Covalent Organic Framework Membranes Research Articles

Phosphorylated Covalent Organic Framework Membranes Toward Ultrafast Single Lithium-Ion Transport.

Developing all-solid-state electrolytes, the chips for lithium-metal batteries, with superior electrochemical and mechanical properties awaitsthe disruptive materials. Herein, ionic covalent organic framework membranes are explored as solid-state electrolytes for single Li+ conduction. In the membrane, the anion groups act as Li+ transporter, determining Li+ binding capacity and releasing ability, whereas the oxygen-containing groups act as Li+ co-transporter, creating relay sites between adjacent Li+ transporters for rapid hopping. The membrane exhibits an unprecedented Li+ conductivity of 1.7 mS cm-1 with a Li+-transference number close to unity at room temperature. Additionally, the membrane possesses high flexibility, low interfacial resistance, and excellent cycling performance at room temperature. This work paves an unprecedented path for the advancement of next-generation Li+ conductors in solid-state electrolytes.

Microwave‐Assisted Fabrication of Highly Crystalline, Robust COF Membrane for Organic Solvent Nanofiltration

AbstractFabrication of crystalline, robust covalent organic framework (COF) membranes based on disorder‐to‐order strategy is promising yet highly challenging. Herein, a microwave‐assisted method for fabricating COF membranes is proposed. Initially, monomers polymerize rapidly on the surface of porous Al2O3 substrate at room temperature to form an amorphous pristine membrane. Subsequently, a microwave field is exerted to trigger fast crystallization, acquiring a crystalline COF membrane within 60 min. The amorphous pristine membrane exhibits a high dissipation factor, indicating excellent microwave absorption capability, which accelerates the dynamic reversible reactions during the microwave treatment and thus ensures a rapid transition from the amorphous to the crystalline state. Owing to the high‐crystallinity and robust structure, the COF membranes exhibit high rejection rates for solute molecules with molecular weights exceeding 700 Da (e.g., Evans blue: 98.7%) and high solvent permeance for organic solvents (e.g., ethanol: 87.8 Lm−2h−1 bar−1, n‐hexane: 222.3 Lm−2h−1 bar−1). Surprisingly, the COF membranes exhibit superior mechanical properties, with Young's modulus of 33.91 ± 3.94 GPa, outperforming previously reported polycrystalline COF membranes and are close to those for inorganic zeolite membranes. The microwave‐assisted COF crystallization method opens a new avenue to fabricating a variety of crystalline membranes for advanced separations.

Molecular Weaving Towards Flexible Covalent Organic Framework Membranes for Efficient Gas Separations.

Covalent organic frameworks (COFs) exhibit considerable potential in gas separations owing to their remarkable stability and tunable pore structures. Nevertheless, their application as gas separation membranes is hindered by limited size-sieving capabilities and poor processability. In this study, we propose a novel molecular weaving strategy that combines hydroxyl polymers and 2D TpPa-SO3H COF nanosheets, achieving high gas separation efficiency. Driven by the strong electrostatic interactions, the hydroxyl chains thread through the COF pores, effectively weaving and assembling the composites to achieve exceptional flexibility and high mechanical strength. The penetrated chains also reduce the effective pore size of COFs, and combined with the "secondary confinement effect" stemming from abundant CO2 sorption sites in the channels, the PVA@TpPa-SO3H membrane demonstrates a remarkable H2 permeance of 1267.3 GPU and an H2/CO2 selectivity of 43, surpassing the 2008 Robson upper bound limit. This facile strategy holds promise for the manufacture of large-area COF-based membranes for small-sized gas separations.

