Menachem Elimelech

Gas separation is central to industrial processes that drive climate mitigation, clean energy, and sustainable technologies. Metal–organic frameworks (MOFs) offer remarkable tunability for adsorption-based separations, yet identifying optimal materials remains challenging due to their vast structural diversity, costly simulations, and the difficulty of achieving a full range of desired properties. Existing machine learning approaches have accelerated screening but often lack generalizability across diverse gas pairs and operating conditions. Herein, we present BiMix-Bench, a curated database comprising ∼125,900 MOFs and five binary gas mixtures. Leveraging this dataset, LightGBM regressor (LGBMR) models are developed to achieve high predictive accuracy for gas uptakes ( R 2 = 0.93 and 0.92) and selectivity ( R 2 = 0.95) under strict robustness controls, including seed randomness and cross validation. Using CO 2 /H 2 as a case study, we evaluate both zero-shot and few-shot transfer performance. While zero-shot predictions provide limited out-of-distribution accuracy, the pretrained LGBMR models can be efficiently adapted with a small number of new simulations ( N = 204) through transfer learning. This data-efficient adaptation enables the rapid identification of top-performing MOFs, which are subsequently validated through grand canonical Monte Carlo simulations. This generalizable and interpretable framework enables scalable, data-driven discovery of advanced adsorbents for complex and evolving separation tasks.

Water reuse is rapidly becoming an integral feature of resilient water systems, where municipal wastewater undergoes advanced treatment, typically involving a sequence of ultrafiltration (UF), reverse osmosis (RO), and an advanced oxidation process (AOP). When RO is used, a concentrated waste stream is produced that is elevated in not only total dissolved solids but also metals, nutrients, and micropollutants that have passed through conventional wastewater treatment. Management of this RO concentrate─dubbed municipal wastewater reuse concentrate (MWRC)─will be critical to address, especially as water reuse practices become more widespread. Building on existing brine management practices, this review explores MWRC management options by identifying infrastructural needs and opportunities for multi-beneficial disposal. To safeguard environmental systems from the potential hazards of MWRC, disposal, monitoring, and regulatory techniques are discussed to promote the safety and affordability of implementing MWRC management. Furthermore, opportunities for resource recovery and valorization are differentiated, while economic techniques to revamp cost-benefit analysis for MWRC management are examined. The goal of this critical review is to create a common foundation for researchers, practitioners, and regulators by providing an interdisciplinary set of tools and frameworks to address the impending challenges and emerging opportunities of MWRC management.

Gas transport in nanoconfined media is fundamental to applications such as gas separation, catalysis, and shale gas extraction. While transport mechanisms in idealized rigid pores or simple fluids are well understood, classifying gas transport in complex soft matter and highly viscous liquids remains challenging. Here, we introduce a quantitative, physically grounded framework for classifying gas transport regimes based on the intrinsic dependence of gas diffusivity on molecular mass. Using molecular dynamics simulations, we systematically examine how gas diffusion coefficients scale with molecular mass across a broad range of nanoconfined media. We define a diffusivity-mass scaling exponent (α) that serves as a mechanistic fingerprint of the transport regime: α values near zero correspond to random diffusion, whereas values approaching -0.5 indicate transport regimes dominated by rigid pore confinement, such as Knudsen, surface, or hopping diffusion. This metric enables the quantitative identification of gas transport mechanisms and captures critical regime transitions of gas nanoflow that have previously been difficult to classify. Further analysis reveals that the molecular mass dependence arises from variations in characteristic step length, governed by molecular momentum and gas-medium interactions. The proposed mass-scaling framework provides a unified and objective criterion for identifying gas transport mechanisms in nanoconfined systems, laying the foundation for a general theory of nanoscale gas transport and enabling more reliable prediction and design of gas-transport materials.

