Anion Exchange Membranes In Electrodialysis Research Articles

Characterization and Performance Evaluation of Digital Light Processing 3D Printed Functional 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.

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

This study presents an investigation on the monovalent/divalent ion permselectivity of anion exchange ionenes, a distinct class of solid polymer polyelectrolytes having cationic fixed charge groups located directly on the polymer backbone, rather than being pendant. These ionenes are commercially available as Aemion® AEMs and are based on hexamethyl-p-terphenyl poly(benzimidazolium) (HMT-PMBI). Monovalent/divalent permselectivity values of four Aemion® membranes of different thickness and ion exchange capacity were determined via electrodialysis in a mixed electrolyte system comprising chloride/sulfate anions. To fully understand the transport phenomena in these materials, ion transport properties were studied in conjunction with water uptake and ionic conductance. When the AEMs were exposed to mixed chloride/sulfate solutions, their water uptake significantly increased compared to purely chloride containing solutions, and the concentration of fixed charge groups in the AEMs consequently decreased. We find that thicker ionenes with lower IEC exhibit the highest permselectivity. The data also reveal that thinner AEMs yield a greater ionic flux loss and decreased permselectivity values, particularly in case of lower IEC AEMs. Surprisingly, despite its low water uptake and high resistance, commercial Selemion AMV possesses lower permselectivity than low IEC ionene-based Aemion® AEMs, which we explain on the basis of the highly tortuous internal morphology of low water content ionenes. Permselectivity-to-resistance ratio values are an order of magnitude higher for low IEC ionenes when compared with high IEC ionenes and Selemion AMV.

Imparting antibacterial adhesion property to anion exchange membrane by constructing negatively charged functional layer

In this work, a series of modified anion exchange membranes (AEMs) has been prepared via self-polymerization of L-dopa on the surface of commercial AEM (Fuji I type), followed by grafting the 4-amino-benzenesulfonic acid monosodium salt (ABS) on L-polydopamine thin layer. The chemical structures of modified AEMs were confirmed by the FTIR and XPS spectra. Our investigation demonstrates that the surface resistance of the modified AEM (L-PDA@ABS-Cl) increases slightly from 1.90 Ω cm2 to 2.37 Ω cm2 and the hydrophilicity of L-PDA@ABS-Cl significantly improves, verifying by the decreased water contact angles from 86.42° to 67.25°. The difference between pristine AEM and L-PDA@ABS-Cl in desalination rates only remained less than 4% throughout the test of 240 min. The modified AEMs have the negatively charged functional layers which produce an electrostatic repulsion effect and negatively charged bacteria cannot attach to the membrane surface. As a result, L-PDA@ABS-Cl shows good performance on fouling resistance, good inhibition effect on Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). Low surface resistance and superior foul inhibition performance indicate the potential application of modified AEMs in electrodialysis (ED).

Fouling dynamics of anion polyacrylamide on anion exchange membrane in electrodialysis

Anion polyacrylamide (APAM) fouling towards anion exchange membrane (AEM) during electrodialysis (ED) of industrial effluent draws recent attention. The fouling dynamics was herein investigated via different approaches in monitoring the variation of the solution and membrane properties. The results showed that conductivity and pH variation is less sensitive than electrochemical signals via transmembrane electric potential (TMEP) and electrochemical impedance spectroscopy (EIS) focusing on micro-scale variation occurred on/near the membrane interface. Initially, the deposition of APAM caused few current transfer limitations as supported by the plateau region for the TMEP profile. Further building-up of APAM fouling gradually hindered the current transfer, resulting into an increased TMEP due to the formation of gel layer on AEM and/or blockage of free volume. Later, the water splitting indicated by an obvious pH variation in dilute and concentrate occurred, which may further accelerate the fouling via modifying the particle size of APAM. EIS successfully distinguished the electrochemical property of membrane bulk and boundary layer, implying the internal fouling is largely responsible for the increased membrane resistance. Future work is suggested to further clarify the external and internal fouling on ion exchange membrane (IEM) and investigate effective fouling mitigation methods in ED.

