Selective adsorption in ion exchange membranes: The effect of solution ion composition on ion partitioning

Electrodialysis is a water desalination technology that enables selective separation of ions, making it a promising solution for sustainable water reuse. The selectivity of the process is mainly determined by the properties of ion exchange membranes that can vary depending on the composition of ions in water, such as water uptake and charge density. In this work, we studied selective adsorption of Na+ and K+ ions in various ion exchange membranes considering the effect of solution ion composition on membrane water volume fraction. For that purpose, we conducted membrane adsorption experiments using solutions with Na+ and K+ ions with different ion compositions including Li+, Ca2+ or Mg2+ ions at different concentrations (0.001 - 0.25 M). The experiments showed that with the total ion concentration and the amount of divalent ions in solution, the membrane water volume fraction decreases while the selective adsorption of the smaller (hydrated) K+ ions over the Na+ ions in the membrane increases. We developed a theoretical framework based on Boublik-Mansoori-Carnahan-Starling-Leland (BMCSL) theory to describe the effect of membrane water volume fraction on selective adsorption of the ions by including volumetric effects, such as size exclusion. The developed framework was used to describe ion partitioning results of the membrane adsorption experiments. In addition, the effect of solution ion composition on selective ion removal during electrodialysis operation was evaluated using experimental data and theoretical calculations. The results of this study show that considering volumetric effects can improve the ion partitioning description in ion exchange membranes for solutions with various ion compositions.

Ultra-Thin Ion Exchange Membranes by Low Ionomer Blending for Energy Harvesting

Exploring the utilization of ion exchange membranes (IEMs) in salinity gradient energy harvesting, a technique that capitalizes on the salinity difference between seawater and freshwater to generate electricity, this study focuses on optimizing PVDF to Nafion ratios to create ultra-thin membranes. Specifically, our investigation aligns with applications such as reverse electrodialysis (RED), where IEMs facilitate selective ion transport across salinity gradients. We demonstrate that membranes with reduced Nafion content, particularly the 50:50 PVDF:Nafion blend, retain high permselectivity comparable to those with higher Nafion content. This challenges traditional understandings of membrane design, highlighting a balance between thinness and durability for energy efficiency. Voltage-current analyses reveal that, despite lower conductivity, the 50:50 blend shows superior short-circuit current density under salinity gradient conditions. This is attributed to effective ion diffusion facilitated by the blend's unique microstructure. These findings suggest that blended membranes are not only cost-effective but also exhibit enhanced performance for energy harvesting, making them promising candidates for sustainable energy solutions. Furthermore, these findings will pave the way for advances in membrane technology, offering new insights into the design and application of ion exchange membranes in renewable energy.

Erythroid anion Exchanger-1 (band 3) transports nitrite for nitric oxide metabolism

Nitrite (NO2-) interacts with hemoglobin (Hb) in various ways to regulate blood flow. During hypoxic vasodilation, nitrite is reduced by deoxyHb to yield nitric oxide (NO). While NO, a hydrophobic gas, could freely diffuse across the cell membrane, how the reactant nitrite anion could permeate through the red blood cell (RBC) membrane remains unclear. We hypothesized that Cl-/HCO3- anion exchanger-1 (AE1; band 3) abundantly embedded in the RBC membrane could transport NO2-, as HCO3- and NO2- exhibit similar hydrated radii. Here, we monitored NO/N2O3 generated from NO2- inside human RBCs by DAF-FM fluorophore. NO2-, not NO3-, increased intraerythrocytic DAF-FM fluorescence. To test the involvement of AE1-mediated transport in intraerythrocytic NO/N2O3 production from nitrite, we lowered Cl- or HCO3- in the RBC-incubating buffer by 20 % and indeed observed slower rise of the DAF-FM fluorescence. Anti-extracellular AE1, but not anti-intracellular AE1 antibodies, reduced the rates of NO formation from nitrite. The AE1 blocker DIDS similarly reduced the rates of NO/N2O3 production from nitrite in a dose-dependent fashion, confirming that nitrite entered RBCs through AE1. Nitrite inside the RBCs reacted with both deoxyHb and oxyHb, as evidenced by 6.1 % decrease in deoxyHb, 14.7 % decrease in oxyHb, and 20.7 % increase in methemoglobin (metHb). Lowering Cl- in the milieu equally delayed metHb production from nitrite-oxyHb and nitrite-deoxyHb reactions. Thus, AE1-mediated NO2- transport facilitates NO2--Hb reactions inside the red cells, supporting NOx metabolism in circulation.

Performance and Stability of Aemion and Aemion+ Membranes in Zero-Gap CO2 Electrolyzers with Mild Anolyte Solutions

The dependence of performance and stability of a zero-gap CO2 electrolyzer on the properties of the anion exchange membrane (AEM) is examined. This work firstly assesses the influence of the anolyte when using an Aemion membrane and then shows that when using 10 mM KHCO3 , a CO2 electrolyzer using a next-generation Aemion+ membrane can achieve lower cell voltages and longer lifetimes due to increased water permeation. The impact of lower permselectivity of Aemion+ on water transport is also discussed. Using Aemion+, a cell voltage of 3.17 V at 200 mA cm-2 is achieved at room temperature, with a faradaic efficiency of >90 %. Stable CO2 electrolysis at 100 mA cm-2 is demonstrated for 100 h, but with reduced lifetime at 300 mA cm-2 . However, the lifetime of the cell at high current densities is shown to be increased by improving water transport characteristics of the AEM and reducing dimensional swelling, as well as by improving cathode design to reduce localized dehydration of the membrane.

