Monovalent Cation Selective Membranes Research Articles

High-Performance Monovalent Selective Cation Exchange Membranes with Ionically Cross-Linkable Side Chains: Effect of the Acidic Groups.

Inspired by the charge-governed protein channels located in the cell membrane, a series of polyether ether ketone-based polymers with side chains containing ionically cross-linkable quaternary ammonium groups and acidic groups have been designed and synthesized to prepare monovalent cation-selective membranes (MCEMs). Three acidic groups (sulfonic acid, carboxylic acid, and phenolic hydroxyl) with different acid dissociation constant (pKa) were selected to form the ionic cross-linking structure with quaternary ammonium groups in the membranes. The ionic cross-linking induced the nanophase separation and constructed ionic channels, which resulted in excellent mechanical performance and high cation fluxes. Interesting, the cation flux of membranes increased as the ionization of acidic groups increase, but the selectivity of MCEMs did not follow the same trend, which was mainly dependent on the affinity between the functional groups and the cations. Carboxyl group-containing MCEMs exhibited the best selectivity (9.01 for Li+/Mg2+), which was higher than that of the commercial monovalent cation-selective CIMS membrane. Therefore, it is possible to prepare stable MCEMs through a simple process using ionically cross-linkable polymers, and tuning acidic groups in the membranes provided an attractive approach to improving the cation flux and selectivity of MCEMs.

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

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

Vertically aligned carbon nanotubes for monovalent cation selective membranes designed by in silico experiments

Many membrane-based ion separation technologies require monovalent cations selectivity. However, available membranes usually show limited specific ion selectivity. In order to provide basic information on the membrane ion selectivity, the confinement effect on narrow pores disrupting the ion hydration is investigated in this work, with the aim to provide a new alternative to classical Cation Exchange Membranes (CEM). Starting from solvated cations structures used as templates and obtained by a quantum approach, Single-Wall Carbon Nanotubes (SWCNT) with ad-hoc designed diameters were chosen to perform in-silico experiments of single-cation permeation through CNT by means of Molecular Dynamics simulations, for which partial charges at nanotube inlets were parametrized through ab initio calculations. Cations' trajectories and energy decomposition analysis suggest that 100 % perm-selectivity towards Na+ with respect to Ca2+ and Mg2+ is virtually attainable with CNT of 1.33 nm diameter. Interestingly, the origin of this behaviour lays on thermodynamics rather than on size exclusion mechanisms. Opposite to polymeric homogeneous CEM, showing higher affinity for multivalent cations, an inverted trend was found for CNT in terms of hydration free energies, where Na+ shows more affinity up to 36 kJ/mol. Finally, total rejection of Cl− was observed in the range of CNT diameters investigated.

Selective removal of sodium ions from greenhouse drainage water – A combined experimental and theoretical approach

High Na+ levels are detrimental for most crops. Selective membranes provide the possibility for the selective removal of Na+ while preserving beneficial ion species. The challenge is to separate two ion species of the same charge. This study evaluates the implementation of an electrodialysis (ED) system equipped with a supported liquid membrane (SLM) and a commercially available monovalent cation-selective membrane (CIMS) in the treatment of greenhouse drainage water. The SLM shows a (minimum) K+ over Na+, Ca2+ and Mg2+ permselectivity of 9, 15 and 30, respectively. Whereas the CIMS holds a high K+ over Ca2+ and Mg2+ permselectivity of 10 and 16, respectively, the K+ over Na+ permselectivity is just 1.3. With the experimentally obtained membrane characteristics at hand, the treatment of drainage water was simulated by a two-steps process with the two membrane types operating in series. Using real-life operational parameters, analysis revealed the optimal configuration and the ability to recover 96% of the K+ and approximately 80% of the water, Ca2+ and Mg2+. Summarized, this study not only shows the efficient separation of two ion species of the same valance but also the implementation of this technology in a real-life application.