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

Hexyl-modified series-connected bipyridine and 1,4-diazabicyclo[2.2.2]octane (DABCO) di-cations functionalized anion exchange membranes (QPPO-DP and QPPO-DABCO respectively) for electrodialysis (ED) were prepared via reactions of brominated poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) and the self-designed quaternization agents containing mono-ionic bipyridine and DABCO moieties. These agents are precursors to the combinations of different di-cations and hydrophobic side chains. QPPO-DP (thickness: 95 μm) and QPPO-DABCO (thickness: 70 μm) with similar functionalization degrees were characterized in terms of ion exchange capacity (IEC), water uptake (WU), swelling ratio (SR), thermal and mechanical stabilities, morphology, area resistance (Rm), transport number (t-) and limiting current density. QPPO-DP and QPPO-DABCO give IECs of 2.27 and 2.63 mmol g−1 respectively owing to the different unit masses of the di-cations. The conjugated aromatic bicyclic structure of bipyridine di-cations offers QPPO-DP higher WU than that of QPPO-DABCO. The relatively weaker hydrophilicity of DABCO di-cations ensures QPPO-DABCO moderate WU, superior Rm and t- (2.77 Ω cm2 and 0.949). Thus, QPPO-DABCO displays more excellent ED performances than those of commercial AMX under the same ED conditions, including current efficiency of 89.58%, salt flux of 81.39 mg m−2 s−1 and reasonable energy consumption of 1.76 kWh kg−1 NaCl, revealing its potential in ED application.

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

The use of electrodialysis (ED) fabricated with proton blocking anion exchange membranes (AEMs) is a promising method to realize waste acid reclamation. To develop AEMs with low proton leakage, in this work, we designed a series of N-methylpiperidine (MPRD)-functionalized poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) cross-linked by weak base 1,3-di-4-piperidylpropane (DiPRD). Our investigations demonstrate the weak base cross-linker could enhance the density of membrane matrix and reduce the hydrophilicity of AEM matrix, which have been verified by thermogravimetric analysis and water absorption with low swelling ratio of 2.43%. Benefit from the unique structure with denser membrane matrix and additional protonated weak base piperidine structure, the acid blocking performance of the optimized crosslinked AEM was enhanced with the H+ concentration of 1.98 M in concentrate cell during 24 h-ED process at current density of 20 mA cm−2 (initial H+ concentration: 1.0 M), outperforming the AEM without cross-linking structure (1.73 M). Nevertheless, it was found that the small-sized proton diffusion in ion-nanochannels of AEM and the aged membrane matrix in ED process with high sulfuric acid concentration resulted in proton leakage to some degree. As a result, a trade-off between the proton blocking and sulfate ion transport of the cross-linked AEMs is achieved. It is believed that this study can provide a guideline for designing advanced proton blocking AEMs.

Stable cycloaliphatic quaternary ammonium-tethered anion exchange membranes for electrodialysis

In this work, we have investigated a series of anion exchange membranes (AEMs) based on brominated poly(2,6-dimethyl-1,6-phenylene oxide) (BPPO) tethered with three saturated heterocyclic quaternary ammonium groups (QAs) of 1-methylpyrrolidine (MPY), N-methylpiperidine (MPRD), and 4-methylmorpholine (MMPH) for electrodialysis (ED) applications, respectively, along with BPPO with trimethylamine (TMA) and Neosepta AMX made for comparison. Our investigations demonstrate that the optimized BPPO-MPRD, having an ion exchange capacity of 1.67 mmol g−1, is highly stable in aqueous KOH (1 mol L−1) with ion exchange capacity retention ratio of 85.1% and hydroxide conductivity retention ratio of 80.3% at 60 °C for over 15 days, relative to other heterocyclic amine decorated AEMs. In ED application process, BPPO-MPRD shows the NaCl removel ratio of 98.8% and energy consumption of 12.58 kWh kg−1, outperforming the Neosepta AMX (97.4% & 15.76 kWh kg−1). The results demonstrate that the as-prepared BPPO-MPRD AEM can be applied in ED.