Unlocking the potential of polymeric desalination membranes by understanding molecular-level interactions and transport mechanisms

Polyamide reverse osmosis (PA-RO) membranes achieve remarkably high water permeability and salt rejection, making them a key technology for addressing water shortages through processes including seawater desalination and wastewater reuse. However, current state-of-the-art membranes suffer from challenges related to inadequate selectivity, fouling, and a poor ability of existing models to predict performance. In this Perspective, we assert that a molecular understanding of the mechanisms that govern selectivity and transport of PA-RO and other polymer membranes is crucial to both guide future membrane development efforts and improve the predictive capability of transport models. We summarize the current understanding of ion, water, and polymer interactions in PA-RO membranes, drawing insights from nanofiltration and ion exchange membranes. Building on this knowledge, we explore how these interactions impact the transport properties of membranes, highlighting assumptions of transport models that warrant further investigation to improve predictive capabilities and elucidate underlying transport mechanisms. We then underscore recent advances in in situ characterization techniques that allow for direct measurements of previously difficult-to-obtain information on hydrated polymer membrane properties, hydrated ion properties, and ion-water-membrane interactions as well as powerful computational and electrochemical methods that facilitate systematic studies of transport phenomena.

Demonstration of a three compartment solar electrolyser with gas phase cathode producing formic acid from CO2 and water using Earth abundant metals

A three compartment solar formic acid generator was built using a Sn on Cu foam cathode and NiFe anode. A bipolar combination of a Fumasep FAD-PET-75 and Nafion 117 membrane was mounted between anode and middle compartment, which was filled with Amberlyst 15H ion exchanger beads. A Fumasep FAD-PET-75 membrane separated the middle compartment from the cathode. The generator was powered with a photovoltaic panel and fed with gaseous CO2 and water. Diluted formic acid solution was produced by flowing water through the middle compartment. Common PV-EC devices are operated using aqueous electrolyte and produce aqueous formate. In our PV-EC device, formic acid is produced straight away, avoiding the need for downstream operations to convert formate to formic acid. The electrolyser was matched with solar photovoltaic cells achieving a coupling efficiency as high as 95%. Our device produces formic acid at a faradaic efficiency of ca. 31% and solar-to-formic acid efficiency of ca. 2%. By producing formic acid from CO2 and water without any need of additional chemicals this electrolyser concept is attractive for use at remote locations with abundant solar energy. Formic acid serves as a liquid renewable fuel or chemical building block.

Development of a New Hydrogel Anion Exchange Membrane for Swine Wastewater Treatment

We developed a proprietary anion exchange membrane (AEM) for wastewater treatment as an alternative to commercial products. Following the successful development of a hydrogel cation exchange membrane on a porous ceramic support, we used the same approach to fabricate an AEM. Different positively charged monomers and conditions were tested, and all AEMs were tested for nitrate and phosphate anion removal from buffers by electrodialysis. The best AEM was tested further with real swine wastewater for phosphate removal by electrodialysis and nitrate removal in a bioelectrochemical denitrification system (BEDS). Our new AEM showed better phosphate removal compared with a commercial membrane; however, due to its low permselectivity, the migration of cations was detected while operating a two-chambered biocathode BEDS in which the membrane was utilized as a separator. After improving the permselectivity of the membrane, the performance of our proprietary AEM was comparable to that of a commercial membrane. Because of the usage of a porous ceramic support, our AEM is self-supporting, sturdy, and easy to attach to various frames, which makes the membrane better suited for harsh and corrosive environments, such as swine and other animal farms and domestic wastewater.

Harvesting Electrical Power during Carbon Capture using Various Amine Solvents

There exists an urgent demand for the advancement of technologies that reduce and capture carbon dioxide (CO₂) emissions to mitigate anthropogenic contributions to climate change. This paper compares the maximum power densities achieved from the combination of reverse electrodialysis (RED) with carbon capture (CC) using various CC solvents. Carbon capture reverse electrodialysis (CCRED) harvests energy from the salinity gradients generated from the reaction of CO₂ with specific solvents, generally amines. To eliminate the requirement of freshwater as an external resource, we took advantage of a semiclosed system that would allow the inputs to be industrial emissions and heat and the outputs to be electrical power, clean emissions, and captured CO₂. We assessed the power density that can be attained using CCRED with five commonly studied CC solvents: monoethanolamine (MEA), diethanolamine (DEA), N-methyldiethanolamine (MDEA), 2-amino-2-methyl-2-propanol (AMP), and ammonia. We achieved the highest power density, 0.94 W m^-2 cell^-1, using ammonia. This work provides a foundation for future iterations of CCRED that may help to incentivize adoption of CC technology.

