Ion exchange enabled selective separation from decontamination to desalination to decarbonization: recent advances and opportunities

Abstract

Fundamentals of ion exchange selective separation based on electrostatic interactions and hydrated ionic radius have been well established, but until the later part of the 20th century, their primary applications were limited to softening and demineralization. With the surge of the environmental movement during the last five decades, novel ion exchange applications based on various selectivity mechanisms have emerged as major contributors toward critical environmental separations. While interests are emerging recently in transforming conventional bulk separation processes (e.g., membranes) to be more selective, a revisit to the selectivity mechanisms of conventional selective processes (e.g., ion exchange) may provide critical insights to facilitate selective separation design. The primary objectives of this review are to present some major developments and progress in using ion exchange selective separation for decontamination, desalination, and decarbonization. Specifically, we present selectivity principles and applications of conventional ion exchangers in softening, novel hybrid anion exchangers in decontaminating oxyanions, weak acid cation exchangers in desalinating and reusing wastewater, and hybrid ligand exchangers in direct air capture of CO₂. Besides selectivity, we highlight the critical role of material regenerability and regeneration process design in transforming ion exchange selective separation from chemical-driven to CO₂ or electricity-driven for better adapting to the carbon-neutral era. We envision this review to inform future selective ion exchange designs to address emerging environmental separation challenges.

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Water impact

Water treatment is increasingly transitioning from bulk separation to selective separation to be more energy and carbon-efficient. Ion exchange is a conventionally selective separation technology that has emerged as a major contributor toward critical environmental separations. A review of the selectivity mechanisms of conventional ion exchange may provide critical insights to facilitate selective separation design.

1. Introduction

Selective separation is emerging as an important approach to address critical environmental challenges. Most environmental problems require the separation of specific ions or compounds from a complex matrix (e.g., wastewater). Non-selective separations would remove as many substances as possible, which consume extra energy or carbon emissions on unnecessary separations. In contrast, selective separation allows target substance removal while preserving nontarget substances in the original matrix. Selective separation is particularly efficient when the target species is in trace amounts, e.g., metals, metalloids, nutrients, and atmospheric CO₂. The National Alliance for Water Innovation (NAWI) in the U.S. has identified precision separation (i.e., selective separation) as one of the priority areas to innovate future environmental solutions.¹ Though the definition and quantitative metrics of selectivity differ depending on materials and application scenarios, the general understanding of selectivity refers to better separation of target species than nontarget species from the original matrix. Interests in improving the selectivity of conventional non-selective separation processes are evolving (e.g., pressure-driven membranes),^2–6 however, innovating conventional selective separation processes (e.g., ion exchange) is sluggish. This review revisits some major developments and progress in using ion exchange selective separation for decontamination, desalination, and decarbonization, aiming to facilitate future selective separation designs.

Enhancing selectivity during separation advances environmental applications such as decontamination, desalination, and decarbonization. Decontamination is one of the most common environmental applications. For example, decontaminating calcium is critical in power plants to prevent boiler scales.⁷ A non-selective separation process like reverse osmosis tends to unnecessarily remove all ions from the feedwater including sodium,⁸ but selective separation like ion exchange allows only calcium removal due to the charge difference between calcium (Ca²⁺) and sodium (Na⁺).⁹ Additionally, enhancing selectivity facilitates “resource recovery” along with “decontamination”. For instance, phosphate is a contaminant in wastewater that stimulates eutrophication once wastewater is discharged into natural water systems.¹⁰ Meanwhile, phosphate is a critical nutrient in fertilizers and existing phosphate mines are depleting.¹¹ Simultaneous phosphate recovery during its decontamination from wastewater will address contamination and resource scarcity in a single step.^12,13 However, non-selective separation processes like chemical precipitation tend to remove phosphate together with many other substances such as metals, preventing the recovery of high-purity phosphate as fertilizers.^14,15 Selective phosphate separation will enhance decontamination efficiency through phosphate-specific interactions and enable phosphate recovery.^16,17 Selectivity is also increasingly demanded for desalination.^18–20 With the intrinsic permeability–selectivity trade-off of desalination membranes, the critical need for increased selectivity, not increased water permeability, has been proposed.³ Highly selective materials are in demand during desalination to selectively remove boron, neutral organic compounds, and as many as possible ions for ultra-pure water production. Additionally, saline water often contains valuable minerals in trace concentration but huge amounts such as lithium (Li⁺).^21–23 Non-selective separation of all dissolved substances together creates barriers to resource mining, thus more thorough separations with tunable selectivity towards different valuable species are beneficial to transform desalination systems to resource recovery systems.^24,25 Besides decontamination and desalination, decarbonization becomes an urgent environmental action to curb global warming.²⁶ Among different decarbonization actions, separating CO₂ from air has emerged as a critical effort.²⁷ Due to a large amount of CO₂ removal demand (e.g., 800 Mt per year in America's Zero Carbon Action plan),²⁸ recovering pure CO₂ for utilization is essential to improve the economic feasibility of decarbonization, which requires high selectivity during CO₂ separation.^29,30 From decontamination to desalination to decarbonization, selective separation plays a vital role and is expected to continue to do so in the upcoming carbon-neutral era.

Ion exchange is a conventional but promising technique for selective separation.³¹ Ion exchangers often carry functional groups with fixed charges in high densities.³² The charged functional groups are balanced by mobile counter ions, which can be exchanged by different ions according to their affinity towards the functional group via electrostatic interactions.³³ In the late 1940s, Boyd et al. reported the fundamental works of ion exchange on equilibrium and kinetics, at which time typical anion and cation exchange resins had already been invented and applied in different industries.^32–34 The fundamental works were carried out because of the high selectivity of ion exchangers towards some trans-uranium elements, which aroused great interest back at that time.^35,36 The hydrated ionic radius was found to be the decisive factor determining the ion selectivity by ion exchangers in those early works. For example, a smaller ionic radius favors higher affinity towards the ion exchanger (Fig. 1a).³⁴ The understanding was later established in two ways:³¹ 1) a smaller hydrated ionic radius indicates a smaller dehydration energy demand during adsorption that requires partial shedding of water molecules; 2) a smaller hydrated ionic radius favors stronger electrostatic interactions between ions and functional groups according to Coulomb's law, because a smaller distance between two charged points results in a stronger electrostatic force, and thus higher affinity by ion exchangers. As a result, ion exchangers naturally allow ion–ion selectivity even between the same charged ions, which has been a critical challenge for selective membrane and electrode development.^5,21,37,38 According to Coulomb's law, ions with higher charges naturally enable stronger electrostatic forces between ions and functional groups, thus ion–ion selectivity between differently charged ions is inherent in ion exchangers. Selectively separating differently charged ions has led to the widely applied ion exchange water softening process due to the higher selectivity for calcium (Ca²⁺) by cation exchangers compared to sodium (Na⁺); however, selectively separating the same charged ions by ion exchangers is underexplored at the process level. The versatile ion–ion selectivity of ion exchangers is promising for continuous exploration.

Fig. 1 Illustration of selectivity principles for different ion exchangers.

To inspire future selective separation designs, this review presents recent advances and opportunities for selective ion exchange separation. Specifically, selectivity principles and applications are elaborated for conventional ion exchangers in softening, novel hybrid anion exchangers in decontaminating oxyanions, weak acid cation exchangers in desalinating and reusing wastewater, and hybrid ligand exchangers in direct air capture of CO₂. We envision this review to inform selectivity designs to address emerging environmental separation challenges and provide insights to design other selective separation materials and processes.

2. The challenge of conventional ion exchange and selectivity reversal

Ion exchange is one of the most common water treatment processes, especially water softening by conventional strong acid cation exchangers. Conventional strong acid cation exchangers have sulfonic acid functional groups (–SO₃⁻) that interact with common cations only through electrostatic interactions.³⁹ According to Coulomb's law, ions with higher charges enable stronger electrostatic forces between ions and functional groups, thus selectively separating differently charged ions such as calcium and sodium is feasible. Conventional ion exchange softening includes two steps:⁴⁰ (1) calcium removal from feedwater such as tap water, natural surface water, and groundwater, by a sodium-form strong acid cation exchanger, during which cation exchangers adsorb calcium and release sodium (forward reaction in eqn (1)); (2) brine regeneration for the calcium-saturated cation exchanger to restore the ion exchange capacity, during which cation exchangers adsorb sodium and release calcium into the spent regenerant (backward reaction in eqn (1). However, the challenge remains on brine regeneration, which requires high concentration sodium salt (e.g., NaCl) that contributes to watershed salinity increase and is increasingly being banned in many regions such as California. Without fundamental and process-level innovation, ion exchange driven by chemical-intensive regeneration faces a critical phasing-out challenge in the coming carbon-neutral era, despite its promising potential in selective separation.^7,41To address the regeneration challenge, a fundamental understanding of the “selectivity reversal” of the cation exchangers must be established, which has been lacking despite the massive practical applications.⁴² For a strong cation exchanger that interacts with common cations only through electrostatic interactions, divalent calcium (Ca²⁺) is naturally more selective by the cation exchanger compared to monovalent sodium (Na⁺). Thus, the calcium removal step is very efficient due to the divalent-ion selectivity. However, the high calcium selectivity hinders resin regeneration, during which monovalent sodium needs to replace divalent calcium bound with the sulfonic acid functional group. Thus, a reversal of the selectivity from the divalent ion to monovalent ion is needed.

