work, using molecular dynamics simulations, we demonstrate that Janus graphene oxide (GO) channels with appropriate geometry and surface charge can serve as highly efficient nanofiltration membranes. We observe that the water permeability of symmetric Janus GO channels is significantly superior to that of asymmetric channels without sacrificing much ion rejection, owing to weakened ion blockage and electrostatic effects. Furthermore, in symmetric Janus GO channels, the transport of water and ions is sensitive to the charge polarity of the channel inner surface, which is realized by tuning the ratio of cationic and anionic functionalization. Specifically, with the increase in cationic functionalization, the water flux decreases monotonously, while ion rejection displays an interesting maximum behavior that indicates desalination optimization. Moreover, the trade-off between water permeability and ion rejection suggests that the Janus GO channels have an excellent desalination potential and are highly tunable according to the specific water treatment requirements. Our work sheds light on the key role of channel geometry and charge polarity in the desalination performance of Janus GO channels, which paves the way for the design of novel desalination devices.

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.

2D Fluorinated Graphene Oxide (FGO)-Polyethyleneimine (PEI) Based 3D Porous Nanoplatform for Effective Removal of Forever Toxic Chemicals, Pharmaceutical Toxins, and Waterborne Pathogens from Environmental Water Samples

Although water is essential for life, as per the United Nations, around 2 billion people in this world lack access to safely managed drinking water services at home. Herein we report the development of a two-dimensional (2D) fluorinated graphene oxide (FGO) and polyethylenimine (PEI) based three-dimensional (3D) porous nanoplatform for the effective removal of polyfluoroalkyl substances (PFAS), pharmaceutical toxins, and waterborne pathogens from contaminated water. Experimental data show that the FGO-PEI based nanoplatform has an estimated adsorption capacity (qm) of ∼219 mg g-1 for perfluorononanoic acid (PFNA) and can be used for 99% removal of several short- and long-chain PFAS. A comparative PFNA capturing study using different types of nanoplatforms indicates that the qm value is in the order FGO-PEI > FGO > GO-PEI, which indicates that fluorophilic, electrostatic, and hydrophobic interactions play important roles for the removal of PFAS. Reported data show that the FGO-PEI based nanoplatform has a capability for 100% removal of moxifloxacin antibiotics with an estimated qm of ∼299 mg g-1. Furthermore, because the pore size of the nanoplatform is much smaller than the size of pathogens, it has a capability for 100% removal of Salmonella and Escherichia coli from water. Moreover, reported data show around 96% removal of PFAS, pharmaceutical toxins, and pathogens simultaneously from spiked river, lake, and tap water samples using the nanoplatform.

Exploring the Knowledge Attained by Machine Learning on Ion Transport across Polyamide Membranes Using Explainable Artificial Intelligence

Recent studies have increasingly applied machine learning (ML) to aid in performance and material design associated with membrane separation. However, whether the knowledge attained by ML with a limited number of available data is enough to capture and validate the fundamental principles of membrane science remains elusive. Herein, we applied explainable artificial intelligence (XAI) to thoroughly investigate the knowledge learned by ML on the mechanisms of ion transport across polyamide reverse osmosis (RO) and nanofiltration (NF) membranes by leveraging 1,585 data from 26 membrane types. The Shapley additive explanation method based on cooperative game theory was used to unveil the influences of various ion and membrane properties on the model predictions. XAI shows that the ML can capture the important roles of size exclusion and electrostatic interaction in regulating membrane separation properly. XAI also identifies that the mechanisms governing ion transport possess different relative importance to cation and anion rejections during RO and NF filtration. Overall, we provide a framework to evaluate the knowledge underlying the ML model prediction and demonstrate that ML is able to learn fundamental mechanisms of ion transport across polyamide membranes, highlighting the importance of elucidating model interpretability for more reliable and explainable ML applications to membrane selection and design.

