Intrapore energy barriers govern ion transport and selectivity of desalination membranes

ERD15 promotes peach and tomato ripening by activating polyamine catabolism

Polyamine oxidase (PAO) is a key enzyme in polyamine (PA) catabolism and plays a vital role during fruit ripening. However, regulatory mechanisms that control PAO expression during maturation remain unclear. This study identifies the transcription factor PpeERD15 through yeast one-hybrid (Y1H) screening with the PpePAO1 promoter. ERD15 (early response to dehydration 15), a member of the early response to dehydration protein family, is known for its role in abiotic stress responses, but its function in fruit ripening remains largely unexplored. Subcellular localization analysis demonstrated that PpeERD15 was localized in both the nucleus and cytoplasm. Y1H and LUC assays confirmed that PpeERD15 directly binds the PpePAO1 promoter. Transient silencing of PpEDR15 in peach fruit downregulated PpePAO1 expression, promoted PA accumulation, inhibited ethylene production, increased fruit firmness, and delayed fruit ripening. Conversely, overexpression of PpeEDR15 upregulated PpePAO1, decreased PA content, promoted ethylene production, reduced fruit firmness, and accelerated fruit ripening. The role of homologous gene of ERD15 was also validated in tomato. This study discovered that PpeEDR15 regulates fruit ripening by promoting PA catabolism via PpePAO1 expression.

Combined Effect of Electrostatic Interaction and Steric Sieving during Ion Transport in Nanochannels

The potential distribution at the membrane-solution interface (Donnan potential) is critical for ion transport in the nanochannel. Herein, a charge-enhanced electrically responsive membrane with a tunable pore size is prepared to investigate the impact of the Donnan effect on ion transport in negative nanochannels of varying sizes. Both the Donnan effect and pore size changes influence ion transport within nanochannels, with an enhanced Donnan effect promoting cation transport while slowing anion permeation. The combined effect of the electrostatic interaction and steric sieving expands the membrane rejection adjustment range from 9.6% to 27.6%, as the applied voltage simultaneously decreases pore size and increases Donnan exclusion. The reduction in pore size enhances the ability of the per unit of Donnan potential to regulate ion transport, and electrostatic exclusion is more pronounced in smaller nanochannels. The same conclusion is confirmed in commercial nanofiltration membranes of varying sizes (NF270 and NF800), but as the nanochannel size decreases further (NF90), the effect of electrostatic interaction on ion transport becomes minimal. Since the nanochannels are too narrow to permit ion transport, size sieving becomes the dominant mechanism for selective separation. This work enhances our understanding of ion transport in nanochannels and supports the development of high-performance nanofiltration membranes.

Direct Quantification of Ion Partitioning and Diffusion Resistances in Reverse Osmosis Membranes via Electrochemical Impedance Spectroscopy

Polyamide (PA) reverse osmosis (RO) membranes are crucial for water desalination and purification, where salt ion transport is governed by partitioning and diffusion through the PA film. Despite extensive research, decoupling these two steps and quantifying their relative contributions remain challenging due to the lack of reliable characterization methods. Here, we develop a rapid, reproducible electrochemical impedance spectroscopy (EIS) protocol incorporating advanced electrical equivalent circuits to directly quantify partitioning and diffusion resistance. Its validity is verified through membrane filtration experiments and activation energy analysis. Our findings reveal that diffusion dominates ion transport resistance, with values 4.5 to 6.0 times higher than partitioning resistance across diverse monovalent cations. However, we discovered a critical concentration-dependent behavior where partitioning resistance becomes increasingly significant at lower electrolyte concentrations, eventually equaling diffusion resistance near 0.1 mM. We also uncovered that the anomalously low rejection of NH4+ of RO membranes stemmed from significantly reduced diffusion resistance, likely due to moderate hydrogen-bonding interactions with membrane pores or its tetrahedral geometry. This quantitative insight into transport resistance mechanisms establishes new design principles for next-generation RO membranes, enabling tailored strategies for applications ranging from high-salinity desalination to the removal of low-concentration micropollutants.

Intrinsic roles of nanosheet characteristics in two-dimensional montmorillonite membranes for efficient Li+/Mg2+ separation

Stacking two-dimensional (2D) nanosheets into lamellar membranes holds great promise in the selective separation of Li+ and Mg2+ from salt-lake brines, but revealing the intrinsic effect of nanosheet properties on the ion transport remains a great challenge. The primary reasons are inevitable emerging defects and changes in surface functional groups during nanosheet preparation. Here, we successfully demonstrated the intrinsic dependence of ion separation on the size and layer charge density of 2D building blocks using defect-free and inherently permanent charged clay nanosheets. The smaller-sized nanosheets readily assembled into lamellar membranes with narrower nanochannel dimension, which facilitated the steric hindrance effect to improve the Li+/Mg2+ selectivity. Experiments and calculations demonstrated the layer charge density-dependent ion separation as well, for which a novel mechanism of intrinsic selective separation driven from the energy barrier difference of ions transport was proposed. Based on the "internal" regulation of the intrinsic nanosheet properties, MMT membranes realized stable and efficient Li+/Mg2+ separation under extreme conditions, multi-cycle and long-term experiments, with an optimal SLi/Mg of 38.9, superior to most of the reported state-of-the-art membranes. This work reveals the intrinsic interplay of nanosheet properties tuning the ion transport and separation, which will inspire the design and development of advanced 2D lamellar membranes, particularly for sustainable and environmental energy exploitation.

Covalent organic framework membranes for lithium extraction: facilitated ion transport strategies to enhance selectivity

The surging global demand for lithium, driven by the proliferation of electric vehicles and energy storage technologies, has exposed significant limitations in conventional lithium extraction methods, including inefficiency and environmental harm. Covalent organic frameworks (COFs) have emerged as a promising platform to address this challenge and enable more sustainable lithium extraction, owing to their unique advantages such as precisely tunable pore sizes, robust stability, and the ability to incorporate functional binding sites for selective ion transport. This review focuses on structural design and functionalization strategies in COFs to optimize lithium-ion separation, highlighting how pore confinement effects, tailored interlayer stacking arrangements, and strategic functional group modifications can dramatically enhance Li+ selectivity over competing ions present in brine solutions. A particular emphasis is placed on the fundamental energy barriers associated with lithium-ion transport. In particular, we discuss how appropriately designed pore environments and lithium-binding functional groups reduce the dehydration energy required for Li+ to enter and traverse COF nanochannels, thereby facilitating faster and more selective Li+ conduction. We also survey recent advancements in COF-based lithium separation technologies, such as high-performance COF membranes and sorbents for extracting lithium from brines and seawater, evaluating their potential, as well as remaining challenges, for sustainable industrial implementation. This review provides a comprehensive understanding of how advanced COF engineering can enable efficient and selective lithium-ion transport, offering valuable insights for the development of next-generation lithium extraction materials and technologies.

