Monovalent Selectivity Research Articles

Sub-nanoporous polyimide membrane with selective and fast K+ transport

Biological ion channels have excellent monovalent ion separation performance, and it is of great significance to develop artificial membrane materials with monovalent ion selectivity based on their mechanism. The latent-track method is a new technology to prepare sub-nanometer channels on polymer membranes by swift heavy ion irradiation. This method has been widely applied in the study of mono/divalent ion separation membranes, but its application in monovalent ion separation membranes still needs to be explored. In this work, polyimide (PI) film was irradiated by swift heavy ion, then water rinsed, and the sub-nanoporous PI membrane with effective channel size in the range of 0.30 and 0.66 nm was fabricated, then its electrical and transport properties were studied. Due to the smaller hydrated diameter and lower hydration energy of K+, the membrane exhibited good selectivity for K+. The selectivity ratio of K+/Li+ is 3.4–3.9, and the maximum K+ flux can reach 0.44 mol m−2 h−1 at 55℃. The sub-nanoporous PI membranes are expected to be applied in salt lake lithium extraction, fine processing of potassium and lithium, and other relevant scenarios.

Leveraging an innovative process integration using highly acid-immune blend membranes for enhanced acid recovery and selective salt separation

We investigate the efficiency and advantage of the integrated diffusion dialysis-electrodialysis (DD-ED) system over the DD process for acid recovery with highly acidic wastewater. The investigation uses various acid (HCl/H2SO4) /salt mixtures encompassing iron, copper and aluminium salts (FeCl2, CuCl2, AlCl3, and FeSO4) as simulated feed. The membrane is prepared using blend mixture of polyvinylidene fluoride (PVDF) and polymethyl methacrylate-co-poly chloromethyl styrene copolymer (PMMA-co-PCMSt) solution by non-solvent induced phase inversion on a nonwoven fabric followed by treatment with different tertiary amines. Representative AEM-Q3 membrane prepared by the post modification of blend membrane with a mixture of N, N, N’, N’- tetramethyl diamino hexane (TMDAH) and trimethyl amine (TMA) (1: 1 w/w %) exhibited the best-balanced performance. The membrane recovered a 84.2% AR (%) of acid with 96.28% purity (%) at a flux (JH+) of 7.08 × 10-4 mol cm-2 h-1 from 3 M HCl and 0.2M FeCl2 mixture making it suitable for further studies. A comprehensive comparative study drawn between the DD and integrated DD-ED showed that there was a 2.3 fold increment in acid recovery for the integrated process over the DD process (for the HCl-FeCl2 system). The membranes have also shown good monovalent (Cl-) and bivalent (SO42-) anion selectivity of 7.8. However, the selectivity was further improved to 12.28 by modification of AEM-Q3 membrane, which is double side cast (referred as AEM-Q3/b). The synthetic strategy of the membrane is easy and scalable and has potential for both acid recovery and selective salt separation by ED process.

Selective mono/divalent ion separation in bifunctional ionomeric capacitive deionization incorporated with counter-charge ionomer layer

In this study, a carbon-based bifunctional ionomeric capacitive deionization (BICDI) system was successfully developed by applying a counter-charge ionomer layer to the BICDI electrode without the need of a monovalent ion exchange membrane (M-IEM) for the selective separation of mono/divalent ions. The counter-charge ionomer layer facilitates the selective separation of mono/divalent ions through a combination of mechanisms involving electrostatic repulsion, steric hindrance, and hydrophobic interactions. Meanwhile, the BICDI electrode matrix serves as an ion transport channel, enabling seamless ion transport and improving the electrosorption capacity of the electrode. SEM and EDS analyses have confirmed the presence of a counter-charge ionomer layer encapsulating the BICDI electrode. Notably, BICDI-C cell with counter-charge ionomer layer exhibits higher Li+ ion adsorption capacity and selectivity (321.6 μmol/g, βMg2+Li+=6.6) compared to commercial monovalent cation exchange membrane CDI (MCDI-C) cell (206.4 μmol/g, βMg2+Li+=1.4). The selectivity of the BICDI-C cell for mono/divalent ions can be optimized by adjusting various testing parameters, including flow rate, voltage, etc. More importantly, this versatile method is not only applicable for the separation of monovalent cation, but can also enhance the monovalent anion adsorption capacity and selectivity of BICDI-A (43.6 μmol/g, βSO42-Cl-=7.3) cell by incorporating a counter-charge ionomer layer onto the BICDI cathode. This approach allows for the efficient separation of both cationic and anionic species, broadening the range of potential applications for BICDI technology.