A high aqueous total concentration leads to the desired selectivity reversal.³¹ To comprehend this phenomenon, Le Chatelier's principle can be used. During an ion exchange reaction, the volume of the ion exchange resin and water remains relatively unaltered. However, the forward reaction in eqn (1) introduces an increase in the molar number of ions, i.e., one mole of calcium was replaced by two moles of sodium in the aqueous phase. Thus, the forward reaction (softening) increases the total aqueous phase molar concentration. According to Le Chatelier's principle, an increase in the total aqueous molar concentration thus favors the backward reaction to diminish the aqueous concentration, during which the ion exchange selectivity shifts from divalent ion-selective (forward reaction favored) to monovalent ion-selective (backward reaction favored). Such a selectivity reversal also exists in anion exchange reactions, e.g., sulfate–chloride exchange by a strong base anion exchange resin.⁴² As a result, existing water softening application by ion exchange is efficient in both step 1 (softening in a low aqueous concentration feedwater, Fig. 1b) and step 2 (regeneration in a high aqueous concentration brine, Fig. 1c) due to the selectivity reversal.

To address the sustainability challenges associated with the high-concentration regenerant, other approaches need to be developed to induce the selectivity reversal, such as modifying the solid-phase ion exchanger.³¹ Similar to the effect of high aqueous concentration, a decrease in the exchanger-phase capacity also favors the backward reaction in eqn (1) to diminish the aqueous phase concentration, because less calcium will be replaced by sodium, and subsequently a lower extent of aqueous concentration increase. The phenomenon also can be comprehended by a charge separation distance theory (Fig. 2).⁴³ A decrease in the exchanger-phase capacity indicates a decrease of the functional groups inside the ion exchanger, thus a longer distance between neighboring functional groups (assuming an even distribution of functional groups). A divalent calcium ion requires two functional groups (–SO₃⁻) in the strong acid cation exchanger for a stable electrostatic interaction. Thus, a longer distance between neighboring functional groups diminishes the affinity towards divalent calcium, and conversely increases the monovalent sodium selectivity. The charge separation distance theory has been used to develop the nitrate-selective ion exchange resin that significantly improves the monovalent nitrate (NO₃⁻) affinity of the anion exchanger compared to divalent sulfate (SO₄²⁻).^44,45 However, decreasing the exchanger capacity reduces the service time of the ion exchange materials, and increases the regeneration frequency, which is a tradeoff requiring further evaluation in practical applications. Considering the huge market of water softening, more fundamental and process-level research is thus needed to innovate ion exchange regeneration.

Fig. 2 Schematic illustration of selectivity reversal caused by the charge separation effect.³¹ Reprinted (adapted) with permission from ref. 31. Copyright 2017 John Wiley and Sons.

3. Proton-selective ion exchanger for CO₂-driven or electricity-driven ion exchange softening, desalination, and water reuse

Different from strong acid cation exchangers that have strong acid functional groups such as sulfonic acid (–SO₃⁻), weak acid cation exchangers carry weak acid functional groups such as carboxylic acid (–COO⁻).⁴⁶ A distinguishing fundamental difference between the two types of functional groups is their acid dissociation constant values (K_a), or pK_a (−log K_a), which led to significantly different proton selectivity of the two types of ion exchangers. Carboxylate functional groups (pK_a ∼ 5) have a high proton-selectivity even under mildly acidic conditions (e.g., 2 < pH < 5), while sulfonic acid functional groups (pK_a ∼ −7) have no proton-selectivity even under strongly acidic conditions (e.g., pH < 2).⁴⁷ The high proton selectivity of weak acid cation exchangers is often considered a drawback in real applications due to a limited working pH range. For example, weak acid cation exchangers cannot perform water softening when feedwater pH is less than 5 because the selectivity of proton surpasses calcium (Fig. 1d).^39,46 However, the unique pH sensitivity and proton selectivity favor the regeneration of weak acid cation exchangers. In contrast to strong acid cation exchangers that require high-concentration brine or acid regeneration, weak acid cation exchangers exhibit great regeneration efficiency even with a mild acidic solution such as pH 3.⁴⁸ The proton-selectivity of weak acid cation exchangers exhibits great potential to transform water softening, desalination, and water reuse applications, e.g., from chemical-driven to CO₂-driven⁴⁹ or electricity-driven processes.⁴⁶

Weak acid cation exchangers enable water softening driven by waste CO₂.⁵⁰ The pH-sensitive calcium removal capacity of a weak acid cation exchanger demonstrated the high proton-selectivity of carboxylate functional groups (Fig. 3a), which allows efficient regeneration even with a pH 3 acidic solution.⁵¹ Producing a mild acidic solution is feasible simply by dissolving CO₂ into water. CO₂-driven ion exchange was proposed and demonstrated to be feasible in the 1960s.⁵² Later on in the 1980s, the CO₂-regenerated ion exchange process (CARIX) was commercialized and used in full plants.^51,53 However, CO₂ regeneration has not been widely adopted since then and the development is sluggish due to the low CO₂ regeneration efficiency and the emergence of membranes as an alternative technique for most ion exchange applications. With the fast development and adoption of membrane technologies for over 40 years, the demand for selectivity during separation gave a chance for ion exchange to serve as an alternative to membranes.⁴⁹ However, the urgent call for climate actions requires ion exchange to rapidly shift away from a chemical-intensive process. Advancing CO₂-driven ion exchange thus is a promising strategic choice to satisfy both the selectivity and carbon-neutral demand.⁵⁴

Fig. 3 (a) pH dependence of cation removal capacity of a weak acid cation exchanger, indicating its high proton selectivity under mild acidic conditions.⁴⁶ Reprinted (adapted) with permission from ref. 46. Copyright 2020 Elsevier. (b) Calculated capacity ratio with different resin functionalization depths (shell/radius ratios) in an individual resin bead.⁵⁴ Reprinted (adapted) with permission from ref. 54. Copyright 2021 American Chemical Society. (c) Schematic of the CO₂-driven ion exchange desalination process illustrating both the desalination and CO₂ regeneration cycle.⁴⁹ Reprinted (adapted) with permission from ref. 49. Copyright 2018 American Chemical Society.

The key challenge of CO₂ regeneration is the low regeneration efficiency caused by the low concentration of proton in the carbonic acid, which requires the cation exchanger to have high proton selectivity and fast regeneration kinetics.⁵⁴ Weak acid cation exchangers provide the desired proton selectivity but still exhibit slow regeneration kinetics. The regeneration kinetics is governed by the intraparticle diffusion kinetics of the ion and counterion, e.g., calcium and proton, respectively, during water softener regeneration.⁴⁷ Furthermore, the diffusion kinetics is regulated by the intraparticle diffusivity of specific ions inside the ion exchanger, and the intraparticle diffusion path length.⁵⁵ The diffusivity of specific ions is determined by the properties of the ion (e.g., ionic size) and the intraparticle diffusion space (e.g., pore size). The ion property is difficult to alter in a decided application, e.g., calcium and proton during water softening. However, the intraparticle diffusion space, i.e., the internal structure of the ion exchanger can be modified during polymer synthesis. A widely adopted modification is the macro-porous ion exchangers, which have a much larger pore size compared to gel-type ion exchangers.⁵⁶ The large pore size improves the ion diffusivity and diffusion kinetics, which has been used in early CO₂-regeneration trials.⁵²

Besides ion diffusivity, shortening the intraparticle diffusion path length is another route to improve the diffusion kinetics.⁵⁵ In the 2000s, cylindrical ion exchange fibers were used in CO₂-regeneration processes, which significantly improved the regeneration efficiency compared to spherical resin beads.⁵⁵ The key feature of ion exchange fiber is the much shorter diameter (e.g., <50 μm), thus a shorter intraparticle diffusion path length, compared to spherical resin beads (e.g., >500 μm).⁵⁷ However, fibers have low capacity and commercial availability compared to resins, hindering further adoption. A recent advance of CO₂ regeneration is using a shallow-shell spherical resin,⁴⁹ which only has functional groups in a shallow shell of the resin bead surface, instead of an even distribution inside the entire bead. The shallow shell structure enables a short intraparticle diffusion path length and minimizes the trade-off of ion exchange capacity. For example, less than 10% capacity loss is enabled with over 30% reduced functionalization depth (intraparticle diffusion path length, Fig. 3b).⁵⁴ A full plant has been constructed and in operation using CO₂-regeneration of shallow-shell weak acid cation exchangers since 2020 in Shandong, China.⁵⁸

