Ion Selectivity Research Articles

How to Compare the Ion Selectivity of Smart Nanopores/ Membranes

How to Compare the Ion Selectivity of Smart Nanopores/ Membranes

Selective divalent/monovalent ion partitioning in cation exchange membranes

The selective partitioning of divalent and monovalent counterions through cation exchange membranes immersed in 2:1 and 1:1 binary salt solutions remains unclear, especially at the molecular level. We present a theoretical framework that quantifies these membranes’ sorption of divalent and monovalent ions, incorporating condensed and free variants to reconcile deviations from the ideal Donnan picture observed in published experimental data. This framework explores the membranes’ global and local selective ion sorption characteristics by assessing the effects of competitive ion condensations, where these counterions compete to bind to the fixed charge groups. First, we calculate the thermodynamic property—the global partition ratio—of the membranes immersed in these binary salt solutions, quantifying the partitioning between the binary salt solutions and the membranes. Our theoretical results successfully capture the published experimental partitioning data for membranes immersed in equimolar and non-equimolar 2:1 and 1:1 binary salt solutions. Furthermore, the Donnan potential of the membranes reciprocally changes with the total concentration of 2:1 and 1:1 external salt solutions. However, for the non-equimolar systems at a constant low total concentration, varying the binary solution concentration ratio of 2:1 to 1:1 salts maintains a strong electric potential intensity due to high ionic imbalance. Second, we quantify the local partitioning of the sorbed counterions that are condensed on the crosslinked polymeric chains and those that are mobile in the interstitial solution of the membrane, a variable that is difficult to access experimentally. Accordingly, we can fractionate the sorbed ions into condensed and free states, which mutually govern the membrane’s kinetic and thermodynamic (global and local partitioning) properties.

PREPARATION AND CHARACTERIZATION OF POLYMER INCLUSION MEMBRANES BASED ON BIODEGRADABLE CELLULOSE/ALGERIAN CLAY FOR HEAVY METALS REMOVAL FROM WASTEWATER

Separation membranes have gained attention as promising options for water and wastewater treatment due to their financial sustainability, and eco-friendliness. However, practical challenges have limited their application in water separation. To overcome these limitations, inorganic-organic hybrid membranes have been developed in this study. The present work deals with two attractive aspects: (i) economical, through the valorization of a local clay (Algerian kaolin), and (ii) environmental, which is based on the membrane selectivity for metal ions. The principal objective of this work is the development of enhanced nanocomposite membranes. It is achieved with low costs, based on cellulose triacetate (CTA) as a polymeric matrix modified by the addition of a lamellar filler, i.e. yellow clay obtained from Jijel, located in the east of Algeria, and plasticized by dioctyl phthalate (DOP). A further objective of this paper was the treatment of wastewater polluted by lead (Pb2+) and cadmium (Cd2+). The prepared membranes were characterized by various characterization techniques, including scanning electron microscopy (SEM), Fourier-transform infrared (FTIR) spectroscopy and thermogravimetric analysis (TGA). All synthetized membranes had an amorphous structure, with homogeneous pore morphology and distribution. Moreover, the presence of nanocomposite clay showed effective integration into the membrane matrix and led to a significant improvement in thermal resistance. These membranes were applied to treat a synthetic aqueous solution contaminated with heavy metals, namely Pb2+ and Cd2+. The results revealed a rejection rate higher than 50%, suggesting the potential effectiveness of a stable and environmentally sustainable polymer inclusion membrane system for water purification.

A Novel Sulfonated Polyimide Composite Membrane Containing a Sulfonated Porous Material for All-Vanadium Redox Flow Batteries.