Resin-Loaded Heterogeneous Polyether Sulfone Ion Exchange Membranes for Saline Groundwater Treatment

Arid areas often contain brackish groundwater that has a salinity exceeding 500 mg/L. This poses several challenges to the users of the water such as a salty taste and damage to household appliances. Desalination can be one of the key solutions to significantly lower the salinity and solute content of the water. However, the technology requires high energy inputs as well as managing waste products. This paper presents the fabrication of ultrafiltration heterogeneous ion exchange membranes for brackish groundwater treatment. Scanning electron microscopy (SEM) images showed a relatively uniform resin particle distribution within the polymer matrix. The mean roughness of the cation exchange membrane (CEM) and anion exchange membrane (AEM) surfaces increased from 42.12 to 317.25 and 68.56 to 295.95 nm, respectively, when resin loading was increased from 1 to 3.5 wt %. Contact angle measures suggested a more hydrophilic surface (86.13 to 76.26° and 88.10 to 74.47° for CEM and AEM, respectively) was achieved with greater resin loading rates. The ion exchange capacity (IEC) of the prepared membranes was assessed using synthetic groundwater in a dead-end filtration system and removal efficiency of K⁺, Mg²⁺, and Ca²⁺ were 56.0, 93.5, and 85.4%, respectively, for CEM with the highest resin loading. Additionally, the anion, NO₃⁻ and SO₄²⁻ removal efficiency was 84.2% and 52.4%, respectively, for the AEM with the highest resin loading. This work demonstrates that the prepared ultrafiltration heterogeneous ion exchange membranes have potential for selective removal for of ions by ion exchange, under filtration conditions at low pressure of 0.05 MPa.

A Comprehensive Analysis of Inorganic Ions and Their Selective Removal from the Reconstituted Tobacco Extract Using Electrodialysis

Many tobacco stalks, dust, and fines are discharged in the tobacco industry, rich in inorganic minerals ions and nicotine salts. The high salinity and nicotine salts are challenging to be addressed by traditional treatment and are a severe threat that ought to be overcome. Thus, proper techniques can regenerate the tobacco stalks into reconstituted tobacco flakes used as cigarette filler. The electrodialysis process has been a viable approach to removing the inorganic ingredients in wastewater. We studied concentration, pH, and co-related influences with the nicotine and sugar/nicotine contents on the desalination performance. The results show that the inorganic ions such as Cl^-, K⁺, Ca²⁺, and Mg²⁺ ions were successfully removed. When the feed concentration ranges from 3 to 15%, the removal ratio of the K⁺ ions is higher than Ca²⁺ and Mg²⁺ ions. As we reported previously, the K⁺ and Ca²⁺ ions are unfavorable for the total particulate matter emission but beneficial to decreasing the HCN delivery in mainstream cigarette smoke. Selective ED is a robust technology to reduce the harmful component delivery in cigarette smoke.

Thin-Reinforced Anion-Exchange Membranes with High Ionic Contents for Electrochemical Energy Conversion Processes

Ion-exchange membranes (IEMs) are a core component that greatly affects the performance of electrochemical energy conversion processes such as reverse electrodialysis (RED) and all-vanadium redox flow battery (VRFB). The IEMs used in electrochemical energy conversion processes require low mass transfer resistance, high permselectivity, excellent durability, and also need to be inexpensive to manufacture. Therefore, in this study, thin-reinforced anion-exchange membranes with excellent physical and chemical stabilities were developed by filling a polyethylene porous substrate with functional monomers, and through in situ polymerization and post-treatments. In particular, the thin-reinforced membranes were made to have a high ion-exchange capacity and a limited degree of swelling at the same time through a double cross-linking reaction. The prepared membranes were shown to possess both strong tensile strength (>120 MPa) and low electrical resistance (<1 Ohm cm²). As a result of applying them to RED and VRFB, the performances were shown to be superior to those of the commercial membrane (AMX, Astom Corp., Japan) in the optimal composition. In addition, the prepared membranes were found to have high oxidation stability, enough for practical applications.

Heat to Hydrogen by RED-Reviewing Membranes and Salts for the RED Heat Engine Concept

The Reverse electrodialysis heat engine (REDHE) combines a reverse electrodialysis stack for power generation with a thermal regeneration unit to restore the concentration difference of the salt solutions. Current approaches for converting low-temperature waste heat to electricity with REDHE have not yielded conversion efficiencies and profits that would allow for the industrialization of the technology. This review explores the concept of Heat-to-Hydrogen with REDHEs and maps crucial developments toward industrialization. We discuss current advances in membrane development that are vital for the breakthrough of the RED Heat Engine. In addition, the choice of salt is a crucial factor that has not received enough attention in the field. Based on ion properties relevant for both the transport through IEMs and the feasibility for regeneration, we pinpoint the most promising salts for use in REDHE, which we find to be KNO3, LiNO3, LiBr and LiCl. To further validate these results and compare the system performance with different salts, there is a demand for a comprehensive thermodynamic model of the REDHE that considers all its units. Guided by such a model, experimental studies can be designed to utilize the most favorable process conditions (e.g., salt solutions).