CO₂-regenerated cation exchangers enable desalination and decontamination by integrating with anion exchangers.⁴⁹ Integrating cation and anion exchangers enables the exchange of both cations and anions in water with desired ions initially loaded on resin. Conventional ion exchange desalination/demineralization processes exchange aqueous cations and anions with protons (H⁺) and hydroxide ions (OH⁻), respectively, which convert aqueous salts into pure water.⁵⁹ However, regeneration of cation and anion exchangers requires a strong acid to provide proton, and a strong base to provide hydroxide ions. To address the challenge, desalination can be achieved alternatively by exchanging aqueous cations and anions with protons (H⁺) and bicarbonate ions (HCO₃⁻), respectively, which convert aqueous salts into carbonic acid (H₂CO₃) and subsequently into CO₂ and water after CO₂ stripping.^50,54 In this way, CO₂ alone allows the regeneration of both cation and anion exchangers by providing protons and bicarbonate ions. During CO₂ regeneration, cation and anion exchange columns connect in sequence (Fig. 3c). The CO₂-pressurized water (carbonic acid, H₂CO₃) flows through the cation exchange column first, in which protons (H⁺) regenerate the cation exchange resin. As previously discussed, this step requires the resin to have high proton selectivity and regeneration kinetics; thus a shallow-shell weak acid cation exchanger is a suitable candidate.⁵⁴ After the cation exchanger regeneration, a high concentration of bicarbonate is produced in the CO₂-pressurized water due to the proton uptake by the cation exchangers. The high concentration of bicarbonate (HCO₃⁻) can subsequently regenerate the anion exchange column. The high-concentration bicarbonate performs similarly to high-concentration brine regenerant, thus conventional anion exchangers can be used, allowing versatile anion removal applications.

The desalination performance has been validated in the field using CO₂ regeneration of a shallow-shell weak acid cation exchanger and a strong base anion exchanger.⁵⁹ Notably, the desalination performance was dramatically different with different column configurations, i.e., anion exchanger proceeds cation exchanger (AX–CX) versus CX–AX. After CO₂ regeneration, around 80% of total dissolved solids (TDS) were removed using the AX–CX configuration, but little desalination was achieved by CX–AX (Fig. 4a). Note that the CX–AX configuration is widely applied in conventional ion exchange softening/demineralization processes, however, it is not suitable for CO₂-regenerated desalination. The reason is due to the required aqueous buffering capacity for continuous cation exchange by the weak acid cation exchanger. With AX–CX, the AX column in front can exchange aqueous anions with the loaded bicarbonate ions, which can neutralize the released protons by the following cation exchanger and buffer the pH change in the CX column. Otherwise if using a CX–AX configuration, the aqueous pH will drop immediately with limited buffering capacity of the feedwater, which prevents the cation exchange by the weak acid cation exchangers due to their high proton-selectivity and pH sensitivity. A similar cation removal performance with TDS removal (Fig. 4b) demonstrated the effect of the column configuration on the cation exchanger, and the anion exchange was not affected by such configuration changes (Fig. 4c). In addition to desalination, decontamination was also feasible by replacing anion exchangers. A nitrate-selective anion exchanger (NSR) has been tested in the CO₂-driven system, which was demonstrated to be feasible for continuous nitrate removal with CO₂ regeneration (Fig. 4d).^54,60–62 A high concentration of bicarbonate is key for regeneration, which demands a high CO₂ pressure, and selective and fast proton uptake by the cation exchanger, highlighting the importance of the proton-selective shallow-shell cation exchanger. Simultaneous nitrate removal and desalination have been demonstrated with CO₂ regeneration, which distinguishes it from conventional brine regeneration (Fig. 4e). Additionally, CO₂ regeneration eliminates additional chemical inputs and enables over 50 times lower salinity in the spent regenerant compared to conventional brine regeneration (Fig. 4f). Other decontamination applications such as phosphate and sulfate are also feasible by using different anion exchangers.⁴⁹ As a result, conventional ion exchange applications including water softening, desalination, and decontamination are promising to be transformed into CO₂-driven processes by leveraging proton-selective cation exchangers. The transformation not only addresses the ion exchange regeneration challenge, but also enables the water sector to contribute to climate actions through CO₂ utilization to facilitate a circular carbon economy.

Fig. 4 Effluent history of (a) TDS, (b) magnesium, and (c) sulfate during two different system configurations using CO₂ regeneration: anion exchanger proceeding the proton-selective cation exchanger (AX–CX) versus CX–AX.⁵⁹ Reprinted (adapted) with permission from ref. 59. Copyright 2020 Elsevier. (d) Repeated nitrate removal cycles from real groundwater with intermittent CO₂ regeneration. Comparison of conductivity in the effluent of (e) treatment cycle and (f) regeneration cycle using CO₂ regeneration versus conventional brine regeneration.⁵⁴ Reprinted (adapted) with permission from ref. 54. Copyright 2021 American Chemical Society.

In addition to CO₂ regeneration, weak acid cation exchangers also enable regeneration driven by electricity. The proton demand during regeneration can be not only provided by carbonic acid (CO₂ dissolution in water) but also by electrochemical water splitting or dissociation. Electrified ion exchange regeneration was reported in the 1950s by Spiegler et al.⁶³ The main mechanism is to produce protons by electrochemically oxidizing water (water splitting: 2H₂O → O₂ + 4H⁺), followed by proton regeneration of cation exchangers. Based on the reactor design, the regeneration can be categorized as in situ (Fig. 5a and b) or ex situ (Fig. 5c and d) regeneration, i.e., whether resins are packed into the electrochemical reactor or not. In the former case (in situ) where resins are packed into the electrochemical reactor, the system is highly integrated for easy operation. However, the resins are often in close contact with the oxidizing electrode which may shorten the resin life (Fig. 5a). In the latter case (ex situ) where the electrochemical reactor and resin columns are separated, the system is flexible in operation but the produced acid transportation increases the operation complexity (Fig. 5c). Electrochemical water dissociation enabled by bipolar membranes (BPM) is another approach to producing protons besides water splitting. Water dissociation directly separates water molecules into protons and hydroxide ions (H₂O → H⁺ + OH⁻), which requires less voltage compared to water splitting and is increasingly being used for acid production.⁶⁴ Electrified ion exchange ex situ regeneration was reported using acid produced by BPM-facilitated water dissociation.⁶⁵ However, the goal of producing high-concentration acid and acid transportation inevitably contributes to high energy consumption. Electrified in situ ion exchange regeneration leveraging proton-selective weak acid cation exchangers and BPM-facilitated water dissociation facilitates addressing the abovementioned challenges.⁶⁶ An electrified ion exchange stripping process (EXS) was reported.⁶⁶ EXS has weak acid cation exchangers packed into an electrochemical reactor with a bipolar membrane (Fig. 5b). Several advantages were reported by EXS compared to BPM-facilitated ex situ regeneration and water splitting-based regeneration: (1) the high proton affinity of the weak acid cation exchanger enables efficient resin regeneration with only a mild acid instead of a high-concentration acid, (2) the BPM avoids close contact between the resin and the electrode that helps prolong the resin and electrode life, and (3) the compact in situ regeneration design enables integrated systems for decentralized applications.

Fig. 5 Illustration of electrified ion exchange by electrochemical water splitting or dissociation. (a) Electrified in situ ion exchange regeneration by water splitting; (b) electrified in situ ion exchange regeneration by BPM-facilitated water dissociation; (c) electrified ex situ ion exchange regeneration by water splitting; (d) electrified ex situ ion exchange regeneration by BPM-facilitated water dissociation.

4. Ligand-selective hybrid anion exchanger: Donnan-principle for oxyanion decontamination and the opportunities for electrified regeneration

Besides electrostatic interactions, introducing Lewis acid–base interactions into ion exchangers opens a new path for selective ion separation.⁶⁷ Many aqueous contaminants are Lewis bases (i.e., electron pair donors), such as oxyanions like arsenic, phosphate, and selenium.^68,69 These contaminants can selectively bind to transition metals through Lewis acid–base interactions, even with a major presence of common aqueous ions such as chloride and sulfate (Fig. 1e). Metal oxide nanoparticles have been demonstrated as effective adsorbents for decontamination applications, such as arsenic removal.⁷⁰ However, the separation of metal oxide nanoparticles from treated water is a practical separation challenge due to their small particle sizes. Supporting metal oxide nanoparticles with different host materials thus becomes a promising strategy, e.g., using activated carbon,⁷¹ chitosan,⁷² alginates,⁷³ and zeolite.⁷⁴ Among different host materials, ion exchange resins stand out due to the Donnan membrane effect,⁷⁵ which significantly enhances arsenic removal by iron oxide nanoparticles hosted inside anion exchange resins.⁷⁶