To improve the battery efficiency and cycling stability of sulfonated polyimide (SPI), a polyphosphazene with built-in -SO3H moieties (PP-SO3H), which is a porous covalent organic framework (COF) material, is facilely synthesized by the polymeric combination of hexachlorocyclotriphosphazene (HCCP) and p-diaminobenzenesulfonic acid. Due to its tunable pore size and flexible molecular design, the COF material can address the trade-off between the conductivity and the ion permeability of ion exchange membranes well, thereby improving the ion selectivity of membranes. The experimental results show that the SPI/PP-SO3H composite membrane has an excellent conductivity (up to 114.8 mS cm-1); the ion selectivity of the SPI/2% PP-SO3H membrane is 11.69 × 104 S min cm-3, which is 2.18 times higher than that of the SPI base membrane. PP-SO3H also improves the SPI membrane's mechanical strength, and the effect of PP-SO3H on SPI intermolecular interactions is analyzed by surface electrostatic potential (ESP) theoretical calculations. The Coulombic efficiency (CE) of the SPI/2% PP-SO3H membrane is 98.92%, the energy efficiency (EE) is 84.1% at a current density of 100 mA cm-2, and the self-discharge time of the SPI/2% PP-SO3H membrane is 3.5 times compared with the SPI base membrane. To measure the cycling stability of the composite membrane, the SPI/2% PP-SO3H membrane is cycled in the VRFB for more than 400 cycles, which is more stable than that of the SPI base membrane. These results show that SPI/2% PP-SO3H composite membranes are viable for VRFB applications.

Osmotic energy conversion in serpentinite-hosted deep-sea hydrothermal vents

Cells harvest energy from ionic gradients by selective ion transport across membranes, and the same principle is recently being used for osmotic power generation from salinity gradients at ocean-river interfaces. Common to these ionic gradient conversions is that they require intricate nanoscale structures. Here, we show that natural submarine serpentinite-hosted hydrothermal vent (HV) precipitates are capable of converting ionic gradients into electrochemical energy by selective transport of Na+, K+, H+, and Cl-. Layered hydroxide nanocrystals are aligned radially outwards from the HV fluid channels, constituting confined nanopores that span millimeters in the HV wall. The nanopores change the surface charge depending on adsorbed ions, allowing the mineral to function as a cation- and anion-selective ion transport membrane. Our findings indicate that chemical disequilibria originating from flow and concentration gradients in geologic environments generate confined nanospaces which enable the spontaneous establishment of osmotic energy conversion.

Mechanisms determining water permeability and ion selectivity of multilayered glycine-functionalized graphene oxide membranes in saline water reverse osmosis processes: A computational investigation

Non-Equilibrium Molecular Dynamics simulations were performed to examine in detail the mechanisms involved in a reverse osmosis process for the removal of monovalent (Na+, Cl−) and divalent (Mg2+) ions from saline water, using multilayered glycine-functionalized graphene oxide (GO) membranes. We varied parameters such as the degree of functionalization of the GO flakes and the structural flexibility of the membranes and examined their performance in a wide range of applied pressure gradients. In all models examined, water permeability was found to be almost 2 orders of magnitude larger compared to that in conventional reverse osmosis (RO) membranes. High structural flexibility of the membranes was found to increase water permeability due to the formation of additional nanochannels that favor faster water transport. Functionalization of the GO flakes by the glycinate groups resulted in a decrease in water permeability due to the formation of a denser hydrogen-bonding network within the membranes. Three main mechanisms were identified regarding ion selectivity. Due to the overall negative charge of the membranes, the strong adsorption of the Mg2+ ions onto the GO flakes resulted in their full rejection. Weaker adsorption of the Na+ ions led only to their partial rejection. At the same time, the electrostatic repulsion of the Cl− ions by the membranes was found to result in almost full removal levels at high pressure gradients. The rejection of the monovalent ions was found to decrease with the structural flexibility of the membranes and increase with the functionalization of the GO flakes, in response to the changes observed in water permeability on these membrane modifications. The pressure-dependent deformability of the structurally flexible membranes combined with charge accumulation close to the membrane's entrance, resulted in a non-monotonic dependence of the monovalent ion rejection on the applied pressure difference.

