The endowment of monovalent selectivity to cation exchange membrane by photo-induced covalent immobilization and self-crosslinking of chitosan

Cation Exchange Membranes and Process Optimizations in Electrodialysis for Selective Metal Separation: A Review

The selective separation of metal species from various sources is highly desirable in applications such as hydrometallurgy, water treatment, and energy production but also challenging. Monovalent cation exchange membranes (CEMs) show a great potential to selectively separate one metal ion over others of the same or different valences from various effluents in electrodialysis. Selectivity among metal cations is influenced by both the inherent properties of membranes and the design and operating conditions of the electrodialysis process. The research progress and recent advances in membrane development and the implication of the electrodialysis systems on counter-ion selectivity are extensively reviewed in this work, focusing on both structure-property relationships of CEM materials and influences of process conditions and mass transport characteristics of target ions. Key membrane properties, such as charge density, water uptake, and polymer morphology, and strategies for enhancing ion selectivity are discussed. The implications of the boundary layer at the membrane surface are elucidated, where differences in the mass transport of ions at interfaces can be exploited to manipulate the transport ratio of competing counter-ions. Based on the progress, possible future R&D directions are also proposed.

Cation Exchange Membranes Coated with Polyethyleneimine and Crown Ether to Improve Monovalent Cation Electrodialytic Selectivity

Developing monovalent cation permselective membranes (MCPMs) with high-efficient permselectivity is the core concern in specific industrial applications. In this work, we have fabricated a series of novel cation exchange membranes (CEMs) based on sulfonated polysulfone (SPSF) surface modification by polyethyleneimine (PEI) and 4′-aminobenzo-12-crown-4 (12C4) codeposited with dopamine (DA) successively, which was followed by the cross-linking of glutaraldehyde (GA). The as-prepared membranes before and after modification were systematically characterized with regard to their structures as well as their physicochemical and electrochemical properties. Particularly, the codeposition sequence of modified ingredients was investigated on galvanostatic permselectivity to cations. The modified membrane (M-12C4-0.50-PEI) exhibits significantly prominent selectivity to Li⁺ ions (PMg2+Li+ = 5.23) and K⁺ ions (PMg2+K+ = 13.56) in Li⁺/Mg²⁺ and K⁺/Mg²⁺ systems in electrodialysis (ED), which is far superior to the pristine membrane (M-0, PMg2+Li+ = 0.46, PMg2+K+ = 1.23) at a constant current density of 5.0 mA·cm⁻². It possibly arises from the synergistic effects of electrostatic repulsion (positively charged PEI), pore-size sieving (distribution of modified ingredients), and specific interaction effect (12C4 ~Li⁺). This facile strategy may provide new insights into developing selective CEMs in the separation of specific cations by ED.

Separation of mono- and di-valent ions from seawater reverse osmosis brine using selective electrodialysis

As water scarcity has become a serious global issue, seawater reverse osmosis (SWRO) is considered as a promising technique to expand traditional water supplies. However, the reject brine from SWRO systems is still a major environmental concern. In this research, the monovalent selective electrodialysis (S-ED) was used to separate and recover one of the primary components, i.e., sodium chloride, from the SWRO brine, thereby avoiding the direct discharge of the brine and achieving the brine valorization. The permselectivity of selective ion-exchange membranes (IEMs) was elucidated by comparing with the standard IEMs in structure-property via membrane characterization techniques. Results indicated that the permselectivity of Selemion CSO membrane was attributed to the positive-charged layer with a low sulfonate/ammonium ratio of 1.28. Whereas the permselectivity of Selemion ASV membrane resulted from the highly cross-linked layer, according to the similar content of the fixed quaternary amines and the shift of the C‑N absorption peak. In addition, the effects of the current density and temperature on the membrane performance were studied, including permselectivity ([Formula: see text] and [Formula: see text]), Na⁺ recovery, and specific energy consumption (E_SEC). Finally, the NaCl-rich brine with the total dissolved solid (TDS) value of 167.5 ± 3.3 g/L was obtained using SWRO brine with the initial TDS of 76.8 g/L. The Na⁺/Mg²⁺ mass ratio of the concentrate was 222.4, compared with the initial value of 9.7 in SWRO brine.

Water Splitting and Transport of Ions in Electromembrane System with Bilayer Ion-Exchange Membrane

Bilayer ion-exchange membranes are mainly used for separating single and multiply charged ions. It is well known that in membranes in which the layers have different charges of the ionogenic groups of the matrix, the limiting current decreases, and the water splitting reaction accelerates in comparison with monolayer (isotropic) ion-exchange membranes. We study samples of bilayer ion-exchange membranes with very thin cation-exchange layers deposited on an anion-exchange membrane-substrate in this work. It was revealed that in bilayer membranes, the limiting current’s value is determined by the properties of a thin surface film (modifying layer). A linear regularity of the dependence of the non-equilibrium effective rate constant of the water-splitting reaction on the resistance of the bipolar region, which is valid for both bilayer and bipolar membranes, has been revealed. It is shown that the introduction of the catalyst significantly reduces the water-splitting voltage, but reduces the selectivity of the membrane. It is possible to regulate the fluxes of salt ions and water splitting products (hydrogen and hydroxyl ions) by changing the current density. Such an ability makes it possible to conduct a controlled process of desalting electrolytes with simultaneous pH adjustment.