Lithium extraction from salt lake brine by four-stage ion-distillation of flow electrode capacitive deionization

Herein, a novel, highly efficient, and low-energy consumption four-stage ion-distillation of FCDI (ID-FCDI) device was developed that combined four commercial monovalent selective membranes with four flow electrode channels for high selectivity to extract Li+ ions from salt lake. It exhibited excellent separation factor (SMg2+Li+ = 11247.27), high enrichment ratio (4.95 times), super purity of Li+ ion solution (99.97 %), and low molar energy consumption (Em = 0.21 kWh mol−1) at mass ratio of Mg2+/Li+ = 1:1. The mathematical model calculation revealed that the excellent selective separation effectiveness of the as-proposed ID-FCDI system is due to the unique property of the monovalent selectivity membranes, in which the transmembrane rate of lithium ions is ten times that of magnesium ions under the same condition. Furthermore, the separation mechanism of the as-proposed ID-FCDI device for Li+/Mg2+ ions was determined by the high electrosorption capacity of the flow electrode for Li+ ions (1.14 times higher than Mg2+ ions), low diffusion resistance (1.413 Ω), and high diffusion coefficient of Li+ ions (2.83 times faster than Mg2+ ions) by electrochemical measurement. On this basis, 3.92 times of lithium was successfully enriched in the natural salt lake brine of Golmud (mass ratio of Mg2+/Li+ = 79.29), and the separation factor was 6307.17 with a 99.64 % purity of Li+ ion solution and an Em of 0.20 kWh mol−1. Finally, the Li2CO3 product (99.66 %) was precipitated via the reaction between Na2CO3 and the enriched Li+ ion solution, consequently fulfilling the battery-level application of the industrial purity requirements. These findings highlight that this device is promising and profitable for lithium extraction from salt lake in industrial production.

Unlocking Direct Lithium Extraction in Harsh Conditions through Thiol-Functionalized Metal-Organic Framework Subnanofluidic Membranes.

Metal-organic framework (MOF) membranes with high ion selectivity are highly desirable for direct lithium-ion (Li+) separation from industrial brines. However, very few MOF membranes can efficiently separate Li+ from brines of high Mg2+/Li+ concentration ratios and keep stable in ultrahigh Mg2+-concentrated brines. This work reports a type of MOF-channel membranes (MOFCMs) by growing UiO-66-(SH)2 into the nanochannels of polymer substrates to improve the efficiency of MOF membranes for challenging Li+ extraction. The resulting membranes demonstrate excellent monovalent metal ion selectivity over divalent metal ions, with Li+/Mg2+ selectivity up to 103 since Mg2+ should overcome a higher energy barrier than Li+ when transported through the MOF pores, as confirmed by molecular dynamics simulations. Under dual-ion diffusion, as the Mg2+/Li+ mole ratio of the feed solution increases from 0.2 to 30, the membrane Li+/Mg2+ selectivity decreases from 1516 to 19, corresponding to the purity of lithium products between 99.9 and 95.0%. Further research on multi-ion diffusion that involves Mg2+ and three monovalent metal ions (K+, Na+, and Li+, referred to as M+) in the feed solutions shows a significant improvement in Li+/Mg2+ separation efficiency. The Li+/Mg2+ selectivity can go up to 1114 when the Mg2+/M+ molar concentration ratio is 1:1, and it remains at 19 when the ratio is 30:1. The membrane selectivity is also stable for 30 days in a highly concentrated solution with a high Mg2+/Li+ concentration ratio. These results indicate the feasibility of the MOFCMs for direct lithium extraction from brines with Mg2+ concentrations up to 3.5 M. This study provides an alternative strategy for designing efficient MOF membranes in extracting valuable minerals in the future.