Advanced membrane contactor coupled with electrodialysis metathesis for efficient carbon dioxide capture and waste salt remediation

Rapid development of society has led to significant increases in carbon dioxide (CO2) emissions, primarily driven by intensified industrial activities and urbanization. Capture and utilization of CO2 not only mitigate greenhouse gas emissions but also support the transition towards a circular economy by recycling CO2 into high-value products. This study proposed a membrane contactor and electrodialysis metathesis (MC-EDM) for CO2 capture, NH4HCO3 generation, and NaHCO3 conversion. The MC facilitated the selective capture of CO2 from gas streams. Concurrently, EDM enhanced the NaHCO3/NH4Cl conversion from NH4HCO3/NaCl through selective ion transport mechanisms. The key influencing factors (e.g., gas flow rate, liquid flow rate, and ammonia concentration) for the MC process were optimized to achieve 1.014 mol/L NH4HCO3 concentration with 88.73 % yield and 78.94 % CO2 removal efficiency. Moreover, the parameters (e.g., solid-liquid ratio, membrane type, operating voltage, and salt component ratio) for the EDM process were optimized to achieve NaHCO3 with a 90.9 % yield and 97.80 % purity. Economic analysis indicated the overall process for NaHCO3 recovery was approximately 0.3267 $/kg. The cyclic experiments demonstrated the stability of MC-EDM for high-value NaHCO3 recovery. Thus, the MC-EDM pathways contribute significantly to reducing greenhouse gas emissions from industrial processes while promoting resource efficiency and economic sustainability.

Ratiometric Fluorescent Probes for Copper Ion Detection and Cuproptosis Studies

Abstract: This study reviews the latest progress of ratiometric fluorescent probes of copper ions from 2022 till now. As a key trace element in biological systems, copper ions play an important role in a variety of biological processes. However, the abnormal change in copper ion concentration is closely related to a variety of diseases, so the development of highly sensitive and highly selective copper ion detection methods is of great significance for scientific research and clinical diagnosis. In this paper, we first introduced the classification of copper ion fluorescence probes, including molecular sensors based on five-membered azocycles, six-membered azocycles, imide groups (R2C=NR'), oxygen and nitrogen functional groups (such as ethers, aldehydes, ketones, esters, carboxylic acids and hydroxyl groups) and group ⅥA elements. Subsequently, the application of nanosensors in the detection of copper ions is further discussed, especially the nanosensors in which copper is used as a reactant, catalyst or alternative reaction. Finally, the current research status of copper ion fluorescence probes is summarized, and the future development direction is prospected in order to provide reference and inspiration for the design of new high-efficiency copper ion detection tools.

Impact of silvicultural practices on the genetic variations between sugar maple (Acer saccharum Marsh.) stands in the Northern Forest, U.S.A

Tree harvesting practices that enhance genetic diversity are essential for the long-term sustainability of our forests. Using eSSRs, which are markers within expressed genes and thus, potentially subjected to natural selection and reveal adaptive potential, this study examines the effects of selection (SE) and shelterwood (SW) management systems using unmanaged (UM) stands as reference on the genetic diversity of sugar maples in the Northern Forest, USA. Comparison between management types shows no difference between SE and SW, but compared to UM, both have lower allelic richness (AR) while only SE has lower expected heterozygosity (HE). Size class comparison shows that seedlings have higher AR and HE than poles and mature trees. Two-way interactions between management types, size classes, and study stands from each of the four states that comprise the Northern Forest region (Maine, New Hampshire, Vermont and New York) were also examined. Results show that seedlings in UM and SW have higher AR compared to all size classes in SE and poles and mature trees in SW, while only seedlings from SW have higher HE compared to poles and mature trees in SE and SW. Only in Maine was there a difference in AR between SW and SE stands per state. Seedlings in New Hampshire and Maine have higher HE, as well as seedlings and mature trees in New York compared to the poles in New Hampshire and mature trees in Maine. Overall, the results indicate genetic erosion in sugar maple stands in the Northern Forest region. This genetic impact may be due to several factors including the intensity of cutting associated with SE and SW systems and size of the canopy gaps created. Higher seedling genetic diversity in SW compared to SE may be due to their more open stand structure that facilitates gene flow and seedling recruitment. When using SE, longer cutting cycle paired with lower level of residual basal area is recommended. This approach allows for a more open stand structure that could facilitate pollen and seed exchanges from the stands, improving their genetic diversity. In both SE and SW systems, leaving high-quality growing stocks will enhance their genetic diversity and ensure their sustainability. This study increases our awareness of the potential genetic impacts of forest harvesting practices and guides the development of recommendations to alleviate genetic erosion in sugar maple stands.