Manipulating Intrapore Energy Barriers in Graphene Oxide Nanochannels for Targeted Removal of Short-Chain PFAS

Removal of per- and polyfluoroalkyl substances (PFAS) from water has become a research topic of interest in recent times. However, it is very challenging to remove short-chain ( Collapse

Key Words

• asymmetrical nanochannels
• energy barriers
• graphene oxide membrane
• host−guest complexation
• nanofiltration
• short-chain PFAS
• β-cyclodextrin

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Affiliation(s)

Eubert Mahofa Nanoscale Science and Engineering Laboratory (NSEL), Department of Mechanical and Aerospace Engineering, Monash University, 20 Research Way, Clayton, VIC 3800, Australia ARC Research Hub for Advanced Manufacturing of 2D Materials, Monash University, 20 Research Way, Clayton, VIC 3800, Australia
Sally El Meragawi Nanoscale Science and Engineering Laboratory (NSEL), Department of Mechanical and Aerospace Engineering, Monash University, 20 Research Way, Clayton, VIC 3800, Australia ARC Research Hub for Advanced Manufacturing of 2D Materials, Monash University, 20 Research Way, Clayton, VIC 3800, Australia
Muhammed A Vilayatteri Nanoscale Science and Engineering Laboratory (NSEL), Department of Mechanical and Aerospace Engineering, Monash University, 20 Research Way, Clayton, VIC 3800, Australia ARC Research Hub for Advanced Manufacturing of 2D Materials, Monash University, 20 Research Way, Clayton, VIC 3800, Australia
Swarit Dwivedi Department of Chemical and Biological Engineering, Monash University, Clayton, VIC 3800, Australia
Manas Ranjan Panda Nanoscale Science and Engineering Laboratory (NSEL), Department of Mechanical and Aerospace Engineering, Monash University, 20 Research Way, Clayton, VIC 3800, Australia ARC Research Hub for Advanced Manufacturing of 2D Materials, Monash University, 20 Research Way, Clayton, VIC 3800, Australia
Petar Jovanović Nanoscale Science and Engineering Laboratory (NSEL), Department of Mechanical and Aerospace Engineering, Monash University, 20 Research Way, Clayton, VIC 3800, Australia ARC Research Hub for Advanced Manufacturing of 2D Materials, Monash University, 20 Research Way, Clayton, VIC 3800, Australia
Adri C T van Duin Department of Mechanical Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States
Benny Freeman Department of Chemical and Biological Engineering, Monash University, Clayton, VIC 3800, Australia
Akshat Tanksale Department of Chemical and Biological Engineering, Monash University, Clayton, VIC 3800, Australia
Mainak Majumder Nanoscale Science and Engineering Laboratory (NSEL), Department of Mechanical and Aerospace Engineering, Monash University, 20 Research Way, Clayton, VIC 3800, Australia ARC Research Hub for Advanced Manufacturing of 2D Materials, Monash University, 20 Research Way, Clayton, VIC 3800, Australia

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Adsorption of natural organic matter and divalent cations onto / inside loose nanofiltration membranes: Implications for drinking water treatment from rejection selectivity perspective

Loose nanofiltration (LNF) membranes hold great promise for the selective rejection of natural organic matter (NOM) while maintaining mineral salts to produce high-quality drinking water. Nevertheless, the rejection selectivity performance is not only determined by the inherent properties of membranes but also influenced by the feed water compositions. This study explored the inevitable adsorption of NOM and inorganic ions onto and inside membrane materials, which in turn altered the charge properties of LNF membranes, thereby affecting the rejection selectivity. Zeta potential measurements, X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry technique were employed to characterize solute adsorption on the membrane surface and within the membrane pores. Filtration experiments using synthetic and natural waters were conducted to assess the contribution of electrostatic effects and evaluate the membrane rejection performance. Results revealed that LNF membrane surfaces during filtration were readily coated by NOM molecules, probably via hydrophobic interactions, which in turn adsorbed divalent cations that actually determined the net charge density on the membrane surface. Additionally, NOM adsorption within the membrane pores largely altered pore charge properties, particularly of the sulfonated polyethersulfone membranes (e.g. NTR7450), where deprotonated sulfonic groups otherwise contributed to a high charge density. These interactions among NOM, divalent cations and membrane materials greatly reduced charge density on the membrane surface and largely diminished charges in pores, leading to decreased rejection of both NOM and mineral salts, as well as the mitigation of co-ion competition effects. Nevertheless, the UA60 membrane, having a molecular weight cut-off of ∼1000 Da, rejected NOM by ∼70 % while maintaining ∼95 % bicarbonate and ∼65 % hardness ions in the treated water, demonstrating fairly good selectivity. These findings offer valuable insights for optimizing LNF membranes to improve the safety, chemical stability and palatability of treated drinking water.

Selective Electrochemical Capture of Monovalent Cations Using Crown Ether-Functionalized COFs

Electrochemical adsorption offers a promising approach for the separation of monovalent cations, which is an important but challenging subject in separation science. However, progress in this area has been hampered by the lack of suitable materials with effective ion selectivity. In this work, we present the synthesis of covalent organic frameworks (COFs) functionalized with a series of crown ethers (NCx-TAB-COFs, x donate 12, 15, 18, indicating the size of crown ether) for the efficient and highly selective electrochemical capture of monovalent cations. In our design, crown ether moieties act as confinement sites, imparting high selectivity for different monovalent cations depending on the cavity dimensions of the crown ether present. COFs electrodes prepared using the novel crown-COFs exhibit superior performance for the selective sequestration of monovalent (alkali metal) cations. Notably, 18-crown-6 ether-substituted COF (NC18-TAB-COF) shows a remarkable selectivity (14.26) for K+ over Na+ and a substantial Rb+/Na+ selectivity of 22.4. Furthermore, NCx-TAB-COFs maintain their remarkable selectivity and capacity under mixed-cation conditions. Density functional theory calculations and molecular dynamics simulations suggest that the unexpectedly high selectivity for larger cations is likely due to diverse binding modes in conjunction with the porous structure of the COFs. Given their lower dehydration-free energies and smaller hydrodynamic radii, K+, Rb+, and Cs+ more readily permeate the confined channels of COFs. In contrast, Na+ and Li+, with higher dehydration-free energies and hydrodynamic radii, diffuse into the NCx-TAB-COFs structure at a much slower rate and are bound predominantly to the surfaces of the COFs.

Dominant Mechanism of Nanofiltration for Chloride/Sulfate Ion Separation in High Salinity Solutions: The Quantification of Pore Size-Influenced Dielectric Exclusion

Nanofiltration membranes attract extensive attention in solute selective separation, especially in resource extraction and recovery. A prevalent strategy to enhance the monovalent and multivalent ion selective separation performance involves modifying the membrane surface charge properties to influence the Donnan exclusion. However, the counterion adsorption and shielding effects are aggravated with increasing ionic strength, which severely weaken the Donnan exclusion. This study revealed that the contribution of Donnan exclusion was fairly moderate to SO42- rejection in high salinity solutions, while it was dielectric exclusion that exerted the most important influence on Cl-/SO42- selective separation with a pore radius at 0.35-0.44 nm (molecular weight cutoff at 180-300 Da). Consequently, we proposed that tailored design of nanofiltration membranes with a precise pore radius to fully utilize the steric and dielectric exclusion instead of increasing membrane charge density is more crucial for monovalent/multivalent ion selective separation in high salinity solutions. Overall, our study reveals the importance of dielectric exclusion and provides new insights into nanofiltration membrane customization and application for ion selective separation in high salinity solutions.

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.

Ion Sieving in Two-Dimensional Membranes from First Principles

A first-principles approach for calculating ion separation in solution through two-dimensional (2D) membranes is proposed and applied. Ionic energy profiles across the membrane are obtained first, where solvation effects are simulated explicitly with machine-learning molecular dynamics, electrostatic corrections are applied to remove finite-size capacitive effects, and a mean-field treatment of the charging of the electrochemical double layer is used. Entropic contributions are assessed analytically and validated against thermodynamic integration. Ionic separations are then inferred through a microkinetic model of the filtration process, accounting for steady-state charge separation effects across the membrane. The approach is applied to Li+, Na+, K+ sieving through a crown-ether functionalized graphene membrane, with a case study of the mechanisms for a highly selective and efficient extraction of lithium from aqueous solutions.