Tunable Anion Exchange Membrane Conductivity and Permselectivity via Non-Covalent, Hydrogen Bond Cross-Linking

Ion exchange membranes (IEMs) are a key component of electrochemical processes that purify water, generate clean energy, and treat waste. Most conventional polymer IEMs are covalently cross-linked, which results in a challenging tradeoff relationship between two desirable properties─high permselectivity and high conductivity─in which one property cannot be changed without negatively affecting the other. In an attempt to overcome this limitation, in this work we synthesized a series of anion exchange membranes containing non-covalent cross-links formed by a hydrogen bond donor (methacrylic acid) and a hydrogen bond acceptor (dimethylacrylamide). We show that these monomers act synergistically to improve both membrane permselectivity and conductivity relative to a control membrane without non-covalent cross-links. Furthermore, we show that the hydrogen bond donor and acceptor loading can be used to tune permselectivity and conductivity relatively independently of one another, escaping the tradeoff observed in conventional membranes.

Spectroelectrochemical Detection of Water Dissociation in Bipolar Membranes

The potentials at which water dissociation occurs in bipolar membranes (BPM) and the relationship between water dissociation and current-voltage curve characteristics are explored using a novel spectroelectrochemical approach in which an anion exchange membrane is doped with a pH indicator. Using this method, we visually detect a pH change in the BPM resulting from OH^- formed during the water dissociation reaction. The color change is measured with a UV/vis spectrometer, while electrochemical characterization of the BPM is performed simultaneously. Additional measurements were performed on BPMs with varying anion and cation exchange membrane layer thickness. Our measurements provide direct evidence of water dissociation occurring within a BPM at cross-membrane potentials below 0.5 V, within the first limiting current density region. We also show that the effects of changing bulk anion and cation exchange layer thickness is highly dependent on the permselectivity of these layers.

Bio-inspired incorporation of phenylalanine enhances ionic selectivity in layer-by-layer deposited polyelectrolyte films

The addition of a common amino acid, phenylalanine, to a Layer-by-Layer (LbL) deposited polyelectrolyte (PE) film on a nanoporous membrane can increase its ionic selectivity over a PE film without the added amino acid. The addition of phenylalanine is inspired by detailed knowledge of the structure of the channelrhodopsins family of protein ion channels, where phenylalanine plays an instrumental role in facilitating sodium ion transport. The normally deposited and crosslinked PE films increase the cationic selectivity of a support membrane in a controllable manner where higher selectivity is achieved with thicker PE coatings, which in turn also increases the ionic resistance of the membrane. The increased ionic selectivity is desired while the increased resistance is not. We show that through incorporation of phenylalanine during the LbL deposition process, in solutions of NaCl with concentrations ranging from 0.1 to 100 mM, the ionic selectivity can be increased independently of the membrane resistance. Specifically, the addition is shown to increase the cationic transference of the PE films from 81.4% to 86.4%, an increase on par with PE films that are nearly triple the thickness while exhibiting much lower resistance compared to the thicker coatings, where the phenylalanine incorporated PE films display an area specific resistance of 1.81 Ω cm2 in 100 mM NaCl while much thicker PE membranes show a higher resistance of 2.75 Ω cm2 in the same 100 mM NaCl solution.

Use of the Microheterogeneous Model to Assess the Applicability of Ion-Exchange Membranes in the Process of Generating Electricity from a Concentration Gradient

The paper shows the possibility of using a microheterogeneous model to estimate the transport numbers of counterions through ion-exchange membranes. It is possible to calculate the open-circuit potential and power density of the reverse electrodialyzer using the data obtained. Eight samples of heterogeneous ion-exchange membranes were studied, two samples for each of the following types of membranes: Ralex CM, Ralex AMH, MK-40, and MA-41. Samples in each pair differed in the year of production and storage conditions. In the work, these samples were named “batch 1” and “batch 2”. According to the microheterogeneous model, to calculate the transport numbers of counterions, it is necessary to use the concentration dependence of the electrical conductivity and diffusion permeability. The electrolyte used was a sodium chloride solution with a concentration range corresponding to the conditional composition of river water and the salinity of the Black Sea. During the research, it was found that samples of Ralex membranes of different batches have similar characteristics over the entire range of investigated concentrations. The calculated values of the transfer numbers for membranes of different batches differ insignificantly: ±0.01 for Ralex AMH in 1 M NaCl. For MK-40 and MA-41 membranes, a significant scatter of characteristics was found, especially in concentrated solutions. As a result, in 1 M NaCl, the transport numbers differ by ±0.05 for MK-40 and ±0.1 for MA-41. The value of the open circuit potential for the Ralex membrane pair showed that the experimental values of the potential are slightly lower than the theoretical ones. At the same time, the maximum calculated power density is higher than the experimental values. The maximum power density achieved in the experiment on reverse electrodialysis was 0.22 W/m², which is in good agreement with the known literature data for heterogeneous membranes. The discrepancy between the experimental and theoretical data may be the difference in the characteristics of the membranes used in the reverse electrodialysis process from the tested samples and does not consider the shadow effect of the spacer in the channels of the electrodialyzer.