Crystal orientation control in angstrom-scale channel membranes for significantly enhanced blue energy harvesting

Achieving high-performance blue energy harvesting from the interface between seawater and river water remains challenging, because the current ion-selective membrane suffers from a trade-off between ion permeability and selectivity. Here, we tackle this issue by manipulating the crystal orientations within metal–organic framework (MOF) MIL-178 membranes, including one with the a-axis perpendicular to the substrate, and the other with the b-axis similarly configured. We show that the MIL-178-a-oriented membrane exhibited over one order of magnitude faster ion transport than the MIL-178-b-oriented membrane, and one-dimensional angstrom-scale (∼3.2 Å) channels endow the former with ultrahigh ion selectivity. Consequently, the MIL-178-a-oriented membrane achieves an ultrahigh power density of ∼9.64 W/m2, with an unprecedented energy conversion efficiency of ∼38.5 % approaching the theoretical upper limit of 50 %, when simulated seawater and river water are mixed, surpassing all the reported non-oriented MOF-based membranes. This study presents a method for finely tuning preferred orientations within polycrystalline MOF membranes, enabling precise engineering of ultrahigh-performance osmotic energy conversion.

Amide/urea-based simple fluorometric receptors for iodide and Hg2+ ions in aqueous medium: Aggregation induced emission and DFT studies

Herein, we report pyrene-tagged amide and urea-based sugar derivatives 1 and 2 in a simple synthetic pathway to recognize I− and Hg2+ ions. Both molecules showed absorbance and fluorescence selectivity towards iodide ions in THF/H2O (7/3, v/v) medium. The selectivity and sensitivity of 2 for iodide ions are superior to 1 due to more H-bond donors in 2. Interestingly, fluorometric receptor 2 exhibited aggregation-induced emission (AIE) at higher pH with a remarkable fluorometric color change. The AIE phenomenon might be explained by the self-association of 2 after forming imine functionality in the alkali medium. The Stern-Volmer plot showed the fluorescence quenching constant of each receptor with an iodide ion and indicated the quenching pathway. The LODs of 1 and 2 for iodide ions were evaluated as 0.84 and 0.17 µM, respectively. The 1:1 binding stoichiometry of 1 or 2 with iodide was found from the Job plot and verified by measuring the complex mass.Further, the complexes of each receptor with I− ions can detect Hg2+ ions selectively by fluorescence turn-on method with low sensitivities (LODs: 0.008 µM for 1 and 0.01 µM for 2). DFT results were used to understand the binding mode of receptors 1 and 2 with iodide ions and the quenching process in the aqueous THF medium. The real application of the receptors was established for the recovery of iodide and Hg2+ ions from natural water samples.

Polyamide nanofiltration membrane modified with defective covalent organic framework interlayer for selective ion sieving

Generally, thin-film composite (TFC) membranes prepared using piperazine (PIP) and trimesoyl chloride (TMC) exhibit good Na2SO4 rejection, but their MgCl2 rejection is not satisfactory. In this study, a defective covalent organic framework (COF) interlayer with residual unreacted amine groups was fabricated via interfacial polymerization (IP) by changing the catalyst concentration. Introduction of the COF layer into a polyamide (PA) separation membrane modified the pore size of the PA layer, thereby improving the rejection of divalent cations. The pore size of the PA layer decreased and the surface morphology of the membrane was converted from nodular to a hybrid morphology of “ridge-and-valleys’’ and nodules, indicating that the residual unreacted amine groups on the COF interlayer successfully reacted with the acyl chloride groups. The optimal membrane (PIP/COF-A/PES) exhibited high rejection of divalent ions (Na2SO4 of 98.3% and MgCl2 of 94.3%), and the membrane showed excellent capability for separating mono/divalent ions. Overall, a method of preparing nanofiltration membranes by modifying the PA layer to enhance the rejection of divalent cations was demonstrated.