Vertically Aligned Carbon Nanotubes for Monovalent Cation Selective Membranes Designed by in Silico Experiments

Vertically Aligned Carbon Nanotubes for Monovalent Cation Selective Membranes Designed by in Silico Experiments

Composite anti-scaling membrane made of interpenetrating networks of nanofibers for selective separation of lithium

The selective separation and extraction of lithium from salt lakes is compromised by the high salt concentration and the presence of competing ions. In this study, a class of novel cation exchange membranes based on Kevlar aramid nanofibers (KANFs) was designed via interpenetrating networks of poly(4-styrenesulfonic acid-co-maleic acid) sodium salt (PSSMA) and amide reaction of 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl (ATTO). Membranes with different PSSMA content were fabricated, and the final membrane denoted as A#PSSMA@KANF-2 achieved a ~1.2 mmol g−1 ion exchange capacity, ~28% water content, ~4.5% swelling rate and ~1.8 Ω cm2 surface electrical resistance. The thin A#PSSMA@KANF-2 membrane (~8 μm thickness) also exhibited a high membrane limiting current density of 32.0 mA cm−2 (in 0.1 M NaCl solution) and an exceptional desalination efficiency (99.9% for NaCl) in electrodialysis. Moreover, compared to some commercial monovalent selectivity cation exchange membranes (CSO and CIMS membranes, which are two commercial monovalent cation selective membranes), the A#PSSMA@KANF-2 membrane was found functional for the separation of Li+/Mg2+ and to have an excellent anti-scaling performance. As the selectivity has potential to be as high or higher as that of commercial membranes, this work provides a promising method to develop membranes with anti-scaling performance for the extraction of lithium from high salt concentrations in salt lakes.

Zwitterion structure membrane provides high monovalent/divalent cation electrodialysis selectivity: Investigating the effect of functional groups and operating parameters

Monovalent cation perm-selective membranes (MCPMs) with enhanced monovalent flux and the capability to reject multivalent cations are critical; however, their realization remains challenging in various industrial applications, such as seawater desalination and wastewater purification. To achieve this criterion, we designed and synthesized a series of MCPMs with a zwitterion structure comprising three quaternary ammonium groups, two carboxylic acids, and one sulfonic acid group. These functional groups act to balance the cation flux and perm-selectivity of the MCPM. The cation fluxes and perm-selectivity of the fabricated MCPMs were investigated for Na+/Mg2+ and Li+/Mg2+ systems. High perm-selectivities of 58.4 and 16.5 were obtained for (PMg2+Na+) and (PMg2+Li+) systems, respectively, using a QPO/DAN-SA-5 membrane. The performance of the MCPMs was compared with the state-of-art commercial monovalent cation selective membrane NeoseptaTM CIMS. The influence of feed concentration and current density on the cation fluxes and perm-selectivity was examined, and the pH was measured to investigate the effect of concentration polarization and water splitting on electrodialysis. Moreover, the presence of various functional groups in the membrane matrix provided additional insights into electrostatic repulsive and interaction effects. The zwitterion structure of the MCPM was capable of tuning the cation flux and perm-selectivity. Our study provides an effective strategy for developing MCPMs with high cation flux and ion perm-selectivity.

Preparation and Characterization of Monovalent Cation Selective Membranes Prepared By a Double Pore-Filled Technique

In order to minimize membrane fouling in electrochemical systems using ion exchange membranes, we are having developed monovalent cation selective membranes prepared by a pore-filled technique. The transparent pore-filled membranes indicated that the porous PE substrate was successfully filled with polymer electrolyte by UV irradiation. The weight ratio of anion polymer electrolyte layer was 0.20 and 0.59. The dimensional stability and area resistance of the monovalent selective membranes increased with the weight content of anion polymer electrolyte. Lab-made monovalent selective membranes showed low ion permeability. However, they have similar permeability ratio between NaCl and MgCl2 (or CaCl2) than CMV, and the flux of the sample was slightly higher than commercial membrane. Further characterizations of the resulting membranes will be performed, and reported in the near future.