Membrane processes offer a promising pathway for selectively separating organics and salts to enable water reuse and resource recovery. While polymeric membranes incorporating macrocyclic molecules that feature amphiphilic nature and tunable cavities are well suited for this purpose, traditional macrocycles with limited reactive sites and uncontrolled diffusion are challenging to be assembled into highly interconnected membranes. Here, we introduce tetra-aldehyde appended calixarene (TACA), a macrocyclic monomer featuring three-dimensional cavity and moderate reactivity, for creating loose-structured nanofilms via unidirectional diffusion assisted interfacial polymerization (UDIP). Precise positioning of the lipophilic TACA at the organic phase boundary allows it to polymerize with aqueous-phase diamines on the hydrogel surface, facilitating an undisturbed environment for controlled polymerization. The resultant thin macrocycle-assembled membranes featuring intrinsic water-facilitated through-cavity exhibited high water permeability of 63.8 L m-2 h-1 bar-1, and exceptional dye/salt selectivity and structural robustness, as evidenced by efficient diafiltration of binary dye/salt mixtures and superior operational stability. This work highlights the potential of macrocycle-assembled membranes for high-salinity organic wastewater treatment. Macrocyclic polymer membranes are well suited for wastewater treatment, though it is challenging to fabricate porous networks using macrocycles. Here the authors report a membrane containing tetraaldehyde appended calixarene using a unidirectional diffusion assisted interfacial polymerization method for a wastewater treatment membrane.

Conventional thin-film composite (TFC) reverse osmosis (RO) membranes experience irreversible performance loss at high temperatures, restricting their use in industries with high-temperature streams, including oil and gas, pharmaceuticals, electronics, power generation, food production, and hybrid desalination plants. However, the mechanisms driving the performance decline of TFC membranes at high temperatures remain poorly understood. Herein, we combine controlled experiments, molecular dynamics simulations, and micromechanical modeling to elucidate TFC failure mechanisms and to evaluate thermally resilient thin-film cross-linked (TFX) composite membrane. Upon exposure to elevated temperatures (>60°C), salt rejection of TFC dropped from ~99 to <90%, with irreversible structural damage in the polysulfone layer, confirmed by scanning electron microscopy. In contrast, the TFX membrane maintained ~99% salt rejection and showed no signs of physical degradation up to 80°C. Our combined analyses revealed that TFC membrane failure arises from irreversible pore expansion in the thermoplastic polysulfone support, leading to polyamide film rupture and delamination. TFX membranes resist thermal deformation, enabling ultrahigh-temperature RO desalination and water reuse.

Covalent organic framework (COF) membranes hold immense potential for aqueous separations, yet their inherently large pore apertures and insufficient film crystallinity often limit their performance, particularly in challenging applications like water desalination. Here, we address these limitations by introducing an acid-modulated interfacial synthesis (AMIS) strategy to precisely engineer an ultramicroporous, highly crystalline Turing COF membrane. A detailed mechanistic investigation reveals that acetic acid forms hydrogen-bonded adducts with the hydrophilic aliphatic linker, oxalyl dihydrazide (ODH), finely tuning both its reactivity and diffusivity during Schiff base condensation with the linker, 1,3,5-triformylphloroglucinol (Tp). This modulated reaction–diffusion behavior not only facilitates the formation of a unique stripe-patterned Turing architecture but also enables sufficient defect self-correction via reaction retardation, yielding a COF film with high crystallinity. The resultant aliphatic ODH–COF membranes exhibit a unique ABC stacking mode and a sub-6-Å pore aperture, validated by experimental data and simulations. These characteristics, working in concert, enable the ODH–COF membranes to achieve record-high NaCl rejection of 99.7% with a water permeance of 0.82 L m –2 h –1 bar –1, surpassing previously reported state-of-the-art COF membranes in pressure-driven separation processes. Coupled with robust fouling resistance and long-term stability, this work substantially advances COF membrane technology for sustainable and efficient water management.