An exploration of AFLC-14-doped ceric phosphate, a novel liquid crystal hybrid ion exchanger: synthesis, ion exchange studies and analytical insights

ABSTRACT In this paper, we present the synthesis and characterisation of an anthraquinone-based discotic liquid crystal (DLC), AFLC-14-doped ceric phosphate, a novel composite ion exchanger which exhibits superior ion exchange properties. The incorporation of AFLC-14 betwixt the layers of ceric phosphate aims to strengthen the material’s thermal property, adsorption capacity, and disinfectant properties along with ion-exchange capabilities. The reported material underwent extensive physico-chemical characterisation using various analytical techniques, including FTIR, UV-vis spectrophotometry, XRD analysis, SEM imaging, TGA/DTA studies and elemental investigation. Additionally, ion-exchange characteristics were investigated, encompassing ion-exchange capacity, concentration dependency, thermal property, adsorption studies for alkaline earth and heavy/transition metal ions, and binary separation experiments. Our findings reveal that the incorporation of a DLC betwixt ceric phosphate matrix significantly improves the exchanger’s overall performance. Notably, the hybrid ion exchanger exhibits remarkable selectivity for mercury (II) ions, indicating its potential for selective ion removal applications in environmental remediation and water purification processes.

Hydrogel ionic diode with ultra-high rectification ratio for ionic circuit

Organisms transmit signals through complex ion systems, and signal transmission in artificial electronic circuits mainly depends on electrons. The development of rectifier ionic diode, grounded in ion transport principles, paves ways for artificial bionic circuit and seamless human–machine interaction. Among its pivotal performance indicators, the ion rectification ratio (IR) is of primal importance, facilitating enhanced ion selectivity, bolstered energy efficiency, and robust, high-performing operations. Here, hybrid hydrogel ionic diode based on chitosan (CS) doped with 5,10,15,20-tetrakis(4-aminophenyl)-21,23H-porphyrin (TAPP)/sodium polyacrylate (PAAs) doped with 5,10,15,20-tetrakis(4-sulfonatophenyl)-21,23H-porphyrin (TPPS) was constructed to achieve a high IR. This design achieved a remarkable feat by elevating the IR from 5.57 to an unprecedented 158.64, surpassing previous benchmarks set by hydrogel-based ionic diodes. Moreover, this study emphasizes the selectivity of polyelectrolytes to different porphyrin structures, as well as the charge properties of the groups carried by porphyrins. It delves into the fundamental mechanisms underpinning their interactions, providing a comprehensive understanding of their behavior. In order to demonstrate the excellent rectification characteristics and practical applicability of the CS-TAPP/PAAs-TPPS ionic diode, we successfully integrated it into the full-wave rectification and logic gate system. The full-wave rectifier can convert a 1.2 V sinusoidal voltage input to a 0.8 V full-wave voltage output. This shows the advantages of high IR ionic diodes. It offers insights for the development of high-performance hydrogel-based ionic diodes, and promotes the advancement of novel biomimetic devices and ionic circuits.

Enhanced Vanadium Redox Flow Battery Performance with New Amphoteric Ion Exchange Membranes.

Vanadium redox flow batteries (VRFBs) depend on the separator membrane for their efficiency and cycle life. Herein, two amphoteric ion exchange membranes are synthesized, based on sulfonic acid group-grafted poly(p-terphenyl piperidinium), for VRFBs. Using ether-free poly(p-terphenyl piperidine) (PTP) as the polymer matrix, and sodium 2-bromoethanesulphonate (ES) and 1,4-butane sultone (BS) as grafting agents, We achieve quaternization of PTP through an environmentally friendly process without alkaline catalysts. PTP-ES and PTP-BS membranes exhibit low area resistance, high H+ permeability, and significantly reduced vanadium ion permeability, leading to exceptional ion selectivity, which is 3.06×106Smincm-3 and 4.34×106Smincm-3, respectively,three orders of magnitude higher than that of Nafion115 (0.27×104Smincm-3). The VRFB with PTP-BS achieves a self-discharge duration of 190h, compared to 86h for Nafion 115. Additionally, under current densities of 40-160mAcm-2, PTP-BS shows coulombic efficiencies of 98.1-99.1% and energy efficiencies of 92.0-82.1%, outperforming Nafion 115. The VRFB with PTP-BS also demonstrates excellent cycle stability and discharge capacity retention over 300 cycles at 100mAcm-2. Therefore, the amphoteric PTP-BS membrane shows remarkable performance, offering significant potential for VRFB applications.