Toward Selective Transport of Monovalent Metal Ions with High Permeability Based on Crown Ether-Encapsulated Metal-Organic Framework Sub-Nanochannels.

Achieving selective transport of monovalent metal ions with high precision and permeability analogues to biological protein ion channels has long been explored for fundamental research and various applications, such as ion sieving, mineral extraction, and energy harvesting and conversion. However, it still remains a significant challenge to construct artificial nanofluidic devices to realize the trade-off effects between selective ion transportation and high ion permeability. In this work, we report a bioinspired functional micropipet with in situ growth of crown ether-encapsulated metal-organic frameworks (MOFs) inside the tip and realize selective transport of monovalent metal ions. The functional ion-selective micropipet with sub-nanochannels was constructed by the interfacial growth method with the formation of composite MOFs consisting of ZIF-8 and 15-crown-5. The resulting micropipet device exhibited obvious monovalent ion selectivity and high flux of Li+ due to the synergistic effects of size sieving in subnanoconfined space and specific coordination of 15-crown-5 toward Na+. The selectivity of Li+/Na+, Li+/K+, Li+/Ca2+, and Li+/Mg2+ with 15-crown-5@ZIF-8-functionalized micropipet reached 3.9, 5.2, 105.8, and 122.4, respectively, which had an obvious enhancement compared to that with ZIF-8. Notably, the ion flux of Li+ can reach up to 93.8 ± 3.6 mol h-1·m-2 that is much higher than previously reported values. Furthermore, the functional micropipet with 15-crown-5@ZIF-8 sub-nanochannels exhibited stable Li+ selectivity under various conditions, such as different ion concentrations, pH values, and mixed ion solutions. This work not only provides new opportunities for the development of MOF-based nanofluidic devices for selective ion transport but also facilitates the promising practical applications in lithium extraction from salt-like brines, sewage treatment, and other related aspects.

Harnessing ion resource recovery: Design of selective cation exchange membranes via a synergistic ionic control method

Effective recovery of valuable ion resources such as lithium is gaining increasing importance. Monovalent selective cation exchange membranes (MCEM) used in electrodialysis have shown great potential, but their development is hampered by the classical trade-offs related to membrane permeability, selectivity and energy efficiency. We reported a new route to design high performance MCEM, utilizing the synergy of ionic control (IC) through cation-π bonding of polydopamine (PDA) and alternating current (AC), which has not been reported before. A set of deliberately controlled membrane fabrication conditions were chosen, with different ionic control (i.e., no cation, K+, Li+) and with/without AC for systematic comparison. Combining with morphology & electrochemical measurements, the membrane with electro-assisted K+ control, i.e., PDAM-K+/AC, exhibited respective 72 % and 51 % higher Li+ and K+ transport rate, higher permselectivity of 9.54 (Li+/Mg2+) and 17.86 (K+/Mg2+), and 3.3-fold lower surface electrical resistance (SER) compared to other modified membranes (e.g., PDAM). Also, PDAM-K+/AC exhibited no sign of scaling, supported by its much increased limiting current density. The results supported the hypothesis of the synergistic effect between cation-π bonding and AC electro-deposition, which altered the polymerization dynamics and produced more ordered membrane structure. The control strategy showed a viable route to fabricate ion exchange membranes for imparting monovalent selectivity, reducing scaling and avoiding energy penalty.

UiO-66 membranes with confined naphthalene disulfonic acid for selective monovalent ion separation

The selective separation of monovalent ions remains a challenge because of the similar sub-nanometer sizes. Metal-organic frameworks (MOFs) are promising candidate membranes for ion selective separation based on the angstrom sized windows for size sieving. Here, to further enhance the monovalent ion selectivity, 1, 5-naphthalenedisulfonic acid tetrahydrate (NTDS) molecules with sulfonate groups are incorporated to offer a binding affinity, generating a UiO-66@NTDS membrane via the window-cavity structure confinement. In addition to the size sieving effect, the charge and affinity interaction can compensate the energy loss induced by the ion dehydration. Thus, the UiO-66@NTDS membranes exhibit elevated separation performance compared to the original UiO-66 membranes under the concentration gradient driven, accompanied by high monovalent cation permeation rates (i.e., 0.3–0.7 mol m−2 h−1) and mono-/divalent cation selectivities (i.e., ∼73 for K+/Mg2+, ∼57 for Na+/Mg2+, and ∼46 for Li+/Mg2+). This work provides guidelines for the development of efficient ion-selective MOF membranes via the combination of size sieving and interaction attraction.

