Influence of membrane structure-dependent water transport on conductivity-permselectivity trade-off and salt/water selectivity in electrodialysis: Implications for osmotic electrodialysis using porous ion exchange membranes

Concentration of High-Salinity Brine Using Single-Stage Membrane Capacitive Deionization

Concentrating saline water is essential for zero liquid discharge (ZLD) of wastewater. However, prevailing membrane-based technologies, such as reverse osmosis (RO) and electrodialysis (ED), can hardly handle high concentration differences (ΔC) in a single stage, where multi-stage operation is needed, which increases the operational difficulties and energy input. However, membrane capacitive deionization (MCDI) is theoretically applicable to high ΔC. This study explored the feasibility of employing an MCDI in brine concentrating and proposed several regulating measures on the electrode's porosity, electrical quantity for charging-discharging, and desorption conditions. Based on the determination of salt and water fluxes, these measures were confirmed to mitigate water transfer across the membrane, thereby facilitating salt transportation for brine concentrating. To address the mass imbalance between adsorbed and desorbed, a novel pre-charge strategy was designed, which enabled successful MCDI continuous operation over 50 cycles. A concentration difference of 161 g/L NaCl was achieved per single stage, which is the highest reported result among RO, ED, and MCDI studies. The concentrating rate was as high as 38.4 g/(m2·h) with a comparative energy consumption at RO and ED. This study demonstrated that MCDI is an optional technology for the future application of brine concentrating in ZLD facilities.

Alkylation as a strategy for optimizing water uptake and enhancing selectivity in polyethyleneimine-based anion-exchange membranes for brine mining via electrodialysis

Brine treatment poses a significant challenge for the growing desalination industry, yet it also holds valuable elements and a substantial amount of water. To efficiently extract these elements and increase water recovery, the development of advanced, highly selective separation technologies is urgently needed. This study addresses this challenge by optimizing polyethyleneimine (PEI)-based anion exchange membranes (AEMs) through an alkylation strategy to enhance water uptake control and ion selectivity. The aim is to achieve the high separation efficiency required for effective reverse osmosis (RO) brine mining via electrodialysis. The careful design of functional amine groups with a mixed composition of alkyl substituents enabled the development of membranes with reduced water uptake and high charge density, providing the best conductivity/selectivity ratio, enhanced ion selectivity, and decreased water-splitting activity. The unmodified PEI-membrane already demonstrated a competitive performance compared to common commercial AEMs membranes used in electrodialysis, such as FujiFilm® AEM Type 1 and 2, Ralex® AM-PP, and Neosepta® AMX. However, the alkylation further improved the performance significantly. Among modified membranes, PEI alkylated with propyl followed by methyl (PEI-Pr-Me) achieved the highest current efficiency of 93 %, while PEI alkylated with a mixture of four (C1C4)n-alkyl groups had the highest Cl⁻/SO42⁻-selectivity coefficients of up to 8.7 and the lowest water transfer across the membrane. This tailored functionalization approach presents a promising pathway for improving AEMs' performance in desalination brine treatment, enabling more efficient water and mineral recovery.

Methodology for Selecting Anion and Cation Exchange Membranes Based on Salt Transport Properties for Bipolar Membrane Fabrication

Bipolar membranes (BPMs) are a unique construction of ion exchange membranes with anion exchange and cation exchange layers in series. Due to the unique transport processes in BPMs, they are becoming an increasingly attractive option for many electrochemical devices, especially in water electrolysis and carbon dioxide reduction. However, because a large number of anion and cation exchange membranes are available, it can be difficult to select the layers for BPM fabrication, particularly when targeting specific properties for use in a device. In this study, a survey of nine anion and nine cation exchange membranes was conducted to assess their steady-state ion transport properties. The primary application of this work is seawater electrolysis; therefore, measurements of salt flux and area resistance in 0.5 mol/L sodium chloride solutions were performed. These measurements displayed a trade-off behavior, with membranes displaying higher area resistance and having a lower salt flux. Conversely, membranes with lower area resistance had a higher salt flux. From these individual membrane results, a methodology was formulated to select component membranes for BPM fabrication, primarily considering their transport characteristics. Three BPMs were fabricated using this methodology. A model was developed to integrate the parameters and ion transport properties measured from individual membranes to predict salt flux and area resistance values for a BPM. Values produced from the model were then compared with experimental salt flux and area resistance BPM measurements. Both the model and experimental salt flux and area resistance BPMs exhibited an area resistance-flux trade-off, like that of the component membranes.