Molecular Weaving Towards Flexible Covalent Organic Framework Membranes for Efficient Gas Separations

AbstractCovalent organic frameworks (COFs) exhibit considerable potential in gas separations owing to their remarkable stability and tunable pore structures. Nevertheless, their application as gas separation membranes is hindered by limited size‐sieving capabilities and poor processability. In this study, we propose a novel molecular weaving strategy that combines hydroxyl polymers and 2D TpPa−SO3H COF nanosheets, achieving high gas separation efficiency. Driven by the strong electrostatic interactions, the hydroxyl chains thread through the COF pores, effectively weaving and assembling the composites to achieve exceptional flexibility and high mechanical strength. The penetrated chains also reduce the effective pore size of COFs, and combined with the “secondary confinement effect” stemming from abundant CO2 sorption sites in the channels, the PVA@TpPa−SO3H membrane demonstrates a remarkable H2 permeance of 1267.3 GPU and an H2/CO2 selectivity of 43, surpassing the 2008 Robson upper bound limit. This facile strategy holds promise for the manufacture of large‐area COF‐based membranes for small‐sized gas separations.

Rapid preparation and mechanism investigation of covalent organic framework membranes by 3D printing based on electrostatic spraying

Covalent organic framework (COF) has great advantages in the field of membrane separation, but the efficient and convenient preparation of COF membranes is still a challenge. Herein, we propose a novel method with green solvent for the preparation of COF membranes based on electrostatic spraying, which can be successfully prepared within 10 min, forming the TpPa COF with a pore size of 1.8 nm. Molecular dynamics simulations demonstrate that electrostatic spraying enhances the reaction rate by lowering the energy barrier and increasing the movement of reacting monomers. Atomization of the solvent in electrostatic spraying causes the rapid volatilization of the solvent, resulting in supersaturation of the generated TpPa, precipitation and crystallization, and promotes the formation of TpPa COF structure. The optimal COF membrane for the separation of Congo red wastewater is prepared by adjusting the voltage, concentration, and spraying time, with selectivity of Na2SO4 for Congo Red attaining 118 and water permeance reaching 70.2 L m-2 h-1 bar-1. This work not only elaborates the membrane preparation mechanism in the electrostatic spraying process, but also greatly shortens the time of COF membrane preparation compared to the traditional method (2–3 days), which contributes to the possibility for practical application of COF membranes.

Guanidinium‐Based Covalent Organic Framework Membranes for Superior Mono‐ and Divalent Cations Separation

Biological membranes exhibit extraordinary efficiency in processing ion permeability and selectivity. However, creating artificial membranes for an ideal ion separation is still challenging due to the subtle distinctions in valence and size among different ions. In the realm of biological recognition, the ultimate selectivity of ion channels is considered to stem from the specific binding interactions and appropriate charge density. Designing artificial membranes to achieve similar performance not only helps the understanding of complex ion transport in bioprocesses but also facilitates critical industrial separations. Inspiring by the remarkable performance in biological systems, a guanidinium‐based covalent organic framework membrane is designed, which exhibits an excellent capability to recognize mono‐/divalent cations, achieving K+/Mg2+ selectivity up to 202 in a mixed salt solution. Furthermore, the membrane displays rapid ion transport owing to the uniform sub‐2 nm channels. The experimental results and molecular dynamics simulations illustrate that the charge‐assisted hydrogen bonding sites and the Cl− counter ions within 1D channels play critical roles in cations sieving. Specifically, divalent ions passing through the positively charged channels need to overcome higher energy barriers than monovalent ions. These findings offer promising avenues for the development of advanced multifunctional membranes for efficient ion separation and sustainable water‐related separations.

Crystalline Covalent organic framework membrane with tailored chargeability for efficient pharmaceutical rejection by in-situ functionalization of polyethylene-imine