Metal-Ion Depletion Impacts the Stability and Performance of Battery Electrode Deionization over Multiple Cycles

Prussian blue hexacyanoferrate (HCF) materials, such as copper hexacyanoferrate (CuHCF) and nickel hexacyanoferrate (NiHCF), can produce higher salt removal capacities than purely capacitive materials when used as electrode materials during electrochemical water deionization due to cation intercalation into the HCF structure. One factor limiting the application of HCF materials is their decay in deionization performance over multiple cycles. By examining the performance of CuHCF and NiHCF electrodes at three different pH values (2.5, 6.3, and 10.2) in multiple-cycle deionization tests, losses in capacity (up to 73% for CuHCF and 39% for NiHCF) were shown to be tied to different redox-active centers through analysis of dissolution of electrode metals. Both copper and iron functioned as active centers for Na⁺ removal in CuHCF, while iron was mainly the active center in NiHCF. This interaction of Na⁺ and active centers was demonstrated by correlating the decrease in performance to the concentration of these metal ions in the effluent solutions collected over multiple cycles at different pHs (up to 0.86 ± 0.14 mg/L for iron and 0.42 ± 0.17 mg/L for copper in CuHCF and 0.38 ± 0.05 mg/L for iron in NiHCF). Both materials were more stable (<11% decay for CuHCF and no decay for NiHCF) when the appropriate metal salt (copper or nickel) was added to the feed solutions to inhibit electrode dissolution. At a pH of 2.5, there was an increased competition between protons and Na⁺ ions, which decreased the Na⁺ removal amount and lowered the thermodynamic energy efficiency for deionization for both electrode materials. Therefore, while an acidic pH provided the most stable performance, a circumneutral pH would be useful to produce a better balance between performance and longevity.

Selective ion transport through three-dimensionally interconnected nanopores of quaternized block copolymer membranes for energy harvesting application

A concentration gradient in an aqueous solution is a promising source of energy that can be converted into electrical energy by an ion-exchange polymer membrane. In concentration-gradient energy harvesters, ion transport through nanoporous channels is an emerging approach to enhance the energy conversion efficiency. Since massive but selective ion transport could be realized through nanochannels, the theoretical calculations predicted that nanoporous membranes can extract significantly larger energy than the conventional non-structured membranes. In this regard, scientists in the field have attempted to produce nanoporous membranes on a macroscopic scale based on 1D, 2D, and 3D materials. However, the fabrication of nanoporous membranes is often accompanied by technical difficulties, which entails high production cost, low throughput, and poor scalability. In this study, we took advantage of the self-segregating properties of block copolymers (BCPs) to address these issues. In particular, the non-solvent-induced phase separation method has been utilized to produce three-dimensionally interconnected nanopores within BCP membranes. In addition, the neutral BCP nanopores' surface was modified with positive charges to allow selective diffusion of anions in concentration-gradient cells. By mounting the porous BCP membranes between two aqueous solutions with different concentrations, we studied the BCP-membrane-mediated energy-harvesting performance.

Nanopore-Based Power Generation from Salinity Gradient: Why It Is Not Viable

In recent years, the development of nanopore-based membranes has revitalized the prospect of harvesting salinity gradient (blue) energy. In this study, we systematically analyze the energetic performance of nanopore-based power generation (NPG) at various process scales, beginning with a single nanopore, followed by a multipore membrane coupon, and ending with a full-scale system. We confirm the high power densities attainable by a single nanopore and demonstrate that, at the coupon scale and above, concentration polarization severely hinders the power density of NPG, revealing the common, yet significant, error in linearly extrapolating single-pore performance to multipore membranes. Through our consideration of concentration polarization, we also importantly show that the development of materials with exceptional nanopore properties provides limited enhancement of practical process performance. For a full-scale NPG membrane module, we find an inherent tradeoff between power density and thermodynamic energy efficiency, whereby achieving a high power density sacrifices the energy efficiency. Furthermore, we derive a simple expression for the theoretical maximum energy efficiency of NPG, showing it is solely related to the membrane selectivity (i.e., S²/2). Through this relation, it is apparent that the energy efficiency of NPG is limited to only 50% (for a completely selective membrane, i.e., S = 1), reinforcing our optimistic full-scale simulations which result in a (practical) maximum energy efficiency of 42%. Finally, we assess the net extractable energy of a full-scale NPG system which mixes river water and seawater by including the energy losses from pretreatment and pumping, revealing that the NPG process-both in its current state of development and in the case of highly optimistic performance with minimized external energy losses-is not viable for power generation.

Experimental Evaluation of Anion Exchange Membranes for the Desalination of (Waste) Water Produced after Polymer-Flooding

Electrodialysis (ED) has been recently proposed to desalinate polymer-flooding produced water (PFPW), a byproduct stream from the oil and gas industry rich in charged polymers. However, process performance is limited by fouling occurring on the ion-exchange membranes, particularly on the anionic ones (AEMs). Thus, this study aimed to correlate the properties of different AEMs with their performance while desalinating PFPW, ultimately evaluating their significance when fouling is to be minimized and operation improved. Six stacks containing different homogeneous and commercially available AEMs were employed to desalinate synthetic PFPW during 8-days ED experiments operated in reversal mode. AEMs recovered from the stacks were analyzed in terms of water uptake, ion-exchange capacity, permselectivity, and area resistance, and compared with virgin AEMs. Relatively small changes were measured for most of the parameters evaluated. For most AEMs, the water uptake and resistance increased, while the ion-exchange capacity (IEC) and permselectivity decreased during operation. Ultimately, AEMs with high area resistance were linked to the fast development of limiting current conditions in the stack, so this property turned out to be the most relevant when desalinating PFPW.