The Donnan membrane effect arises from the inability of ions to diffuse out from one phase in a heterogeneous system, which can regulate the equilibrium ion distribution in different phases.⁷⁵ The conventional Donnan membrane effect was studied by introducing large-size ions into one side of a two-chamber reactor separated by a semi-permeable membrane.⁷⁷ Due to the inability of the large-size ion to diffuse from one side to another side across the membrane, the equilibrium distribution of other diffusible ions can be significantly altered in the two chambers. Similarly, ion exchange resins have a high density of charged functional groups that are unable to diffuse out from the resin phase, which alters the ion distribution in the resin phase and aqueous phase in a water–resin mixed heterogeneous system.⁷⁸ The water–resin system does not have a physical membrane, however, the Donnan membrane effect exists and distinguishes resins from other host materials for metal oxide nanoparticles. The hybrid anion exchanger has iron oxide nanoparticles doped inside an anion exchanger that has a high density of positively charged functional groups. Due to the immobility of the functional groups, mobile anion concentration (including arsenic) inside the anion exchanger is much higher than the aqueous phase due to the Donnan membrane effect (Fig. 6a).⁷⁹ In contrast, a cation exchanger has a high density of negatively charged functional groups, which will reject anions (Fig. 6b). As a result, the doped iron oxide nanoparticles confront elevated concentrations of anions compared to the aqueous phase in a hybrid anion exchanger, and form Lewis acid–base interactions with arsenic, enabling significantly enhanced arsenic removal capacity compared to a hybrid cation exchanger (Fig. 6c) and to iron oxide nanoparticles alone without a host (Fig. 6d).⁸⁰ The synergy of ion selectivity and the Donnan membrane effect results in the hybrid anion exchanger being one of the most effective arsenic decontamination materials, which has been commercialized since the early 2000s and has been used in full plants (Fig. 6d).^17,76,81

Fig. 6 Schematic illustrating the “Donnan membrane effect” with resins: (a) enhanced permeation of anions into the hybrid sorbent in the presence of non-diffusible cations (anion exchanger) and (b) exclusion of anions from the hybrid sorbent in the presence of non-diffusible anions (cation exchanger). HFO: hydrated ferric oxide nanoparticles.⁶⁷ (c) Comparison of arsenic removal by a hybrid anion exchanger (HAIX) and hybrid cation exchange (HCIX). Reprinted (adapted) with permission from ref. 63. Copyright 2005 American Chemical Society. (d) Arsenic removal by granulated ferric oxide (GFO) versus HAIX during a pilot run before a full plant installation (shown in the picture) in Sahuarita, Arizona.³¹ Reprinted (adapted) with permission from ref. 31. Copyright 2017 John Wiley and Sons.

Hybrid anion exchangers have been field-validated in the Indian subcontinent and have continuously served arsenic-free drinking water in some communities for over 10 years.⁸² However, the regeneration challenge stands out as similar to other ion exchange processes. Regeneration efficiency has been identified as a tradeoff along with increasing selectivity.¹² Hybrid anion exchangers have high selectivity toward Lewis's base anions owing to the high affinity, which creates difficulties during regeneration when desorption of Lewis's base anions is needed.⁴⁶ Regeneration is particularly important when the Lewis's base anion has recovery values, such as phosphate.^83,84 Selective trace phosphate removal by hybrid anion exchangers is a promising technique and significant research progress has been made to improve phosphate selectivity by using different metal oxide nanoparticles, such as from iron to lanthanum.^85,86 However, the regeneration approach by using strong base solutions remains the same for decades, and becomes more difficult along with improved selectivity, e.g., requiring heating in addition to high-concentration base solutions for La-based hybrid anion exchangers.⁸⁵

Transforming chemical-driven decontamination applications into electricity-driven is feasible by improving the regenerability of hybrid anion exchangers based on their pH sensitivity.⁴⁶ Metal oxide nanoparticles inside the hybrid anion exchanger enhance the selectivity towards Lewis's base anions, but with a strong dependency on the aqueous pH.⁸⁷ Different metal oxide nanoparticles have different pK_a values that can regulate the surface charge based on aqueous pH. The surface charge of iron oxide nanoparticles shifts from primarily neutral at pH 7 to negative at pH 11, resulting in selective phosphate removal at neutral pH, but strong phosphate desorption at mild basic pH.⁸³ Mild basic solution can be readily produced with electrochemical approaches, such as water splitting.^46,65,88,89 With a simple electrochemical reactor containing two electrodes and an ion exchange membrane (Fig. 7a), the electrolyte pH can readily be elevated to pH 11 even with a low current density. The pH changes remain similar with decreasing chemical inputs in the electrolyte (Fig. 7b), providing a pathway for reducing the demand for chemical inputs. The desorption of phosphate from iron-based hybrid anion exchangers has been demonstrated with electrochemically produced pH 11 solutions, and 50% capacity was maintained for multiple cycles.⁴⁶ Though there was a capacity loss, the required concentration of base solutions dropped 100–1000 times (from above pH 13 to pH 11), enabling electricity-driven selective phosphate separation. Note that electrochemical base production is energy-intensive if aiming at producing high concentrations of hydroxide ions. However, turning a pH 7 solution to pH 11 is very efficient via electrochemical water splitting, making electricity-driven hybrid anion exchanger regeneration a promising technique in selective decontamination applications.⁴⁶ Regeneration tests comparing the hybrid anion exchanger (HAIX, with iron oxide nanoparticles) and the parent resin (SBA, without iron oxide nanoparticles) confirmed that the regenerability by a mild base solution (e.g., pH 11) was purely induced by the doped iron oxide nanoparticles, and the regeneration was caused by the solution pH, not the background ions (Fig. 7c and d). The results revealed that iron oxide nanoparticles not only improved the material selectivity towards ligand anions, but also improved the regenerability and enabled a new pathway for electrified ion exchange regeneration.

Fig. 7 (a) Electrochemical reactor setup for electro-assisted mild base production and (b) transient pH changes in the cathode chamber with different electrolyte concentrations. Capacities of (c) hybrid anion exchanger (HAIX synthesized by SBA dopped with ferric oxide nanoparticles, FeOnp) and (d) strong base anion exchanger (SBA, without ferric oxide nanoparticles) after regeneration with different regenerants. The same color but different fill patterns indicate the same solution before and after electrochemical water spitting for pH elevation (e.g., Eff. = treated effluent, Catho-Eff = catholyte made from the effluent). Regenerant pH rather than background ions dominated HAIX regeneration and confirmed that FeOnp primarily caused elevated regeneration efficiency of HAIX under pH 11. Error bars represent one standard deviation; error bars not shown are smaller than symbols.⁴⁶ Reprinted (adapted) with permission from ref. 46. Copyright 2020 Elsevier.

5. Selective process design to expand the capability of hybrid anion exchangers: Bio-Nano-IX process for selective selenium separation

Besides arsenic and phosphate, selenium is another oxyanion that contributes to water contamination. Selenate, as the most stable selenium oxide, is highly soluble and often the dominant species of selenium in natural water. Meanwhile, in many bodies of selenium-contaminated water, sulfate (SO₄²⁻) is present, and often at nearly two to three orders of magnitude greater than selenate (SeO₄²⁻). The high concentration of sulfate is the biggest hurdle to achieving selective selenium separation because both selenate and sulfate are divalent charged ions with similar molecular structures.⁹⁰ Removing selenate from the background of sulfate by physical–chemical processes remains an engineering challenge.⁶⁹

The capability of a hybrid anion exchanger in selective selenium separation diminishes because the predominant species selenate, Se(VI), is not a ligand. To address the challenge, process design can be leveraged to achieve selective separation and expand the capability of hybrid anion exchangers in addressing non-ligand oxyanion separation challenges. Chen et al. designed a tailored hybrid biological-ion exchange separation process (Bio-Nano-IX process, Fig. 8a)⁶⁹ and provided a new pathway to address the selenium challenge. During the first step of the Bio-Nano-IX process, a facultative bacterium under anoxic conditions converts Se(VI) or SeO₄²⁻ to Se(IV) or HSeO₃⁻ in the biological column. With the conversion, the total dissolved selenium concentration remains the same (Fig. 8b), but the non-ligand selenate, Se(VI), was converted to a ligand anion, selenite, Se(IV). Then in the second step, the hybrid anion exchanger selectively removed selenite in the presence of high-concentration sulfate due to the Lewis acid–base interactions between metal oxide nanoparticles and selenite, providing selenium-safe water and ensuring that sulfate concentration remains unchanged. The virtually comparable selenium effluent histories (Fig. 8c) confirmed that the hybrid biological-ion exchange separation process is sustainable with intermittent regenerations, demonstrating the feasibility of hybrid anion exchangers for selectively separating selenium by converting its non-ligand species to ligand species through process design. For practical applications in the future, the Bio-Nano-IX process still needs to address the regeneration challenge of the hybrid anion exchanger that requires alkali solutions. Integrating electricity-driven hybrid anion exchanger regeneration may be a promising optimization direction for the Bio-Nano-IX process.

Fig. 8 (a) Process schematic illustrating the two-step hybrid biological-ion exchange separation process combining bioreduction of selenate to selenite and subsequent selective sorption of selenite onto hybrid anion exchangers without being influenced by sulfate. (b) Bar chart illustrating total selenium and relative distribution of selenate and selenite in the feed and treated water collected after 500-bed volumes. (c) Selenium effluent histories for three consecutive column runs of the hybrid biological-ion exchange process.⁶⁹ Reprinted (adapted) with permission from ref. 65. Copyright 2021 American Chemical Society.