The nanofiltration (NF) membrane with better hydrophilicity, uniform pore size distribution, and dually charged properties is highly desirable to improve the pharmaceutical rejection, especially for the neutral and positively charged pharmaceuticals. Herein, a TpPa membrane was first in-situ crystallized via PTSA-mediated interfacial catalytic polymerization (ICP) strategy using 1,3,5-triformylphloroglucinol (Tp) and p-phenylenediamine (Pa), followed by post-functionalization with PEI to narrow pore size, improve hydrophilicity, and tailor membrane charges to enhance pharmaceutical rejection. PTSA was used as a catalyst to enhance the crystallinity of the TpPa membrane. The PEI introduction narrowed the pore radius from 0.382 nm to 0.272 nm, improved surface hydrophilicity from 74.8°to 37.0°, and shifted surface charge from -19.25 mV to 11.15 mV. This PEI-functionalized TpPa (TpPa-PEI) layers exhibited heterogeneous charges on both sides with a positively charged top and negatively charged bottom. MgCl2 rejection increased from 13.67% to 83.0% without sacrificing water permeance. Additionally, pharmaceutical rejection and the water permeance of the optimal TpPa-PEI membrane exceeded those of the TpPaIP-PEI membrane fabricated without PTSA by about 3.3 and 1.3 times, respectively. Furthermore, compared to the pristine TpPa membranes, the substantially enhanced electropositivity of the optimal TpPa-PEI membrane led to 3-5 times increase in positively charged pharmaceutical rejection (94.1% for propranolol, 97.2% for sulpiride, and 71.2% for metformin). The synergy of negatively charged bottom TpPa-PEI layers and reduced pore size maintained sulfadiazine rejection of 62.1%. Mechanistic study confirmed that PEI penetrated 100 nm into the TpPa layer and cross-linked with the aldehyde groups, leading to tailored chargeability, improved hydrophilicity, and reduced pore size. Via a PEI cross-linking strategy, sub-nanometer channels of crystalline COF layers can be rationally designed, featuring precisely tailored chargeability and hydrophilicity, promising functionalization of the membrane pores and remarkably robust for water reuse.

Interfacially Fabricated Covalent Organic Framework Membranes for Film‐Based Fluorescence Humidity Sensors and Moisture Driven Actuators

AbstractRapid, on‐site measurement of ppm‐level humidity in real time remains a challenge. In this work, we fabricated a few micrometer thick, β‐ketoenamine‐linked covalent organic framework (COF) membrane via interfacially confined condensation of 1,3,5‐tris‐(4‐aminophenyl)triazine (TTA) with 1,3,5‐tri‐formylphloroglucinol (TP). Based on the super‐sensitive and reversible response of the COF membrane to water vapor, we developed a high‐performance film‐based fluorescence humidity sensor, depicting unprecedented detection limit of 0.005 ppm, fast response/recovery (2.2 s/2.0 s), and a detection range from 0.005 to 100 ppm. Remarkably, more than 7,000‐time continuous tests showed no observable change in the performance of the sensor. The applicability of the sensor was verified by on‐site and real‐time monitoring of humidity in a glovebox. The superior performance of the sensor was ascribed to the highly porous structure and unique affinity of the COF membrane to water molecules as they enable fast mass transfer and efficient utilization of the water binding sites. Moreover, based on the remarkable moisture driven deformation of the COF membrane and its composition with the known polyimide films, some conceptual actuators were created. This study brings new ideas to the design of ultra‐sensitive film‐based fluorescent sensors (FFSs) and high‐performance actuators.

Interfacially Fabricated Covalent Organic Framework Membranes for Film-Based Fluorescence Humidity Sensors and Moisture Driven Actuators.

Rapid, on-site measurement of ppm-level humidity in real time remains a challenge. In this work, we fabricated a few micrometer thick, β-ketoenamine-linked covalent organic framework (COF) membrane via interfacially confined condensation of 1,3,5-tris-(4-aminophenyl)triazine (TTA) with 1,3,5-tri-formylphloroglucinol (TP). Based on the super-sensitive and reversible response of the COF membrane to water vapor, we developed a high-performance film-based fluorescence humidity sensor, depicting unprecedented detection limit of 0.005 ppm, fast response/recovery (2.2 s/2.0 s), and a detection range from 0.005 to 100 ppm. Remarkably, more than 7,000-time continuous tests showed no observable change in the performance of the sensor. The applicability of the sensor was verified by on-site and real-time monitoring of humidity in a glovebox. The superior performance of the sensor was ascribed to the highly porous structure and unique affinity of the COF membrane to water molecules as they enable fast mass transfer and efficient utilization of the water binding sites. Moreover, based on the remarkable moisture driven deformation of the COF membrane and its composition with the known polyimide films, some conceptual actuators were created. This study brings new ideas to the design of ultra-sensitive film-based fluorescent sensors (FFSs) and high-performance actuators.