High Freshwater Flux Solar Desalination via a 3D Plasmonic Evaporator with an Efficient Heat-Mass Evaporation Interface

Passive solar desalination with interfacial heating is a promising technique to utilize solar energy to convert seawater into fresh water through evaporation and condensation. However, the current freshwater flux of solar desalination is much below industrial requirements (> 20 L m-2  h-1 ). Herein, it is demonstrated that a 3D plasmonic evaporator with an efficient heat-mass evaporation interface (HM-EI) achieves a freshwater flux of 29.1 L m-2  h-1 for 3.5 wt.% NaCl, which surpasses the previous solar evaporators and approaches the level of reverse osmosis (the highest installed capacity in industrial seawater desalination technology). The realization of high freshwater flux solar desalination comes from the efficient HM-EI comprising a grid-like plasmonic macrostructure for enhanced energy utilization in heat properties and a large-pore microstructure for accelerated ion transport in mass properties. This work provides a new direction for designing next-generation solar evaporators with high freshwater flux for industrial requirements.

Mechanisms and models for water transport in reverse osmosis membranes: history, critical assessment, and recent developments

Water scarcity is one of the greatest societal challenges facing humanity. Reverse osmosis (RO) desalination, a widely used membrane-based technology, has proven to be effective to augment water supply in water-stressed regions of our planet. However, progress in the design and development of RO membranes has been limited. To significantly enhance the performance of RO membranes, it is essential to acquire a deep understanding of the membrane separation and transport mechanisms. In this tutorial review, we cover the pivotal historical developments in RO technology, examine the chemical and physical properties of RO membrane materials, and critically review the models and mechanisms proposed for water transport in RO membranes. Based on recent experimental and computational findings, we conduct a thorough analysis of the key transport models-the solution-diffusion and pore-flow models-to assess their validity for accurately describing water transport in RO membranes. Our analysis involves examining the experimental evidence in favor of the solution-diffusion mechanism. Specifically, we explain whether the water content gradient within the membrane, cited as evidence for the key assumption in the solution-diffusion model, can drive a diffusive transport through RO membranes. Additionally, we review the recent molecular dynamics simulations which support the pore-flow mechanism for describing water transport in RO membranes. We conclude by providing future research directions aimed at addressing key knowledge gaps in water transport phenomena in RO membranes, with the goal of advancing the development of next-generation RO membranes.

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

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

Extreme Ion-Transport Inorganic 2D Membranes for Nanofluidic Applications

Inorganic 2D materials offer a new approach to controlling mass diffusion at the nanoscale. Controlling ion transport in nanofluidics is key to energy conversion, energy storage, water purification, and numerous other applications wherein persistent challenges for efficient separation must be addressed. The recent development of 2D membranes in the emerging field of energy harvesting, water desalination, and proton/Li-ion production in the context of green energy and environmental technology is herein discussed. The fundamental mechanisms, 2D membrane fabrication, and challenges toward practical applications are highlighted. Finally, the fundamental issues of thermodynamics and kinetics are outlined along with potential membrane designs that must be resolved to bridge the gap between lab-scale experiments and production levels.

The validity of the continuum modeling limit in a single pore flows to the molecular scale

The discrete characteristics of molecules become dominant in the molecular regime when the surface-to-volume ratio becomes very high. Using the well-established continuum approach is questionable due to this dominant behavior. Due to the lack of perfect modeling of such a small-scale system, the experimentalist must rely on the trial and error method. Here we analyze the water transport mechanism through a nanoporous graphene membrane at the molecular level by adopting the classical molecular dynamics (MD) simulation. The results for SPC/E water molecules were compared with those obtained for liquid argon atoms and continuum Sampson's equation predictions. We find that the effect of local variants such as density layering, interatomic forces, slip velocity, and geometric boundary conditions become exponentially dominant with decreasing nanopore size. Consequently, the continuum assumptions break down at 1.5 nm pore diameter due to neglecting the dominant local properties.