6. Hybrid ligand exchanger: ligand-selective ion exchange for direct air capture of CO₂ and seawater regeneration

Instead of doping metal oxide nanoparticles inside the resin without affecting the original functional group in hybrid anion exchangers, using metal ions to modify the existing functional groups to form hybrid ligand exchangers is another way to regulate ion selectivity for ion exchange separation (Fig. 1f).^68,91,92 Hybrid ligand exchangers were proposed for the decontamination of anionic ligands in the 1990s by anchoring copper ions onto chelating polymeric resins with nitrogen donor atoms as functional groups.⁹³ By choosing appropriate nitrogen-containing functional groups, copper ions can be anchored to the functional groups through Lewis acid–base interactions, leaving the positive charges of copper ions unnaturalized. The anchored copper ions thus become anion exchange sites that enable the selective removal of ligand anions via both electrostatic and Lewis acid–base interactions, such as phosphate, arsenate, and selenite.^94,95

Besides decontamination, the hybrid ligand exchanger enables high-capacity decarbonization that significantly advances direct air capture of CO₂.⁹⁶ By loading copper ions onto a weak base anion exchanger with polyamine functional groups, copper ions were stably anchored onto polyamine with the two positive charges unneutralized. The copper(II) ions thus serve as anion exchange sites enabling the binding of hydroxide ions, and the subsequent adsorption of CO₂ as bicarbonate/carbonate through acid–base reactions (Fig. 9a).⁹⁶ After exhaustion, the polyamine-copper based hybrid sorbent can be efficiently regenerated through conventional aqueous regenerant containing salts such as sodium chloride, through anion exchange reactions between bicarbonate/carbonate and chloride. Following a mild base solution rinse after salt regeneration, the sorbent can be used in multiple cycles for CO₂ capture. Two improvements were made by the polyamine-copper based hybrid sorbent compared to conventional amine-based solid sorbents, e.g., the original anion exchanger with polyamine functional groups but without copper. First, polyamine-copper sorbent offered nearly 2–3 times higher capacity than purely polyamine sorbents.⁹⁷ The dramatic capacity increase after copper loading is because of the transformation of weak base polyamine sites into strong base sites by copper. Weak base sites exhibit pH-dependent anion adsorption capacity, which drops with increasing pH from 3–6. However, strong base sites are not pH-dependent, thus exhibiting stable and higher capacity than weak base sites across the entire pH range from 3–11 (Fig. 9b). At ambient CO₂ partial pressure, the equilibrium aqueous pH is typically around 4–6 which causes the higher capacity of the polyamine-copper sorbent than purely polyamine sorbent. Second, aqueous salt regeneration of the polyamine-copper sorbent facilitates sustainable CO₂ capture using seawater. Conventional amine-based solid sorbents use temperature or pressure-swing approaches for regeneration, which is energy-intensive. Anion exchange-based aqueous salt regeneration avoids the demand of temperature or pressure swings, and the required salt solution can be readily provided by seawater. Atlantic seawater was tested and demonstrated to be feasible for regenerating the polyamine-copper sorbent and enabling multiple-cycle CO₂ capture (Fig. 9c and d). The used seawater regenerant containing extra bicarbonate can be released back into the ocean to help reverse the ocean acidification or for subsequent CO₂ recovery. Notably, besides salt solution regeneration, the polyamine-copper based hybrid sorbent also can be regenerated via temperature swing similar to conventional amine-based solid sorbents,^27,98 enabling versatile operations in real applications.

Fig. 9 (a) Schematics of individual steps of the gradual progression of CO₂ sorption by the polyamine–Cu(II) complex. (1: CO₂ dissolution, 2: transport of non-ionized H₂CO₃ inside the ion exchanger, and 3: rapid neutralization with OH⁻ followed by selective binding of HCO₃⁻). (b) Capacity comparison of Polyam-N-Cu²⁺ and its parent resin Polyam-N at different pH conditions, evidencing the transformation of the weak base nitrogen site into strong base site by copper loading. (c) CO₂ effluent histories for direct air capture of CO₂ during 15 consecutive cycles with Polyam-N-Cu²⁺ and (d) elution histories of alkalinity from the CO₂-saturated Polyam-N-Cu²⁺ during regeneration using seawater from the Atlantic Ocean.⁹⁶ Reprinted (adapted) from ref. 93, an open-access article.

With all the advances of decarbonization through the polyamine-copper hybrid ligand exchanger, the challenge remains on the conversion of the copper sites to hydroxide-ion form after salt regeneration.⁹⁶ Future research on decentralized hydroxide ion production driven by electricity or other clean energy is needed to facilitate the technology adoption.⁶⁶ Due to the ligand property of hydroxide ions, the exchange of chloride ions on the copper site (after salt regeneration) by hydroxide ions is also selective and efficient. Thus, a mild base solution at pH 11–12 is likely sufficient, which is promising to reduce energy consumption through decentralized hydroxide ion production.^46,66 Detailed economic analysis and life cycle assessment also need to be performed for future validations.

7. Designing selective ion exchange processes to minimize ion exchange regeneration

Besides transforming ion exchange regeneration from conventional chemical-driven to CO₂ or electricity-driven, designing selective ion exchange processes by minimizing or eliminating regeneration is another promising route to address regeneration challenges. The origin of the regeneration demand is the use of ion exchange materials as both separation and storage media, e.g., target ions are separated from wastewater and simultaneously stored in ion exchange resins. Avoiding the storage step is key to minimizing or eliminating regeneration. Two methods can potentially avoid the storage step: combining ion exchange separation with a species-transformation process, or using ion exchange membranes instead of resins. Species-transformation processes such as biological nitrate reduction can convert ionic species into gas, thus minimizing ion storage in ion exchange resins. Ion exchange membranes have limited ion storage capacity compared to resins, thus enabling ion separation through ion transport across the membrane without much retention or storage. A biological nitrate-selective ion exchange process (BIO-NIX)⁶² and an ion exchange membrane-enabled Donnan dialysis process⁹⁹ have demonstrated the feasibility of minimizing/eliminating ion exchange regeneration via process design. The BIO-NIX process combines a fixed-bed biological denitrification (BIO) column with a nitrate-selective ion exchange (NIX) column in sequence. The BIO column contains highly porous ceramic materials for hosting denitrifying bacteria, which enable the transformation of over 90% nitrate into nitrogen gas. The transformation minimizes ion storage in the NIX column that contains nitrate-selective ion exchange resins. Meanwhile, selective nitrate separation enabled by the NIX column significantly enhances the resilience of the BIO column under unstable operating conditions (e.g., carbon shortage, influent nitrate surge, and flowrate jump). As a result, the BIO-NIX system achieved resilient nitrate removal for over two months without any intermittent resin regeneration.⁶² The ion exchange membrane-enabled Donnan dialysis process⁹⁹ leverages electrochemical potential as the driving force to transport phosphate ions from one side (wastewater) to another side (draw solution) of an anion exchange membrane. The use of an ion exchange membrane eliminated the regeneration process when using resins. Several recent advances of Donnan dialysis include using ligands to enable extracting phosphate from alum-laden waste activated sludge instead of wastewater,⁹⁹ using tubular anion- and cation-exchange membranes to simultaneously recover ammonium and phosphate from urine as struvite,¹⁰⁰ and more.¹⁰¹ While these new processes are promising to minimize or eliminate regeneration, more work is needed beyond proof-of-concept demonstration to facilitate field implementations.

Challenges and opportunities

Ion exchange selective separation advances decontamination, desalination, and decarbonization, but several challenges remain and bring opportunities for innovating the design of materials and processes. For material design, regenerability should be carefully considered along with selectivity. Due to the promising demand for selective separation, ion exchange selectivity is expected to continue evolving, from calcium, arsenic, phosphate, nitrate, and CO₂ to many other species. However, regenerability is often a tradeoff of high selectivity due to the requirement of desorbing high-affinity species from functional sites. pH-sensitive functionality has set an example for tunable ion affinity control to balance selectivity and regenerability,⁴⁶ but future fundamental research to develop other tunable functionalities is needed. For process design, shifting away from chemical-intensive regeneration is an urgent task due to the emerging bans of brine discharge and the climate action calls. Envisioning the next-generation ion exchange selective separation requires a novel process design not driven by conventional chemicals such as brine, strong acids, and strong bases. CO₂-driven ion exchange still requires a chemical input, however, the demand for CO₂ is promising to facilitate CO₂ utilization and subsequently benefit CO₂ capture.⁵⁴ Besides CO₂ capture and utilization, CO₂ sequestration has been reported in the CO₂-driven ion exchange softening system by precipitating CO₂ as calcium carbonate using calcium selectively separated from hard water.⁴⁸ Thus, ion exchange selective separation can contribute to the entire technology chain of CO₂ capture, utilization, and sequestration (CCUS). The CO₂ economy is expected to continue developing, thus CO₂-driven ion exchange represents a possible process model for the carbon-neutral era. Electrification is also a trend for the carbon-neutral era. Many sectors such as transportation are transforming as electricity-driven because of the climate benefits. Thus, electricity-driven ion exchange is an important development direction to avoid phasing out of the ion exchange selective separation techniques.^102–107 The combination of electrochemical acid–base production with pH-sensitive ion exchangers is a good start for developing electricity-driven ion exchange selective separation processes,^46,66 however, challenges remain in energy efficiency improvement and technology adoption. Besides CO₂-driven and electricity-driven process design, a more diverse technology portfolio is also needed to better adopt the carbon-neutral era. With the transformation of ion exchange selective separation, many other processes also can be innovated. For example, CO₂-driven and electricity-driven ion exchange have been reported as anti-scaling pretreatment for membrane processes due to the versatile selectivity towards scaling ions (sulfate, calcium, silica, etc.).^42,47,59,108 Due to these challenges and opportunities, we envision this review on fundamental principles and real applications of ion exchange selective separation to provide insights for future material and process design in advancing selective separation.