Recovery of phosphorus and metallic nickel along with HCl production from electroless nickel plating effluents: The key role of three-compartment photoelectrocatalytic cell system

A three-compartment photoelectrocatalytic (PEC) cell system combined with ion exchange and chemical precipitation was proposed to recover phosphorus and nickel from electroless nickel plating effluents containing hypophosphite (H₂PO₂^-) and nickel ions (Ni²⁺). Ion exchange was used to concentrate and separate Ni²⁺ and H₂PO₂^-. As a key unit, the established PEC system consisted of TiO₂/Ni-Sb-SnO₂ photoanode and Ti cathode. With 25.8 mM NaH₂PO₂ and 500 mM NiCl₂, 100 % H₂PO₂^- was oxidized to PO₄^3- in the anode cell, 78 % Ni²⁺ was recovered as metallic Ni in the cathode cell, and 900 mM HCl was obtained in the middle cell within 24 h at 3.0 V. Based on quenching experiments and ESR technique, OH radicals were mainly responsible for H₂PO₂^- oxidation. In situ Raman spectroscopy indicated that Ni²⁺ initially reacted with OH^- to form α-Ni(OH)₂, which was gradually reduced to metallic Ni. Fortunately, a slight pH decrease in the cathode cell in the three-compartment cell system was beneficial for Ni²⁺ reduction to Ni°. The obtained PO₄^3- was recovered by chemical precipitation. Finally, recovery of phosphorus and metallic nickel along with HCl production from an actual electroless nickel plating effluents in terms of efficiency, cost-benefit, and stability assessment were demonstrated.

Understanding Mono- and Bivalent Ion Selectivities of Nanoporous Graphene Using Ionic and Bi-ionic Potentials

Nanoporous graphene displays salt-dependent ion permeation. In this work, we investigate the differences in Donnan potentials arising between reservoirs, separated by a perforated graphene membrane, containing different cations. We compare the case of monovalent cations interacting with nanoporous graphene with the case of bivalent cations. This is accomplished through both measurements of membrane potential arising between two salt reservoirs at different concentrations involving a single cation (ionic potential) and between two reservoirs containing different cations at the same concentration (bi-ionic potential). In our present study, Donnan dialysis experiments involve bivalent MgCl₂, CaCl₂, and CuCl₂ as well as monovalent KCl and NH₄Cl salts. For all salts, except CuCl₂, clear Donnan and diffusion potential plateaus were observed at low and high salt concentrations, respectively. Our observations show that the membrane potential scaled to the Nernst potential for bivalent cations has a lower value (≈50%) than for monovalent cations (≈72%) in the Donnan exclusion regime. This is likely due to the adsorption of these bivalent cations on monolayer graphene. For bivalent cations, the diffusion regime is reached at a lower ionic strength compared to the monovalent cations. For Mg²⁺ and Ca²⁺, the membrane potential does not seem to depend upon the type of ions in the entire ionic strength range. A similar behavior is observed for the KCl and NH₄Cl membrane potential curves. For CuCl₂, the membrane potential curve is shifted toward lower ionic strength compared to the other two bivalent salts and the Donnan plateau is not observed at the lowest ionic strength. Bi-ionic potential measurements give further insight into the strength of specific interactions, allowing for the estimation of the relative ionic selectivities of different cations based on comparing their bi-ionic potentials. This effect of possible ion adsorption on graphene can be removed through ion exchange with monovalent salts.

Intensification of nickel recovery from water using an electrically driven hybrid process: continuous electropermutation

Process intensification through the combined use of electrodialysis (ED) and ion-exchange resin (IER) hybrid process, called continuous electropermutation (CEP), was employed to remove Ni(II) cations from water. To carry out this process, Amberjet 1200 H cation-exchange resin was introduced into the feed compartment of the ED cell. The applied electrical field improves the mobility of species and ensures a continuous resin activation which is a main drawback in IER process. Furthermore, the IER incorporated in the ED cell enhances the conductivity of the feed water, therefore it extends the range of ED which could be applied for the recovery of ions from very low concentration wastewaters. The effects of some factors such as the type of regenerating electrolyte, current density, quantity of resin incorporated in the conducting space and concentration of Ni(II) at the inlet were investigated. The efficiency of CEP and ED for Ni(II) removal was expressed in terms of recovery rate and concentration factor. In CEP process, recovery rates of 99% were found with a 40 ppm Ni(II) concentration and an applied current density of 2 mA.cm^-2 resulting in an outlet Ni(II) concentration lower than 1 ppm, against 73.69% in conventional ED. Moreover, in CEP Ni(II) cation was recovered in receiver compartment more than the feed solution with concentration factor more than 10 against 0.39 in ED. On the other hand, the voltage of ED cell was found to increase due to the lower conductivity in the feed compartment compared with that of CEP. In CEP, the highest concentration factor was found at an applied current density of 2.7 mA.cm^-2 which reached 41.26. Finally, with increasing Ni(II) feed inlet concentration, there was a trade-off between obtaining a high Ni(II) concentration in the receiver compartment and a low Ni(II) concentration at the outlet of feed compartment.

