Ion-Exchange Resins: Solutions for a Wide Range of Challenges

A large selection of ion-exchange resins is available today for a constantly growing variety of applications. In order to choose the appropriate resin for a specific application, it is important to carefully evaluate the range of resin and process properties and parameters

Various inorganic and organic materials of both natural and synthetic origin, among them clays, peat, zeolites or metal silicates, are prone to exchange bound ions with other ions from a surrounding liquid phase.

This article focuses on ion-exchange (IEX) resins [1] — that is, functionalized organic polymers. In the early days, these were mainly phenol-formaldehyde polymers, but today, IEX resins are most importantly derived from vinylbenzenes or acrylates. Besides ion exchange, some of these materials are also able to function as an absorber for uncharged, polar and even nonpolar molecules, which further widens the application opportunities.

Four main types of IEX resins have been developed over time. They are categorized [2] as strong and weak acid cation-exchange resins (SAC/WAC) on one hand and strong and weak basic anion-exchange resins (SBA/WBA) on the other hand.

Since they were first produced back in the late 1930s in Wolfen, Saxony-Anhalt, Germany [3], polystyrene-based IEX resins have been employed in a variety of applications. Softening and demineralization of water have been of focal importance for decades and still play a major role, not only for industrial applications — for example, in power-plant cooling circuits [4], for the preparation of ultrapure water for use in medical applications and in the food, beverage and semiconductor industries — but also for municipal water treatment and household use.

IEX resins based on polystyrene and polyacrylates account for the great majority of products in today’s global markets. For industrial applications, polystyrene-based resins are often favored due to their better stability, leading to longer service life. These resins can handle high and also variable flowrates, as well as acids and bases in fairly high concentrations.

 

Competing technologies

Even for demineralization, quite a few technologies are available today besides ion exchange. Reverse osmosis (RO), for example, can also remove dissolved ionic substances quite efficiently [5]. The membranes employed, however, are frequently susceptible to fouling and can have difficulties when dealing with variable flowrates. Very low ion concentrations in permeates can only be achieved with difficulties at the price of repeated, energy-consuming treatment.

Electrodeionization (EDI) [6], as another example, requires a relatively high-energy input and also has difficulties in obtaining water resistivity above 16 MΩ, as is required for ultra-pure water. Silica is especially difficult to remove in one single step. EDI systems generally have a very low tolerance for hardness ions and organic matter due to blocking of the membranes. Furthermore, maintenance and replacement cannot easily be split into a device and an active component as is possible with an IEX system.

In contrast to IEX with resins that can be tailored to be highly selective (see below), both RO and EDI exhibit only very limited ion selectivity, if any. Therefore, both the latter methods can only remove the dissolved ion contents as a whole. In all these respects, an IEX resin system exhibits superior properties that make it favorable whenever one or more of the requirements mentioned above are crucial.

However, IEX resins also have limitations. Although the stability against oxidative stress is significantly better with IEX compared to RO and EDI, oxidizing agents may markedly limit the service life especially of anion exchange resins. Due to their polymeric backbone, operating temperatures for most types of polystyrene-based resins are limited to approximately 140°C (SAC/WAC) and 70°C (SBA/WBA, chloride form) or 40–45°C (SBA/WBA, OH form). SBA and WBA resins on acrylate basis are more sensitive to elevated temperatures. Even in chloride form they should not be employed at temperatures above 40°C.

 

Capabilities of IEX resins

Besides the four functionalization classes of IEX resins mentioned above, additional subclasses can be identified that contain special functional groups, such as bi- or polydentate groups, which are capable of forming chelating complexes with enhanced selectivity. Additionally, certain resins may also allow for non-ionic interactions with substrates, thus establishing co-operative binding modes (Figure 1).

Selectivity

Di- versus monovalent ions. A key property of modern IEX resins is their selectivity, which goes far beyond that simply for anions or cations in general. Resins can easily discriminate between monovalent and divalent ions, for example, due to a markedly different binding strength. These differences in selectivity usually increase with a higher degree of cross-linking, that is, a higher share of divinylbenzene added as cross-linking agent during polymerization, as shown in Table 1 [7].

For trivalent ions, selectivity differences may be even more pronounced. Special IEX resins exhibit small bead size and very fast kinetics. Therefore, even a partial separation of rare-earth ions, namely of lanthanum, cerium, praseodymium and neodymium, from other rare earth, earth alkali and aluminum ions is possible [8]. Separations of this kind are considered to be the most difficult of all.