Conflicts of interest

There are no conflicts to declare.

References

1. M. S. Mauter and P. S. Fiske, Desalination for a circular water economy, Energy Environ. Sci., 2020, 13(10), 3180–3184.
2. H. B. Park, J. Kamcev, L. M. Robeson, M. Elimelech and B. D. Freeman, Maximizing the right stuff: The trade-off between membrane permeability and selectivity, Science, 2017, 356(6343), 1138–1148.
3. J. R. Werber, A. Deshmukh and M. Elimelech, The Critical Need for Increased Selectivity, Not Increased Water Permeability, for Desalination Membranes, Environ. Sci. Technol. Lett., 2016, 3(4), 112–120.
4. R. M. DuChanois, C. J. Porter, C. Violet, R. Verduzco and M. Elimelech, Membrane Materials for Selective Ion Separations at the Water–Energy Nexus, Adv. Mater., 2021, 2101312(38), 1–18.
5. R. Epsztein, R. M. DuChanois, C. L. Ritt, A. Noy and M. Elimelech, Towards single-species selectivity of membranes with subnanometre pores, Nat. Nanotechnol., 2020, 15(6), 426–436.
6. Y. Zhao, T. Tong, X. Wang, S. Lin, E. M. Reid and Y. Chen, Differentiating Solutes with Precise Nanofiltration for Next Generation Environmental Separations: A Review, Environ. Sci. Technol., 2021, 55(3), 1359–1376.
7. P. Clauwaert, J. De Paepe, F. Jiang, B. Alonso-Fariñas, E. Vaiopoulou and A. Verliefde, et al., Electrochemical tap water softening: A zero chemical input approach, Water Res., 2020, 169, 115263.
8. P. Čuda, P. Pospíšil and J. Tenglerová, Reverse osmosis in water treatment for boilers, Desalination, 2006, 198(1–3), 41–46.
9. J. Li, S. Koner, M. German and A. K. Sengupta, Aluminum-Cycle Ion Exchange Process for Hardness Removal: A New Approach for Sustainable Softening, Environ. Sci. Technol., 2016, 50(21), 11943–11950.
10. B. E. Rittmann, B. Mayer, P. Westerhoff and M. Edwards, Capturing the lost phosphorus, Chemosphere, 2011, 84(6), 846–853.
11. T. A. Larsen, M. E. Riechmann and K. M. Udert, State of the art of urine treatment technologies: A critical review, Water Res.: X, 2021, 13, 100114.
12. B. Wu, J. Wan, Y. Zhang, B. Pan and I. M. C. Lo, Selective Phosphate Removal from Water and Wastewater using Sorption: Process Fundamentals and Removal Mechanisms, Environ. Sci. Technol., 2020, 4(1), 50–66.
13. Y. Lei, B. Song, R. D. Van Der Weijden, M. Saakes and C. J. N. Buisman, Electrochemical Induced Calcium Phosphate Precipitation: Importance of Local pH, Environ. Sci. Technol., 2017, 51(19), 11156–11164.
14. K. S. Le Corre, E. Valsami-Jones, P. Hobbs and S. A. Parsons, Impact of calcium on struvite crystal size, shape and purity, J. Cryst. Growth, 2005, 283(3–4), 514–522.
15. O. Lahav, M. Telzhensky, A. Zewuhn, Y. Gendel, J. Gerth and W. Calmano, et al., Struvite recovery from municipal-wastewater sludge centrifuge supernatant using seawater NF concentrate as a cheap Mg(II) source, Sep. Purif. Technol., 2013, 108, 103–110.
16. R. Liu, L. Chi, X. Wang, Y. Wang, Y. Sui and T. Xie, et al., Effective and selective adsorption of phosphate from aqueous solution via trivalent-metals-based amino-MIL-101 MOFs, Chem. Eng. J., 2019, 357, 159–168.
17. S. Sengupta and A. Pandit, Selective removal of phosphorus from wastewater combined with its recovery as a solid-phase fertilizer, Water Res., 2011, 45(11), 3318–3330.
18. J. R. Werber, C. O. Osuji and M. Elimelech, Materials for next-generation desalination and water purification membranes, Nat. Rev. Mater., 2016, 1(5), 16018.
19. K. Chang, H. Luo and G. M. Geise, Water content, relative permittivity, and ion sorption properties of polymers for membrane desalination, J. Membr. Sci., 2019, 574, 24–32.
20. N. Nadav, Boron removal from seawater reverse osmosis permeate utilizing selective ion exchange resin, Desalination, 1999, 124(1–3), 131–135.
21. C. Liu, Y. Li, D. Lin, P. C. Hsu, B. Liu and G. Yan, et al., Lithium Extraction from Seawater through Pulsed Electrochemical Intercalation, Joule, 2020, 4(7), 1459–1469.
22. T. Ryu, Y. Haldorai, A. Rengaraj, J. Shin, H. J. Hong and G. W. Lee, et al., Recovery of Lithium Ions from Seawater Using a Continuous Flow Adsorption Column Packed with Granulated Chitosan-Lithium Manganese Oxide, Ind. Eng. Chem. Res., 2016, 55(26), 7218–7225.
23. Z. Li, C. Li, X. Liu, L. Cao, P. Li and R. Wei, et al., Continuous electrical pumping membrane process for seawater lithium mining, Energy Environ. Sci., 2021, 14(5), 3152–3159.
24. H. Fan, Y. Huang and N. Y. Yip, Advancing ion-exchange membranes to ion-selective membranes: principles, status, and opportunities, Front. Environ. Sci. Eng., 2023, 17(2), 1–27.
25. D. M. Miller, K. Abels, J. Guo, K. S. Williams, M. J. Liu and W. A. Tarpeh, Electrochemical Wastewater Refining: A Vision for Circular Chemical Manufacturing, J. Am. Chem. Soc., 2023, 145(36), 19422–19439.
26. K. S. Lackner, Capture of carbon dioxide from ambient air, Eur. Phys. J.: Spec. Top., 2009, 176(1), 93–106.
27. E. S. Sanz-Pérez, C. R. Murdock, S. A. Didas and C. W. Jones, Direct Capture of CO2 from Ambient Air, Chem. Rev., 2016, 116(19), 11840–11876.
28. S. D. S. Network, America's Zero Carbon Action Plan, 2020.
29. A. Kätelhön, R. Meys, S. Deutz, S. Suh and A. Bardow, Climate change mitigation potential of carbon capture and utilization in the chemical industry, Proc. Natl. Acad. Sci. U. S. A., 2019, 166(23), 11187–11194.
30. C. Hepburn, E. Adlen, J. Beddington, E. A. Carter, S. Fuss and N. Mac Dowell, et al., The technological and economic prospects for CO2 utilization and removal, Nature, 2019, 575(7781), 87–97.
31. A. K. SenGupta, Ion Exchange in Environmental Processes, Wiley, Hoboken, New Jersey, 2017.
32. G. E. Boyd, J. Schubert and A. W. Adamson, The Exchange Adsorption of Ions from Aqueous Solutions by Organic Zeolites. III. Performance of Deep Adsorbent Beds under Non-equilibrium Conditions, J. Am. Chem. Soc., 1947, 69(11), 2818–2829.
33. G. E. Boyd, J. Schubert and A. W. Adamson, The Exchange Adsorption of Ions from Aqueous Solutions by Organic Zeolites. II. Kinetics, J. Am. Chem. Soc., 1947, 69(11), 2818–2829.
34. G. E. Boyd, J. Schubert and A. W. Adamson, The Exchange Adsorption of Ions from Aqueous Solutions by Organic Zeolites. I. Ion-exchange Equilibria 1, J. Am. Chem. Soc., 1947, 69(11), 2818–2829.
35. E. R. Tompkins, J. X. Khym and W. E. Cohn, Ion-Exchange as a Separations Method. I. The Separation of Fission-Produced Radioisotopes, Including Individual Rare Earths, by Complexing Elution from Amberlite Resin 1, J. Am. Chem. Soc., 1947, 69(11), 2769–2777.
36. E. Rona, Exchange Reactions of Uranium Ions in Solution, J. Am. Chem. Soc., 1950, 72(10), 4339–4343.
37. T. Luo, S. Abdu and M. Wessling, Selectivity of ion exchange membranes: A review, J. Membr. Sci., 2018, 555, 429–454.
38. T. Le, X. Chen, H. Dong, W. Tarpeh, A. Perea-Cachero and J. Coronas, et al., An Evolving Insight into Metal Organic Framework-Functionalized Membranes for Water and Wastewater Treatment and Resource Recovery, Ind. Eng. Chem. Res., 2021, 60(19), 6869–6907.