Carbon-doped metal oxide interfacial nanofilms for ultrafast and precise separation of molecules

Membranes with molecular-sized, high-density nanopores, which are stable under industrially relevant conditions, are needed to decrease energy consumption for separations. Interfacial polymerization has demonstrated its potential for large-scale production of organic membranes, such as polyamide desalination membranes. We report an analogous ultrafast interfacial process to generate inorganic, nanoporous carbon-doped metal oxide (CDTO) nanofilms for precise molecular separation. For a given pore size, these nanofilms have 2 to 10 times higher pore density (assuming the same tortuosity) than reported and commercial organic solvent nanofiltration membranes, yielding ultra-high solvent permeance, even if they are thicker. Owing to exceptional mechanical, chemical, and thermal stabilities, CDTO nanofilms with designable, rigid nanopores exhibited long-term stable and efficient organic separation under harsh conditions.

Influence of water quality factors on cake layer 3D structures and water channels during ultrafiltration process

The three-dimensional (3D) structure of the cake layer, which could be influenced by water quality factors, plays a significant role in the ultrafiltration (UF) efficiency of water purification. However, it remains challenging to precisely reveal the variation of cake layer 3D structures and water channel characteristics. Herein, we systematically report the variation in the cake layer 3D structure at the nanoscale induced by key water quality factors and reveal its influence on water transport, in particular the abundance of water channels within the cake layer. In comparison with pH and Na+, Ca2+ played more significant role in determining cake layer structures. The sandwich-like cake layer, which was induced by the asynchronous deposition of humic acids and sodium alginate (SA), shifted to an isotropic structure when Ca2+ was present due to the Ca2+ bridging. In comparison with the sandwich-like structure, the isotropic cake layer has higher fractions of free volume (voids) and more water channels, leading to a 147% improvement in the water transport coefficient, 60% reduction in the cake layer resistance, and 21% increase in the final membrane specific flux. Our work elucidates a structure-property relationship where improving the isotropy of the cake layer 3D structure is conducive to the optimization of water channels and water transport within cake layers. This could inspire tailored regulation strategies for cake layers to enhance the UF efficiency of water purification.

Pressure-driven distillation using air-trapping membranes for fast and selective water purification

Membrane technologies that enable the efficient purification of impaired water sources are needed to address growing water scarcity. However, state-of-the-art engineered membranes are constrained by a universal, deleterious trade-off where membranes with high water permeability lack selectivity. Current membranes also poorly remove low-molecular weight neutral solutes and are vulnerable to degradation from oxidants used in water treatment. We report a water desalination technology that uses applied pressure to drive vapor transport through membranes with an entrapped air layer. Since separation occurs due to a gas-liquid phase change, near-complete rejection of dissolved solutes including sodium chloride, boron, urea, and N-nitrosodimethylamine is observed. Membranes fabricated with sub-200-nm-thick air layers showed water permeabilities that exceed those of commercial membranes without sacrificing salt rejection. We also find the air-trapping membranes tolerate exposure to chlorine and ozone oxidants. The results advance our understanding of evaporation behavior and facilitate high-throughput ultraselective separations.

Correlating the Diffusion of Water to Performance in Model Reverse Osmosis Polyamides with Controlled Crosslink Densities

We systematically reduce the cross-link density of a PA network based on m-phenylene diamine by substituting a fraction of the trifunctional trimesoyl chloride cross-linking agent with a difunctional isophthaloyl analog that promotes chain extension, in order to elucidate robust design cues for improving the polyamide (PA) separation layer in reverse osmosis (RO) membranes for desalination. Thin films of these model PA networks are fully integrated into a composite membrane and evaluated in terms of their water flux and salt rejection. By incorporating 15 mol % of the difunctional chain extender, we reduce the cross-link density of the network by a factor of two, which leads to an 80 % increase in the free or unreacted amine content. The resulting swelling of the PA network in liquid water increases by a factor of two accompanied by a 30 % increase in the salt passage through the membrane. Surprisingly, this leads to a 30 % decrease in the overall permeance of water through the membrane. This conundrum is resolved by quantifying the microscopic diffusion coefficient of water inside the PA network with quasi-elastic neutron scattering. In the highest and lowest cross-link density networks, water shows strong signatures of confined diffusion. At short length scales, the water exhibits a translational diffusion that is consistent with the jump-diffusion mechanism. This translational diffusion coefficient is approximately five times slower in the lowest cross-linked density network, consistent with the reduced water permeance. This is interpreted as water molecules interacting more strongly with the increased free amine content. Over longer length scales the water diffusion is confined, exhibiting mobility that is independent of length scale. The length scales of confinement from the quasi-elastic neutron scattering experiments at which this transition from confined to translational diffusion occurs is on the order of (5 to 6) Å , consistent with complementary X-ray scattering, small angle neutron scattering, and positron annihilation lifetime spectroscopy measurements. The confinement appears to come from heterogeneities in the average inter-atomic distances, suggesting that diffusion occurs by water bouncing between chains and occasionally sticking to the polar functional groups. The results obtained here are compared with similar studies of water diffusion through both rigid porous silicates and ion exchange membranes, revealing robust design cues for engineering high-performance RO membranes.