Selective electrodialysis: Targeting nitrate over chloride using PVDF-based AEMs

The depletion of natural resources and the escalating environmental concerns associated with pollution necessitate innovative approaches for sustainable resource management. In this study, we investigated the selective separation of nitrate from chloride in electrodialysis (ED) using a recently introduced PVDF-based anion-exchange membrane (PVDF-50) and two commercially available membranes (ACS and AMX from Neosepta). Experimental ED data show that the membrane PVDF-50 presents a higher value of nitrate over chloride selectivity compared with the two commercial membranes, the highest reported in literature.However, recognizing that completely preventing the permeation of chloride ions into the concentrate stream was not feasible, we explored the potential of a system in which anion-exchange membranes presenting different monovalent selectivity were alternated. In this approach, referred to as nitrate-selectrodialysis (NO3-SED), the different selectivity of PVDF-50 and AMX membranes was leveraged to demonstrate the possibility of increasing nitrate concentration, while simultaneously reducing chloride levels in a stream. This approach proves highly advantageous in applications where mitigating chloride contamination is a significant concern while recovering nitrate.

Shearing Liquid-Crystalline MXene into Lamellar Membranes with Super-Aligned Nanochannels for Ion Sieving.

Ion-selective membranes are crucial in various chemical and physiological processes. Numerous studies have demonstrated progress in separating monovalent/multivalent ions, but efficient monovalent/monovalent ion sieving remains a great challenge due to their same valence and similar radii. Here, this work reports a two-dimensional (2D) MXene membrane with super-aligned slit-shaped nanochannels with ultrahigh monovalent ion selectivity. The MXene membrane is prepared by applying shear forces to a liquid-crystalline (LC) MXene dispersion, which is conducive to the highly-ordered stacking of the MXene nanosheets. The obtained LC MXene membrane (LCMM) exhibits ultrahigh selectivities toward Li+ /Na+ , Li+ /K+ , and Li+ /Rb+ separation (≈45, ≈49, and ≈59), combined with a fast Li+ transport with a permeation rate of ≈0.35 mol m-2 h-1 , outperforming the state-of-the-art membranes. Theoretical calculations indicate that in MXene nanochannels, the hydrated Li+ with a tetrahedral shape has the smallest diameter among the monovalent ions, contributing to the highest mobility. Besides, the weakest interaction is found between hydrated Li+ and MXene channels which also contributes to the ultrafast permeation of Li+ through the super-aligned MXene channels. This work demonstrates the capability of MXene membranes in monovalent ion separation, which also provides a facile and general strategy to fabricate lamellar membranes in a large scale.

Surface-Modified Pore-Filled Anion-Exchange Membranes for Efficient Energy Harvesting via Reverse Electrodialysis.

In this study, novel pore-filled anion-exchange membranes (PFAEMs) modified with polypyrrole (PPy) and reduced graphene oxide (rGO) were developed to improve the energy harvesting performance of reverse electrodialysis (RED). The surface-modified PFAEMs were fabricated by varying the contents of PPy and rGO through simple spin coating and chemical/thermal treatments. It was confirmed that the PPy and PPy/rGO layers introduced on the membrane surface did not significantly increase the electrical resistance of the membrane and could effectively control surface characteristics, such as structural tightness, hydrophilicity, and electrostatic repulsion. The PPy/rGO-modified PFAEM showed excellent monovalent ion selectivity, more than four times higher than that of the commercial membrane (AMX, Astom Corp., Tokyo, Japan). This means that the PPy/rGO layer can effectively reduce the permeation of multivalent ions with a high charge intensity and a relatively large hydration radius compared to monovalent ions. The results of evaluating the performance of the surface-modified PFAEMs by applying them to a RED cell revealed that the decrease in potential difference occurring in the membrane was reduced by effectively suppressing the uphill transport of multivalent ions. Consequently, the PPy/rGO-modified membrane exhibited a 5.43% higher power density than the AMX membrane.