Industry-leading polyamide membranes are thin-film composites produced via interfacial polymerization (IP) at an alkane-water interface. However, the current fabrication method results in suboptimal membrane microstructure and compromised performance due to insufficient control of mass and heat transfer within the interfacial reaction zone. Furthermore, the fabrication process utilizes volatile alkane solvents, contributing to a significant environmental burden. Here, we report an IP strategy at an ionic liquid/water interface to synchronously achieve kinetic and thermodynamic control of the interfacial reaction, thereby optimizing the microstructure of polyamide membranes. The high viscosity and low volatility of the ionic liquid facilitate the integration of the industrial doctor blading technique into the IP process, enabling rapid, eco-friendly, and scalable polyamide membrane production. The resulting membrane exhibits an unprecedented combination of high pure water permeance (25.8 LMH/bar) and excellent salt (sodium sulfate) rejection (96.54%), surpassing the performance of commercial benchmark polyamide membranes. This facile fabrication strategy paves the way for the design and production of next-generation, high-performance thin-film composite membranes. Polyamide membranes are often fabricated using interfacial polymerization methods, though these methods can compromise membrane structure and performance. Here the authors design a polymerization method using ionic liquid and a doctor blading method to optimize membrane fabrication.

Membrane brine concentration (MBC) offers the ability to concentrate brines at lower energy and cost compared to thermal brine concentration (TBC), thus expanding the global opportunities for minimum and zero liquid discharge (M/ZLD). Ultrahigh-pressure reverse osmosis (UHPRO) and osmosis-assisted RO (OARO) MBC processes operate between 80 and 120 bar, depending on the specific process configuration. Herein, we show that operating commercial RO membranes on top of commercial permeate carriers (PCs) at such high pressures leads to both RO membrane compaction and embossing, which initially cause reduced water permeability and elevated permeate backpressure, respectively. Eventually, the polyamide (and polysulfone) layer can rupture, resulting in the irreversible loss of salt rejection at applied pressures of 80 to 120 bar. Optical and SEM images confirmed through-thickness mechanical imprinting that aligned with the structural features of the PCs, establishing a clear causal relationship between embossing and performance decline. Finally, we designed an embossing-free permeate carrier (EFPC) composed of two soft tricot mesh layers sandwiching a rigid stainless-steel mesh. This EFPC eliminated embossing-enhanced permeate backpressure and membrane damage, enabling commercial RO membranes to sustain stable water flux and salt rejection up to 120 bar. Overall, this study demonstrates that advancements in UHPRO/OARO technology must incorporate both compaction-resistant membranes and EFPCs to fully realize the potential of MBC technology.

Fabricating covalent organic framework (COF) membranes with molecular size cutoffs matching theoretical pore sizes is essential for selective angstrom-scale aqueous separations. We report a dual-regulation interfacial polymerization strategy to fabricate COF membranes with pore sizes approaching theoretical values, using Brønsted acid and organobase in separate phases to synchronously control polymerization and self-healing, as supported by molecular simulations of monomer diffusion and liquid chromatography–mass spectrometry analysis for component tracing. The dual-regulation COF membranes achieve a selectivity of 267 in single-salt test and an actual selectivity of 234 for K + /Mg 2+ in binary systems, demonstrating a threefold increase in mono/divalent cation separation compared to single-phase–regulated membranes. Additionally, we elucidate the untrapped and trapped transport of hydrated monovalent and divalent cations within the confined cavities through molecular dynamics simulations. This work provides an alternative approach to COF membrane fabrication and advances their application in precise sieving for water purification and resource recovery.

In this study, we present a class of thin-film crosslinked (TFX) composite reverse osmosis (RO) membranes that resist physical compaction at ultrahigh pressures (up to 200 bar). Since RO membranes experience compaction at virtually all pressure ranges, the ability to resist compaction has widespread implications for RO membrane technology. The process described herein involves crosslinking a phase inverted porous polyimide (PI) support membrane followed by interfacial polymerization of a polyamide layer, thereby forming a fully thermoset composite membrane structure. We explore a range of phase inversion membrane formation parameters such as PI concentration, solvent-cosolvent ratios, coagulation bath composition, and crosslinking methods in addition to interfacial polymerization reaction chemistry and conditions. Overall, TFX membranes exhibit significantly less compaction compared to hand-cast and commercial high-pressure RO membranes, experiencing less than 10% decline in water permeance and maintaining salt rejection over 99% for NaCl solutions up to 180,000 mg/L with 200 bar applied pressure.