Pd2+-coordinated polybenzimidazole membranes with fast and selective ion transport for alkaline aqueous organic redox flow battery

The advancement of aqueous organic redox-flow batteries (AORFBs) is impeded by the lack of efficient, low-resistance, and highly selective ion-conducting membranes (ICMs). Although polybenzimidazole (PBI) is one of the most promising low-cost non-fluorinated ion-conducting membranes, it remains challenging to design stable PBI membranes with fast and selective ion transport channels. Here, we engineer the chemical structure of PBI and regulate the ion transport channels to prepare highly conductive and selective Pd2+-coordinated membranes with grafting and crosslinking structures. In this design, the positively charged quaternary ammonium groups on the side chain improve the conductivity of OH−, while the continuous cross-linked network formed by ionic bonds between quaternary ammonium groups and deprotonated imidazole groups significantly enhances the membrane's mechanical properties. Furthermore, the coordination of Pd2+ with PBI and plasticization of PBI chain by trifluoroacetate anions expand the molecular free volume while inducing local contraction. Consequently, the QPBI-xPBI-Pd membrane enables fast charge carrier transport and suppresses the diffusion of active substances. The AORFBs assembled with the QPBI-10PBI-Pd membranes exhibit high coulombic efficiency (99.6 %) and energy efficiency (77.67 %) at 100 mA cm−2, maintaining excellent stability for 500 cycles. This study provides an innovative strategy to design high-performance PBI membranes for RFBs, thereby advancing the viability of RFBs in the realm of large-scale energy storage technologies.

Modular Customized Biomimetic Nanofluidic Diode for Tunable Asymmetric Ion Transport.

Artificial ion diodes, inspired by biological ion channels, have made significant contributions to the fields of physics, chemistry, and biology. However, constructing asymmetric sub-nanofluidic membranes that simultaneously meet the requirements of easy fabrication, high ion transport efficiency, and tunable ion transport remains a challenge. Here, a direct and flexible in situ staged host-guest self-assembly strategy is employed to fabricate ion diode membranes capable of achieving zonal regulation. Coupling the interfacial polymerization process with a host-guest assembly strategy, it is possible to easily manipulate the type, order, thickness, and charge density of each module by introducing two oppositely charged modules in stages. This method enables the tuning of ion transport behavior over a wide range salinity, as well as responsive to varying pH levels. To verify the potential of controllable diode membranes for application, two ion diode membranes with different ion selectivity and high charge density are coupled in a reverse electrodialysis device. This resulted in an output power density of 63.7W m-2 at 50-fold NaCl concentration gradient, which is 12 times higher than commercial standards. This approach shows potential for expanding the variety of materials that are appropriate for microelectronic power generation devices, desalination, and biosensing.

Polyfunctional and Multisensory Bio-Ionoelastomers Enabled by Covalent Adaptive Networks With Hierarchically Dynamic Bonding.

Developing versatile ionoelastomers, the alternatives to hydrogels and ionogels, will boost the advancement of high-performance ionotronic devices. However, meeting the requirements of bio-derivation, high toughness, high stretchability, autonomous self-healing ability, high ionic conductivity, reprocessing, and favorable recyclability in a single ionoelastomer remains a challenging endeavor. Herein, a dynamic covalent and supramolecular design, lipoic acid (LA)-based dynamic covalent ionoelastomer (DCIE), is proposed via melt building covalent adaptive networks with hierarchically dynamic bonding (CAN-HDB), wherein lithium bonds aid in the dissociation of ions and the integration of dynamic disulfide metathesis, lithium bonds, and binary hydrogen bonds enhances the mechanical performances, self-healing capability, reprocessing, and recyclability. Therefore, the trade-off among mechanical versatility, ionic conductivity, self-healing capability, reprocessing, and recyclability is successfully handled. The obtained DCIE demonstrates remarkable stretchability (1011.7%), high toughness (3877kJm-3), high ionic conductivity (3.94×10-4Sm-1), outstanding self-healing capability, reprocessing for 3D printing, and desirable recyclability. Significantly, the selective ion transport endows the DCIE with multisensory feature capable of generating continuous electrical signals for high-quality sensations towards temperature, humidity, and strain. Coupled with the straightforward methodology, abundant availability of LA and HPC, as well as multifunction, the DCIEs present new concept of advanced ionic conductors for developing soft ionotronics.