The Potential of Electrodialysis with Mediating Solution (EDM) for Eliminating Alkaline Scaling: Experimental Validation and Mechanistic Elucidation

Alkaline scaling in the cathode chambers of conventional electrodialysis (ED) stacks presents significant challenges when desalinating solutions containing divalent cations. This scaling, resulting from the combined effects of water electrolysis and the migration of divalent cations from the feedwater into the catholyte, further extends from the cathode chamber to the surfaces of both the cation exchange membrane (CEM) and the anion exchange membrane (AEM) in the adjacent dilute chamber. This study aims to mitigate alkaline scaling, without pretreatment or antiscalant dosing, by optimizing the ED stack design to restrict divalent cation transport toward the cathode. We evaluated three ED stack configurations, each forming the cathode chamber with a distinct ion transport control mechanism: (1) a monovalent selective cation exchange membrane (SCEM), (2) a bipolar membrane (BPM), and (3) a mediating solution chamber adjacent to the cathode chamber (EDM). Our results indicated that stacks employing the SCEM or BPM partially restricted divalent cation migration but remained vulnerable to scaling under higher feed salinities, due to weakened Donnan exclusion within the SCEM, and strong internal ion polarization at the BPM interface. In contrast, the EDM stack exhibited superior antiscaling performance by combining strong Donnan exclusion through an AEM with ionic buffering in the mediating solution chamber, effectively blocking cation transport and eliminating conditions conducive to scaling. Additionally, the EDM stack maintained low electrical resistance and high operational stability, making it a simple, efficient, and cost-effective solution for scaling mitigation in ED systems.

Contribution of the transmembrane electric potential to the set voltage in a single-anion exchange membrane electrodialysis-cell and the role of solution conditions

The transmembrane electric potential (TMEP) drives ion transport via electromigration across ion exchange membranes (IEMs) during electrodialysis (ED). For ED operation, a voltage is either measured, or set to remain constant, between two electrodes on either side of a single IEM (simplified ED-cell) or a stack of IEMs (bench-scale ED system). The set/measured voltage has been used in the literature to approximate the TMEP in simplified ED-cells assuming that other elements between the electrodes (e.g., solutions, boundary layers, concentration gradient) contribute negligibly to the measured/set voltage; however, there is no experimental evidence in the literature comprehensively evaluating the accuracy of this assumption. Accordingly, our objectives were to (i) determine the contribution of the TMEP to the set voltage in a simplified ED-cell under operationally relevant solution conditions, and (ii) understand the role of solution conditions on the potential drop contributions from each element between reference electrodes. We studied sodium salts of eight anions (inorganic and organic) and three desalination levels at a set voltage of 0.4 V. Results showed that the set voltage was not a good approximation of the TMEP for any solution condition which was primarily attributed to the substantial potential drop from the solutions. The TMEP also varied substantially depending on solute identity and concentration. Additionally, the TMEP decreased substantially as the desalination level increased from 0% to 75%, which was attributed to the increase in potential drops due to the boundary layers and open circuit voltage. The reported findings provide important insights into the effective driving force of ion transport via electromigration when operating ED at a set voltage.

Feasibility study of electrodialysis as an ammonium reuse process for covering the nitrogen demand of an industrial wastewater treatment plant

Electrodialysis (ED) is a cost-effective membrane technology used is a variety of fields for desalination and concentration. This feasibility study explores the potential of ED as an NH4-N recovery technology from anaerobic digestate liquor (ADL), and the use of the concentrate as a nitrogen source in an industrial wastewater treatment plant (WWTP). Three neighboring WWTPs were the focus of this study: Two municipal WWTPs A and B, operating anaerobic sludge stabilization, and a pulp & paper WWTP C, utilizing urea as a nitrogen source. Two-stage bench-scale experiments with the municipal ADL from WWTP A and WWTP B were conducted, and performance indicators were determined. A concentration of approximately 10 g NH4-N/L and 15 g NH4-N/L was obtained in stages 1 and 2, respectively. The NH4-N removal was above 85 % in all experiment, while recovery varied between 25 and 95 %. The specific energy consumption (SEC) was on average 12.9 kWh/kg NH4-N. Moreover, mass and energy balances in a model WWTP demonstrated that an ED side-stream treatment for NH4-N removal coupled with microfiltration (MF) pre-treatment results in a net energy gain, also without the added benefit of the ED concentrate as a nitrogen source.