Preparation and Characterization of Monovalent Cation Selective Membranes Prepared by a Layer-By-Layer Pore-Filled Technique

Membrane fouling is one of fatal barriers in electrochemical water treating technologies using ion exchange membranes (IEM) such as electrodeionization (EDI), Electrodialysis (ED), capacitive deionization (CDI), and reverse electrodialysis (RED). Calcium and Magnesium ions are representative foulants which make cake-like scale on the IEMs, leading to high resistance and low durability. In order to minimize membrane fouling in systems using IEMs, we are having developed monovalent cation selective membranes prepared by a layer-by-layer pore-filled technique. Anion exchangeable electrolytes are pore-filled on both surfaces of a semi-pore-filled cation exchange membrane (layer-by-layer). A porous PE substrate (<20 um) is used to be filled with electrolytes and crosslinking agents and to be polymerized. Monovalent cation selectivity against divalent ions, calcium and magnesium ions, of the resulting membranes was evaluated using 1N MgCl2, CaCl2 and NaCl solutions.

Nanofibrous composite membranes (NFCMs) for mono/divalent cations separation

The development of cation-exchange membrane with the ability to effectively separate mono/divalent ions is of crucial importance to various industrial applications such as wastewater purification and seawater desalination. However, it remains a big challenge to fabricate monovalent cation selective membrane maintaining both of high ion flux and good permselectivity. In this study, nanofibrous composite membranes (NFCMs) containing -N+(CH3)3 and -SO3- groups are fabricated by impregnating bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) electrospun nanofibrous mats into sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) (SPPO) solution and followed by quaternization of the bromomethyl groups. The unique nanofibrous composite structure results in low electrical resistance, high limiting current density (ilim) and significant improvement in dimensional stability and ion flux. In addition, due to the introduced quaternized electrospun nanofibrous mats, monovalent cations can be separated from divalent cations by their difference of electrostatic repulsion force. Compared with the commercial monovalent cation selective membrane (CSO), the optimized membrane (NQS1) in this study shows better performance with the ion flux of 2.96×10−8molcm−2 s−1 and the permselectivity of 1.62 in an electrodialysis process (feed solution: 0.1molL−1 NaCl /MgCl2). According to the excellent performances, our nanofibrous composite membranes are expected to be a promising candidate for Na+/Mg2+ separation.

Capacitive deionization (CDI) integrated with monovalent cation selective membrane for producing divalent cation-rich solution

In this study, a newly developed capacitive deionization (MCDI) integrated with a monovalent cation permselective exchange membrane was evaluated for selective removal of monovalent and divalent cations. A variety of solution chemistries including cation composition, total dissolved solids (TDS), and feedwater pH were investigated. The removal selectivity was worsened when TDS concentration increased and pH decreased. Based on the experimental observations in this study, the optimum operating conditions (i.e., time, voltage, and flow rate) were recommended for maximum selectivity. The energy consumption of MCDI process was compared with NF process by collecting NF plant data from the previous literature. It was found that the MCDI process was more effective for selective removal of monovalent cations, and used less energy than the NF process under optimum conditions. Based on the results, a novel concept of employing MCDI process producing Ca2+-rich water as a post-treatment integrated with typical NF/LPRO softening process was proposed for preventing the corrosion in the pipe distribution system.

Mono-valent cation selective membranes for electrodialysis by introducing polyquaternium-7 in a commercial cation exchange membrane

Ion exchange membranes are essential for electrodialysis. However, the presence of multivalent ions, such as Ca2+, Mg2+, or CO32−, SO42−, may result in a detrimental risk of membrane scaling. Mono-valent cation selective membranes may solve this problem. In this study, the surfactant N,N-dimethyl-N-2-propenyl-2-propene-1-ammonium chloride-2-propenamide (poly-quaternium-7, PQ7) is used to modificate the cation exchange membranes. Different concentrations of degraded polyquaternium-7 and sodium hydroxide are investigated to determine the optimal point. The composition and characteristics of the membranes were determined with Fourier transform infrared spectroscopy (FTIR), X-ray photo-electron spectroscopy (XPS), Scanning electron microscopy (SEM) and Atomic Force Microscopy (AFM). Hydrophilic and functional groups of the membrane were determined by the water uptake, contact angle and ion exchange capacity. Current–voltage curves were measured to characterize the transport properties of membranes. The obtained results showed that the limiting current was reduced while the Ohmic and electro-convection resistances were increased. Diffusion dialysis experiments have demonstrated that leakage of modified membrane is lower. Furthermore, a series of electrodialysis experiments was conducted to evaluate the monovalent selectivity of the unmodified and modified CEMs. The leakage of Zn2+ was decreased from 22.0% to 14.2% and the leakage of Ca2+ and Mg2+ were both decreased while the membranes are used in seawater concentration. The obtained results indicate that the membrane has an excellent performance, particularly with respect to selectivity.