The endowment of monovalent anion selectivity and antifouling property to cross-linked ion-exchange membranes by constructing amphoteric structure

Ion exchange membranes (IEMs) with high monovalent-ion selectivity show promising application in ion resource recovery. In this work, we report a kind of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO)-based on amphoteric ion exchange membranes (AIEMs) having cross-linked structure. Therein, four imidazolium-terminated aliphatic chains (2IM-nC, n = 3, 6, 9, and 12) with varied number of −CH2− were used as crosslinker for cross-linking membrane matrix. Small-angle X-ray scattering tests indicate that the ion cluster sizes of the four AIEMs are evaluated with values of 0.378, 0.384, 0.388 to 0.408 nm, which possibly render the enhanced mono-valent anion selectivity of as-prepared AIEMs. Tested in the electrodialysis (ED) filled with a mixture solution of 0.05 M NaCl + 0.05 M Na2SO4, at current densities of 2.5 mA·cm−2 and 5.0 mA·cm−2, the PSO42-Cl- values of cross-linked AIEM having longer alkyl chains are 20.41 and 22.51, respectively, which are higher than AIEM with shorter cross-linker alkyl chain under the same conditions (3.5). Due to the hydrophilic surface and hydrophilic/negatively charged zwitterionic surface of AIEM, act as useful barriers via physical blocking and electrostatic repulsion, respectively, the anti-fouling performance of IEMs is improved simultaneously. In view of the superior performance, it is believed that the structure–property relationship of AIEM developed here can serve as a guideline for advanced IEMs.

Numerical Modeling Unlocks Remarkable Ion Selectivity of Capacitive Deionization

Ion-selective water treatment is an important frontier in water research, as for many applications removing all ions indiscriminately leads to significant extra energy and downstream re-ionization costs. Capacitive deionization (CDI) is under intensive investigations for ion-selective treatment of polluted feedwaters [1]. CDI has the remarkable feature of being not only highly selective, but additionally dynamically tuneable to adjust, real-time, to varying feedwater composition and produced water targets [1]. However, we will show that to further develop CDI technologies to achieve desired separations, in feedwaters of several competing anions and cations, requires detailed numerical models. Such models couple ion transport theory to nanopore electrosorption, and often include pH dynamics. We here describe our recent work exploring the limits of ion-ion selectivity by capacitive deionization with inexpensive nanoporous carbon electrodes. We show how theory enabled us to achieve in the lab remarkable selectivity and a diverse set of separations, such as “perfect” divalent cation selectivity[2], monovalent ion selectivity[3], and removal of amphoteric pollutant species such as boric acid and nutrient species[4]. We show using strong-acid functionalized electrodes that the same two-electrode system can be used for either excellent divalent selectivity, or long-lasting monovalent ion selectivity, depending on cell operational parameters [2,3]. Our work furthers the argument that membraneless CDI, based on inexpensive and easily-scalable porous carbon electrodes, can address a wide variety of important applications in water treatment.

Polyaniline encapsulated sulphonated graphene oxide based cation exchange membrane for electrodialytic separation of mono- and bi-valent ions

Polyaniline encapsulated sulphonated graphene oxide (SGO-PANI) was prepared by ion exchange process. The SGO-PANI was incorporated in sulphonated poly(ether ether ketone) (SPEEK) matrix to fabricate the mono-valent selective cation exchange membrane. The structural morphology of SGO-PANI was studied by FTIR, XPS, SEM and XRD spectra. Resultant SPEEK@SGO-PANI membrane exhibited excellent stabilities and physicochemical and electrochemical properties suitable for electrodialysis (ED) applications. Encapsulation of strong acid with a weak base seems to improve electrostatic repulsion between membrane surfaces and of bi-valent ions, which are responsible for significant flux reduction. Chronopotentiometric and current–voltage responses of SPEEK@SGO-PANI membrane confirmed its ion-conducting nature along with 3.32 mA cm−2 limiting current density (Ilim). Reported SPEEK@SGO-PANI membrane showed a high electrodialytic flux of Na+ compared to bi-valent ions (Mg2+, Ca2+, and Cd2+). Extremely low energy consumption and high current efficiency (81.18%) at 6.0 V applied voltage for Na+ (0.10 M) revealed the promising nature of the reported membrane. Observed mono-valent ion selectivity aroused due to improved electrostatic repulsion of bi-valent ions with the membrane surface.