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.

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.

Zwitterionic Nanochannels in Covalent Organic Framework Membranes for Improved Flux and Antifouling Property.

Zwitterionic membranes demonstrate excellent antifouling property in water purification. The covalent organic frameworks (COFs), due to the ordered channels and abundant organic functional groups, have distinct superiority in constructing zwitterionic surfaces.Here, the zwitterionic COF membrane is prepared with precise framework structures and uniform charge distribution. The negatively charged 4,4'-diaminobiphenyl-2,2'-sisulphonic acid sodium (SA) and positively charged ethidium bromide (EB) fragments are used to react with 1,3,5-triformylphloroglucinol (TP) at the gas-liquid interface to prepare zwitterionic COF membrane. The complementary charged fragments in the inter-layer and inner-layer facilitate the formation of continuous and tight hydration layer on the membrane surface and pore walls to resist the adsorption of pollutants. The zwitterionic COF membrane effectively resists both negatively charged bovine serum albumin and positively charged lysozyme pollutants with flux recovery ratio (FRR) of 97% and 85%, respectively. Furthermore, the regular nano-channels and balanced interactions between water and surface/pore walls of the zwitterionic membrane result in outstanding permeability of up to 146 L m-2 h-1 bar-1 and excellent dye/salt separation selectivity. The water permeation and antifouling mechanism of membranes are elucidated by experimental and molecular dynamics calculation. Zwitterionic COF membranes can find promising applications in preparing high-performance antifouling membranes.

Beta-Cyclodextrin-Modified Covalent Organic Framework Nanochannel for Electrochemical Chiral Recognition of Amino Acids.

The chiral recognition and separation of enantiomers are of great importance for biological research and the pharmaceutical industry. Preparing homochiral materials with adjustable size and chiral binding sites is beneficial for achieving an efficient chiral recognition performance. Here, a homochiral covalent organic framework membrane modified with β-cyclodextrin (CD-COF) was constructed, which was subsequently utilized as an electrochemical sensor for the enantioselective sensing of tryptophan (Trp) molecules. The preferential adsorption of l-Trp over d-Trp at the β-CD sites can enhance the surface charge density and hydrophilicity of the CD-COF membrane, resulting in an increased transmembrane ionic current. Trp enantiomers with concentrations down to 0.28 nM can be effectively discriminated. The l-/d-Trp recognition selectivity increases with the Trp concentration and reaches a value of 19.2 at 1 mM. The selective adsorption of l-Trp to the CD-COF membrane will also hinder its transport, resulting in a l-/d-Trp permeation selectivity of 15.3. This study offers a new strategy to construct homochiral porous membranes and achieve efficient chiral sensing and separation.

Spatial Size Manipulation of 1D/2D Channels in Covalent Organic Framework Membranes Through Dopamine Chemistry for Ion Separations

AbstractCovalent organic framework (COF) membranes feature with well‐developed 1D in‐plane pores and parallelly arranged 2D interlayer gallery, presenting promising platform for precise separations. However, it remains a formidable challenge to construct and regulate membrane channels at angstrom scale. Herein, pH‐sensitive dopamine is taken advantage to elaborately engineer the spatial size of 1D/2D channels in COF membranes for the separations of alkali metal ions. Acid treatment allows monomolecular dopamine to segment 1D in‐plane pores of COF membrane, achieving ultramicroporous regulation from 1.25 nm to 0.71 nm, which enables high selectivity of 18.7 for K+/Li+ separation. Molecular dynamics simulations reveal the higher dehydration degree, weaker channel‐cation interaction and faster diffusion coefficient of K+ than Li+. For alkaline treatment, dopamine self‐polymerizes to form nanoparticles between COF layers, which enlarges the 2D interlayer channels from 0.33 nm to 0.45 nm in COF membrane, enabling high‐permeance ion/molecule separations. The water permeance increases 86.7% to 404 L m−2 h−1 bar−1, without the sacrifice of membrane sieving ability. Both cation separation and ion/molecule separation performances outperform the current state‐of‐the‐art membranes. This dopamine‐mediated channel engineering strategy may provide the new insights for the design of membrane channels in precise separations.