Specific ions. Appropriately functionalized resins can even be tailored to preferentially bind a specific ion, allowing for a partial or even almost complete separation of mixtures [9]. Most often, this selectivity is due to chelating functional groups, that is, ionic or polar groups that can establish more than one (ionic) contact to the substrate ion in the course of complexation (Figure 2).

As an example, the removal of calcium and magnesium ions is possible even from concentrated brine using a special SAC resin with small beads [10]. The absence of earth alkali cations is a crucial requirement in chlor-alkali electrolysis in order to prevent blocking of the cell membranes and enhance the efficiency of the electrolysis. The concentration of these divalent ions needs to be reduced down to the single-digit parts-per-billion (ppb) range for this purpose. In a similar process, calcium in a wide concentration range can also be separated from lithium brine by ion exchange during the production of battery-grade lithium salts. Even lithium itself can be recovered in the form of lithium aluminates from brine with special resins that have been modified with aluminum salts [11].

Selective binding of ions can also be a solution for medical treatment, as exemplified by IEX resins that — after oral administration — selectively bind potassium ions and can thus be use to treat hyperkalemia [12, 13]. The resin binds potassium in the digestive tract and is then excreted in a loaded form. Further medical applications include the controlled release of active pharmaceutical ingredients over an extended period of time. IEX resins can also be employed as excipients in medical formulations [14], for example, as a taste masking agent in orally administered drugs [15] containing, for instance, antibiotics with a bitter taste or nicotine for smoking cessation. The mechanism involves the drug being initially bound to the resin. The complexation retains structural integrity in the neutral oral environment. Under the acidic environment in the stomach, the medicine is then released from the resin after being replaced by protons [16].

An application that has gained substantial interest recently in the course of the fight against climate change is direct air capture (DAC) of carbon dioxide facilitated by SBA or WBA resins [17, 18]. This is one of several carbon capture and storage (CCS) technologies that have been proposed and implemented to reduce CO₂ emissions from point sources.

 

Hydrophobic interactions

Additionally, IEX resins can even bind uncharged molecules by adsorption. This is due to weak polar interactions and can, for example, be employed to separate micro-pollutants containing aromatic rings, such as active pharmaceutical ingredients, pesticides or non-ionic detergents (Figure 3) during municipal wastewater treatment [19–21]. The IEX resin is more efficient than activated carbon with high loading capacity, high mechanical stability and exhibits fast exchange kinetics, which allows the use of small, compact filters instead of large columns.

Cooperative binding situations can also occur. In such cases, a substrate molecule is simultaneously bound to the IEX resin by means of ionic and hydrophobic interactions (Figure 4). Such a behavior is observed during removal of long-chain PFAS (per- and polyfluoroalkyl substances), for example, perfluorononanoic acid (PFNA) from wastewater. PFAS molecules usually consist of a polar “head” (carboxylic acid) and a nonpolar “tail” (per- or polyfluorinated carbon chain). While the former is bound to the IEX resin via Coulombic attraction, the latter establishes weak interactions with aromatic π-electron systems of the polystyrene backbone of the resin.

 

Selection parameters for resins

Even if there are many characteristics that determine which resin is best suited for a particular application, some basic parameters will be discussed here due to their general importance, namely uniformity, morphology, bead size and the life-cycle sequence.

Uniformity. For more than four decades now, specialized polymerization processes are available for the production of resins of uniform particle size (monodispersed resins) [22]. These resins offer significant advantages over heterodispersed products, including the following:

• Fewer fine and fewer coarse beads leading to less ion leakage and better regeneration performance
• Higher operating capacity due to more uniform flow over the surface and less tendency for channel formation
• Lower pressure drop due to the existence of evenly wide, unblocked channels between the beads to enable high flowrates
• Higher mechanical stability due to homogeneous, optimized functionalization — longer service life, less generation of fines, which would increase the pressure drop
• Higher osmotic shock stability — especially important for macroporous chelating resins, for example when a chelating resin loaded with calcium as obtained from brine polishing for chlor-alkali electrolysis (see above) is regenerated with hydrochloric acid and afterwards conditioned with caustic soda solution. The latter causes osmotic stress resulting from a 60% increase in volume.