Amine-Modified ZIF Composite Membranes: Regulated Nanochannel Interactions for Enhanced Cation Transport and Precise Separation

Electromembrane water treatment technologies are attracting attention for their energy efficiency and precise separation of counterions. However, ion-exchange membranes exhibit low ionic conductance and selectivity for ions with similar charges. In this study, we developed a novel ZIF-8 composite membrane with amine-modified nanochannels through an in situ PEI-assisted seeding and secondary growth method. An integral and uniform selective layer was formed, and the amine-modified nanochannels induced differential transport of Li+, Na+, K+, and Mg2+ via the dehydration-hydration process. The composite membrane possessed a lower energy barrier for Na+ transport (Ea = 13 kJ mol-1) compared to Mg2+ (Ea = 17 kJ mol-1), showing a Na+ flux of 3.7 × 10-8 mol·cm-2·s-1 and a Na+/Mg2+ permselectivity of 52 (∼60 times higher than the commercial membrane). The physicochemical and electrochemical properties of the composite membranes were systematically characterized, revealing the significant role of the Mg2+ layer in increasing Mg2+ repulsion and facilitating Na+ diffusion. Besides, DFT simulation and interaction energy calculation elucidated that a moderate binding energy and compensation effect between ions and nanochannels, which can be precisely regulated by PEI incorporation, are crucial for the favorable passage of Na+ while maintaining high Mg2+ rejection. The membrane also demonstrated performance stability during a 5-day test and maintained high selectivity across varying salinity and pH conditions. This work advances the development of efficient cation separation membranes for sustainable desalination and resource recovery.

Engineering of Covalent Organic Framework Nanosheet Membranes for Fast and Efficient Ion Sieving: Charge-Induced Cation Confined Transport

Artificial membranes with ion-selective nanochannels for high-efficiency mono/divalent ion separation are of great significance in water desalination and lithium-ion extraction, but they remain a great challenge due to the slight physicochemical property differences of various ions. Here, the successful synthesis of two-dimensional TpEBr-based covalent organic framework (COF) nanosheets, and the stacking of them as consecutive membranes for efficient mono/divalent ion separation is reported. The obtained COF nanosheet membranes with intrinsic one-dimensional pores and abundant cationic sites display high permeation rates for monovalent cations (K+, Na+, Li+) of ≈0.1-0.3 mol m-2 h-1, while the value of divalent cations (Ca2+, Mg2+) is two orders of magnitude lower, resulting in an ultrahigh mono/divalent cation separation selectivity up to 130.4, superior to the state-of-the-art ion sieving membranes. Molecular dynamics simulations further confirm that electrostatic interaction controls the confined transport of cations through the cationic COF nanopores, where multivalent cations face i) strong electrostatic repulsion and ii) steric transport hindrance since the large hydrated divalent cations are retarded due to a layer of strongly adsorbed chloride ions at the pore wall, while smaller monovalent cations can swiftly permeate through the nanopores.

Approaching infinite selectivity in membrane-based aqueous lithium extraction via solid-state ion transport

As the gap between lithium supply and demand continues to widen, the need to develop ion-selective technologies, which can efficiently extract lithium from unconventional water sources, grows increasingly crucial. In this study, we investigated the fundamentals of applying a solid-state electrolyte (SSE), typically used in battery technologies, as a membrane material for aqueous lithium extraction. We find that the anhydrous hopping of lithium ions through the ordered and confined SSE lattice is highly distinct from ion migration through the hydrated free volumes of conventional nanoporous membranes, thus culminating in unique membrane transport properties. Notably, we reveal that the SSE provides unparalleled performance with respect to ion-ion selectivity, consistently demonstrating lithium ion selectivity values that are immeasurable by even the part-per-billion detection limit of mass spectrometry. Such exceptional selectivity is shown to be the result of the characteristic size and charge exclusion mechanisms of solid-state ion transport, which may be leveraged in the design of next-generation membranes for resource recovery.

MXene Membranes Inserted with Tannic Acid Etched MOF Nanocrystals for Ultrafast Water Permeation: Elucidating the Water Transport Mechanism in Nanoconfined Interlaminar Channels

Utilizing pore and interlayer engineering within nanoconfined interlaminar channels represents an ingenious approach to design highly permselective MXene (Ti3C2TX) membranes. Herein, the tannic acid (TA) etched ZIF-8 (TZIF-8) nanocrystals with hollow structures were effectually inserted into the interlayer spacing of MXene membranes. First, the density functional theory (DFT) results demonstrated the reaction mechanism between TA and ZIF-8. Then, the underlying mechanism of enhanced water-adsorptive properties for MXene/TZIF-8 membrane was due to the higher binding energy of water/TZIF-8 system than that of water/ZIF-8 system, elucidated by molecular dynamic simulation. Furthermore, the low mass transfer resistance and abundant mass transfer pathways of the MXene/TZIF-8 membrane were comprehensively proved by various experimental conclusions, characterizations and simulation calculations. As a result, the optimal MXene/TZIF-8 membrane exhibited high water permeance and concurrently satisfactory separation efficacy toward various oil/water emulsions. This work is anticipated to deepen the comprehension of high-efficiency water transport along interbedded nanochannels in MXene membranes.

Pulsatile Ion Transport in Nanofiltration Membranes Coupled with Electrically Tunable Pore and Hydroxyl Electrostatic Interactions

Pulsatile ion transport facilitates the adjusted transfer of substances, meeting the requirements for the gradient and timed separation of multiple components in membrane processes. Responsive nanofiltration membranes are thus currently receiving widespread attention but face limitations due to their narrow performance adjustment range. Herein, hydroxyl functional groups were introduced into electrically responsive nanofiltration membranes to broaden the adjustment range of separation performance through a combination of pore size sieving and functional group interactions, resulting in a greater change in rejection and flux compared to the original membrane. Membrane pore size is regulated by polypyrrole volume changes and becomes more variable when the cation's hydration radius is smaller. Although the hydroxyl group did not affect the charge transfer or volume change capacity of polypyrrole, it enhanced ion-pore interactions during ion transport, which was particularly pronounced in smaller nanochannels. The size effect of functional group interactions more strongly enhances the transmembrane energy barrier in the reduced state compared with the oxidized state, ultimately resulting in greater modulation of performance. This coupling strategy provides insights into the design of responsive membranes, offering the potential to achieve gradient separation of various solutes.

Machine Learning-Aided Inverse Design and Discovery of Novel Polymeric Materials for Membrane Separation

Polymeric membranes have been widely used for liquid and gas separation in various industrial applications over the past few decades because of their exceptional versatility and high tunability. Traditional trial-and-error methods for material synthesis are inadequate to meet the growing demands for high-performance membranes. Machine learning (ML) has demonstrated huge potential to accelerate design and discovery of membrane materials. In this review, we cover strengths and weaknesses of the traditional methods, followed by a discussion on the emergence of ML for developing advanced polymeric membranes. We describe methodologies for data collection, data preparation, the commonly used ML models, and the explainable artificial intelligence (XAI) tools implemented in membrane research. Furthermore, we explain the experimental and computational validation steps to verify the results provided by these ML models. Subsequently, we showcase successful case studies of polymeric membranes and emphasize inverse design methodology within a ML-driven structured framework. Finally, we conclude by highlighting the recent progress, challenges, and future research directions to advance ML research for next generation polymeric membranes. With this review, we aim to provide a comprehensive guideline to researchers, scientists, and engineers assisting in the implementation of ML to membrane research and to accelerate the membrane design and material discovery process.