Water Scarcity convened to discuss advancements and challenges in water desalination and reuse in Washington, DC, for a two-day meeting on September 10-11, 2024.

The general mechanisms of silica scaling through the polymerization of silicic acid at supersaturation have been predominantly studied in solutions. However, the pathway of silica polymerization occurring directly on surfaces, leading to silica precipitation, remains largely unexplored despite its wide-ranging implications for biomineralization processes, green material synthesis, and scaling in various engineered systems. In this study, we analyze the kinetics of silica polymerization from oversaturated solutions onto surfaces functionalized with various types of self-assembled monolayers (SAMs) or reverse osmosis (RO) membranes using a quartz crystal microbalance with dissipation. Upon contact with oversaturated silicic acid, the rate of silica polymerization on amine-terminated surfaces is nearly 6 times higher than that on carboxyl-, hydroxyl-, or methyl-SAMs. Silica polymerization on the surface of RO membranes over extended periods spontaneously transitions from a moderate to an accelerated regime, which corresponds to a structural transformation in silica scaling from the isotropic growth of aggregated particles to a gel-like glassy layer. Additionally, the presence of calcium ions in solutions significantly promotes silica scaling on membrane surfaces along with an increase in the viscoelastic properties of the formed scale layer. Our findings provide mechanistic insights into the molecular interactions between oversaturated silicic acid and functionalized surfaces, highlighting the critical roles of surface functional groups and coexisting ions in silica polymerization for scale formation on engineered surfaces.

The goal of achieving energy-efficient, precise molecular separations has motivated interest in developing and employing porous crystalline frameworks as membrane materials. Covalent organic frameworks (COFs) are ordered crystalline matrices composed of covalently bonded organic monomers and are synthesized via reversible reticular chemistry. COFs possess high porosity, structural tunability, and chemical and thermal stability, making them ideally suited for emerging, high-value membrane separation processes, such as ion separations, organic solvent nanofiltration, and gas separations. Although a range of COF membranes have been fabricated and tested in the past decade, these membranes are primarily polycrystalline, weakly crystalline, and/or discontinuous, resulting in suboptimal performance. In this review, we identify the properties that make COFs well-suited as membrane materials, while critically outlining the shortcomings of existing disordered COF membranes. We then highlight the recent emergence of highly crystalline, continuous, oriented two-dimensional COF membranes as a promising path forward for highly selective molecular separations. These continuous, oriented COF membranes exhibit tunable one-dimensional nanochannels, allowing for ultrafast molecular transport and precise species selectivity, thereby expanding the set of separations that can be practically achieved with membrane systems. We discuss synthesis and modification techniques that result in continuous, oriented COF membranes and evaluate the performance of such membranes for a variety of molecular separations. We conclude by identifying ongoing challenges in the development of COF membranes and outlining the future of their applications in molecular separations, which will necessarily rely on advancements in the synthesis of continuous, oriented membranes.

Pressure-driven separations with nanoporous membranes, such as reverse osmosis and nanofiltration, play a vital role in addressing water scarcity and enabling resource recovery. Understanding water or solvent transport in membrane pores is essential for advancing membrane separation technologies. A key question in transport modeling is to establish a relationship between solvent permeability and membrane porous structure properties, such as porosity or pore size. The nano- and subnanometer pores in polymeric membranes such as reverse osmosis and nanofiltration membranes are highly tortuous and dynamically connected, which challenges the conventional methods of transport modeling. This study addresses this challenge by developing a theoretical framework to describe solvent transport through membranes with dynamic and disordered porous structure. Specifically, we propose a lattice model to describe the pore network, while preserving the viscous nature of solvent permeation. We further establish a relationship between solvent permeability and membrane porosity or pore size, which is validated by molecular dynamics simulations and experimental data. By integrating this relationship into the solution-friction model, we define pore connectivity and local friction coefficient to quantify the impact of pore structure on solvent permeability. Our analysis highlights the dominant influence of pore connectivity on the permeability of reverse osmosis and nanofiltration membranes, particularly when the pore size approaches the dimensions of solvent molecules. Overall, this study provides critical insights into water and solvent transport mechanisms in nanoporous membranes, opening the door for strategies to substantially enhance membrane performance.