Cake layer 3D structure regulation to optimize water channels during Al-based coagulation-ultrafiltration process

The variation in cake layer three-dimensional (3D) structures and related water channel characteristics induced by coagulation pretreatment remains unclear; however, gaining such knowledge will aid in improving ultrafiltration (UF) efficiency for water purification. Herein, the regulation of cake layer 3D structures (3D distribution of organic foulants within cake layers) by Al-based coagulation pretreatment was analyzed at the micro/nanoscale. The sandwich-like cake layer of humic acids and sodium alginate induced without coagulation was ruptured, and foulants were gradually uniformly distributed within the floc layer (toward an isotropic structure) with increasing coagulant dosage (a critical dosage was observed). Furthermore, the structure of the foulant-floc layer was more isotropic when coagulants with high Al₁₃ concentrations were used (either AlCl₃ at pH 6 or polyaluminum chloride, in comparison with AlCl₃ at pH 8 where small-molecular-weight humic acids were enriched near the membrane). These high Al₁₃ concentrations lead to a 48.4% higher specific membrane flux than that seen for UF without coagulation. Molecular dynamics simulations revealed that with increasing Al₁₃ concentration (Al₁₃: 6.2% to 22.6%), the water channels within the cake layer were enlarged and more connected, and the water transport coefficient was improved by up to 54.1%, indicating faster water transport. These findings demonstrate that facilitating an isotropic foulant-floc layer with highly connected water channels by coagulation pretreatment with high-Al₁₃-concentration coagulants (having a strong ability to complex organic foulants) is the key issue in optimizing the UF efficiency for water purification. The results should provide further understanding of the underlying mechanisms of coagulation-enhancing UF behavior and inspire precise design of coagulation pretreatment to achieve efficient UF.

When self-assembly meets interfacial polymerization

Interfacial polymerization (IP) and self-assembly are two thermodynamically different processes involving an interface in their systems. When the two systems are incorporated, the interface will exhibit extraordinary characteristics and generate structural and morphological transformation. In this work, an ultrapermeable polyamide (PA) reverse osmosis (RO) membrane with crumpled surface morphology and enlarged free volume was fabricated via IP reaction with the introduction of self-assembled surfactant micellar system. The mechanisms of the formation of crumpled nanostructures were elucidated via multiscale simulations. The electrostatic interactions among m-phenylenediamine (MPD) molecules, surfactant monolayer and micelles, lead to disruption of the monolayer at the interface, which in turn shapes the initial pattern formation of the PA layer. The interfacial instability brought about by these molecular interactions promotes the formation of crumpled PA layer with larger effective surface area, facilitating the enhanced water transport. This work provides valuable insights into the mechanisms of the IP process and is fundamental for exploring high-performance desalination membranes.