39. M. Coca, S. Mato, G. González-Benito, M. Ángel Urueña and M. T. García-Cubero, Use of weak cation exchange resin Lewatit S 8528 as alternative to strong ion exchange resins for calcium salt removal, J. Food Eng., 2010, 97(4), 569–573.
40. J. E. Greenleaf and A. K. SenGupta, Carbon Dioxide Regeneration of Ion Exchange Resins and Fibers: A Review, Solvent Extr. Ion Exch., 2012, 30(4), 350–371.
41. O. Kavvada, W. A. Tarpeh, A. Horvath and K. L. Nelson, Life-cycle cost and environmental assessment of decentralized nitrogen recovery using ion exchange from source-separated urine through spatial modeling, Environ. Sci. Technol., 2017, 51(21), 12061–12071.
42. R. C. Smith and A. K. SenGupta, Integrating Tunable Anion Exchange with Reverse Osmosis for Enhanced Recovery During Inland Brackish Water Desalination, Environ. Sci. Technol., 2015, 49(9), 5637–5644.
43. H. Dong, C. S. Shepsko, M. German and A. K. SenGupta, Hybrid nitrate selective resin (NSR-NanoZr) for simultaneous selective removal of nitrate and phosphate (or fluoride) from impaired water sources, J. Environ. Chem. Eng., 2020, 8(4), 103846.
44. S. Ebrahimi and D. J. Roberts, Bioregeneration of single use nitrate selective ion-exchange resin enclosed in a membrane: Kinetics of desorption, Sep. Purif. Technol., 2015, 146, 268–275.
45. D. Clifford and W. J. Weber Jr., The determinants of divalent/monovalent selectivity in anion exchangers, React. Polym., Ion Exch., Sorbents, 1983, 1(2), 77–89.
46. H. Dong, L. Wei and W. A. Tarpeh, Electro-assisted regeneration of pH-sensitive ion exchangers for sustainable phosphate removal and recovery, Water Res., 2020, 184, 116167.
47. H. Dong, Z. Wu, M. J. Liu and W. A. Tarpeh, The role of intraparticle diffusion path length during electro-assisted regeneration of ion exchange resins: Implications for selective adsorbent design and reverse osmosis pretreatment, Chem. Eng. J., 2021, 407, 127821.
48. H. Dong, T. Lin and A. K. SenGupta, Field validation of multifunctional ion exchange process for reverse osmosis pretreatment and phosphate recovery during impaired water reuse, J. Water Process Eng., 2020, 36, 101347.
49. H. Dong, C. S. Shepsko, M. German and A. K. SenGupta, Hybrid Ion Exchange Desalination (HIX-Desal) of Impaired Brackish Water Using Pressurized Carbon Dioxide (CO2) as the Source of Energy and Regenerant, Environ. Sci. Technol. Lett., 2018, 5(11), 701–706.
50. W. H. Höll, CARIX process—A novel approach to desalination by ion exchange, Ion Exchange Technology: Advances in Pollution Control, 1995, pp. 271–313, https://www.taylorfrancis.com/chapters/edit/10.1201/9780367813345-7/carix-process%E2%80%94a-novel-approach-desalination-ion-exchange-wolfgang-h%C3%B6ll.
51. W. H. Höll and K. Hagen, Municipal Drinking Water Treatment by the CARIX Ion Exchange Process, in Ion Exchange Advances, ed. M. J. Slater, SpringerNetherlands, Dordrecht, 1992, pp. 136–143.
52. R. Kunin and B. Vassiliou, Regeneration of Carboxylic Cation Exchange Resins with Carbon Dioxide, Ind. Eng. Chem. Prod. Res. Dev., 1963, 2(1), 1–3.
53. W. H. Höll, Treatment of drinking water by the CARIX ion exchange process: Experience of two years of operation of the first full-scale plant, in Proc 49th Int Water Conf, Pittsburgh, PA, 1988, 24, pp. 80–88.
54. H. Dong, H. Chen and A. K. SenGupta, CO2 Utilization for Water Treatment: Ion Exchange Nitrate Removal Driven by CO2 without Producing Spent Brine Regenerant, ACS ES&T Water, 2021, 1(10), 2275–2283.
55. J. E. Greenleaf and A. K. Sengupta, Environmentally benign hardness removal using ion-exchange fibers and snowmelt, Environ. Sci. Technol., 2006, 40(1), 370–376.
56. P. Li and A. K. Sengupta, Intraparticle diffusion during selective sorption of trace contaminants: The effect of gel versus macroporous morphology, Environ. Sci. Technol., 2000, 34(24), 5193–5200.
57. S. Padungthon, J. E. Greenleaf and A. K. Sengupta, Carbon dioxide sequestration through novel use of ion exchange fibers (IX-fibers), Chem. Eng. Res. Des., 2011, 89(9), 1891–1900.
58. The first ion treatment sewage system in China settled in Rongcheng City. [cited 2023 Sep 7]. Available from: https://www.rongcheng.gov.cn/art/2020/4/15/art_40746_2303362.html.
59. H. Dong, M. German, L. Tian and A. K. SenGupta, Multifunctional ion exchange pretreatment driven by carbon dioxide for enhancing reverse osmosis recovery during impaired water reuse, Desalination, 2020, 485, 114459.
60. D. A. Clifford, Nitrate Removal From Water Supplies by Ion Exchange, EPA, 1978.
61. D. Clifford and X. Liu, Biological denitrification of spent regenerant brine using a sequencing batch reactor, Water Res., 1993, 27(9), 1477–1484.
62. H. Dong, H. Chen, C. Shepsko and A. SenGupta, Resilient Nitrate Removal by a Biological Nitrate-Selective Ion Exchange (BIO-NIX) Process, ACS ES&T Water, 2023, 3(9), 2989–2995.
63. K. S. Spiegler, On the Electrochemistry of Ion-Exchange Resins A Review of Recent Work, J. Electrochem. Soc., 1953, 100(11), 303C–316C.
64. R. Pärnamäe, S. Mareev, V. Nikonenko, S. Melnikov, N. Sheldeshov and V. Zabolotskii, et al., Bipolar membranes: A review on principles, latest developments, and applications, J. Membr. Sci., 2021, 617, 118538.
65. J. R. Davis, Y. Chen, J. C. Baygents and J. Farrell, Production of Acids and Bases for Ion Exchange Regeneration from Dilute Salt Solutions Using Bipolar Membrane Electrodialysis, ACS Sustainable Chem. Eng., 2015, 3(9), 2337–23342.
66. H. Dong, C. M. Laguna, M. J. Liu, J. Guo and W. A. Tarpeh, Electrified Ion Exchange Enabled by Water Dissociation in Bipolar Membranes for Nitrogen Recovery from Source-Separated Urine, Environ. Sci. Technol., 2022, 56(22), 16134–16143.
67. L. Cumbal and A. K. SenGupta, Arsenic Removal Using Polymer-Supported Hydrated Iron(III) Oxide Nanoparticles: Role of Donnan Membrane Effect, Environ. Sci. Technol., 2005, 39(17), 6508–6515.
68. D. Zhao, A. K. SenGupta and Y. Zhu, Trace contaminant sorption through polymeric ligand exchange, Ind. Eng. Chem. Res., 1995, 34, 2676–2684.
69. H. Chen, C. Shepsko and A. K. SenGupta, Use of a Novel Bio-Nano-IX Process to Remove SeO 4 2– or Se(VI) from Contaminated Water in the Presence of Competing Sulfate (SO 4 2–), ACS ES&T Water, 2021, 1(8), 1859–1867.
70. Q. Zhang, B. Pan, W. Zhang, B. Pan, Q. Zhang and H. Ren, Arsenate removal from aqueous media by nanosized Hydrated Ferric Oxide (HFO)-loaded polymeric sorbents: Effect of HFO loadings, Ind. Eng. Chem. Res., 2008, 47(11), 3957–3962.
71. T. A. Saleh, M. Tuzen and A. Sari, Magnetic activated carbon loaded with tungsten oxide nanoparticles for aluminum removal from waters, J. Environ. Chem. Eng., 2017, 5(3), 2853–2860.
72. J. S. Yamani, S. M. Miller, M. L. Spaulding and J. B. Zimmerman, Enhanced arsenic removal using mixed metal oxide impregnated chitosan beads, Water Res., 2012, 46(14), 4427–4434.
73. P. Praipipat, M. M. El-Moselhy, K. Khuanmar, P. Weerayutsil, T. T. Nguyen and S. Padungthon, Enhanced defluoridation using reusable strong acid cation exchangers in Al3+ form (SAC-Al) containing hydrated Al(III) oxide nanoparticles, Chem. Eng. J., 2017, 314, 192–201.