Anisotropic Nanofluidic Ionic Skin for Pressure-Independent Thermosensing

Ionic skin can mimic human skin to sense both temperature and pressure simultaneously. However, a significant challenge remains in creating precise ionic skins resistant to external stimuli interference when subjected to pressure. In this study, we present an innovative approach to address this challenge by introducing a highly anisotropic nanofluidic ionic skin (ANIS) composed of carboxylated cellulose nanofibril (CNF)-reinforced poly(vinyl alcohol) (PVA) nanofibrillar network achieved through a straightforward one-step hot drawing method. The inherent anisotropic nanostructures endowed the ANIS with a modulus (20.9 ± 4.9 MPa) comparable to that of human cartilage and skin, alongside higher fracture energy (41.4 ± 0.3 kJ/m2) and fatigue threshold (1360 J/m2). Incorporating carboxylated CNF not only improves the negative potential but also increases the ionic conductivity of ANIS up to 0.001 S/cm, even at very low ionic concentration (1.0 × 10-6 M). Furthermore, the ANIS exhibits pressure-independent temperature sensitivity due to its high deformation-resistant performance. Thus, this work introduces a facile strategy for fabricating ANIS with pressure-independent thermosensing properties, promising prospects for practical healthcare applications.

Modulation of Ion Transport in Nanopores Using Polyethylene Glycol

Ion transport in nanopores is crucial for various biological and technological processes, exhibiting unique behaviors compared to bulk solutions. In this study, we systematically explore how polyethylene glycol (PEG) modulates ion transport within a conical nanopore. Our experiments reveal that introducing PEG into the ionic solution induces a reversal in ion current rectification (ICR). We further investigate the impact of PEG concentration, molecular weight, nanopore size, and cation type on ion transport. Additionally, we assess three different configurations of PEG introduction, identifying diffusive flow driven by an asymmetric cation distribution within the nanopore as a dominant transport mechanism. Our results confirm that the interactions between PEG and cations significantly affect ion transport properties. These findings advance our understanding of macromolecular crowding effects on ion transport and suggest potential applications in iontronic devices and biomolecule sensing.

Heterogeneous Covalent Organic Framework Membranes Mediated by Polycations for Efficient Ions Separation

Precise ions sieving at angstrom-scale is gaining tremendous attention thanks to its significant impact at the water-energy nexus. Herein, a novel polycation-modulated interfacial polymerization (IP) strategy is developed to prepare a heterogeneously charged covalent organic frameworks (COFs) membrane. Cationic poly(diallyldimethylammonium chloride) (PDDA) regulates the growth and assembly of anionic COFs nanosheets, which thus provides a negative, smooth top surface and positive, rough bottom surface, indicating the presence of heterogeneously charged angstrom-scale channels through the membrane. Experiments and simulations are conducted to understand the facilitated ions transport behavior relative to specific interactions raised by heterogeneously charged channels and angstrom-scale steric hinderance as well, rendering the membrane with robust mono-/divalent cations sieving capabilities. The selectivity (61.6) of Li+ to Mg2+ in mixed saline under the continuous cross-flow filtration mode is superior to most of the reported nanofiltration membranes. This polycation-mediated interfacial polymerization strategy offers a compelling opportunity to develop versatile heterogeneously charged COF membranes for exquisite ion sieving.

Engineering Ion Affinity of Zr-MOF Hybrid PDMS Membranes for the Selective Separation of Na+/Ca2

Ion-selective separation, especially Na+/Ca2+ separation, is of significant importance in the realms of biomimetic research and the fabrication of biomimetic devices, underscoring the pivotal role that sodium and calcium ions play in cellular metabolism. However, the analogous ionic radii and charge densities shared by sodium and calcium ions significantly impede their effective discrimination, presenting formidable challenges for the precise engineering of ion separation materials, such as separation membranes. In this study, a polydimethylsiloxane (PDMS) separation membrane hybridized with zirconium-based metal-organic frameworks (UiO-66, UiO-66-NO2 and UiO-66-NH2) was constructed. Through the meticulous design of the MOF functional groups, the material's affinity for specific ions was modulated, thereby achieving efficient Na+/Ca2+ separation. Notably, the PDMS integrated with amino-modified Zr-MOF exhibited an efficacious selective separation of Na+ and Ca2+ ions. The interaction between the amino group of UiO-66-NH2 and Ca2+ gave rise to the observed superior selectivity toward Ca2+ cations and enhanced separation efficiencies of up to 64% compared to pristine PDMS for UiO-66-NH2-embedded membranes.

Sodium-Ion Traps in a Two-Dimensional Confined Channel for Effective Water and Ion Selective Transport

Nanoconfined interlayer channels in two-dimensional (2D) laminates are promising for the construction of novel permselective membranes. Controlling the interlayer spacing and modifying the interlayer chemical microenvironment are effective for high-efficiency separation. However, manipulating ion-diffusion energy barriers in confined channels is challenging, and their role in selective water-ion transport is unclear. This study engineered sodium-ion traps (SITs) in confined graphene oxide (GO) interlayer channels by incorporating N-phenylaza-15-crown-5 (NPCE) to regulate the ion-diffusion energy barriers. Partial reduction of GO enhanced membrane stability and enabled precise adjustment of interlayer spacing, while the addition of NPCE further constricted the free space and entrapped sodium ions by increasing their transport energy barriers. The optimized membrane with NPCE-dominated SITs achieved mono/monovalent-ion efficient sieving (K+/Na+ ≈ 10.2, and Li+/Na+ ≈ 6.7) and rejection of Na2SO4 (∼99.0%) and NaCl (∼90.0%), while maintaining stable performance for >1500 h. The findings provide new insights into manipulating transport energy barriers and modifying other 2D material laminates.

Supramolecular-coordinated nanofiltration membranes with quaternary-ammonium Cyclen for efficient lithium extraction from high magnesium/lithium ratio brine

Ion-selective membranes (ISM) with sub-nanosized pore channels hold significant potential for applications in saline wastewater treatment and resource recovery. Herein, novel synergistic ion channels featuring bi-periodic structures were constructed through the coordination of functional Cyclen (quaternary_1,4,7,10-tetraazacyclododecane, Q_Cyclen) and Cu2+-m-Phenylenediamine (Cu2+-MPD) to develop supramolecular membranes for lithium extraction. The exterior quaternary ammonium-rich sites exhibit a significant Donnan exclusion effect, resulting in tremendous mono/divalent (Li+/Mg2+) ion selectivity; while the interior regular-confined channels of Cyclen yield a fast vehicular pathway, facilitating water molecules and Li+ ion-selective transport. The optimized membrane exhibited an increased water permeance of 19.2 L·m-2·h-1·bar-1 and simultaneously promoted Li+/Mg2+ selectivity (achieving a selectivity of 18.5 under a Mg2+/Li+ mass ratio of 30), surpassing the trade-off limit of conventional nanofiltration membranes. Due to the acquired excellent Li+/Mg2+ selectivity, lithium extraction from simulated salt-lake brines was successfully achieved through a two-stage nanofiltration process, reducing the Mg2+/Li+ mass ratio from 40 to 1.1. This work validates the applicability of macrocyclic with intrinsic sub-nanosized channels and desired multifunctionality for developing high-performance ISM for efficient lithium separation and beyond.

Covalent organic framework membranes with vertically aligned nanorods for efficient separation of rare metal ions

Covalent organic frameworks (COFs) have emerged as promising platforms for membrane separations, while remaining challenging for separating ions in a fast and selective way. Here, we propose a concept of COF membranes with vertically aligned nanorods for efficient separation of rare metal ions. A quaternary ammonium-functionalized monomer is rationally designed to synthesize COF layers on porous substrates via interfacial synthesis. The COF layers possess an asymmetric structure, in which the upper part displays vertically aligned nanorods, while the lower part exhibits an ultrathin dense layer. The vertically aligned nanorods enlarge contact areas to harvest water and monovalent ions, and the ultrathin dense layer enables both high permeability and selectivity. The resulting membranes exhibit exceptional separation performances, for instance, a Cs+ permeation rate of 0.33 mol m-2 h-1, close to the value in porous substrates, and selectivities with Cs+/La3+ up to 75.9 and 69.8 in single and binary systems, highlighting the great potentials in the separation of rare metal ions.