Relative contributions of mobility and partitioning to volatile fatty acid flux during electrodialysis

Economically valuable volatile fatty acids (VFAs) are sustainably produced via fermentation processes. To use VFAs downstream, they must be recovered using technologies like electrodialysis (ED). Solute transport properties (i.e., partition coefficient (K), diffusion coefficient (D), and permeability (P=KD)) govern flux in ED. Therefore, to advance understanding of VFA flux through anion exchange membranes (AEMs) in ED, we aimed to elucidate the relative contributions of VFA partitioning and mobility to their flux. Accordingly, for VFAs of different sizes (C1-C5) and inorganic anions (Cl-, Br-), we measured their fluxes during ED, permeabilities, and partition coefficients, and calculated the diffusion coefficients. We then evaluated the correlations between flux and transport properties and between transport properties and anion physicochemical properties. Results showed VFA flux had a strong correlation with permeability (R2=0.94, p<0.01), consistent with flux described by the Nernst-Planck equation. Further, while there was a negative correlation between VFA flux and partition coefficient (R2=0.46, p=0.21), there was a positive correlation between VFA flux and diffusion coefficient (R2=0.95, p<0.01) which showed VFA mobility governed VFA flux. We observed a negative correlation between VFA diffusion coefficient and carbon-chain length which was attributed to steric hindrance, and a positive correlation between partition coefficient and carbon chain-length which we attributed to hydrophobicity and polarizability. This work provides fundamental insight on interactions between VFAs and AEMs which affect anion flux during ED.

Toward a Circular Lithium Economy with Electrodialysis: Upcycling Spent Battery Leachates with Selective and Bipolar Ion-Exchange Membranes

Recycling spent lithium-ion batteries offers a sustainable solution to reduce ecological degradation from mining and mitigate raw material shortages and price volatility. This study investigates using electrodialysis with selective and bipolar ion-exchange membranes to establish a circular economy for lithium-ion batteries. An experimental data set of over 1700 ion concentration measurements across five current densities, two solution compositions, and three pH levels supports the techno-economic analysis. Selective electrodialysis (SED) isolates lithium ions from battery leachates, yielding a 99% Li-pure retentate with 68.8% lithium retention, achieving relative ionic fluxes up to 2.41 for Li+ over transition metal cations and a selectivity of 5.64 over monovalent cations. Bipolar membrane electrodialysis (BMED) converts LiCl into high-purity LiOH and HCl, essential for battery remanufacturing and reducing acid consumption via acid recycling. High current densities reduce ion leakage, achieving lithium leakage as low as 0.03%, though hydronium and hydroxide leakage in BMED remains high at 11-20%. Our analysis projects LiOH production costs between USD 1.1 and 3.6 per kilogram, significantly lower than current prices. Optimal SED and BMED conditions are identified, emphasizing the need to control proton transport in BMED and improve cobalt-lithium separation in SED to enhance cost efficiency.

Novel Crosslinked Anion Exchange Membranes Based on Thermally Cured Epoxy Resin: Synthesis, Structure and Mechanical and Ion Transport Properties

Limitations in existing anion exchange membranes deter their use in the efficient treatment of industrial wastewater effluent. This work presents an approach to fabricating novel anion-conducting membranes using epoxy resin monomers like hydrophobic or hydrophilic diglycidyl ether and quaternized polyethyleneimine (PEI). Manipulating the diglycidyl ether nature, the quantitative composition of the copolymer and the conditions of quaternization allows control of the physicochemical properties of the membranes, including water uptake (20.0-330%), ion exchange capacity (1.5-3.7 mmol/g), ionic conductivity (0.2-17 mS/cm in the Cl form at 20 °C), potentiostatic transport numbers (75-97%), as well as mechanical properties. A relationship was established between copolymer structure and conductivity/selectivity trade-off. The higher the quaternized polyethyleneimine, diluent fraction, and hydrophilicity of diglycidyl ether, the higher the conductivity and the lower the permselectivity. Hydrophobic diglycidyl ether gives a much better conductivity/selectivity ratio since it provides a lower degree of hydration than hydrophilic diglycidyl ether. Different mesh and non-woven reinforcing materials were also examined. The developed membranes demonstrate good stability in both neutral and acidic environments, and their benchmark characteristics in laboratory electrodialysis cells and batch-mode dialysis experiments are similar to or superior to, commercial membranes such as Neosepta© AMX, FujiFilm© Type1, and Fumasep FAD-PET.