Property characterization and mechanism analysis on organic fouling of structurally different anion exchange membranes in electrodialysis

The physicochemical and structural properties of the AMX (homogeneous AEM), AMA (semi-homogeneous AEM) and HAEM (heterogeneous AEM) were characterized systematically to examine their effects on membrane fouling from sodium dodecyl sulfate (SDS) in electrodialysis. Results demonstrated that the fouling of three AEMs was closely related to their membrane structure, and the denser membrane structure caused more SDS accumulated on the membrane surface to make the severer membrane fouling. The existence of SDS in the dilute solution caused the desalination rate of AMX, AMA and HAEM decreased by 42.3%, 15.3% and 9.2%, respectively. Different fouling layer was formed on the surface of AMX and AMA except HAEM, which was demonstrated firstly by element mapping of membrane cross-section. Electrochemical impedance spectroscopy (EIS) suggested the ionic migration through the fouled AMX was restricted completely, different from the transmembrane migration of partial ions through the fouled AMA and HAEM. Compared with the fouled AMA and HAEM, the performance of the fouled AMX caused by SDS was more difficult to be recovered due to its denser membrane structure.

Effect of plasma treatment on the surface properties and antifouling performance of homogeneous anion exchange membrane

A homogeneous anion exchange membrane (AEM) with quaternary ammonium group was treated by low temperature plasma using different feed gases to improve the antifouling performance of AEMs in electrodialysis (ED). X-ray photoelectron spectroscopy analysis indicated that nitrogen-and oxygen-containing functionalities were incorporated, respectively, on the membrane surface after N-2 and O-2 plasma treatments. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) analyses indicated that the surface morphology and surface roughness of the plasma-treated AEMs exhibited various changes because of the different etching effects of the two gases. Contact angle and zeta potential measurement confirmed that the surface hydrophilicity and negative charge density of the plasma-treated AEMs improved significantly and variously, which were related to the difference of the feed gases. No obvious changes occurred in the electrical resistance and ED performance of the plasma-treated AEMs, compared with the pristine AEM. The antifouling performance of the plasma-treated AEMs for sodium dodecylbenzenesulfonate (SDBS) improved significantly. Moreover, the O-2-treated AEM exhibited better antifouling property than the N-2-treated AEM. Results indicated that the high surface negative charge of the O-2-treated AEM was crucial to the improvement of the antifouling performance for SDBS.

Gypsum scaling of anion exchange membranes in electrodialysis

Scaling of homogeneous AMV and heterogeneous MA-40 anion exchange membranes (AEM) by gypsum (CaSO4·2H2O) was investigated during electrodialysis (ED) at constant current conditions. The effects of scale development on counter-ion flux, electrical potential difference across the scaled-membrane, the overall electrical stack resistance and the extent of water splitting, were studied as a function of the current density and flow velocity of the solution in the relevant compartments.It was found that scale grows mainly in the interior of MA-40 while for the homogeneous AMV membrane scale grows mainly on the membrane surface facing the concentrate compartment. This implies that the homogeneous membrane could be more easily cleaned and its properties almost fully retained, compared with the case of the heterogeneous membrane where the internal scale was strongly attached to the internal matrix domains. These findings have important implications for the use of ED in the treatment of highly concentrated solutions and as part of a chain of treatment towards zero liquid discharge (ZLD).

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

The fouling of anion exchange membranes (AEMs) during an electrodialysis (ED) is a serious problem. Membrane fouling is often classified into organic fouling, inorganic fouling (scaling) and biofouling. In our previous studies, we reduced organic fouling by modifying the membrane surface with dopamine solution, which formed a negatively charged, hydrophilic polydopamine (PDA) layer on the membrane surface. PDA coating also increased monovalent anion permselectivity. Improvement in monovalent anion selectivity is expected to decrease scale formation and consequently improves anti-inorganic fouling properties. In this work, we investigated the effect of PDA coating on biofouling of AEM. The anti-adhesive properties were evaluated with a static adhesion test using Pseudomonas putida (P. putida). It was found that the anti-adhesive properties were effectively improved by surface modification with 0.1kg/m3 dopamine solution. Moreover, we evaluated biofouling during ED operation, for the first time, by measuring the time elapsed before biofouling occurred using a P. putida suspension as the feed solution. The surface was then observed by scanning electron microscope (SEM) after biofouling experiment. The results showed that a direct electric current application suppressed bacterial attachment on the AEM surface in addition to the effect of membrane surface modification with PDA.

