Hexyl-modified series-connected bipyridine and DABCO di-cations functionalized anion exchange membranes for electrodialysis desalination

The selective separation of chloride ions (Cl−) from fluoride ions (F−) is a critical concern in various industries due to the potential risks of F− on public health and industrial operations. However, Cl−/F− separation is known to be technologically challenging, considering they have the same charge and valency. This study explores the design of anion exchange membranes (AEMs) in electrodialysis to achieve Cl−/F− separation based on the difference in their hydration energy. Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) polymers brominated at varying reaction temperatures were employed as the basic AEM material, enabling fine-tuning of the hydrophobic properties of the resulting membranes through selective bromination at the benzyl or aryl positions and the subsequent quaternization with tertiary amines of different chain lengths, ultimately enhancing their selectivity. The developed AEMs are tested in electrodialysis involving a binary Cl−/F− mixture. The obtained membranes exhibited remarkable selectivity, reaching as high as 12.5 ± 1.0, surpassing the performance of commercial monovalent selective AEMs. The results demonstrate the potential of Cl−/F− separation by electrodialysis and highlight the significance of tailored membrane design in achieving the separation of ions with the same valency.

Removing halide ions from wastewater and industrial effluents is crucial to eliminating their potential risks to human health, ecosystems, and industrial operations. However, conventional techniques for this process are inefficient and have severe drawbacks, including the use of hazardous chemicals, secondary pollution, and increased costs. Utilizing monovalent selective anion exchange membranes (AEMs) in electrodialysis has emerged as an effective solution for separating halide ions from sulfate-rich solutions. This review presents the recent progress, applications, and future prospects of this method, elucidates the principles underlying monovalent selectivity in AEMs, provides an overview of AEM membrane materials and preparation methods, and addresses the impacts of electrodialysis operating conditions on halide removing processes, such as current density, flow rate, pH, and stack design. Existing challenges and recognized gaps, such as complexities in solution composition, membrane stability concerns, insufficient consideration of operational factors, and limitations in modeling, demanding further efforts in this field are also presented. Overcoming these hurdles necessitates a focused approach involving material and membrane design, in-depth exploration of solution dynamics, better operational understanding, and the application of advanced modeling techniques. Effectively addressing these challenges holds the potential to notably amplify the efficiency and efficacy of electrodialysis in mitigating halide ion pollution in sulfate-rich solutions.

Electrodialysis (ED), an ion exchange membrane-based desalination method is particularly notable for its application in treating brackish water and ion separations [1]. In an ED system, anion exchange membranes (AEMs) that increase the permselectivity of anions is crucial to enhancing the separation efficiency of an ED process, requiring not only high ions selectivity, but also a low ionic resistance, low fouling, and good chemical stability. Polymers based on poly[2,2’-(2,2",4,4",6,6"-hexamethyl-p-terphenyl-3,3"-diyl)-5,5’-bibenzimidazole] (HMT-PMBI) exhibit enhanced chemical stability and modifiable properties based on the degree of methylation (dm). HMT-PMBI with 75% degree of methylation (HMT-PMBI 75dm) exhibit exceptionally large mono/divalent permselectivity but relatively lower ionic conductivity (high resistance) [2]; higher degrees of methylation, such as 89% (HMT-PMBI 89dm), increases ionic conductivity at the expense of decreased permselectivity [2]. Therefore, in this work, we examined the formation of bilayers HMT-PMBI having two different degrees of methylation, 75dm and 89 dm, and thus two different properties. When the 89dm layer of the bilayer 75dm/89dm membrane faced the cathode, an improved permselectivity (= 4.3 for 23 µm) was noted compared to single layer 89dm (= 0.9 for 23 µm). Additionally, fouling resistance to a surfactant (sodium dodecyl benzenesulfonate, SDBS) exhibited a significant improvement, with a transition time of 18.4 minutes, around thirteen times greater than the single layer 75dm membrane (transition time of 1.37 min). In conclusion, the bilayer configuration membrane effectively brought the advantage of the higher hydrophilicity of the 89dm layer to enhance conductivity and fouling resistance while preserving the permselectivity afforded by 75dm layer. This work highlights that significant advances in the field of membrane technology for ED applications may potentially be achieved through bilayer and multilayer systems of ion-containing polymers of similar types but different ion exchange capacities, balancing individual layers of differing ionic conductivity and fouling resistance, and making use of inherent permselectivity interfaces that lie between multiplayers.[1] M.A. Alkhadra, X. Su, M.E. Suss, H. Tian, E.N. Guyes, A.N. Shocron, K.M. Conforti, J.P. De Souza, N. Kim, M. Tedesco, K. Khoiruddin, I.G. Wenten, J.G. Santiago, T.A. Hatton, M.Z. Bazant, Electrochemical Methods for Water Purification, Ion Separations, and Energy Conversion, Chem Rev 122 (2022) 13547–13635. https://doi.org/10.1021/acs.chemrev.1c00396.[2] A.S. Gangrade, B. Tusi, P.C. Ghosh, S. Holdcroft, High monovalent/divalent permselectivity and low ionic resistance of ionene-based anion exchange membranes in electrodialysis, J Memb Sci 685 (2023). https://doi.org/10.1016/j.memsci.2023.121906. Figure 1