Long-Term Robustness and Failure Mechanisms of Electrochemical Stripping for Wastewater Ammonia Recovery

Nitrogen in wastewater has negative environmental, human health, and economic impacts but can be recovered to reduce the costs and environmental impacts of wastewater treatment and chemical production. To recover ammonia/ammonium (total ammonia nitrogen, TAN) from urine, we operated electrochemical stripping (ECS) for over a month, achieving 83.4 ± 1.5% TAN removal and 73.0 ± 2.9% TAN recovery. With two reactors, we recovered sixteen 500-mL batches (8 L total) of ammonium sulfate (20.9 g/L TAN) approaching commercial fertilizer concentrations (28.4 g/L TAN) and often having >95% purity. While evaluating the operation and maintenance needs, we identified pH, full-cell voltage, product volume, and water flux into the product as informative process monitoring parameters that can be inexpensively and rapidly measured. Characterization of fouled cation exchange and omniphobic membranes informs cleaning and reactor modifications to reduce fouling with organics and calcium/magnesium salts. To evaluate the impact of urine collection and storage on ECS, we conducted experiments with urine at different levels of dilution with flush water, extents of divalent cation precipitation, and degrees of hydrolysis. ECS effectively treated urine under all conditions, but minimizing flush water and ensuring storage until complete hydrolysis would enable energy-efficient TAN recovery. Our experimental results and cost analysis motivate a multifaceted approach to improving ECS's technical and economic viability by extending component lifetimes, decreasing component costs, and reducing energy consumption through material, reactor, and process engineering. In summary, we demonstrated urine treatment as a foothold for electrochemical nutrient recovery from wastewater while supporting the applicability of ECS to seven other wastewaters with widely varying characteristics. Our findings will facilitate the scale-up and deployment of electrochemical nutrient recovery technologies, enabling a circular nitrogen economy that fosters sanitation provision, efficient chemical production, and water resource protection.

Desalting Plasma Protein Solutions by Membrane Capacitive Deionization

Plasma protein therapies are used by millions of people across the globe to treat a litany of diseases and serious medical conditions. One challenge in the manufacture of plasma protein therapies is the removal of salt ions (e.g., sodium, phosphate, and chloride) from the protein solution. The conventional approach to remove salt ions is the use of diafiltration membranes (e.g., tangential flow filtration) and ion-exchange chromatography. However, the ion-exchange resins within the chromatographic column as well as filtration membranes are subject to fouling by the plasma protein. In this work, we investigate the membrane capacitive deionization (MCDI) as an alternative separation platform for removing ions from plasma protein solutions with negligible protein loss. MCDI has been previously deployed for brackish water desalination, nutrient recovery, mineral recovery, and removal of pollutants from water. However, this is the first time this technique has been applied for removing 28% of ions (sodium, chloride, and phosphate) from human serum albumin solutions with less than 3% protein loss from the process stream. Furthermore, the MCDI experiments utilized highly conductive poly(phenylene alkylene)-based ion exchange membranes (IEMs). These IEMs combined with ionomer-coated nylon meshes in the spacer channel ameliorate Ohmic resistances in MCDI improving the energy efficiency. Overall, we envision MCDI as an effective separation platform in biopharmaceutical manufacturing for deionizing plasma protein solutions and other pharmaceutical formulations without a loss of active pharmaceutical ingredients.