One-pot solvent-free synthesis of cross-linked anion exchange membranes for electrodialysis

Solvent-free strategy has attracted a broad interest in preparation of ion exchange membranes in recent years stemming from the no need of organic solvents and facilitated construction of highly crosslinking networks. However, post-functionalization is commonly required for further introducing ion-exchange groups. In this study, we report a one-pot solvent-free synthesis of cross-linked anion exchange membranes (AEMs) without the need for post-functionalization. We firstly dissolved brominated poly (2, 6-dimethyl-1, 4-phenylene oxide) (BPPO) in liquid monomers mixture of 4-vinylbenzyl chloride (VBC) and styrene without any organic solvent, then added appropriate amount of N-vinylimidazole and N-methylimidazole to introduce imidazolium groups into both of polyelectrolyte and solvents. The transparent, robust and crosslinked AEMs were obtained by the thermal crosslinking of the unsaturated moieties during the membrane forming process. This approach, distinct from the classical post-functionalization processes, performs crosslinking and functionalization simultaneously. Particularly, imidazolium cations locating on cross-links endow the resultant membrane a high concentration of charge carriers (ion-exchange capacity) for target low resistance, while maintain a high mechanical stability.

Nitrate Selectivity and Transport Properties of a Novel Anion Exchange Membrane in Electrodialysis

The synthesis and characterization of a novel heterogeneous anion exchange membrane with the aim of improving performance and increasing its nitrate selectivity was investigated. The lab-made membrane has a superior performance (higher nitrate selectivity, faster conductivity reduction and nitrate removal) at low voltages relative to that of a commercial membrane.As an ideal ion exchange membrane should have a high retention of co-ions and for some applications, selectivity between counter ions, the selectivity of the membrane was investigated in two ways: the selectivity between co-ions/counter-ions and the selectivity between two counter ions in electrodialysis. Activated carbon particles were used as an additive in order to enhance the properties and performance of the membrane. The retention of co-ions (selectivity of membrane towards co- ion/counter- ions), IEC, FIC and conductivity were enhanced through addition of activated carbon to anion exchange resin by a ratio of up to 1/100. A high conductivity reduction of feed solution was obtained at activated carbon to resin ratio of 1/100. However, the activated carbon has a negative effect on the membrane selectivity between counter ions (nitrate and chloride) and the highest selectivity was obtained in the lack of activated carbon. The lab made membrane has low resistance and it was found that at low voltages the lab-made membrane (without activated carbon) had a nitrate to chloride selectivity of 4.68 in electrodialysis.

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

Electrodialysis (ED) is one of the useful membrane processes to produce a drinking water from underground water, brine water and other water sources. In ED, the monovalent anion selectivity is a very important property of anion exchange membranes (AEMs) to remove harmful ions like NO3− and F− ions from feed solutions and to protect the precipitation of scale. In this paper, we theoretically discussed the effect of parameters on permselectivity and suggested ways to improve the monovalent anion selectivity of ED system using the equivalent circuit model. The results showed that a decrease in the thickness of diluted compartment was very important to properly reflect the membrane properties on the permselectivity. It was also shown that the multivalent anion selectivity of an AEM was increased by increasing the positive charge density of the AEM. On the other hand, the monovalent anion selectivity was improved by modifying the AEM with a negatively charged layer. Finally, we discussed a method for estimating the monovalent anion selectivity of ED system.