Two-Dimensional Graphene-Based Potassium Channels Built at an Oil/Water Interface.

Graphene-based laminar membranes exhibit remarkable ion sieving properties, but their monovalent ion selectivity is still low and much less than the natural ion channels. Inspired by the elementary structure/function relationships of biological ion channels embedded in biomembranes, a new strategy is proposed herein to mimic biological K+ channels by using the graphene laminar membrane (GLM) composed of two-dimensional (2D) angstrom(Å)-scale channels to support a simple model of semi-biomembrane, namely oil/water (O/W) interface. It is found that K+ is strongly preferred over Na+ and Li+ for transferring across the GLM-supported water/1,2-dichloroethane (W/DCE) interface within the same potential window (-0.1-0.6 V), although the monovalent ion selectivity of GLM under the aqueous solution is still low (K+/Na+~1.11 and K+/Li+~1.35). Moreover, the voltammetric responses corresponding to the ion transfer of NH4+ observed at the GLM-supported W/DCE interface also show that NH4+ can often pass through the biological K+ channels due to their comparable hydration-free energies and cation-π interactions. The underlying mechanism of as-observed K+ selective voltammetric responses is discussed and found to be consistent with the energy balance of cationic partial-dehydration (energetic costs) and cation-π interaction (energetic gains) as involved in biological K+ channels.

Valorization of Seawater Reverse Osmosis Brine by Monovalent Ion-Selective Membranes through Electrodialysis.

This paper proposes the use of monovalent selective electrodialysis technology to concentrate the valuable sodium chloride (NaCl) component present in seawater reverse osmosis (SWRO) brine for direct utilization in the chlor-alkali industry. To enhance monovalent selectivity, a polyamide selective layer was fabricated on commercial ion exchange membranes (IEMs) through interfacial polymerization (IP) of piperazine (PIP) and 1,3,5-Benzenetricarbonyl chloride (TMC). The IP-modified IEMs were characterized using various techniques to investigate changes in chemical structure, morphology, and surface charge. Ion chromatography (IC) analysis showed that the divalent rejection rate was more than 90% for IP-modified IEMs, compared to less than 65% for commercial IEMs. Electrodialysis results demonstrated that the SWRO brine was successfully concentrated to 14.9 g/L NaCl at a power consumption rate of 3.041 kWh/kg, indicating the advantageous performance of the IP-modified IEMs. Overall, the proposed monovalent selective electrodialysis technology using IP-modified IEMs has the potential to provide a sustainable solution for the direct utilization of NaCl in the chlor-alkali industry.

Synthesis of nanofiltration membranes with enhanced monovalent and divalent selectivity using β-cyclodextrin, tannic acid and ZIF-8

β-cyclodextrin (β-CD) and tannic acid (TA) together as an organic phase and zeolitic imidazolate framework-8 (ZIF-8) as an additive can be used along with trimesoyl chloride (TMC) as a monomer to synthesize nanofiltration (NF) membranes. Those membranes have better separation performance and provide higher flux. In this study, a new NF membrane was synthesized using them to investigate the effects of monomer concentration, substrate morphology, and reaction temperature on membrane performance. The results showed that when the concentrations of both β-CD and TA were 1 wt%, the membranes had the highest selectivity (2.99) for monovalent and divalent salts. A 10-fold increase in TMC concentration (0.05 to 0.5 wt%) optimized membrane selectivity and flux. When both spongy and cavernous structures were present in the substrate, the membranes could achieve >85 % removal of monovalent and divalent salts. After the introduction of ZIF-8, the roughness of the membrane increased, and the removal rate of monovalent salts decreased significantly, resulting in a salt selectivity of 7.97 and a flux up to 27.81 ± 1.17 L/(m2 h). The growth of ZIF-8 on the membrane surface was assisted by an increase in the reaction temperature.