Ion Resource Recovery via Electrodialysis Fabricated with Poly(Arylene Ether Sulfone)-Based Anion Exchange Membrane in Organic Solvent System

Ion resource recovery from organic wastewater is beneficial for achieving emission peaks and carbon neutrality targets. Advanced organic solvent-resistant anion exchange membranes (AEMs) for treating organic wastewater via electrodialysis (ED) are of significant interest. Herein, a kind of 3D network AEM based on poly(arylene ether sulfone) cross-linked with a flexible cross-linker (DBH) for ion resource recovery via ED in organic solvent system is reported. Investigations demonstrate that the as-prepared AEMs show excellent dimensional stability in 60% DMSO (aq.), 60% ethanol (aq.), and 60% acetone (aq.), respectively. For example, the optimized AEM shows very low swelling ratios of 1.04-1.10% in the organic solvents. ED desalination ratio can reach 99.1% after exposure of the AEM to organic solvents for 30 days, and remain > 99% in a mixture solution containing organic solvents and 0.5 m NaCl. Additionally, at a current density of 2.5 mA cm-2, the optimized AEM soaked in organic solvents for 30 days shows a high perm-selectivity (Cl-/SO4 2-) of 133.09 (vs 13.11, Neosepta ACS). The superior ED performance is attributed to the stable continuous sub-nanochannels within AEM confirmed by SAXS, rotational energy barriers, etc. This work shows the potential application of cross-linked AEMs for resource recovery in organic wastewater.

Fate of organic micropollutants during brackish water desalination for drinking water production in decentralized capacitive electrodialysis

Capacitive electrodialysis (CED) is an emerging and promising desalination technology for decentralized drinking water production. Brackish water, often used as a drinking water source, may contain organic micropollutants (OMPs), thus raising environmental and health concerns. This study investigated the transport of OMPs in a fully-functional decentralized CED system for drinking water production under realistic operational conditions. Eighteen environmentally-relevant OMPs (20 µg L-1) with different physicochemical properties (charge, size, hydrophobicity) were selected and added to the feed water. The removal of OMPs was significantly lower than that of salts (∼94%), mainly due to their lower electrical mobility and higher steric hindrance. The removal of negatively-charged OMPs reached 50% and was generally higher than that of positively-charged OMPs (31%), whereas non-charged OMPs were barely transported. Marginal adsorption of OMPs was found under moderate water recovery (50%), in contrast to significant adsorption of charged OMPs under high water recovery (80%). The five-month operation demonstrated that the CED system could reliably produce water with low salt ions and TOC concentrations, meeting the respective WHO requirements. The specific energy consumption of the CED stack under 80% water recovery was 0.54 kWh m-3, which is competitive to state-of-the-art RO, ED, and emerging MCDI in brackish water desalination. Under this condition, the total OPEX was 2.43 € m-3, of which the cost of membrane replacement contributed significantly. Although the CED system proved to be a robust, highly adaptive, and fully automated technology for decentralized drinking water production, it was not highly efficient in removing OMPs, especially non-charged OMPs.

Sustainable Lithium Recovery from Hypersaline Salt-Lakes by Selective Electrodialysis: Transport and Thermodynamics

Evaporative technology for lithium mining from salt-lakes exacerbates freshwater scarcity and wetland destruction, and suffers from protracted production cycles. Electrodialysis (ED) offers an environmentally benign alternative for continuous lithium extraction and is amenable to renewable energy usage. Salt-lake brines, however, are hypersaline multicomponent mixtures, and the impact of the complex brine-membrane interactions remains poorly understood. Here, we quantify the influence of the solution composition, salinity, and acidity on the counterion selectivity and thermodynamic efficiency of electrodialysis, leveraging 1250 original measurements with salt-lake brines that span four feed salinities, three pH levels, and five current densities. Our experiments reveal that commonly used binary cation solutions, which neglect Na+ and K+ transport, may overestimate the Li+/Mg2+ selectivity by 250% and underpredict the specific energy consumption (SEC) by a factor of 54.8. As a result of the hypersaline conditions, exposure to salt-lake brine weakens the efficacy of Donnan exclusion, amplifying Mg2+ leakage. Higher current densities enhance the Donnan potential across the solution-membrane interface and ameliorate the selectivity degradation with hypersaline brines. However, a steep trade-off between counterion selectivity and thermodynamic efficiency governs ED's performance: a 6.25 times enhancement in Li+/Mg2+ selectivity is accompanied by a 71.6% increase in the SEC. Lastly, our analysis suggests that an industrial-scale ED module can meet existing salt-lake production capacities, while being powered by a photovoltaic farm that utilizes <1% of the salt-flat area.