【Original Contribution】 Effect of the Membrane Surface on Performance Improvements to Anion Exchange Membranes for Electrodialysis through Layer-by-layer Deposition

【Original Contribution】 Effect of the Membrane Surface on Performance Improvements to Anion Exchange Membranes for Electrodialysis through Layer-by-layer Deposition

Simultaneous improvement of the monovalent anion selectivity and antifouling properties of an anion exchange membrane in an electrodialysis process, using polyelectrolyte multilayer deposition

Selectivity for monovalent anions is very important property of anion exchange membranes (AEMs) for making drinking water along with high antifouling property. Generally, standard AEMs show higher selectivity for multivalent anions than monovalent anions. We simultaneously improved the monovalent anion selectivity and antifouling potential of standard AEMs in electrodialysis using layer-by-layer (LbL). Poly(sodium 4-styrene sulfonate) (PSS) was used as a polyanion, with poly(allylamine hydrochloride) as a polycation. The monovalent anion selectivity was evaluated using the transport number ratio between sulfate and chloride ions. The antifouling potential of the AEMs was evaluated using the time elapsed before the onset of fouling using sodium dodecilbenzene sulfonate as a foulant. The monovalent anion selectivity of the PSS-top-layer membrane improved with the number of layers. This was attributed to the increase in the total excess negative charge within the LbL layers. The antifouling potential was also improved by the LbL deposition, although the optimum condition was different from that for the monovalent anion selectivity. This was attributed to the increased hydrophilicity and the negatively charged membrane surface. Modified AEMs with an odd number of LbL layers (PSS top layer) above 15 had sufficient monovalent anion selectivity for practical use, and showed high antifouling potential.

The influence of current density on the electrochemical properties of anion-exchange membranes in electrodialysis of phenylalanine solution

Electrochemical properties of heterogeneous anion-exchange membranes can be indicators of their fouling during electrodialysis of phenylalanine solutions. The current–voltage curves of the membrane MA-40 make it possible to reveal the difference in the limiting current values for fresh and fouled membranes. Increase of membrane voltage with time characterizes the development of fouling in the electromembrane system. Membrane conductivity decreases significantly in solutions containing phenylalanine. Overlimiting current conditions lead to “washing out” of amino acid from membrane phase and therefore help to avoid fouling.

Fouling of anion-exchange membranes in electrodialysis of aromatic amino acid solution

Ion-exchange membranes undergo fouling by aromatic amino acids during the electrodialysis demineralization process. It is shown that fouling of heterogeneous anion-exchange membrane MA-41 occurs at current densities close to the limiting values. As a result, membrane conductivity decreases significantly in solutions containing phenylalanine and sodium chloride. IR spectra confirm the presence of amino acid anions and bipolar ions in membrane phase after electrodialysis demineralization of phenylalanine containing solutions. Contact angle measurements of membranes equilibrated in solution containing aromatic amino acid reveal an increase of membrane surface hydrophobicity.

Effects of silica sol on ion exchange membranes: Electrochemical characterization of anion exchange membranes in electrodialysis of silica sol containing-solutions

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.

Fouling mitigation of anion exchange membrane by zeta potential control

The feasibility of fouling mitigation of anion exchange membranes (AEMs) in the presence of humate was studied by adding three different types of water-soluble polymers, i.e., poly(acrylic acid) (PAA), poly(vinyl alcohol) (PVA), and poly(ethylene imine) (PEI), during electrodialysis (ED) desalination. Measurement of zeta potential of the humate used in this study showed highly negative potential (about −30 mV), implying that the humate had a strong fouling potential on the AEMs in ED. Of the three water-soluble polymers, PEI showed a positive zeta potential (about +14 mV) and is able to form an interpolymer complex with the humate. PAA and PVA hardly formed interpolymer complexes with humate due to electrostatic repulsion. The PEI–humate mixture with a volume ratio of 1:20 (PEI:humate) showed zero zeta potential, and a complexed humate with zero surface charge was formed, resulting in no fouling effects on the AEMs. Accordingly, the desalting ED experiments with PEI showed improved ED performance. Further, black colloids formed in the mixture did not cause the cell resistance to increase.