Today, most of the monodispersed resins are based on polystyrene as polymeric backbone. Polyacrylate resins are mostly heterodispersed due to the lack of an economically feasible production process. However, membrane emulsification processes were developed in recent years for this purpose [23]. Currently, most acrylate resins belong to the WAC class of resins, where this is not of crucial importance.

Morphology. Basically, two types of resin can be distinguished in terms of morphology (Figure 6). In gel-type or microporous resins, on one hand, the bead surface is covered by a gel layer containing the functional groups that are easily available for ion exchange. The micropores are usually less than 2 nm in diameter. Gel-type resins exhibit high operative capacities. Typical applications include acid-catalyzed reactions with gel-type SAC resins such as dehydrations, (co-)condensations, esterifications and Friedel-Crafts-type alkylations [24]. However, their surface is sensitive to fouling, induced, for example, by natural organic matter (NOM), which makes access to the functional groups more difficult.

In macroporous resins, on the other hand, not only the bead surface, but also wide channels of more than 50 nm in diameter within the bead are equipped with functional groups. This leads to a markedly increased active surface and enhanced mechanical stability of the beads. Because of their high mechanical and osmotic stability, they are employed in a variety of processes, including those in non-aqueous solvents, such as for binding heavy metal ions. Whenever a high total capacity and therefore a high degree of cross-linking is required and at the same time stable resins are needed, there is no way around macroporous resins. Even relatively large contaminant molecules, such as NOM, can be adsorbed in the pores and are subsequently liberated during regeneration.

Macroporous, strongly basic anion-exchange resins based on a cross-linked polyacrylate can be tailored to exhibit a special pore structure and resin matrix. They are then ideally suited for the capture of high-molecular-weight compounds, for example, for the treatment and purification of products derived from biomass. This means that liquid sugar syrups or complex process solutions, such as fermentation broths, can be purified and treated. As an example, the naturally occurring glycosaminoglycan polymer heparin, which is used to prevent blood coagulation, can be extracted and thus purified with such a resin [25, 26].

Bead size. The particle size of monodispersed resins can be adjusted with high precision by means of continuous bead formation through a perforated plate. In aqueous suspensions containing monomer droplets of uniform size, the resin beads are then formed by means of polymerization (Figure 7). This method allows beads of different sizes to be created in a flexible and reproducible manner [27, 28].

To obtain ultrapure metals — ultimately through electrolytic separation — by means of hydrometallurgy, interfering foreign ions have to be removed right down to the trace level. This presents special requirements regarding selectivity, capacity and exchange speed. The size of the resin beads plays a key role here.

Macroporous resins with a sponge-like structure and a large inner surface area are usually employed here. The polymer beads in standard resin types measure between 0.5 and 0.7 mm in diameter. In addition to the type of functional groups in the polymer, their suitability for a specific separation task depends on their number and a range of other properties and characteristics. Process parameters, such as the pH value, temperature and flowrate, also influence the separation performance.

Small resin beads (monodispersed small, MDS) with a diameter of just 0.3 to 0.4 mm exhibit very different properties and characteristics than standard-sized beads. Thanks to their smaller size and, in turn, shorter diffusion paths, they exhibit faster kinetics during exchange and regeneration. Their high packing density makes them ideal for chromatographic separation. They also have higher capacity utilization and, in turn, longer service lives with lower chemical requirements for regeneration. However, the higher packing density also results in greater pressure loss.

A comparison of the loading performance (Figure 8) of an iminodiacetic acid (IDA) chelating resin with MDS beads (left) with copper ions (blue) shows clear differences with respect to standard monodispersed resin (MD, middle) and heterodispersed resin (HD, right) with a wider grain size distribution. In addition to superior retention, the MDS resin exhibits a sharp, precisely defined limit zone of adsorption. This prevents a premature breakthrough observed especially with HD resin.

These beneficial properties can be leveraged for various tasks, such as lithium brine purification where small-size beads significantly reduce calcium. Ultrapure lithium brine obtained in this way is needed mainly for electrolysis in order to protect cell membranes from scale precipitation.

The resin-in-pulp (RIP) process [29] imposes quite different requirements with respect to bead size. In such a process, an ion-containing suspension of ore slurry is initially mixed with the resin beads. After a contact period during which the resin absorbs the ions, the resin is separated again. To increase efficiency, multiple vessels are positioned in a cascade arrangement, and the ore suspension is treated with the exchange resin in counterflow continuousRIP (cRIP; Figure 9). The RIP process is a useful alternative to fixed-bed ion exchange in columns. This is particularly advantageous when the substrate is a suspension or dispersion instead of a clear solution.