Improvement of Li/Mg monovalent ion selectivity of cation-exchange membranes by incorporation of cerium or zirconium phosphate particles

To improve the selectivity of cation-exchange membranes to the transfer of lithium with respect to magnesium during the electrodialysis desalination of lithium and magnesium sulfates solutions, the surface of a commercial cation-exchange membrane based on sulfated polystyrene was modified with cerium(III, IV) and zirconium phosphates. Upon incorporation of phosphate particles, the Li/Mg selectivity coefficients of the membranes increased up to 113%.

Lithium-ion extraction using electro-driven freestanding graphene oxide composite membranes

Lithium-ion extraction and recovery from unconventional aqueous sources by utilizing membranes as a separation barrier are potential technologies to address the challenge of increased lithium demand. Separating lithium from other competing metal ions is challenging due to their low selectivity when using conventional membranes as selective barriers. Graphene oxide (GO) nanofluidic membranes possess a higher monovalent ion permeation rate than multivalent ions, however, the selectivity of lithium over other monovalent ions is low. In this study, we created freestanding GO-based membranes with enhanced Li+/monovalent ions selectivity to concentrate and recover lithium from aqueous solutions containing competing monovalent ions (e.g., K+ and Na+). Specifically, sulfonate groups are introduced into the membrane matrices by intercalating the spacing agent, polystyrene sulfonate (PSS), into the adjacent GO nanosheets. Additionally, neutral and positively charged spacing agents are also used for comparison, and to further investigate the transport mechanisms. The Li+ permeation rate of GOM-S (PSS-incorporated GO membrane) is higher than the other GO-based membranes in a single salt solution. In a mixed salt solution, the Li+/K+ selectivity of GOM-S depends on concentration and we observed the highest value, 3.43, when using 0.5 M LiCl and 0.5 M KCl as the feed (high) concentrations and 0.001 M LiCl and 0.001 M KCl as permeate (low) concentrations. We observed that as permeate concentration decreased the selectivity increased and the pH values from 3 to 10 did not change the selectivity. DFT calculations show that the Li–SO3H pair has smaller binding energy, facilitating lithium-ion transport along the membrane nanochannel. Also, the solution concentration is crucial to the competitive mixed ionic transport, which results from the preferential surface transport induced by the electrical double layer (EDL) extension within the confined membrane nanochannels.

Enhancing the Total Salt Adsorption Capacity and Monovalent Ion Selectivity in Capacitive Deionization Using a Dual-Layer Membrane Coating Electrode

Capacitive deionization (CDI) is highly attractive for seawater desalination, while the trade-off between total salt adsorption capacity (SAC) and monovalent ion selectivity is a big challenge for current electrodes, especially facing solution containing organics. Herein, a simple approach was used to prepare a novel dual-layer membrane coating electrode (D-MCE) by depositing ion exchange polymers onto activated carbon fiber (ACF) to overcome this issue. Performances of the ACF, single-layer MCE, anion exchange membrane ACF, and D-MCE were investigated. Compared to ACF, the single-layer MCE showed an increased total SAC from 435.7 to 593.0 μmol/g with a reduced Cl–/SO42– selectivity from 0.9 to 0.4. The introduction of the membrane increased total SAC to 524.3 μmol/g and decreased Cl–/SO42– selectivity to 0.7. The D-MCE exhibited an improved total SAC to 679.5 μmol/g by inhibiting the co-ion repulsion and Faradaic reaction. Meanwhile, the D-MCE enabled fast Cl– diffusion and presented a high selectivity (3.8). After 50 cycles, the D-MCE exhibited an almost unchanged total SAC and selectivity even in the presence of organics. The narrowed pores and negative outermost layer endowed the D-MCE with an outstanding anti-fouling behavior, which well explained its stable performance. These findings confirmed the high potential of the D-MCE in addressing the trade-off and provided valuable insights into the design of the CDI electrode for desalination.