Biological sodium channels efficiently discriminate between same–charge ions with similar hydration shells. However, achieving precise ion selectivity and high throughput in artificial ion channel fabrication remains challenging. Here, we investigate angstrom–scale channels in 15-crown-5 (15C5) functionalized COF membranes for fast, selective ion transport. Due to crown ether recognition of sodium ions, channels in DHTA-Hz-15C5 membranes selectively facilitate Na+ transport, further enhanced by the hydroxyl-enriched COF skeleton. A Na+/K+ selectivity of 58.31 is achieved with 9.33 mmol m−2 h–1 permeance, significantly exceeding current membranes and resembling biological channels. Theoretical simulations indicate one–dimensional COF channels facilitate transport, while crown ether recognition makes the Na+ energy barrier significantly lower than K⁺, enabling ultrahigh selectivity with high Na⁺ permeability. This promotes COFs for efficient single-ion transport and advances crown ether ion selectivity in nano-restricted environments. Discriminating between ions with the same charge and similar hydration shells with artificial ion channels is challenging. Here, the authors produced crown ether functionalized covalent organic framework membranes for fast and selective sodium transport.

Reverse osmosis (RO) membranes are essential for desalination and water reuse, yet their permeability declines in high-pressure applications due to membrane compaction. This study investigates the structural and functional responses of commercial brackish, seawater, and high-pressure RO membranes at applied pressures up to 120 bar using a multiscale, nondestructive in operando scanning electron microscopy ( i SEM) imaging platform. The i SEM technique reveals progressive densification across the composite membrane structure, which correlates with observed declines in water and solute permeance. To quantify these structural changes with greater fidelity, we combined X-ray computed tomography with AI-based segmentation enabling precise analysis of pore size distribution and thickness of the polysulfone support layer. Compared to traditional thresholding, AI segmentation accurately delineates material phases and void spaces, enhancing the reproducibility and resolution of morphological assessments. The results demonstrate that compaction-induced reductions in porosity and thickness strongly impact membrane transport properties. These findings provide mechanistic insights into the compaction behavior of RO membranes and underscore the potential for advanced imaging and AI-driven data analysis to guide the design of next-generation membranes with improved mechanical resilience and operational longevity.

Ion-exchange membranes (IEMs) are critical to electrochemical ion-separation technologies for environmental and energy applications. Although advancements in separation performance have been frequently reported, significant challenges in evaluating mixed-salt IEM performance under electro-driven conditions remain largely overlooked. Here, we present a new method for the rapid and standardized characterization of ion transport properties for IEMs in mixed-salt systems. Using a modified Goldman-Hodgkin-Katz equation, we demonstrate that individual ion permeability can be obtained from IEM conductance measurements with mixed-salt linear sweep voltammetry experiments. Under low-current conditions, the ion permeability ratio can be converted into different metrics for quantification of ion–ion selectivity including current-based and flux-based selectivity. Our findings also reveal substantial differences between ion transport properties of IEMs under single-salt and mixed-salt conditions, highlighting the critical role of competing ions in transport experiments. Overall, this new approach provides an efficient framework to evaluate IEM performance in environmentally relevant conditions, facilitating cross-membrane performance comparison and establishing the foundation to elucidate the structure-property-performance relationships for IEMs in mixed-salt systems.

Conventional electrochemical reactors for nitrate reduction typically suffer from limited reaction efficiency when applied for real-world water treatment due to poor utilization of electrocatalytic active sites. Here, we applied nanoporous electrofiltration to intensify atomic utilization by incorporating single-atom catalysts into an electrified membrane for reducing low-concentration nitrate to ammonia under realistic water conditions. We enhance the exposure of single atoms in nanopores by coating the catalysts on a carbon nanotube-interwoven membrane framework. Electrofiltration intensifies the transport and adsorption of nitrate in confined nanopores with highly exposed single-atom active sites to enhance reduction. The membrane enables a superior ammonia turnover frequency of 15.1 grams of nitrogen per gram of metal per hour, up to four orders of magnitude higher than that reported in the literature, under both high removal efficiency and Faradaic efficiency of over 86% when treating influents with a low nitrate concentration of 100 milligrams of nitrogen per liter in a residence time on the order of seconds.