High monovalent/divalent permselectivity and low ionic resistance of ionene-based anion exchange membranes in electrodialysis

To change the permselectivity between halogen ions through anion exchange membranes in electrodialysis, the anion exchange membranes were modified: impregnation of compounds which have ether bonds into the membrane and formation of a cationic polysoap layer on the membrane surface, led to control of the hydrophilicity of the membrane. Evaluation was performed by measurement of the transport numbers of fluoride, bromide and iodide ions relative to chloride ions. The permeation of the less hydrated bromide anions decreased and that of the strongly hydrated fluoride anions increased relative to chloride ions by impregnating compounds containing ether bonds into the membranes, which was a consequence of the increase in the hydrophilicity of the membranes. Notably, permeation of chloride ions through the membrane exceeded that of bromide ions in the presence of ethylene glycols, which is the opposite behaviour of those of conventional anion exchange membranes. On the contrary, to examine the effect of decreasing the hydrophilicity on the permselectivity between halogen ions, a hydrophobic layer [poly(N-dodecyl 4-vinylpyridinium bromide)] was formed on the desalting side of the membrane surface. Although fluoride ions permeated only with difficulty through the membrane owing to formation of the layer, permeation of bromide and iodide ions was enhanced by the layer. In particular, the transport number of iodide ions relative to chloride ions reached 35.5 (without the layer 4.8). Controlling the hydrophilicity of the anion exchange membranes is an effective method to change the permeation behavior of halogen ions through anion exchange membranes.

Stable cycloaliphatic quaternary ammonium-tethered anion exchange membranes for electrodialysis

The fouling potential of the negatively charged silica sol in electrodialysis (ED) by adsorption on the surface of an anion exchange membrane was investigated. Since the fouling potential is related to the physical and electrochemical properties of the silica sol and anion exchange membranes, it is important to characterize the properties of silica sol and membranes. The surface charge of silica sol was investigated by the electrophoretic mobility and its isoelectric point was determined as pH 3. The commercial anion exchange membranes were characterized in terms of exchange capacity, water content, the zeta potential and the electrochemical properties of the membranes using impedance spectroscopy to predict the effects on the electrodialysis performances. Among the characterized properties, exchange capacity and some electrochemical properties of the anion exchange membranes were rather improved after ED experiments. In the electrodialysis of solution containing silica sol, deposition of the silica sol did not decrease the desalting performance of the anion exchange membranes because of loosely packed cake layer on the membrane surface.

Theoretical study of the permselectivity of an anion exchange membrane in electrodialysis

Fouling dynamics of anion polyacrylamide on anion exchange membrane in electrodialysis

Gypsum scaling of anion exchange membranes in electrodialysis

With the rapid development of 3D printing technologies, more attention has been focused on using 3D printing for the fabrication of membranes. This study investigated the application of digital light processing (DLP) 3D printing combined with quaternization processes to develop dense anion exchange membranes (AEMs) for electrodialysis (ED) separation of Cl− and SO42− ions. It was discovered that at optimal curing times of 40 min, the membrane pore density was significantly enhanced and the surface roughness was reduced, and this resulted in an elevation of desalination rates (97.5–98.7%) and concentration rates (165.8–174.1%) of the ED process. Furthermore, increasing the number of printed layers improved the membranes’ overall polymerization and performance, with double-layer printing showing superior ion flux. This study also highlights the impact of the polyethylene glycol diacrylate (PEGDA) molecular weight on membrane efficacy, where PEGDA-700 outperformed PEGDA-400 in ion transport capabilities and desalination efficiency. Additionally, higher 4-vinylbenzyl chloride (VBC) content improved the quaternary ammonium group concentration and membrane conductivity, and hence elevated the ED performance. Under optimized conditions, DLP 3D printed membranes demonstrated exceptional selectivity of 24.0 for Cl−/SO42− and a selective purity of 81.4%. With a current density of 400 A/m2, the current efficiency and energy consumption were in the range of 82.4% to 99.7%, and 17.2 to 25.4 kW‧h‧kg−1, respectively, showcasing the potential of advanced manufacturing techniques in creating efficient and functional ion exchange membranes.

Effect of polydopamine coating and direct electric current application on anti-biofouling properties of anion exchange membranes in electrodialysis

Exploring the acid enrichment application of piperidinium-functionalized cross-linked poly(2,6-dimethyl-1,4-phenylene oxide) anion exchange membranes in electrodialysis

Change in Transport Properties of Anion-Exchange Membranes in the Presence of Ethylene Glycols in Electrodialysis

Fouling mitigation of anion exchange membrane by zeta potential control