Li+/Mg2+ separation by membrane separation: The role of the compensatory effect

Molecular Design of Positively Charged 3D Covalent-Organic Framework Membranes for Li+/Mg2+ Separation

3D covalent-organic framework (3D COF) membranes have unique features such as smaller pore sizes and more interconnected networks compared with 2D COF counterparts. However, the complicated and unmanageable fabrication hinders their rapid development. Molecular simulation, which can efficiently explore the structure-performance relationship of membranes, holds great promise in accelerating the development of 3D COF membranes. In this study, a series of 3D-COF membranes (TFPM-Pa-X) is designed with different charge densities (fully charged, partially charged, and neutral) and interpenetration numbers (2-, 3-, 4-, and 5-fold), subsequently investigate their contributions to Li+/Mg2+ separation through molecular simulation. Membrane morphology and pore size are found to strongly depend on the charged density and interpenetration number. The pore size and Cl- ion density play a crucial role in governing membrane separation performance. TFPM-Pa-X membrane with a smaller interpenetration number and a higher charge density promotes Li+/Mg2+ separation. The fully charged 2-fold interpenetrated membrane has superior performance in breaking the trade-off between the flux of Li+ (JLi +) and the selectivity of Li+ over Mg2+ (SLi + /Mg 2+). This study may facilitate the rational design of new 3D COF membranes for high-performance ion separation.

Regulation of Ion Binding Sites in Covalent Organic Framework Membranes for Enhanced Selectivity under High Ionic Competition

The strategic spatial positioning of ion affinity sites within biological ion channels and their cooperative binding with the targeted ions are pivotal for enhancing ion recognition and ensuring exceptional selectivity in high ionic competition scenarios. However, the application of these principles to artificial ion channels remains largely unexplored. Herein, we present a series of covalent organic framework (COF) membranes, engineered with oxygen functional groups aligned along the rims of oriented COF pore channels of varying sizes to achieve a precise spatial arrangement of ion affinity sites. A notable COF membrane, featuring subnanometer pores decorated alternately with carbonyl and amide groups, demonstrated outstanding selectivity, achieving a Li/Mg selectivity ratio of 513 under equal mole and electrodialysis conditions. Impressively, as the Mg/Li ratio in the source solution increased to 16.6, the selectivity ratio rose to 833, significantly exceeding the reductions typically seen in conventional selective electrodialysis and nanofiltration methods. Both simulation and experimental analyses indicate that this exceptional selectivity stems from the cooperative binding between the oxygen functional groups and Li+ ions within the confined nanochannels, facilitating the preferential transport of Li+ ions. These findings provide a promising approach for designing selective ion extraction systems that function effectively in highly competitive environments.

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.

Constructing coral reef-like imprinted structure on molecularly imprinted nanocomposite membranes based on nanospheres with hydrophilic multicores for selective separation of acteoside

Molecularly imprinted nanocomposite membranes (MINMs) have shown great superiority in selective separation of acteoside (ACT) from phenylethanoid glycosides in Cistanche tubulosa. Herein, ACT-based MINMs (A-MINMs) with coral reef-like imprinted structure were proposed and developed for specifically separating ACT molecules. The nanospheres with hydrophilic multicores (NHMs) were introduced into polyvinylidene fluoride (PVDF) powders to obtain NHMs@PVDF membranes by a phase inversion method. Subsequently, the designed coral reef-like imprinted structure was constructed on NHMs@PVDF membrane-based surface. The A-MINMs with coral reef-like imprinted structure had dendritic and porous properties, which helped to form abundant ACT-imprinted cavities and sites of A-MINMs. In addition, the hydrophilic multicores and void space of NHMs together formed the affinity capture cages for hydrophilic ACT, enhancing rebinding capacity and permselectivity of A-MINMs. Therefore, A-MINMs with coral reef-like imprinted structure exhibited the high rebinding capacity (110.95 mg/g), rebinding selectivity of 5.15 and outstanding permselectivity of 10.04 toward ACT molecules. Moreover, the designed strategy of A-MHIMs with coral reef-like imprinted structure provides a new feasible method for permselectivity separation of other bioactive components.

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.