Advanced electrochemical membrane technologies for near-complete resource recovery and zero-discharge of urine: Performance optimization and evaluation

The depletion of nutrient sources in fertilizers demands a paradigm shift in the treatment of nutrient-rich wastewater, such as urine, to enable efficient resource recovery and high-value conversion. This study presented an integrated bipolar membrane electrodialysis (BMED) and hollow fiber membrane (HFM) system for near-complete resource recovery and zero-discharge from urine treatment. Computational simulations and experimental validations demonstrated that a higher voltage (20 V) significantly enhanced energy utilization, while an optimal flow rate of 0.4 L/min effectively mitigated the negative effects of concentration polarization and electro-osmosis on system performance. Within 40 min, the process separated 90.13% of the salts in urine, with an energy consumption of only 8.45 kWh/kgbase. Utilizing a multi-chamber structure for selective separation, the system achieved recovery efficiencies of 89% for nitrogen, 96% for phosphorus, and 95% for potassium from fresh urine, converting them into high-value products such as 85 mM acid, 69.5 mM base, and liquid fertilizer. According to techno-economic analysis, the cost of treating urine using this system at the lab-scale was $6.29/kg of products (including acid, base, and (NH4)2SO4), which was significantly lower than the $20.44/kg cost for the precipitation method to produce struvite. Excluding fixed costs, a net profit of $18.24/m3 was achieved through the recovery of valuable products from urine using this system. The pilot-scale assessment showed that the net benefit amounts to $19.90/m3 of urine, demonstrating significant economic feasibility. This study presents an effective approach for the near-complete resource recovery and zero-discharge treatment of urine, offering a practical solution for sustainable nutrient recycling and wastewater management.

Covalent Organic Framework Membranes with Regulated Orientation for Monovalent Cation Sieving

Continuous covalent organic framework (COF) thin membranes have garnered broad concern over the past few years due to their merits of low energy requirements, operational simplicity, ecofriendliness, and high separation efficiency in the application process. This study marks the first instance of fabricating two distinct, self-supporting COF membranes from identical building blocks through solvent modulation. Notably, the precision of the COF membrane's separation capabilities is substantially enhanced by altering the pore alignment from a random to a vertical orientation. Within these confined channels, the membrane with vertically aligned pores and micron-scale stacking thickness demonstrates rapid and selective transportation of Li+ ions over Na+ and K+ ions, achieving Li+/K+ and Li+/Na+ selectivity ratios of 38.7 and 7.2, respectively. This research not only reveals regulated orientation and layer stacking in COF membranes via strategic solvent selection but also offers a potent approach for developing membranes specialized in Li+ ion separation.

Optimizing selectivity via membrane molecular packing manipulation for simultaneous cation and anion screening

Advancing membranes with enhanced solute-solute selectivity is essential for expanding membrane technology applications, yet it presents a notable challenge. Drawing inspiration from the unparalleled selectivity of biological systems, which benefit from the sophisticated spatial organization of functionalities, we posit that manipulating the arrangement of the membrane's building blocks, an aspect previously given limited attention, can address this challenge. We demonstrate that optimizing the face-to-face orientation of building blocks during the assembly of covalent-organic-framework (COF) membranes improves ion-π interactions with multivalent ions. This optimization leads to extraordinary selectivity in differentiating between monovalent cations and anions from their multivalent counterparts, achieving selectivity factors of 214 for K+/Al3+ and 451 for NO3-/PO43-. Leveraging this attribute, the COF membrane facilitates the direct extraction of NaCl from seawater with a purity of 99.57%. These findings offer an alternative approach for designing highly selective membrane materials, offering promising prospects for advancing membrane-based technologies.

The Hidden Role of the Dielectric Effect in Nanofiltration: A Novel Perspective to Unravel New Ion Separation Mechanisms

Nanofiltration (NF) membranes play a critical role in separation processes, necessitating an in-depth understanding of their selective mechanisms. Existing NF models predominantly include steric and Donnan mechanisms as primary mechanisms. However, these models often fail in elucidating the NF selectivity between ions of similar dimensions and the same valence. To address this gap, an innovative methodology was proposed to unravel new selective mechanisms by quantifying the nominal dielectric effect isolated from steric and Donnan exclusion through fitted pore dielectric constants by regression analysis. We demonstrated that the nominal dielectric effect encompassed unidentified selective mechanisms of significant relevance by establishing the correlation between the fitted pore dielectric constants and these hindrance factors. Our findings revealed that dehydration-induced ion-membrane interaction, rather than ion dehydration, played a pivotal role in ion partitioning within NF membranes. This interaction was closely linked to the nondeformable fraction of hydrated ions. Further delineation of the dielectric effect showed that favorable interactions between ions and membrane functional groups contributed to entropy-driven selectivity, which is a key factor in explaining ion selectivity differences between ions sharing the same size and valence. This study deepens our understanding of NF selectivity and sheds light on the design of highly selective membranes for water and wastewater treatment.

Ion-Ion Selectivity of Synthetic Membranes with Confined Nanostructures

Synthetic membranes featuring confined nanostructures have emerged as a prominent category of leading materials that can selectively separate target ions from complex water matrices. Further advancements in these membranes will pressingly rely on the ability to elucidate the inherent connection between transmembrane ion permeation behaviors and the ion-selective nanostructures. In this review, we first abstract state-of-the-art nanostructures with a diversity of spatial confinements in current synthetic membranes. Next, the underlying mechanisms that govern ion permeation under the spatial nanoconfinement are analyzed. We then proceed to assess ion-selective membrane materials with a focus on their structural merits that allow ultrahigh selectivity for a wide range of monovalent and divalent ions. We also highlight recent advancements in experimental methodologies for measuring ionic permeability, hydration numbers, and energy barriers to transport. We conclude by putting forth the future research prospects and challenges in the realm of high-performance ion-selective membranes.

Separation of Mono-/Divalent Ions via Controlled Dynamic Adsorption/Desorption at Polythiophene Coated Carbon Surface with Flow-Electrode Capacitive Deionization

Capacitive deionization for selective separation of ions is rarely reported since it relies on the electrostatic attraction of oppositely charged ions with no capability to distinguish ions of different valent states. Using molecular dynamic simulation, a screening process identified a hybrid material known as AC/PTh, which consists of activated carbon with a thin layer of polythiophene (PTh) coating. By utilizing AC/PTh as electrode material implementing the short-circuit cycle (SCC) mode in flow-electrode capacitive deionization (FCDI), selective separation of mono-/divalent ions can be realized via precise control of dynamic adsorption and desorption of mono-/divalent ions at a particular surface. Specifically, AC/PTh shows strong interaction with divalent ions but weak interaction with monovalent ions, the distribution of divalent ions can be enriched in the electric double layer after a couple of adsorption-desorption cycles. At Cu2+/Na+ molar ratio of 1:40, selectivity toward divalent ions can reach up to 110.3 in FCDI SCC mode at 1.0 V. This work presents a promising strategy for separating ions of different valence states in a continuously operated FCDI device.

Built-in Electric Fields in Heterostructured Lamellar Membranes Enable Highly Efficient Rejection of Charged Mass

Separation membranes with homogeneous charge channels are the mainstream to reject charged mass by forming electrical double layer (EDL). However, the EDL often compresses effective solvent transport space and weakens channel-ion interaction. Here, built-in electric fields (BIEFs) are constructed in lamellar membranes by assembling the heterostructured nanosheets, which contain alternate positively-charged nanodomains and negatively-charged nanodomains. We demonstrate that the BIEFs are perpendicular to horizontal channel and the direction switches alternately, significantly weakening the EDL effect and forces ions to repeatedly collide with channel walls. Thus, highly efficient rejection for charged mass (salts, dyes, and organic acids/bases) and ultrafast water transport are achieved. Moreover, for desalination on four-stage filtration option, salt rejection reaches 99.9 % and water permeance reaches 19.2 L m-2 h-1 bar-1. Such mass transport behavior is quite different from that in homogeneous charge channels. Furthermore, the ion transport behavior in nanochannels is elucidated by validating horizontal projectile motion model.