Zwitterionic covalent organic framework membranes for efficient liquid molecular separations

Covalent organic frameworks (COFs) featuring uniform topological structure and devisable functionality have emerged as promising membrane materials. The design and precise manipulation of COF membranes with advanced spatial structure to achieve efficient liquid molecular separations are of great necessity. Herein, zwitterionic COF membranes have been in-situ fabricated on porous polymeric substrates using an interfacial polymerization modification strategy. The continuous defect-free COF membranes with two-dimensional in-plane dominant growth can be achieved by optimizing the fabrication parameters including reaction time, monomer concentration, and catalyst concentration. Subsequent zwitterionic modification thereon not only favors the formation of hydrophilic surface but also improves the sieving capability by sheltering effect. Attributed to the synergistic contribution, the optimized zwitterionic COF membrane possesses a superior separation factor of 2839 with the water content in the permeate up to 99.7 wt%, while maintaining a comparable permeation flux of 3309 g m−2 h−1 during the ethanol dehydration process, outperforming most of other representative membranes. Furthermore, the excellent durability of the zwitterionic COF membrane in the ethanol dehydration process and its efficient separation performance towards other alcohol dehydration systems demonstrate its potential practical applications. The easy scalability of the fabrication and regulation method offers crucial guidance for the engineering of advanced COF membranes in efficient liquid molecular separations.

Cell membrane-inspired COF/AAO hybrid nanofluidic membrane with ion current rectification properties for ultrasensitive detection of E. coli

Nanofluidic membranes hold great prospects in sensitive and rapid detection of bioanalysts, while remaining lacking robust and general approaches for specificity. In this work, we introduce a cell membrane-inspired nanofluidic membrane with excellent ion current rectification properties as a simple and robust platform for label-free and ultrasensitive biosensing. The asymmetric nanofluidic membrane is prepared by coupling a two-dimensional covalent organic framework (COF) membrane to an anodic aluminum oxide (AAO) nanochannel via hydrogen bonding. The prepared COF/AAO membrane presents selective ion transport due to the asymmetric structure and anion selectivity of COF. With the large surface area and abundant functional modification sites of COF, further grafting of the recognition unit can be easily achieved to mimic the recognition process of receptors and ligands on cell membranes. As a proof of concept, the application in bacterial detection is demonstrated by covalently bonded Concanavalin A as recognition units. The highly amplified ionic current based on ion current rectification properties of the COF/AAO membrane allows sensitive and selective bioanalysis. Specifically, the proposed asymmetric nanofluidic membrane presents a linear detection range from 10 to 106 CFU/mL with an ultralow detection limit of 7 CFU/mL for E. coli. This work reveals the great potential of COF/AAO hybrid nanofluidic membrane as a sensor for rapid and precise bioanalysis.

Highly Anion-Conductive Viologen-Based Two-Dimensional Polymer Membranes as Nanopower Generators.

Two-dimensional polymers (2DPs) and their layer-stacked 2D covalent organic frameworks (2D COFs) membranes hold great potential for harvesting sustainable osmotic energy. The nascent research has yet to simultaneously achieve high ionic flux and selectivity, primarily due to inefficient ion transport dynamics. This is directly related to ultrasmall pore size (<3 nm), much smaller than the duple Debye length in the diluted electrolyte (6-20 nm), as well as low charge density (<4.5 mC m-2). Here, we introduce a π-conjugated viologen-based 2DP (V2DP) membrane possessing a large pore size of 4.5 nm, strategically enhancing the overlapping of the electric double layer, coupled with an exceptional positive surface charge density (~6 mC m-2). These characteristics enable the membrane to facilitate high anion flux while maintaining ideal selectivity. Notably, V2DP membranes realize an impressive current density of 5.5×103 A m-2, surpassing benchmarks set by previously reported nanofluidic membranes. In the practical application scenario involving the mixing of artificial seawater and river water, the V2DP membranes exhibit a considerable ion transference number of 0.70 towards Cl-, contributing to an outstanding power density of ~55 W m-2. Theoretical calculations reveal the important role of the large quantity of anion transport sites, which act as binding sites evenly located in the positively charged N-containing pyridine rings. These binding sites enable kinematic coupling and decoupling between anions and the V2DP skeleton, establishing a continuous Cl- ion transport pathway. This work demonstrates the great promise of large-area ultrathin 2DP membranes featuring highly organized charged ion transport networks when applied for osmotic energy conversion.