Enhancing ion selectivity by tuning solvation abilities of covalent-organic-framework membranes

Understanding the molecular-level mechanisms involved in transmembrane ion selectivity is essential for optimizing membrane separation performance. In this study, we reveal our observations regarding the transmembrane behavior of Li+ and Mg2+ ions as a response to the changing pore solvation abilities of the covalent-organic-framework (COF) membranes. These abilities were manipulated by adjusting the lengths of the oligoether segments attached to the pore channels. Through comparative experiments, we were able to unravel the relationships between pore solvation ability and various ion transport properties, such as partitioning, conduction, and selectivity. We also emphasize the significance of the competition between Li+ and Mg2+ with the solvating segments in modulating selectivity. We found that increasing the length of the oligoether chain facilitated ion transport; however, it was the COF membrane with oligoether chains containing two ethylene oxide units that exhibited the most pronounced discrepancy in transmembrane energy barrier between Li+ and Mg2+, resulting in the highest separation factor among all the evaluated membranes. Remarkably, under electro-driven binary-salt conditions, this specific COF membrane achieved an exceptional Li+/Mg2+ selectivity of up to 1352, making it one of the most effective membranes available for Li+/Mg2+ separation. The insights gained from this study significantly contribute to advancing our understanding of selective ion transport within confined nanospaces and provide valuable design principles for developing highly selective COF membranes.

Function-Led Design of Covalent-Organic-Framework Membranes for Precise Ion Separation

Insufficient access to clean water and resources has emerged as one of the most pressing issues affecting people globally. Membrane-based ion separation has become a focal point of research for the generation of fresh water and the extraction of energy elements. This Review encapsulates recent advancements in the selective ion transport of covalent organic framework (COF) membranes, accomplished by strategically pairing diverse monomers to create membranes with various pore sizes and environments for specific purposes. We first discuss the merits of using COF materials as a basis for fabricating membranes for ion separation. We then explore the development of COF membranes in areas such as desalination, acid recovery, and energy element extraction, with a particular emphasis on the fundamental principles of membrane design. Lastly, we address both theoretical and practical challenges, as well as potential opportunities in the targeted design of ion-selective membranes. The goal of this Review is to stimulate future investigative efforts in this field, which is of significant scientific and strategic importance.

Comparison of the Mg2+-Li+ Separation of Different Nanofiltration Membranes

Nanofiltration application for the separation of Mg2+-Li+ from salt-lake brines was attempted in the present work. Four different nanofiltration membranes identified in the manuscript as DL, DK, NF-270, and NF-90 were used to treat salt brine with a magnesium to lithium ratio (MLR) of 61, additionally contaminated by the other ions such as Na+, K+, Ca2+, etc. The effect of the dilution factor, operating pressure, circulation rate, and feed pH were assessed to identify the optimal operating conditions for each membrane based on the retention efficiency of each ion. The results showed an insignificant effect of Ca2+ on the retention performance of Mg2+-Li+. Na+ and K+ had a smaller hydration radius and larger diffusion coefficient, which competed with Li+ and altered the separation of Mg2+-Li+. Under the optimal conditions (dilution factor: 40; operating pressure: 1.2 MPa; circulation flow rate: 500 L/h; pH: 7), the retention efficiency of lithium was as low as 5.17%, separation factor (SF) was as low as 0.074, and the MLR in the permeate reduced to 0.088.

Lithium Pyrene Squarate Covalent Organic Frameworks for Efficient Lithium and Magnesium Separation from Salt Water

The increasing pressure for lithium resources from the electric vehicle and nuclear energy industries means that new technologies to separate Mg²⁺ from Li⁺ from salt water are in demand. To address this need, we fabricated lithium pyrene squarate covalent organic frameworks (Li-SQCOFs) to separate Mg²⁺/Li⁺ mixtures from salt water. We optimized the effect of the electrolyte and the amount of the adsorbent and then carried out a kinetics study on the adsorbent recovery at various pH levels using both batch and continuous flow adsorption methods. Li-SQCOF was found to have excellent selectivity for solutions containing a mixture of Mg²⁺/Li⁺ ions. This work represents a unique path for the separation of Mg²⁺/Li⁺ through direct adsorption using a covalent organic framework (COF). The COF-supported ultrafiltration bed made in this study gave a Mg²⁺ separation flux of 60.5 h^-1 m^-2.