Water transport in reverse osmosis membranes is governed by pore flow, not a solution-diffusion mechanism

We performed nonequilibrium molecular dynamics (NEMD) simulations and solvent permeation experiments to unravel the mechanism of water transport in reverse osmosis (RO) membranes. The NEMD simulations reveal that water transport is driven by a pressure gradient within the membranes, not by a water concentration gradient, in marked contrast to the classic solution-diffusion model. We further show that water molecules travel as clusters through a network of pores that are transiently connected. Permeation experiments with water and organic solvents using polyamide and cellulose triacetate RO membranes showed that solvent permeance depends on the membrane pore size, kinetic diameter of solvent molecules, and solvent viscosity. This observation is not consistent with the solution-diffusion model, where permeance depends on the solvent solubility. Motivated by these observations, we demonstrate that the solution-friction model, in which transport is driven by a pressure gradient, can describe water and solvent transport in RO membranes.

Host-Guest Recognition Boosts Biomimetic Mono/Multivalent Cation Separation

Biomimetic ion permselective membranes with ultrahigh ion permeability and selectivity represent a research frontier in ion separation, yet the successful fabrication of such membranes remains a formidable challenge. Here, we demonstrate a 4-sulfocalix[4]arene (4-SCA)-modified graphene oxide (GO) membrane that shows extraordinary performance in separating mono-from multivalent cations, as well as having reversible pH-responsiveness. The resulting 4-SCA-modified GO (SCA-GO) membrane preferentially transports potassium ions (K⁺) over radionuclide cations (Co²⁺, UO₂²⁺, La³⁺, Eu³⁺, and Th⁴⁺). The ion selectivities are an order of magnitude higher than that of the unmodified GO membrane. Theoretical calculations and experimental investigations demonstrate that the much-improved ion selectivity arises from the specific recognition between 4-SCA and radionuclide cations. The transport of multivalent radionuclides is impeded by a binding-obstructing mechanism from the host-guest interactions. Interestingly, the host-guest interactions are responsive to the protonation/deprotonation transformation of the 4-SCA. Therefore, the SCA-GO membrane mimics pH-regulated ion selective behavior found in biological ion channels. Our strategy of designing a biomimetic permselective GO membrane may allow efficient nuclear wastewater treatment and, more importantly, deepen our understanding of biomimetic ion transport mechanisms.

Supercomputing and machine learning-aided optimal design of high permeability seawater reverse osmosis membrane systems

Concentration polarization (CP) should limit the energy and cost reducing benefits of high permeability seawater reverse osmosis (SWRO) membranes operating at a water flux higher than normal one. Herein, we propose a multiscale optimization framework coupling membrane permeability, feed spacer design (sub-millimeter scale) and system design (meter scale) via computational fluid dynamics and system level modeling using advanced supercomputing in conjunction with machine learning. Simulation results suggest energy consumption could be reduced by 27.5% (to 1.66 kWh m^-3) predominantly through the use of high permeability SWRO membranes (12.2%) and a two-stage design (14.5%). Without optimization, CP approaches 1.52 at the system inlet, whereas the optimized CP is limited to 1.20. This work elucidates the optimized permeability, module design, operating scheme and benefits of high permeability SWRO membranes in seawater desalination.

High-Performance Polyamide Reverse Osmosis Membrane Containing Flexible Aliphatic Ring for Water Purification

A reverse osmosis (RO) membrane with a high water permeance and salt rejection is needed to reduce the energy requirement for desalination and water treatment. However, improving water permeance while maintaining a high rejection of the polyamide RO membrane remains a great challenge. Herein, we report a rigid-flexible coupling strategy to prepare a high-performance RO membrane through introducing monoamine with a flexible aliphatic ring (i.e., piperidine (PPR)) into the interfacial polymerization (IP) system of trimesoyl chloride (TMC) and m-phenylenediamine (MPD). The resulted polyamide film consists of a robust aromatic skeleton and soft aliphatic-ring side chain, where the aliphatic ring optimizes the microstructure of polyamide network at a molecular level. The obtained membranes thereby showed an enhanced water permeance of up to 2.96 L·m^-2 h^-1 bar^-1, nearly a 3-fold enhancement compared to the control group, meanwhile exhibiting an ultrahigh rejection toward NaCl (99.4%), thus successfully overcoming the permeability-selectivity trade-off limit. Furthermore, the mechanism of the enhanced performance was investigated by molecular simulation. Our work provides a simple way to fabricate advanced RO membranes with outstanding performance.