Photo-Induced Geometry and Polarity Gradients in Covalent Organic Frameworks Enabling Fast and Durable Molecular Separations

Azobenzene, which activates its geometric and chemical structure under light stimulation enables noninvasive control of mass transport in many processes including membrane separations. However, producing azobenzene-decorated channels that have precise size tunability and favorable pore wall chemistry allowing fast and durable permeation to solvent molecules, remains a great challenge. Herein, an advanced membrane that comprises geometry and polarity gradients within covalent organic framework (COF) nanochannels utilizing photoisomerization of azobenzene groups is reported. Such functional variations afford reduced interfacial transfer resistance and enhanced solvent-philic pore channels, thus creating a fast solvent transport pathway without compromising selectivity. Moreover, the membrane sets up a densely covered defense layer to prevent foulant adhesion and the accumulation of cake layer, contributing to enhanced antifouling resistance to organic foulants, and a high recovery rate of solvent permeance. More importantly, the solvent permeance displays a negligible decline throughout the long-term filtration for over 40 days. This work reports the geometry and polarity gradients in COF channels induced by the conformation change of branched azobenzene groups and demonstrates the strong capability of this conformation change in realizing fast and durable molecular separations.

Electrostatic-induced ion-confined partitioning in graphene nanolaminate membrane for breaking anion-cation co-transport to enhance desalination

Constructing nanolaminate membranes made of two-dimensional graphene oxide nanosheets has gained enormous interest in recent decades. However, a key challenge facing current graphene-based membranes is their poor rejection for monovalent salts due to the swelling-induced weak nanoconfinement and the transmembrane co-transport of anions and cations. Herein, we propose a strategy of electrostatic-induced ion-confined partitioning in a reduced graphene oxide membrane for breaking the correlation of anions and cations to suppress anion-cation co-transport, substantially improving the desalination performance. The membrane demonstrates a rejection of 95.5% for NaCl with a water permeance of 48.6 L m-2 h-1 bar-1 in pressure-driven process, and it also exhibits a salt rejection of 99.7% and a water flux of 47.0 L m-2 h-1 under osmosis-driven condition, outperforming the performance of reported graphene-based membranes. The simulation and calculation results unveil that the strong electrostatic attraction of membrane forces the hydrated Na+ to undergo dehydration and be exclusively confined in the nanochannels, strengthening the intra-nanochannel anion/cation partitioning, which refrains from the dynamical anion-cation correlations and thereby prevents anions and cations from co-transporting through the membrane. This study provides guidance for designing advanced desalination membranes and inspires the future development of membrane-based separation technologies.

Reversible pH-Gated MXene Membranes with Ultrahigh Mono-/Divalent-Ion Selectivity

Artificial ion channel membranes hold high promise in water treatment, nanofluidics, and energy conversion, but it remains a great challenge to construct such smart membranes with both reversible ion-gating capability and desirable ion selectivity. Herein, we constructed a smart MXene-based membrane via p-phenylenediamine functionalization (MLM-PPD) with highly stable and aligned two-dimensional subnanochannels, which exhibits reversible ion-gating capability and ultrahigh metal ion selectivity similar to biological ion channels. The pH-sensitive groups within the MLM-PPD channel confers excellent reversible Mg2+-gating capability with a pH-switching ratio of up to 100. The mono/divalent metal-ion selectivity up to 1243.8 and 400.9 for K+/Mg2+ and Li+/Mg2+, respectively, outperforms other reported membranes. Theoretical calculations combined with experimental results reveal that the steric hindrance and stronger PPD-ion interactions substantially enhance the energy barrier for divalent metal ions passing through the MLM-PPD, and thus leading to ultrahigh mono/divalent metal-ion selectivity. This work provides a new strategy for developing artificial-ion channel membranes with both reversible ion-gating functionality and high-ion selectivity for various applications.

Advancing Lithium-Magnesium Separation: Pioneering Swelling-Embedded Cation Exchange Membranes Based on Sulfonated Poly(ether ether ketone)

With the continuous advancement of electrodialysis (ED) technology, there arises a demand for improved monovalent cation exchange membranes (CEMs). However, limitations in membrane materials and structures have resulted in the low selectivity of monovalent CEMs, posing challenges in the separation of Li+ and Mg2+. In this investigation, a designed CEM with a swelling-embedded structure was created by integrating a polyelectrolyte containing N-oxide Zwitterion into a sulfonated poly(ether ether ketone) (SPEEK) membrane, leveraging the notable solubility characteristic of SPEEK. The membranes were prepared by using N-oxide zwitterionic polyethylenimine (ZPEI) and 1,3,5-benzenetrlcarbonyl trichloride (TMC). The as-prepared membranes underwent systematic characterization and testing, evaluating their structural, physicochemical, electrochemical, and selective ED properties. During ED, the modified membranes demonstrated notable permeability selectivity for Li+ ions in binary (Li+/Mg2+) systems. Notably, at a constant current density of 2.5 mA cm-2, the modified membrane PEI-TMC/SPEEK exhibited significant permeability selectivity ( P Mg 2 + Li + = 5.63 ) in the Li+/Mg2+ system, while ZPEI-TMC/SPEEK outperformed, displaying remarkable permeability selectivity ( P Mg 2 + Li + = 12.43 ) in the Li+/Mg2+ system, surpassing commercial monovalent cation-selective membrane commercial monovalent cation-selective membrane (CIMS). Furthermore, in the Li+/Mg2+ binary system, Li+ flux reached 9.78 × 10-9 mol cm-2 s-1 for ZPEI-TMC/SPEEK, while its Mg2+ flux only reached 2.7 × 10-9 mol cm-2 s-1, showing potential for lithium-magnesium separation. In addition, ZPEI-TMC/SPEEK was tested for performance and stability at high current densities. This work offers a straightforward preparation process and an innovative structural approach, presenting methodological insights for the advancement of lithium and magnesium separation techniques.

Polyamide Membranes with Tunable Surface Charge Induced by Dipole-Dipole Interaction for Selective Ion Separation

Nanofiltration (NF) has the potential to achieve precise ion-ion separation at the subnanometer scale, which is necessary for resource recovery and a circular water economy. Fabricating NF membranes for selective ion separation is highly desirable but represents a substantial technical challenge. Dipole-dipole interaction is a mechanism of intermolecular attractions between polar molecules with a dipole moment due to uneven charge distribution, but such an interaction has not been leveraged to tune membrane structure and selectivity. Herein, we propose a novel strategy to achieve tunable surface charge of polyamide membrane by introducing polar solvent with a large dipole moment during interfacial polymerization, in which the dipole-dipole interaction with acyl chloride groups of trimesoyl chloride (TMC) can successfully intervene in the amidation reaction to alter the density of surface carboxyl groups in the polyamide selective layer. As a result, the prepared positively charged (PEI-TMC)-NH2 and negatively charged (PEI-TMC)-COOH composite membranes, which show similarly high water permeance, demonstrate highly selective separations of cations and anions in engineering applications, respectively. Our findings, for the first time, confirm that solvent-induced dipole-dipole interactions are able to alter the charge type and density of polyamide membranes and achieve tunable surface charge for selective and efficient ion separation.