Effects of fixed charge group physicochemistry on anion exchange membrane permselectivity and ion transport

Understanding the effects of polymer chemistry on membrane ion transport properties is critical for enabling efforts to design advanced highly permselective ion exchange membranes for water purification and energy applications. Here, the effects of fixed charge group type on anion exchange membrane (AEM) apparent permselectivity and ion transport properties were investigated using two crosslinked AEMs. The two AEMs, containing a similar acrylonitrile, styrene and divinyl benzene-based polymer backbone, had either trimethyl ammonium or 1,4-dimethyl imidazolium fixed charge groups. Membrane deswelling, apparent permselectivity and ion transport properties of the two AEMs were characterized using aqueous solutions of lithium chloride, sodium chloride, ammonium chloride, sodium bromide and sodium nitrate. Apparent permselectivity measurements revealed a minor influence of the fixed charge group type on apparent permselectivity. Further analysis of membrane swelling and ion sorption, however, suggests that less hydrophilic fixed charge groups more effectively exclude co-ions compared to more hydrophilic fixed charge groups. Analysis of ion diffusion properties suggest that ion and fixed charge group enthalpy of hydration properties influence ion transport, likely through a counter-ion condensation, ion pairing or binding mechanism. Interactions between fixed charge groups and counter-ions may be stronger if the enthalpy of hydration properties of the ion and fixed charge group are similar, and suppressed counter-ion diffusion was observed in this situation. In general, the hydration properties of the fixed charge group may be important for understanding how fixed charge group chemistry influences ion transport properties in anion exchange membranes.

Improving the Thermodynamic Energy Efficiency of Battery Electrode Deionization Using Flow-Through Electrodes

Ion intercalation electrodes are being investigated for use in mixed capacitive deionization (CDI) and battery electrode deionization (BDI) systems because they can achieve selective ion removal and low energy deionization. To improve the thermodynamic energy efficiency (TEE) of these systems, flow-through electrodes were developed by coating porous carbon felt electrodes with a copper hexacyanoferrate composite mixture. The TEE for ion separation using flow-through electrodes was compared to a system using flow-by electrodes with the same materials. The flow-through BDI system increased the recoverable energy nearly 3-fold (0.009 kWh m^-3, compared to a 0.003 kWh m^-3), which increased the TEE from ∼6% to 8% (NaCl concentration reduction from 50 to 42 mM; 10 A m^-2, 50% water recovery, and 0.5 mL min^-1). The TEE was further increased to 12% by decreasing the flow rate from 0.50 to 0.25 mL min^-1. These findings suggest that, under similar operational conditions and materials, flow-through battery electrodes could achieve better energy recovery and TEE for desalination than flow-by electrodes.

Custom-Made Ion Exchange Membranes at Laboratory Scale for Reverse Electrodialysis

Salinity gradient power is a renewable, non-intermittent, and neutral carbon energy source. Reverse electrodialysis is one of the most efficient and mature techniques that can harvest this energy from natural estuaries produced by the mixture of seawater and river water. For this, the development of cheap and suitable ion-exchange membranes is crucial for a harvest profitability energy from salinity gradients. In this work, both anion-exchange membrane and cation-exchange membrane based on poly(epichlorohydrin) and polyvinyl chloride, respectively, were synthesized at a laboratory scale (255 c m 2) by way of a solvent evaporation technique. Anion-exchange membrane was surface modified with poly(ethylenimine) and glutaraldehyde, while cellulose acetate was used for the cation exchange membrane structural modification. Modified cation-exchange membrane showed an increase in surface hydrophilicity, ion transportation and permselectivity. Structural modification on the cation-exchange membrane was evidenced by scanning electron microscopy. For the modified anion exchange membrane, a decrease in swelling degree and an increase in both the ion exchange capacity and the fixed charge density suggests an improved performance over the unmodified membrane. Finally, the results obtained in both modified membranes suggest that an enhanced performance in blue energy generation can be expected from these membranes using the reverse electrodialysis technique.

Highly Selective Transport of Alkali Metal Ions by Nanochannels of Polyelectrolyte Threaded MIL-53 Metal Organic Framework

Conventional ion-exchange polymeric membranes have limited selectivity due to their nonuniform and unstable structures. The rigid, regular, high porosity of metal organic framework (MOF) generally provides MOF membrane with exclusion/sieving effect but lack of electrostatic screening. Here we report for the first time a nonbiological highly selective MOF membrane with polyelectrolyte threaded in the nanochannel of metal organic framework (polyelectrolyte∼MOF) and its selective transport of alkali metal cations. Poly(sodium vinyl sulfonated-co-acrylic acid)∼MIL-53(Al) is prepared on anodic aluminum oxide substrate via steps of MOF MIL-53(Al) growth followed by in situ polymerization. The poly(VS-co-AA)∼MIL-53(Al) membrane demonstrates highly specific selectivity in transport of alkali metal cations. Rate of ion transport correlates inversely with the hydrated diameter of the ion reaching a low limiting rate near 0.7 nm hydrated diameter. Charge exclusion is demonstrated with blockage of anion transport under a concentration gradient. The highly uniform porous nanostructure of MOF and ionic function of polyelectrolyte offers the MOF membrane with synergistic selectivity based on exclusion forces of the framework and Coulomb forces from fixed charges of polyelectrolytes in nanochannels.