Bipolar Membrane Electrodialysis for Cleaner Production of Diprotic Malic Acid: Separation Mechanism and Performance Evaluation

Bipolar membrane electrodialysis (BMED) is a promising process for the cleaner production of organic acid. In this study, the separation mechanism of BMED with different cell configurations, i.e., BP-A, BP-A-C, and BP-C (BP, bipolar membrane; A, anion exchange membrane; C, cation exchange membrane), to produce diprotic malic acid from sodium malate was compared in consideration of the conversion ratio, current efficiency and energy consumption. Additionally, the current density and feed concentration were investigated to optimize the BMED performance. Results indicate that the conversion ratio follows BP-C > BP-A-C > BP-A, the current efficiency follows BP-A-C > BP-C > BP-A, and the energy consumption follows BP-C < BP-A-C < BP-A. For the optimized BP-C configuration, the current density was optimized as 40 mA/cm² in consideration of low total process cost; high feed concentration (0.5-1.0 mol/L) is more feasible to produce diprotic malic acid due to the high conversion ratio (73.4-76.2%), high current efficiency (88.6-90.7%), low energy consumption (0.66-0.71 kWh/kg) and low process cost (0.58-0.59 USD/kg). Moreover, a high concentration of by-product NaOH (1.3497 mol/L) can be directly recycled to the upstream process. Therefore, BMED is a cleaner, high-efficient, low energy consumption and environmentally friendly process to produce diprotic malic acid.

Selective separation of monovalent anions by PPy/pTS membrane electrodes in redox transistor electrodialysis

Selective separation of nitrate over chloride is crucial for eutrophication mitigation and nitrogen resource recovery but remains a challenge due to their similar ionic radius and the same valence. Herein, a polypyrrole membrane electrode (PME) was fabricated by polymerization of pyrrole (Py) and p-toluenesulfonate (pTS), which was used as a working electrode in redox transistor electrodialysis. The anions in the source solution were first incorporated into the PME at reduction potentials and then released to receiving solution at oxidation potentials. Pulse widths and potentials were optimized to maximize the ion separation performance of PME, resulting in the improvement of NO₃^-/Cl^- separation factor up to 6.93. The ion distributions in various depths of PME indicated that both NO₃^- and Cl^- were incorporated into PME at negative potentials. Then, NO₃^- was preferentially released from PME at positive potentials, but most Cl^- was retained. This was ascribed to the high binding energy between Cl^- and PPy/pTS structure, which was 51.4% higher than that between NO₃^- and PPy/pTS structure. Therefore, the higher transport rate of NO₃^- in comparison with Cl^- was achieved, leading to a high NO₃^- selectivity over Cl^-. This work provides a promising avenue for the selective separation of nitrate over chloride, which may contribute to nitrogen resource recycling and reuse.

A comprehensive review of capacitive deionization technology with biochar-based electrodes: Biochar-based electrode preparation, deionization mechanism and applications

The recently developed techniques for desalination and wastewater treatment are costly and unsustainable. Therefore, a cost-effective and sustainable approach is essential to achieve desalination through wastewater treatment. Capacitive deionization (CDI), an electrochemical desalination technology, has been developed as a novel water treatment technology with great potential. The electrode material is one of the key factors that promotes the development of CDI technology and broadens the scope of CDI applications. Biochar-based electrode materials have attracted increasing attention from researchers because of their advantages, such as environmentally friendly, economical, and renewable properties. This paper reviews the methods for preparing biochar-based electrode materials and elaborates on the mechanism of CDI ion storage. We then summarize the applications of CDI technology in water treatment, analyze the mechanism of pollutant removal and resource recovery, and discuss the applicability of different CDI configurations, including hybrid CDI systems. In addition, the paper notes that environmentally friendly green activators that facilitate the development of pore structure should be developed more often to avoid the adverse environmental impact. The development of ion-selective electrode materials should be enhanced and it is necessary to comprehensively assess the impact of heteroatoms on selective ion removal and CDI performance. Electrooxidation of organic pollutants should be further promoted to achieve organic degradation by extending to redox reactions.