Advanced Covalent Organic Framework-Based Membranes for Recovery of Ionic Resources

Membrane technology has shown a viable potential in conversion of liquid-waste or high-salt streams to fresh waters and resources. However, the non-adjustability pore size of traditional membranes limits the application of ion capture due to their low selectivity for target ions. Recently, covalent organic frameworks (COFs) have become a promising candidate for construction of advanced ion separation membranes for ion resource recovery due to their low density, large surface area, tunable channel structure, and tailored functionality. This tutorial review aims to analyze and summarize the progress in understanding ion capture mechanisms, preparation processes, and applications of COF-based membranes. First, the design principles for target ion selectivity are illustrated in terms of theoretical simulation of ions transport in COFs, and key properties for ion selectivity of COFs and COF-based membranes. Next, the fabrication methods of diverse COF-based membranes are classified into pure COF membranes, COF continuous membranes, and COF mixed matrix membranes. Finally, current applications of COF-based membranes are highlighted: desalination, extraction, removal of toxic metal ions, radionuclides and lithium, and acid recovery. This review presents promising approaches for design, preparation, and application of COF-based membranes in ion selectivity for recovery of ionic resources.

Molecular modeling of thin-film nanocomposite membranes for reverse osmosis water desalination

The scarcity of freshwater resources is a major global challenge causedby population and economic growth. Water desalination using a reverse osmosis (RO) membrane is a promising technology to supply potable water from seawater and brackish water. The advancement of RO desalination highly depends on new membrane materials. Currently, the RO technology mainly relies on polyamide thin-film composite (TFC) membranes, which suffer from several drawbacks (e.g., low water permeability, permeability-selectivity tradeoff, and low fouling resistance) that hamper their real-world applications. Nanoscale fillers with specific characteristics can be used to improve the properties of TFC membranes. Embedding nanofillers into TFC membranes using interfacial polymerization allows the creation of thin-film nanocomposite (TFNC) membranes, and has become an emerging strategy in the fabrication of high-performance membranes for advanced RO water desalination. To achieve optimal design, it is indispensable to search for reliable methods that can provide fast and accurate predictions of the structural and transport properties of the TFNC membranes. However, molecular understanding of permeability-selectivity characteristics of nanofillers remains limited, partially due to the challenges in experimentally exploring microscopic behaviors of water and salt ions in confinement. Molecular modeling and simulations can fill this gap by generating molecular-level insights into the effects of nanofillers' characteristics (e.g., shape, size, surface chemistry, and density) on water permeability and ion selectivity. In this review, we summarize molecular simulations of a diverse range of nanofillers including nanotubes (carbon nanotubes, boron nitride nanotubes, and aquaporin-mimicking nanochannels) and nanosheets (graphene, graphene oxide, boron nitride sheets, molybdenum disulfide, metal and covalent organic frameworks) for water desalination applications. These simulations reveal that water permeability and salt rejection, as the major factors determining the desalination performance of TFNC membranes, significantly depend on the size, topology, density, and chemical modifications of the nanofillers. Identifying their influences and the physicochemical processes behind, via molecular modeling, is expected to yield important insights for the fabrication and optimization of the next generation high-performance TFNC membranes for RO water desalination.

Ultrathin Covalent Organic Framework Membranes Prepared by Rapid Electrophoretic Deposition

Covalent organic frameworks (COFs) are a disruptive material platform for various novel applications including nanofiltration for water purification due to their excellent physicochemical features. Nevertheless, the currently available approaches for preparing COF membranes need stringent synthesis conditions, prolonged fabrication time, and tedious post-processing, leading to poor productivity. Herein, a simple and efficient layer-by-layer stacking assembly strategy is developed based on electrophoretic deposition (EPD) to rapidly generate ionic COF membranes due to the uniform driving force for nanosheet assembly. A new two-cell EPD design avoids the usual EPD problems such as bubbles and acidic/alkaline microenvironments in the near-electrode region in aqueous EPD processes. Ultrathin COF membranes with homogenous structures can be produced within several minutes. Consequently, the prepared COF membranes exhibit outstanding permselectivity and possess good stability and anti-pressure ability due to their uniform architecture and unique chemical composition.