Membrane compaction is inherent in pressure-driven membrane processes, resulting in a decrease in porosity and pore size of polymeric membranes as solvent flow compresses the porous structure of the polymer. The compaction of pores reduces solvent permeability and significantly impacts separation performance. Despite the importance of membrane compaction, its fundamental mechanisms have not been well studied. In this study, we combine well-controlled experiments and theory to analyze the relationship between pressure and porosity profiles within membranes. Specifically, we stack six porous films in a customized dead-end cell and measure the solvent content of each film immediately after pressure-driven solvent permeation tests. We show that, when a viscous solvent permeates through the stacked membranes, membrane porosity continuously decreases from the feed side to the permeate side. Compaction results in ∼25% reduction in solvent content (or porosity) at the membrane permeate side under 10 bar applied pressure and up to ∼50% reduction under 40 bar. We further analyze the stress-strain behavior of solvent-swollen films under mechanical compression and compare it to the compacted porosity in the solvent permeation tests. Our analysis reveals that the compression pressure in the permeation tests corresponds to the reduction in hydrostatic pressure within the membrane. In addition, we investigate the compaction behavior when membranes with varying pore sizes are stacked. Our results show that solvent permeating from tight-to-loose films induces significantly greater compaction than flow in the loose-to-tight direction. The compaction in tight-to-loose membrane stacking causes a 27% greater loss in membrane solvent content compared to the reverse arrangement. Notably, this study demonstrates that porosity gradients resulting from compaction cannot be interpreted as concentration gradients that drive diffusive solvent transport. Overall, our findings can inform the design of membranes with improved resistance to compaction by guiding material selection, processing techniques, and structural optimization.

Nanoplastics (NPs) are emerging aqueous pollutants, posing risks to drinking water safety and human health. However, conventional coagulants, widely employed in water treatment plants globally, are ineffective at removing NPs. Here, we present an in-situ Fe(III) method based on the simultaneous use of Fe(II) coagulant and an oxidant to enhance conventional coagulation by altering the nanostructure of Fe-based precipitates in flocs for efficient NP removal. Unlike the nanospheres formed by conventional Fe(III) coagulation, which are weakly attached to the NP surface, nanosheets formed by our approach can fully encapsulate NPs, achieving efficient nanonet trapping with a flexible mesh structure. In-situ formed nanosheets exhibit faster agglomeration, higher removal rate, and stronger anti-interference ability. The practical viability of our approach was proven in different natural water samples, where the inhibition for NP removal by various constituents of natural organic matter was effectively reduced. Theoretical calculations demonstrate that crystal structure differences between such nanosheets and nanospheres change short-range forces, thereby enhancing NP removal. Overall, this concept of modifying the nanoscale crystal structure of flocs offers valuable insights into enhanced coagulation processes, with broad applications in water treatment and environmental systems, and provides a promising solution to the critical challenge of NP removal.

Lithium-ion batteries are becoming more ubiquitous, increasing lithium demand. Lithium carbonate, the most common form of industrial lithium, is primarily produced from lithium-rich brines concentrated using solar evaporation ponds. This extraction method is environmentally damaging and slow, taking a year or more to sufficiently concentrate. To meet the increasing demand, new technologies are being developed which can facilitate lithium recovery and shorten production timelines. This work examines two promising separations technologies, ion exchange resins due to their low cost and intercalation electrodes due to their high selectivity, and applies them to two potential alternative lithium sources — seawater reverse osmosis concentrate, and oil and gas produced water — to evaluate their efficacy. We develop process trains for each technology and analyze them using technoeconomic analysis and lifecycle assessment to determine economic and environmental feasibility. Further, technical improvements and process train modifications are examined to determine the impact on lithium carbonate cost. Our results show that currently ion exchange resins are the cheaper technology for recovering lithium, and that oil and gas produced water can produce lithium carbonate for as little as $14.96 kgLi 2 CO 3 −1 while recovering 34.3 % of lithium in solution, making it economically viable as a recovery option. Given sufficient technical improvements, intercalation electrode production costs for lithium carbonate may also be competitive. The preferred technology for minimizing the environmental impact is dependent on water source, but the ion exchange resin paired with oil and gas produced water has the lowest environmental impact overall, producing only 17.7 kgCO 2 -eq kgLi 2 CO 3 −1 .