Enhanced Mg2+/Li+ separation performance of polyelectrolyte multilayers nanofiltration membranes modified by trimethylamine N-oxide zwitterions

Polyelectrolyte multilayer nanofiltration (NF) membranes have garnered attention for their potential in Mg2⁺/Li⁺ separation applications, particularly due to their positively charged nature and the versatility offered by the layer-by-layer (LbL) self-assembly technique. However, these membranes often face significant challenges in balancing high ion selectivity with adequate water permeability. Addressing these limitations, this study introduces a novel approach by grafting a zwitterionic material, trimethylamine N-oxide (TMAO), onto polyacrylamide hydrochloride (PAH), creating a new cationic polyelectrolyte (TPAH). By cyclically depositing sodium polystyrene sulfonate (PSS) and TPAH onto a polyethersulfone (PES) substrate, we have successfully fabricated a high-performance NF membrane (i.e., (PSS/TPAH)n) that demonstrates enhanced Mg2⁺/Li⁺ selectivity with high water permeability. TMAO features shorter distances between its positive and negative charge groups and smaller dipoles compared to other zwitterions. These characteristics enhance its ability to resistance to salt effects and transfer charge from amine oxide to water molecule. Furthermore, the grafting of TMAO enhanced the hydrophilicity of TPAH and regulated the charge distribution of the polyelectrolyte. Compared to the control membrane (PSS/PAH)n, the optimized membrane (PSS/TPAH)n showed reduced rejection of LiCl from 60.9 ± 2.7 % to 35.9 ± 1.5 %, while that of MgCl2 increased from 94.3 ± 1.6 % to 96.1 ± 0.7 %. Additionally, water permeability improved from 9.2 ± 0.9 LMH/bar to 16.1 ± 0.5 LMH/bar. Notably, the membranes exhibited an improved selectivity for Mg2+/Li+ ions in synthetic saline, reaching a maximum of 30.5 ± 1.5, highlighting its potential for practical separation applications. Density functional theory simulations indicate that TMAO not only enhances Donnan effect and intensify ion dehydration of polyelectrolyte multilayers NF membranes but also increases their hydrophilicity. In general, these polyelectrolyte multilayers NF membranes exhibit considerable promise for multiple applications, such as seawater pretreatment, groundwater softening and lithium extraction.

Capping-grafting synergistic strategy for the preparation of high-performance Mg2+/Li+ separation nanofiltration membranes

Polyethyleneimine (PEI) nanofiltration (NF) membranes demonstrate remarkable effectiveness in separating lithium and magnesium from salt lakes, attributed to their positively charged surface. However, undesired permeability resulting from densely cross-linked structures limits the further industrial applications and development of PEI NF membranes. Herein, a high permeable Mg2+/Li+ separation NF membrane with stronger internal and surface positive charge was fabricated using 1-Aminopyridinium iodide (1-AI) for the first time through a capping-grafting synergistic strategy. On the one hand, the monoamine character of 1-AI can realize the end-capping (EC) effect on the acyl chloride monomers in the interfacial polymerization process, which can contribute to a reduction in the cross-linking degree of the membrane. On the other hand, 1-AI with a quaternary ammonium group as the surface grafting (SG) agent can significantly improve the surface positive charge property of the membrane. The synergistic effect of capping-grafting not only rendered the membrane looser and effectively enhanced the surface positive charge of the membrane, but also increased the depth of grafting modification, leading to a notable rise in the internal positive charge of the membrane. After the capping-grafting treatment, the water permeance of the PEI-EC/SG membrane reached 19.8 L·m−2·h−1·bar−1, approximately 4.5 times that of the original PEI membrane, while keeping an ideal MgCl2 rejection rate (97.1%) and exhibiting good Mg2+/Li+ selectivity (27.7). This work demonstrates the advantages of the capping-grafting synergistic method in regulating membrane structure for improving permeability as well as the surface and internal charge properties of the membrane for enhancing ion selectivity.