Mechanism Understanding of Li-ion Separation Using A Perovskite-Based Membrane

Lithium ions play a crucial role in the energy storage industry. Finding suitable lithium-ion-conductive membranes is one of the important issues of energy storage studies. Hence, a perovskite-based membrane, Lithium Lanthanum Titanate (LLTO), was innovatively implemented in the presence and absence of solvents to precisely understand the mechanism of lithium ion separation. The ion-selective membrane's mechanism and the perovskite-based membrane's efficiency were investigated using Molecular Dynamic (MD) simulation. The results specified that the change in the ambient condition, pH, and temperature led to a shift in LLTO pore sizes. Based on the results, pH plays an undeniable role in facilitating lithium ion transmission through the membrane. It is noticeable that the hydrogen bond interaction between the ions and membrane led to an expanding pore size, from (1.07 Å) to (1.18-1.20 Å), successfully enriching lithium from seawater. However, this value in the absence of the solvent would have been 1.1 Å at 50 °C. It was found that increasing the temperature slightly impacted lithium extraction. The charge analysis exhibited that the trapping energies applied by the membrane to the first three ions (Li⁺, K⁺, and Na⁺) were more than the ions' hydration energies. Therefore, Li⁺, K⁺, and Na⁺ were fully dehydrated, whereas Mg²⁺ was partially dehydrated and could not pass through the membrane. Evaluating the membrane window diameter, and the combined effect of the three key parameters (barrier energy, hydration energy, and binding energy) illustrates that the required energy to transport Li ions through the membrane is higher than that for other monovalent cations.

Study on Spacing Regulation and Separation Performance of Nanofiltration Membranes of GO

Graphene oxide (GO) membranes have attracted significant attention in the field of water processing in recent years due to their unique characteristics. However, few reports focus on both membrane stability and the “trade-off” effect. In this study, a series of aliphatic diamines (1, 2-ethylenediamine, 1, 4-butanediamine, and 1, 6-hexamethylenediamine) of covalent crosslinked GO were used to prepare diamine-modified nanofiltration membranes, BPPO/AX-GO, with adjustable layer spacing using the vacuum extraction−filtration method. Moreover, Ax-GO-modified nanofiltration membranes modified with adipose diamine had higher layer spacing, lower mass-transfer resistance, and better stability. When the number of carbon atoms was 5, the best layer spacing was reached, and when the number of carbon atoms was greater than 4, the modified membrane nanosheets more easily accumulated. With the increase in layer spacing, the water flux of the composite film increased to 26.27 L/m2·h·bar. Meanwhile, adipose diamine crosslinking significantly improved the stability of GO films. The interception sequence of different valence salts in the composite membrane was NaCl > Na2SO4 > MgSO4, and the rejection rate of bivalent salts was higher than that of monovalent salts. The results can provide some experimental basis and research ideas for overcoming the “trade-off” effect of a lamellar GO membrane.

A fundamental study on selective extraction of Li+ with dibenzo-14-crown-4 ether: Toward new technology development for lithium recovery from brines

The present study has proposed a selective Li⁺ extraction process using a novel extractant of dibenzo-14-crown-4 ether functionalized with an alkyl C16 chain (DB14C4-C16) synthesized based on the ion imprinting technology (IIT). Theoretical analysis of the possible complexes formed by DB14C4-C16 with Li⁺ and the competing ions of Na⁺, K⁺, Ca²⁺ and Mg²⁺ was performed through density functional theory (DFT) modeling. The Gibbs free energy change of the complexes of metal ions with DB14C4-C16 and water molecules were calculated to be -125.81 and -166.01 kJ/mol for lithium, -55.73 and -117.77 kJ/mol for sodium, and -196.02 and -291.52 kJ/mol for magnesium, respectively. Furthermore, the solvent extraction experiments were carried out in both single Li⁺ and multi-ions containing solutions, and the results delivered a good selectivity of DB14C4-C16 towards Li⁺ over the competing ions, showing separation coefficients of 68.09 for Ca²⁺-Li⁺, 24.53 for K⁺-Li⁺, 16.32 for Na⁺-Li⁺, and 3.99 for Mg²⁺-Li⁺ under the optimal conditions. The experimental results are generally in agreement with the theoretical calculations.