Anion Exchange Membranes with Dynamic Redox-Responsive Properties

Redox-responsive anion exchange membranes were developed using photoinitiated free-radical polymerization and reversible oxidation and reduction of viologen. The membranes were formulated using poly(ethylene glycol diacrylate) and diurethane dimethacrylate oligomers, dipentaerythritol penta-/hexa-acrylate cross-linker, photoinitiators, and 4-vinylbenzyl chloride as precursors for functionalization. In the membrane, 4,4'-bipyridine reacted with the 4-vinylbenzyl chloride residues, and subsequently, unreacted amines were methylated with iodomethane to obtain viologen as both the ion carrier and redox-responsive group. Upon oxidation, viologen supports two cations, where the reduced form only contains one cation. Thus, the redox responsiveness changed the membrane ionicity by a factor of 2. The area-specific resistance of the membranes in the oxidized, +2, state was lower than in the reduced, +1, state. The resistance increased between 40.6 ± 0.1 and 111.6 ± 0.1%, depending on membrane thickness, with the most significant increment being a resistance change from 4.88 × 10^-4 Ω m² in the oxidized state to 1.03 × 10^-3 Ω m² in the reduced state. Membrane permselectivity in the reduced, +1, state was between 15.9 ± 0.1 and 26.5 ± 0.01% lower than in the oxidized, +2, state, with no change in water uptake, spanning an average of 0.87 ± 0.02 in the oxidized state to an average of 0.7 ± 0.01 in the reduced state. Upon reduction, membrane ion-exchange capacity decreases, increasing ionic resistance and decreasing membrane permselectivity due to a reduction in fixed charge concentration without a measurable change in water uptake. This trend is not generally observed for ion-exchange membranes and explains that the changes in transport properties result from changes in ionicity, not water uptake or domain size. The reversibility and stability of the stimuli responsiveness were confirmed by the absence of transport property changes after redox cycling.

Electro-Forward Osmosis

The impact of ion migration induced by an electrical field on water flux in a forward osmosis (FO) process was examined using a thin-film composite (TFC) membrane, held between two cation exchange membranes. An applied fixed current of 100 mA (1.7 mA cm^-2) was sustained by the proton flux through the TFC-BW membrane using a feed of 34 mM NaCl, and a 257 mM NaCl draw solution. Protons generated at the anode were transported through the cation exchange membrane and into the draw solution, lowering the pH of the draw solution. Additional proton transport through the TFC-BW membrane also lowered the pH of the feed solution. The localized accumulation of the protons on the draw side of the TFC-BW membrane resulted in high concentration polarization modulus of 1.41 × 10⁵, which enhanced the water flux into the draw solution (5.56 LMH at 100 mA), compared to the control (1.10 LMH with no current). These results using this electro-forward osmosis (EFO) process demonstrated that enhanced water flux into the draw solution could be achieved using ion accumulation induced by an electrical field. The EFO system could be used for FO applications where a limited use of draw solute is necessary.

Modelling nanofiltration of electrolyte solutions

This review critically examines current models for nanofiltration (NF) of electrolyte solutions. We start from linear irreversible thermodynamics, we derive a basic equation set for ion transfer in terms of gradients of ion electrochemical potentials and transmembrane volume flux. These equations are extended to the case of significant differences of thermodynamic forces across the membrane (continuous version of irreversible thermodynamics) and solved in quadratures for single salts and trace ions added to single salts in the case of macroscopically-homogeneous membranes. These solutions reduce to (quasi)analytical expressions in the popular Spiegler-Kedem approximation (composition-independent phenomenological coefficients), which we extend to the case of trace ions. This enables us to identify membrane properties (e.g. ion permeances, ion reflection coefficients, electrokinetic charge density) that control its performance in NF of multi-ion solutions. Further, we specify the phenomenological coefficients of irreversible thermodynamics in terms of ion partitioning, hindrance and diffusion coefficients for the model of straight cylindrical capillaries. The corresponding expressions enable assessment of the applicability of the popular nanopore model of NF. This model (based on the use of macroscopic approaches at nanoscale) leads to a number of trends that have never been observed experimentally. We also show that the use of the Born formula (frequently employed for the description of dielectric exclusion) hardly leads to meaningful values of solvent dielectric constant in membrane pores because this formula disregards the very solvent structure whose changes are supposed to bring about the reduction of dielectric permittivity in nanopores. We conclude that the effect should better be quantified in terms of ion excess solvation energies in the membrane phase. As an alternative to the nanopore description of NF, we review recent work on the development of an advanced engineering model for NF of multi-ion solutions in terms of a solution-diffusion-electromigration mechanism. This model (taking into account spontaneously arising transmembrane electric fields) captures several trends observed experimentally, and the use of trace ions can provide model parameters (ion permeances in the membrane) from experiment. We also consider a recent model (ultrathin barrier layers with deviations from local electroneutrality) that may reproduce observed feed-salt concentration dependences of membrane performance in terms of concentration-independent properties like excess ion solvation energies. Due to its complexity, practical modelling of nanofiltration will probably be performed with advanced engineering models for the foreseeable future. Although mechanistic studies are vital for understanding transport and developing membranes, future simulations in this area will likely need to depart from typical continuum models to provide physical insight. For enhancing the quality of modelling input, it is essential to improve the control of concentration polarization in membrane test cells.