Selectivity of Transport Processes in Ion-Exchange Membranes: Relationship with the Structure and Methods for Its Improvement

Nowadays, ion-exchange membranes have numerous applications in water desalination, electrolysis, chemistry, food, health, energy, environment and other fields. All of these applications require high selectivity of ion transfer, i.e., high membrane permselectivity. The transport properties of ion-exchange membranes are determined by their structure, composition and preparation method. For various applications, the selectivity of transfer processes can be characterized by different parameters, for example, by the transport number of counterions (permselectivity in electrodialysis) or by the ratio of ionic conductivity to the permeability of some gases (crossover in fuel cells). However, in most cases there is a correlation: the higher the flux density of the target component through the membrane, the lower the selectivity of the process. This correlation has two aspects: first, it follows from the membrane material properties, often expressed as the trade-off between membrane permeability and permselectivity; and, second, it is due to the concentration polarization phenomenon, which increases with an increase in the applied driving force. In this review, both aspects are considered. Recent research and progress in the membrane selectivity improvement, mainly including a number of approaches as crosslinking, nanoparticle doping, surface modification, and the use of special synthetic methods (e.g., synthesis of grafted membranes or membranes with a fairly rigid three-dimensional matrix) are summarized. These approaches are promising for the ion-exchange membranes synthesis for electrodialysis, alternative energy, and the valuable component extraction from natural or waste-water. Perspectives on future development in this research field are also discussed.

Codeposition Modification of Cation Exchange Membranes with Dopamine and Crown Ether To Achieve High K+ Electrodialysis Selectivity

Surface modification has been proven to be an effective approach for ion exchange membranes to achieve separation of counterions with different valences by altering interfacial construction of membranes to improve ion transfer performance. In this work, we have fabricated a series of novel cation exchange membranes (CEMs) by modifying sulfonated polysulfone (SPSF) membranes via codeposition of mussel-inspired dopamine (DA) and 4'-aminobenzo-15-crown-5 (ACE), followed by glutaraldehyde cross-linking, aiming at achieving selective separation of specific cations. The as-prepared membranes before and after modification were systematically characterized in terms of their structural, physicochemical, electrochemical, and electrodialytic properties. In the electrodialysis process, the modified membranes exhibit distinct perm selectivity to K⁺ ions in binary (K⁺/Li⁺, K⁺/Na⁺, K⁺/Mg²⁺) and ternary (K⁺/Li⁺/Mg²⁺) systems. In particular, at a constant current density of 5.0 mA·cm^-2, modified membrane M-co-0.50 shows significantly prominent perm selectivity [Formula: see text] in the K⁺/Mg²⁺ system and M-co-0.75 exhibits remarkable performance in the K⁺/Li⁺ system [Formula: see text], superior to commercial monovalent-selective CEM (CIMS, [Formula: see text], [Formula: see text]). Besides, in the K⁺/Li⁺/Mg²⁺ ternary system, K⁺ flux reaches 30.8 nmol·cm^-2·s^-1 for M-co-0.50, while it reaches 25.8 nmol·cm^-2·s^-1 for CIMS. It possibly arises from the effects of pore-size sieving and the synergistic action of electric field driving and host-guest molecular recognition of ACE and K⁺ ions. This study can provide new insights into the separation of specific alkali metal ions, especially on reducing influence of coexisting cations K⁺ and Na⁺ on Li⁺ ion recovery from salt lake and seawater.

Preparation of Two-Layer Anion-Exchange Poly(ethersulfone) Based Membrane: Effect of Surface Modification

The present work deals with the surface modification of a commercial microfiltration poly(ethersulfone) membrane by graft polymerization technique. Poly(styrene-co-divinylbenzene-co-4-vinylbenzylchloride) surface layer was covalently attached onto the poly(ethersulfone) support layer to improve the membrane electrochemical properties. Followed by amination, a two-layer anion-exchange membrane was prepared. The effect of surface layer treatment using the extraction in various solvents on membrane morphological and electrochemical characteristics was studied. The membranes were tested from the point of view of water content, ion-exchange capacity, specific resistance, permselectivity, FT-IR spectroscopy, and SEM analysis. It was found that the two-layer anion-exchange membranes after the extraction using tetrahydrofuran or toluene exhibited smooth and porous surface layer, which resulted in improved ion-exchange capacity, electrical resistance, and permselectivity of the membranes.

Enhanced conductivity of monovalent cation exchange membranes with chitosan/PANI composite modification

The application of electrodialysis (ED) for desalination requires the use of natural seawater or river water, in which the presence of multivalent ions is inevitable.