3D Covalent Organic Framework Membranes for Molecular Separations.

Membrane separation has become an indispensable separation technology in social production, playing an important role in drug production, water purification, etc. The key core of membranes lies in achieving efficient and precise sieving between substances. As a result, a typical trade-off arises: highly permeable membranes usually sacrifice selectivity and vice versa. To address this dilemma, long-term research has focused on comprehensive understanding and modelling of synthetic membranes at various scales. A significant advancement in this arena is the advent of three-dimensional covalent organic framework (3D COF) membranes, a novel category of long-range ordered porous organic polymer materials. Characterized by an abundance of interconnected channels, diverse pore wall properties, tunable structures, and robust thermal and chemical resilience, 3D COF membranes offer a promising approach for efficient substance separation. This review undertakes a meticulous investigation of the synthesis and physicochemical properties of 3D COF membranes, accentuating the underlying design principles, fabrication methods, and application attempts. A comprehensive assessment of their research trajectory and current standing in the field of membrane processes is provided. The review culminates in a forward-looking outlook, summarizing future research directions and highlighting the substantial potential of this innovative work to shape the future of efficient membrane separation processes.

Alkoxy-functionalized covalent organic Framework membranes for efficient molecular sieving with high water permeance

The presence of various trace water-soluble organic pollutants, such as dyes, poses significant potential risks to human health and ecosystems. Membrane technology presents a promising and sustainable approach for their removal from aqueous environments. However, the amorphous structure of most commercial membranes makes it challenging to achieve effective separation with high water flux. Here, we present a novel series of hydrophilic COF membranes with different alkoxy sidechain lengths for the first time. These alkoxy sidechain functional COF membranes exhibit efficient rejection of water-soluble organic dyes, including rose bengal (> 50 %), congo red (> 98 %), brilliant blue R250 (> 99 %), and alcian blue (> 99 %). Moreover, these membranes demonstrate a high water flux exceeding 291.8 L m-2h−1 bar−1 and exceptional long-term operational stability lasting at least 2880 min. The superior performance of COF membranes is attributed to their optimal pore size, excellent hydrophilicity, and high crystallinity. This finding offers a promising approach for effectively eliminating water-soluble organic pollutants, thereby ensuring the safety and sustainability of the water supply.

Polyelectrolyte-modified covalent organic framework membranes for multivalent cation removal

Covalent organic framework (COF) has become a promising membrane material in chemical separations. However, the application of COF membranes for ion separation is still challenging, due to the mismatch between the pore size of COFs (1–5 nm) with the diameter of hydrated ions (<1 nm). Herein, a cationic polyelectrolyte surface modification strategy of COF membranes was proposed, by assembling a cationic polyelectrolyte (PEI, PDDA, PAH) layer on the surface of an anionic COF (TpPa-SO3H) membrane via dip coating. The cationic polyelectrolytes on the membrane surface offered abundant positive charges and reduced the surface pore size, leading to the synergistic enhancement of the size sieving and the electrostatic repulsion. Accordingly, effective rejection of multivalent cations was realized with a significantly increased rejection rate of 97.1 % for MgCl2, compared to that of TpPa-SO3H membrane (10.3 %). Furthermore, the polyelectrolyte-modified COF membrane exhibited desirable separation performance for heavy metal ions in water. The rejection rates for heavy metal salts, including CrCl3, CdCl2, CuCl2, PbCl2 and NiCl2, reached 98.5 %, 97.3 %, 97.0 %, 96.2 %, and 96.0 %, respectively, and remained above 95 % during the 7-day long-term operation test.