During this process, the majority of metal ions from the slurry are bound to the resin and can be separated from this when the resin is regenerated. In the field of hydrometallurgy, ion-exchange processes such as these are increasingly replacing the decanting of suspensions in large water tanks, because this not only requires a great deal of space, but is also extremely time- and cost-intensive.

Mechanically robust ion exchangers are needed for separating and transferring the resin as efficiently as possible. This helps prevent premature resin breakage during extraction. A sufficient size difference between resin and ore slurry particles is also essential for efficient separation. Because of this, monodispersed resins with a larger particle diameter of 0.85 mm (XL) and heterodispersed resins with an even larger average particle diameter of 1.1 mm (±0.1, XXL) have been developed (see Ref. 30 and 31 for XL resin type; and Ref. 32 and 33 for XXL resin type).

Lifecycle sequence. The ability to be regenerated is a key advantage of IEX resins over other materials that can just act as adsorbers and have to be disposed of after single use. In most cases, for example, when employed in softening or demineralization of water, in the preparation of makeup water or in condensate polishing in industrial water-steam circuits, IEX resins can be regenerated many times, resulting in a service life of several years.

There are, however, applications where regeneration may be inefficient or even disadvantageous. The former could be true in cases where the resin quantities employed are small and no on-site regeneration systems are available. If external regeneration is feasible, it should be decided on according to economic standards. Regeneration is usually also omitted in cases where only trace amounts of ions are removed from large volumes and the service life of the resin is therefore extremely long. This might be true for final polishing mixed-bed IEX systems in the production of ultrapure water. In this case, the original quality of the delivery form can no longer be restored, at least on site. For economic reasons, it therefore does not make sense to set up and maintain a regeneration unit.

Regeneration may be disadvantageous when hazardous contaminants are bound to the IEX resin so that a significant concentration, that is volume reduction, of the hazardous waste has already taken place. If then the resin would be regenerated, a relatively small volume of resin would give rise to a larger volume of contaminated regeneration and washing solutions, which would have to be disposed of subsequently. This might be the case, for example, for resins loaded with mercury from fluegas scrubbing or with those loaded with radioactive cations from nuclear-power plants. In addition, such cations are bound very tightly, which would make regeneration impossible or at least not economical.

Innovative regeneration protocols, however, could help to recycle valuable resins even after loading hazardous substances onto them. One example is the regeneration of SBA resins which have been used to trap per- and polyfluoroalkyl substances (PFAS). In these cases, regeneration can be achieved by treatment with aqueous methanol containing a small amount of sodium chloride [ 34]. After regeneration, the resin can be reused and the methanol can be stripped off the regeneration solution, leaving behind only a very small amount of PFAS, salt and water.

 

Outlook

Although the basic principles of ion exchange facilitated by resins are known for more than a century now, the development is still ongoing. Improvements in selectivity, capacity and stability have been achieved over time and are likely to continue in the future. Not the least, future development will also be triggered by newly emerging, challenging fields of application. Improved recycling methods for spent battery materials, catalytic processes for the circular economy or advanced biomedical applications could be conceivable options. The industrial production of the first acrylate- and polystyrene-based resins from renewable feedstock or recyclates can be considered a milestone on the path to improved sustainability. The first representatives of this class of sustainable resins have just become available, not only in lab or pilot quantities [ 35], but on an industrial scale [36].

Edited by Gerald Ondrey

 

Acknowledgements

Many thanks to Dr. Thomas Schmidt for many fruitful discussions and editorial contributions.

All figures courtesy of Lanxess Deutschland GmbH.

 

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Author

Stefan Hilger is the manager global technical marketing of the Business Unit Liquid Purification Technologies, Lanxess Deutschland GmbH (Kennedyplatz 1, 50569 Cologne, Germany; Phone: +49-221-8885-0). He has more than 30 years working experience in the field of water treatment, specializing in IEX systems for the past 20 years. He worked for several companies designing, constructing and implementing water-treatment system before joining Lanxess as a technical marketing manager for IRX resins and Bayoxide adsorbers in industrial and drinking water treatment.