Lanthanide transport in angstrom-scale MoS2-based two-dimensional channels

Rare earth elements (REEs), critical to modern industry, are difficult to separate and purify, given their similar physicochemical properties originating from the lanthanide contraction. Here, we systematically study the transport of lanthanide ions (Ln3+) in artificially confined angstrom-scale two-dimensional channels using MoS2-based building blocks in an aqueous environment. The results show that the uptake and permeability of Ln3+ assume a well-defined volcano shape peaked at Sm3+. This transport behavior is rooted from the tradeoff between the barrier for dehydration and the strength of interactions of lanthanide ions in the confinement channels, reminiscent of the Sabatier principle. Molecular dynamics simulations reveal that Sm3+, with moderate hydration free energy and intermediate affinity for channel interaction, exhibit the smallest dehydration degree, consequently resulting in the highest permeability. Our work not only highlights the distinct mass transport properties under extreme confinement but also demonstrates the potential of dialing confinement dimension and chemistry for greener REEs separation.

Regulating ion affinity and dehydration of metal-organic framework sub-nanochannels for high-precision ion separation

Membrane consisting of ordered sub-nanochannels has been pursued in ion separation technology to achieve applications including desalination, environment management, and energy conversion. However, high-precision ion separation has not yet been achieved owing to the lack of deep understanding of ion transport mechanism in confined environments. Biological ion channels can conduct ions with ultrahigh permeability and selectivity, which is inseparable from the important role of channel size and "ion-channel" interaction. Here, inspired by the biological systems, we report the high-precision separation of monovalent and divalent cations in functionalized metal-organic framework (MOF) membranes (UiO-66-(X)2, X = NH2, SH, OH and OCH3). We find that the functional group (X) and size of the MOF sub-nanochannel synergistically regulate the ion binding affinity and dehydration process, which is the key in enlarging the transport activation energy difference between target and interference ions to improve the separation performance. The K+/Mg2+ selectivity of the UiO-66-(OCH3)2 membrane reaches as high as 1567.8. This work provides a gateway to the understanding of ion transport mechanism and development of high-precision ion separation membranes.

Two-Dimensional Sulfonate-Functionalized Metal-Organic Framework Membranes for Efficient Lithium-Ion Sieving

Two-dimensional (2D) membranes have shown promising potential for ion-selective separation but often suffer from the trade-off between permeability and selectivity. Herein, we report an ultrathin 2D sulfonate-functionalized metal-organic framework (MOF) membrane for efficient lithium-ion sieving. The narrow pores with angstrom precision in the MOF assist hydrated ions to partially remove the hydration shell, according to different hydration energies. The abundant sulfonate groups in the MOF channels serve as hopping sites for fast lithium-ion transport, contributing to a high Li-ion permeability. Then, the difference in affinity of the Li+, Na+, K+, and Mg2+ ions to the terminal sulfonate groups further enhances the Li-ion selectivity. The reported ultrathin MOF membrane overcomes the trade-off between permeability and selectivity and opens up a new avenue for highly permselective membranes.

Advancing Ion Selective Membranes with Micropore Ion Channels in the Interaction Confinement Regime

Ion exchange membranes allowing the passage of charge-carrying ions have established their critical role in water, environmental, and energy-relevant applications. The design strategies for high-performance ion exchange membranes have evolved beyond creating microphase-separated membrane morphologies, which include advanced ion exchange membranes to ion-selective membranes. The properties and functions of ion-selective membranes have been repeatedly updated by the emergence of materials with subnanometer-sized pores and the understanding of ion movement under confined micropore ion channels. These research progresses have motivated researchers to consider even greater aims in the field, i.e., replicating the functions of ion channels in living cells with exotic materials or at least targeting fast and ion-specific transmembrane conduction. To help realize such goals, we briefly outline and comment on the fundamentals of rationally designing membrane pore channels for ultrafast and specific ion conduction, pore architecture/chemistry, and membrane materials. Challenges are discussed, and perspectives and outlooks are given.

Unifying the Conversation: Membrane Separation Performance in Energy, Water, and Industrial Applications

Dense polymer membranes enable a diverse range of separations and clean energy technologies, including gas separation, water treatment, and renewable fuel production or conversion. The transport of small molecular and ionic solutes in the majority of these membranes is described by the same solution-diffusion mechanism, yet a comparison of membrane separation performance across applications is rare. A better understanding of how structure-property relationships and driving forces compare among applications would drive innovation in membrane development by identifying opportunities for cross-disciplinary knowledge transfer. Here, we aim to inspire such cross-pollination by evaluating the selectivity and electrochemical driving forces for 29 separations across nine different applications using a common framework grounded in the physicochemical characteristics of the permeating and rejected solutes. Our analysis shows that highly selective membranes usually exhibit high solute rejection, rather than fast solute permeation, and often exploit contrasts in the size and charge of solutes rather than a nonelectrostatic chemical property, polarizability. We also highlight the power of selective driving forces (e.g., the fact that applied electric potential acts on charged solutes but not on neutral ones) to enable effective separation processes, even when the membrane itself has poor selectivity. We conclude by proposing several research opportunities that are likely to impact multiple areas of membrane science. The high-level perspective of membrane separation across fields presented herein aims to promote cross-pollination and innovation by enabling comparisons of solute transport and driving forces among membrane separation applications.

Electronanofiltration Membranes with a Bilayer Charged Structure Enable High Li+/Mg2+ Selectivity

Achieving separation of lithium and magnesium with similar radii is crucial for the current lithium extraction technology from salt lakes, which usually possess a high lithium-to-magnesium ratio. Herein, we proposed the facile sequential interfacial polymerization (SIP) approach to construct electronanofiltration membranes (ENFMs) with a bilayer charged structure consisting of a high positively charged surface and a negatively charged sublayer. The trimesoyl chloride (TMC) concentration was adjusted to enhance the -COOH content and negative charge of the polyamide sublayer to promote Li+ migration, and then the quaternized polyethylenimine was introduced to the membrane surface by the SIP process to increase the positive charge density on the surface of the ENFMs, which would block the migration of Mg2+ and enhance the Li+/Mg2+ selectivity of the ENFMs. The optimal quaternary-modified ENFMs achieved outstanding selectivity for Li+/Mg2+ (49.85) and high Li+ flux (4.10 × 10-8 mol cm-2 s-1) at a current density of 10 mA cm-2. Moreover, in simulated brines with low lithium concentration and high Mg2+/Li+ ratio, the optimal ENFMs also displayed elevated Li+/Mg2+ selectivity (>45), highlighting the substantial promise of the membranes for practical applications.

Membrane Design Principles for Ion-Selective Electrodialysis: An Analysis for Li/Mg Separation

Selective electrodialysis (ED) is a promising membrane-based process to separate Li+ from Mg2+, which is the most critical step for Li extraction from brine lakes. This study theoretically compares the ED-based Li/Mg separation performance of different monovalent selective cation exchange membranes (CEMs) and nanofiltration (NF) membranes at the coupon scale using a unified mass transport model, i.e., a solution-friction model. We demonstrated that monovalent selective CEMs with a dense surface thin film like a polyamide film are more effective in enhancing the Li/Mg separation performance than those with a loose but highly charged thin film. Polyamide film-coated CEMs when used in ED have a performance similar to that of polyamide-based NF membranes when used in NF. NF membranes, when expected to replace monovalent selective CEMs in ED for Li/Mg separation, will require a thin support layer with low tortuosity and high porosity to reduce the internal concentration polarization. The coupon-scale performance analysis and comparison provide new insights into the design of composite membranes used for ED-based selective ion-ion separation.