74. F. Kong, Y. Zhang, H. Wang, J. Tang, Y. Li and S. Wang, Removal of Cr(VI) from wastewater by artificial zeolite spheres loaded with nano Fe–Al bimetallic oxide in constructed wetland, Chemosphere, 2020, 257, 127224.
75. S. Sarkar, A. K. Sengupta and P. Prakash, The Donnan membrane principle: Opportunities for sustainable engineered processes and materials, Environ. Sci. Technol., 2010, 44(4), 1161–1166.
76. S. Sarkar, L. M. Blaney, A. Gupta, D. Ghosh and A. K. SenGupta, Use of ArsenXnp, a hybrid anion exchanger, for arsenic removal in remote villages in the Indian subcontinent, React. Funct. Polym., 2007, 67(12 SPEC ISS), 1599–1611.
77. F. G. Donnan, Theory of membrane equilibria and membrane potentials in the presence of non-dialysing electrolytes. A contribution to physical-chemical physiology, J. Membr. Sci., 1995, 100(1), 45–55.
78. P. Puttamraju and A. K. SenGupta, Evidence of tunable on-off sorption behaviors of metal oxide nanoparticles: Role of ion exchanger support, Ind. Eng. Chem. Res., 2006, 45(22), 7737–7742.
79. L. Cumbal, J. Greenleaf, D. Leun and A. K. Sengupta, Polymer supported inorganic nanoparticles: characterization and environmental applications, React. Funct. Polym., 2003, 54, 167–180.
80. M. German, H. Seingheng and A. K. SenGupta, Mitigating arsenic crisis in the developing world: Role of robust, reusable and selective hybrid anion exchanger (HAIX), Sci. Total Environ., 2014, 488–489(1), 547–553.
81. B. Pan, J. Wu, B. Pan, L. Lv, W. Zhang and L. Xiao, et al., Development of polymer-based nanosized hydrated ferric oxides (HFOs) for enhanced phosphate removal from waste effluents, Water Res., 2009, 43(17), 4421–4429.
82. M. S. German, T. A. Watkins, M. Chowdhury, P. Chatterjee, M. Rahman and H. Seingheng, et al., Evidence of Economically Sustainable Village-Scale Microenterprises for Arsenic Remediation in Developing Countries, Environ. Sci. Technol., 2019, 53(3), 1078–1086.
83. L. M. Blaney, S. Cinar and A. K. SenGupta, Hybrid anion exchanger for trace phosphate removal from water and wastewater, Water Res., 2007, 41(7), 1603–1613.
84. S. Wang, Z. Zhang, T. Jiao, T. Wang, Q. Du and Q. Zhang, et al., Selective removal of phosphate in waters using a novel of cation adsorbent: Zirconium phosphate (ZrP) behavior and mechanism, Chem. Eng. J., 2013, 221, 315–321.
85. Y. Zhang, B. Pan, C. Shan and X. Gao, Enhanced Phosphate Removal by Nanosized Hydrated La(III) Oxide Confined in Cross-linked Polystyrene Networks, Environ. Sci. Technol., 2016, 50(3), 1447–1454.
86. Y. Zhang, X. She, X. Gao, C. Shan and B.-C. Pan, Unexpected Favorable Role of Ca 2+ in Phosphate Removal by Using Nanosized Ferric Oxides Confined in Porous Polystyrene Beads, Environ. Sci. Technol., 2019, 53(1), 365–372.
87. M. J. Demarco, A. K. Sengupta and J. E. Greenleaf, Arsenic removal using a polymeric/inorganic hybrid sorbent, Water Res., 2003, 37, 164–176.
88. A. V. Lalwani, H. Dong, L. Mu, K. Woo, H. A. Johnson and M. A. Holliday, et al., Selective aqueous ammonia sensors using electrochemical stripping and capacitive detection, AIChE J., 2021, 67(12), 3344.
89. M. C. Sauer, P. F. Southwick, K. S. Spiegler and M. R. J. Wyllie, Electrical Conductance of Porous Plugs - Ion Exchange Resin-Solution Systems, Ind. Eng. Chem., 1955, 47(10), 2187–2193.
90. L. N. Pincus, H. E. Rudel, P. V. Petrović, S. Gupta, P. Westerhoff and C. L. Muhich, et al., Exploring the Mechanisms of Selectivity for Environmentally Significant Oxo-Anion Removal during Water Treatment: A Review of Common Competing Oxo-Anions and Tools for Quantifying Selective Adsorption, Environ. Sci. Technol., 2020, 54(16), 9769–9790.
91. B. Clark, G. Gilles and W. A. Tarpeh, Resin-Mediated pH Control of Metal-Loaded Ligand Exchangers for Selective Nitrogen Recovery from Wastewaters, ACS Appl. Mater. Interfaces, 2022, 14(20), 22950–22964.
92. B. Clark and W. A. Tarpeh, Selective Recovery of Ammonia Nitrogen from Wastewaters with Transition Metal-Loaded Polymeric Cation Exchange Adsorbents, Chem. – Eur. J., 2020, 26(44), 10099–10112.
93. D. Zhao, A. K. SenGupta and L. Stewart, Selective Removal of Cr(VI) Oxyanions with a New Anion Exchanger, Ind. Eng. Chem. Res., 1998, 37(11), 4383–4387.
94. D. Zhao and A. K. SenGupta, Ligand Separation with a Copper(II)-Loaded Polymeric Ligand Exchanger, Ind. Eng. Chem. Res., 2000, 39(2), 455–462.
95. D. Zhao and A. K. Sengupta, Ultimate Removal of Phosphate From Wastewater Using a New Class of Polymeric Ion Exchangers, Water Res., 1998, 32(5), 1613–1625.
96. H. Chen, H. Dong, Z. Shi and A. K. SenGupta, Direct air capture (DAC) and sequestration of CO2: Dramatic effect of coordinated Cu(II) onto a chelating weak base ion exchanger, Sci. Adv., 2023, 9(10), 1–10.
97. T. Wang, K. S. Lackner and A. Wright, Moisture swing sorbent for carbon dioxide capture from ambient air, Environ. Sci. Technol., 2011, 45(15), 6670–6675.
98. S. Choi, J. H. Drese and C. W. Jones, Adsorbent materials for carbon dioxide capture from large anthropogenic point sources, ChemSusChem, 2009, 2(9), 796–854.
99. U. Shashvatt, F. Amurrio and L. Blaney, Ligand-Enabled Donnan Dialysis for Phosphorus Recovery from Alum-Laden Waste Activated Sludge, Environ. Sci. Technol., 2022, 56(19), 13945–13953.
100. H. Chen, U. Shashvatt, F. Amurrio, K. Stewart and L. Blaney, Sustainable nutrient recovery from synthetic urine by Donnan dialysis with tubular ion-exchange membranes, Chem. Eng. J., 2023, 460, 141625.
101. H. Chen, M. Rose, M. Fleming, S. Souizi, U. Shashvatt and L. Blaney, Recent advances in Donnan dialysis processes for water/wastewater treatment and resource recovery: A critical review, Chem. Eng. J., 2023, 455, 140522.
102. D. K. Hubler, J. C. Baygents and J. Farrell, Sustainable electrochemical regeneration of copper-loaded ion exchange media, Ind. Eng. Chem. Res., 2012, 51(40), 13259–13267.
103. K. S. Spiegler, On the Electrochemistry of Ion-Exchange Resins, J. Electrochem. Soc., 1953, 100(11), 303C.
104. M. Li, C. Feng, Z. Zhang, R. Zhao, X. Lei and R. Chen, et al., Application of an electrochemical-ion exchange reactor for ammonia removal, Electrochim. Acta, 2009, 55(1), 159–164.
105. N. J. Bridger, C. P. Jones and M. D. Neville, Electrochemical ion exchange, J. Chem. Technol. Biotechnol., 1991, 50(4), 469–481.
106. Q. Shu, L. Legrand, P. Kuntke, M. Tedesco and H. V. M. Hamelers, Electrochemical Regeneration of Spent Alkaline Absorbent from Direct Air Capture, Environ. Sci. Technol., 2020, 54(14), 8990–8998.
107. Y. Chen, J. R. Davis, C. H. Nguyen, J. C. Baygents and J. Farrell, Electrochemical Ion-Exchange Regeneration and Fluidized Bed Crystallization for Zero-Liquid-Discharge Water Softening, Environ. Sci. Technol., 2016, 50(11), 5900–5907.
108. M. S. German, H. Dong, A. Schevets, R. C. Smith and A. K. SenGupta, Field validation of self-regenerating reversible ion exchange-membrane (RIX-M) process to prevent sulfate and silica fouling, Desalination, 2019, 469, 114093.

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