Preparation of monovalent cation selective membranes through annealing treatment

Different from enhancing permselectivity of cation exchange membrane (CEM) by the common surface modification, this study reports an annealing treatment strategy to enhance the density of PVA-based CEM by tuning crystallinity, which can improve pore-size sieving effect for ions. X-ray diffraction (XRD) suggests that these membranes possess controllable crystallinity ranging from 11.8% to 43.9%. AFM analysis implies that after annealing treatment crystalline regions increase remarkably and distribute uniformly in membrane. When they are applied to electrodialysis (ED) for acid recovery from H+/Zn2+ solution, it is experimentally evidenced that the permselectivity (PH+Zn2+) and Zn2+ leakage are dependent on crystallinity. The optimal membrane with a crystallinity of 43.9% exhibits a favorable permselectivity of 0.007 between Zn2+ and H+ and only 0.01% of Zn2+ leakage, which are better than that of monovalent cation exchange membrane modified by surface modification.

Ionic transport phenomenon across sol–gel derived organic–inorganic composite mono-valent cation selective membranes

Organic–inorganic composite mono-valent cation selective membranes (MCSMs) were prepared by sol–gel under acidic conditions, in which sulfonic acid groups were introduced at the inorganic segment. Studies on physicochemical and electrochemical properties revealed their excellent mechanical, thermal, and oxidative stabilities, high conductivity, ion-exchange capacity, permselectivity for mono-valent cations, ionic diffusion and water transport number. These properties suggested the suitability of MCSMs, especially Si-65%, for electro-separation of Na + from Ca 2+, Mg 2+, and Fe 3+. The effect of electrolyte solution on the characteristics of the current–voltage ( i – v ) curve in MCSM was studied based on the concentration polarization. Electro-transport of different ions in terms of plateau length and concentration profiles for different ions in the solution phase, diffusion boundary layer and membrane phase were presented. Information obtained from i – v curve analysis were validated by electrodialysis (ED) experiments for individual or mixed electrolyte solutions. Electro-transport efficiency and separation factor of different ions for MCSM and Nafion117 (N117) membranes were compared, which suggested suitability of MCSMs for separating cations.

Separation of sodium ions from trivalent chromium by electrodialysis using monovalent cation selective membranes

Trivalent chromium Cr(III) in wastewaters produced by leather tanning processes must be treated before discharge in the environment. Electrodialysis was studied for this application. Cr(III) ion is separated from sodium ion by using modified cation-exchange membranes. The membrane modification consists of a polyethylenimine layer electrodeposited on the membrane surface. This layer is positively charged in acidic media and repels multivalent ions while monovalent ions cross the membrane. The modified membrane in this study was a Nafion ® 324 membrane. The transfer of chromium, sodium, calcium, magnesium, chloride and sulphate ions from a mixture was investigated. The pH must be regulated in order to avoid chromium hydroxide precipitation in the dilute chamber. The behaviour of sulphate chloride system is unusual for the AMX membrane. Adsorption of PEI on the membrane surface is assumed to explain this behaviour. The overall current efficiency was close to 96–98% for cations and anions.

Transport and separation of Ag+ and Zn2+ by donnan dialysis through a monovalent cation selective membrane

Donnan Dialysis of Ag+ and Zn2+ was investigated through a cation exchange membrane (CMS Neosepta) when a proton concentration difference was maintained between the two sides of the membrane. Developed for the production of brine from sea water, CMS Neosepta showed a higher permeability to monovalent than to bivalent cations. Several physico-chemical parameters have been determined (electrical resistance, membrane potential, sorption of electrolytes, Zn2+ and Ag+ diffusion coefficients). The flux of Ag+ and the diffusion potential in the membrane increase with HNO3 concentrations. Ag+ and Zn2+ can be separated because of the preferential membrane transfer for Ag+.