Membrane-based processes, such as reverse osmosis (RO) and nanofiltration (NF), are widely used for water purification and desalination due to their high energy efficiency and exceptional solute-water selectivity. Nevertheless, the fundamental, molecular-level mechanisms governing ion selectivity are still not fully understood. This study explores ion selectivity in polyamide desalination membranes, focusing on the partitioning and diffusion mechanisms of co-ions and counterions. Our experimental and molecular simulation results reveal that electrostatic interactions play a key role in impeding co-ion partitioning while enhancing their diffusion. The results further suggest that ion selectivity is predominantly controlled by the partitioning step, particularly the selective partitioning of co-ions. This finding highlights the importance of focusing on ion partitioning at the water-membrane interface to improve membrane ion-ion selectivity. In addition, our results point out to a trade-off between partitioning and diffusion, requiring careful tuning of these processes. Overall, this study provides the scientific foundation for molecular design of membranes with high ion-ion selectivity.

Single-ion-selective membranes are indispensable for efficient ion separations in environmental, energy, and biomedical technologies. Inspired by biological ion channels, this work harnessed the selective and reversible ion binding features of ionophores to fabricate an ultrathin, ionophore-based K + -selective polyamide membrane through molecular layer-by-layer (m-LbL) polymerization with 18-crown-6-functionalized monomers. Compared with Cs +, Li +, and Mg 2+, K + exhibited the highest binding energy to 18-crown-6, facilitating its transport over the competing cations across the sub-10 nm polyamide film in a binary salt mixture. The need for competitive binding for selective K + transport was further demonstrated through investigations of ion selectivity at varying concentration ratios between K + and competing cations. Additionally, we extended the Nernst–Planck equation to describe individual ion flux in a binary system, identifying factors that govern ion transport. Our findings demonstrate the potential of selective single-ion transport enabled by preferential ion binding, showing promise for the development of biomimetic ion-selective polymeric membranes.

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.

This study explores the compaction behavior of thin-film composite reverse osmosis (TFC RO) membranes for different combinations of transmembrane pressure (TMP) and transmembrane water flux. Operating a crossflow system at constant feed pressure (60 bar) but different feed solution osmotic pressures enabled adjusting the TMP─the difference between hydraulic and osmotic pressure─and water flux. The extent of membrane compaction increases as TMP (and flux) increases. Both commercial and hand-cast TFC RO membranes showed substantial compaction at high TMP (up to 30% compaction at 50 bar TMP) compared to less than 10% at 10 bar TMP. Scanning electron microscope (SEM) images reveal a direct relationship between TMP and polysulfone (PSU) support layer compaction, while molecular dynamics (MD) simulations confirmed decreased porosity and reduced thickness in the polyamide (PA) active layer as TMP increases. Combined findings from wet-testing and MD simulations confirm a hydraulic pressure drop occurs across both the PA active layer and the meso-to-macro-porous support layer; higher TMP exacerbates compaction in both layers resulting in lower water permeability but higher water flux, observed salt rejection, and salt permeability. Transitioning from high TMP to low TMP or vice versa did not notably alter the extent of membrane compaction. This observation is attributed to the highly cross-linked PA active layer's ability to recover after pressure is released, whereas the compaction in the PSU support layer is largely irreversible. While TMP dictates the overall pressure gradient, our findings suggest that flux-induced frictional forces play a crucial role in compaction dynamics. Specifically, higher flux generates additional drag forces on the polymer matrix of both the PSU support layer and the PA selective layer, intensifying structural deformation. Overall, our findings offer critical insights into the mechanisms of membrane compaction, providing a foundation for optimizing RO membrane performance and advancing next-generation membrane technologies.