Discovering Molecular Coordination Environment Trends for Selective Ion Binding to Molecular Complexes Using Machine Learning

The design of ion-selective materials with improved separation efficacy and efficiency is paramount, as current technologies fail to meet real-world deployment challenges. Selectivity in these materials can be informed by local ion binding in confined membrane ion channels. In this study, we utilize a data-driven approach to investigate design features in small molecular complexes coordinating ions as simplified models of ion channels. We curate a data set of 563 alkali metal coordinating molecular complexes (i.e., with Li+, Na+, or K+) from the Cambridge Structural Database and calculate differential ion binding energies using density functional theory. Using this information, we probe when and why structures favor exchange with alternate ions. Our analysis reveals that energetic preferences are related to ion size but are largely due to chemical interactions rather than structural reorganization. We identify unique trends in the selectivity for Li+ over other alkali ions, including the presence of N coordination atoms, planar coordination geometry, and small coordinating ring sizes. We use machine learning models to identify the key contributions of both geometric and electronic features in predicting selective ion binding. These physical insights offer preliminary guidance into the design of optimal membranes for ion selectivity.

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.

Ceramic thin-film composite membranes with tunable subnanometer pores for molecular sieving

Ceramic membranes are a promising alternative to polymeric membranes for selective separations, given their ability to operate under harsh chemical conditions. However, current fabrication technologies fail to construct ceramic membranes suitable for selective molecular separations. Herein, we demonstrate a molecular-level design of ceramic thin-film composite membranes with tunable subnanometer pores for precise molecular sieving. Through burning off the distributed carbonaceous species of varied dimensions within hybrid aluminum oxide films, we created membranes with tunable molecular sieving. Specifically, the membranes created with methanol showed exceptional selectivity toward monovalent and divalent salts. We attribute this observed selectivity to the dehydration of the large divalent ions within the subnanometer pores. As a comparison, smaller monovalent ions can rapidly permeate with an intact hydration shell. Lastly, the flux of neutral solutes through each fabricated aluminum oxide membrane was measured for the demonstration of tunable separation capability. Overall, our work provides the scientific basis for the design of ceramic membranes with subnanometer pores for molecular sieving using atomic layer deposition.

Subnanoscale spatially confined heterogeneous Fenton reaction enables mineralization of perfluorooctanoic acid

Superoxide radical (•O2-) is capable of degrading perfluorinated compounds that are persistent in nature and cannot be removed by biological or advanced oxidation treatments, but the inherent drawback is the negligible reactivity of •O2-in aqueous phases due to the hydration effect. Here, we explored an innovative way to make use of •O2- by modulating a partial hydration state through spatial confinement control. We demonstrated this idea by conducting heterogeneous Fenton reaction with layered iron oxychloride (FeOCl) catalyst, wherein •O2-radicals produced and confined within the catalyst structure (interlayer spacing of 7.92 Å) showed defluorination effect dealing with perfluorooctanoic acid (PFOA) as model compound. The defluorination combined with advanced oxidation achieved mineralization. Mechanism study revealed that the confinement frustrated the hydration shell of •O2-with coordination number reduced from 3.3 (for bulk phase) to 1.89, and thereby changed its orbital electron properties and enhanced the nucleophilic ability. We further demonstrated a compact FeOCl membrane reactor with highly efficient degradation of PFOA (kobs up to 1.2 min-1) and cost-effective mineralization (2 × 10-6 $ per mgC), operated under ultrafiltration reaction mode. Our findings highlight the great interest of developing spatial confinement technology to modulate •O2--based reactions, as well as the feasibility of combining confinement catalyst structures with heterogeneous Fenton reaction to achieve the mineralization treatment goal.

Nanocomposite Hydrogel Engineered Janus Membrane for Membrane Distillation with Robust Fouling, Wetting, and Scaling Resistance

Membrane distillation (MD) is considered to be rather promising for high-salinity wastewater reclamation. However, its practical viability is seriously challenged by membrane wetting, fouling, and scaling issues arising from the complex components of hypersaline wastewater. It remains extremely difficult to overcome all three challenges at the same time. Herein, a nanocomposite hydrogel engineered Janus membrane has been facilely constructed for desired wetting/fouling/scaling-free properties, where a cellulose nanocrystal (CNC) composite hydrogel layer is formed in situ atop a microporous hydrophobic polytetrafluoroethylene (PTFE) substrate intermediated by an adhesive layer. By the synergies of the elevated membrane liquid entry pressure, inhibited surfactant diffusion, and highly hydratable surface imparted by the hydrogel/CNC (HC) layer, the resultant HC-PTFE membrane exhibits robust resistance to surfactant-induced wetting and oil fouling during 120 h of MD operation. Meanwhile, owing to the dense and hydroxyl-abundant surface, it is capable of mitigating gypsum scaling and scaling-induced wetting, resulting in a high normalized flux and low distillate conductivity at a concentration factor of 5.2. Importantly, the HC-PTFE membrane enables direct desalination of real hypersaline wastewater containing broad-spectrum foulants with stable vapor flux and robust salt rejection (99.90%) during long-term operation, demonstrating its great potential for wastewater management in industrial scenarios.

Relative Insignificance of Polyamide Layer Selectivity for Seawater Electrolysis Applications

Low-cost polyamide thin-film composite (TFC) membranes are being explored as alternatives to cation exchange membranes for seawater electrolysis. An optimal membrane should have a low electrical resistance to minimize applied potentials needed for water electrolysis and be able to block chloride ions present in a seawater catholyte from reaching the anode. The largest energy loss associated with a TFC membrane was the Nernstian overpotential of 0.74 V (equivalent to 37 Ω cm2 at 20 mA cm-2), derived from the pH difference between the anolyte and catholyte and not the membrane ohmic overpotential. Based on analysis using electrochemical impedance spectroscopy, the pristine TFC membrane contributed only 5.00 Ω cm2 to the ohmic resistance. Removing the polyester support layer reduced the resistance by 79% to only 1.04 Ω cm2, without altering the salt ion transport between the electrolytes. Enlarging the pore size (∼5 times) in the polyamide active layer minimally impacted counterion transport across the membrane during electrolysis, but it increased the total concentration of chloride transported by 60%. Overall, this study suggests that TFC membranes with thinner but mechanically strong supporting layers and size-selective active layers should reduce energy consumption and the potential for chlorine generation for seawater electrolyzers.

Evaluating the potential of nanofiltration membranes for removing ammonium, nitrate, and nitrite in drinking water sources

Advanced drinking water treatment process using nanofiltration (NF) membranes has gained attention recently because it removes many challenging constituents in contaminated surface waters, such as dissolved organics and heavy metals. However, much literature has reported high variations and uncertainties of NF membranes for removing nitrogen compounds in the contaminated water-ammonium (NH4+), nitrates (NO3-), and nitrites (NO2-). This study aimed to identify the ability of commercial NF membranes to remove NH4+, NO2-, and NO3- and clarify the mechanisms underlying their transport through NF membranes. This was examined by evaluating their rejection by three commercial NF membranes using artificial and actual river waters under various conditions (variable permeate flux, temperature, pH, and ionic strength). Ammonium commonly showed the highest removal among the three nitrogen compounds, followed by nitrites and nitrates. Interestingly, ammonium removal varied considerably from 6% to 86%, depending on the membrane type and operating conditions. The results indicated that the selected nitrogen compounds (NH4+, NO2-, and NO3-) could be highly rejected depending on the clearance between their hydrated radius and the membrane's pore walls. Further, the rejection of the lowest molecular-weight nitrogen compound (NH4+) could be higher than NO2- and NO3- due to its highest energy barrier and larger hydrated radius. This study suggests that compliance with the drinking water regulations of NH4+, NO2-, and NO3- can be reliably achieved by selecting appropriate membrane types and predicting the range of their removal under various feed water quality and operating conditions.