Ion Exchange

• Type of physical science: Chemistry
• Field of study: Chemical methods

Ion exchange is the reversible exchange of ions of the same charge between solutions and an insoluble solid in contact with them. It has many important uses, including water softening, the refining of chemicals, toxic waste treatment, and chemical analysis.

Overview

An understanding of ion exchange first requires an understanding of the terms “ion” and “electrolyte.” An electrolyte is any substance that—dissolved in water (and some other solvents that will not be described here)—produces solutions that conduct electricity. The ability of electrolyte solutions to conduct electricity is a result of the fact that the dissolution of the electrolyte causes it to break apart (ionize) into charged particles, called ions. Two types of ions form in this way: positively charged cations and negatively charged anions. For example, the electrolyte table salt (NaCl) ionizes to produce an equal number of positive and negative charges by forming equal numbers of cations (Na⁺, sodium ions) and anions (Cl^-, chloride ions), as required by the law of electrical neutrality.

In nature, water that contains dissolved electrolytes often undergoes interactions with solids wherein some of the ions it contains are exchanged for other types of like-charged ions that are present on the surfaces—and sometimes in the interiors—of the solids involved. When ion exchange occurs, the law of electrical neutrality is again observed. Here, this means that the total number of charges that leave the solution and enter the solid involved equals the number of charges that leave the solid and enter the solution.

Ion exchange has been occurring on the Earth as long as liquid water has existed. It first received scientific recognition in the 1850s, when Henry S. Thompson and J. Thomas Way studied the fate of ammonia in soil and found that manure fertilizer applied to soil lost ammonia to the soil and gave up potassium and calcium in exchange. Later, other agricultural researchers found that many of the other components of fertilizers were taken up by the soil via ion exchange.

The utilization of ion exchange and the understanding of its chemical bases began in the 1930s, with the use of sulfonated coal and the synthesis of the organic chemical polymers called cation and anion exchange resins in the Central Research Laboratory of the British Department of Scientific and Industrial Research. At first, the polymers were an interesting curiosity. By 1940, however, interest in the United States and other industrial nations produced a widespread effort at synthesizing polymers of use for industrial ion exchange. This led to the many different chemicals now utilized for societally valuable ion exchange processes and to their many applications.

Basically, all ion exchange resins are giant, chainlike molecules (polymers) that are extensively cross-linked. Cross-linking produces polymers possessing latticelike structures.

Anionic or cationic portions (groups) are attached chemically to the polymers during their synthesis. The cation exchangers all contain chemically attached anionic (negatively charged) groups and act by exchanging oppositely charged cations. The anion exchangers contain chemically attached cationic (positively charged) groups and act by exchanging oppositely charged anions.

The attached groups in the lattice-like, polymeric structure of an ion exchanger is called fixed ions. Bound loosely to each fixed ion in a functional ion exchanger is an oppositely charged, exchangeable counterion, added previously. The counterions leave the exchanger, changing places with the oppositely charged ions that enter it from solutions with which ion exchange is desired. In many cases, ion exchange occurs only at the surface of an ion exchanger.

The most desired ion exchangers, however, are designed to let ions enter the polymer particles to exchange with counterions in their interior portions. This ability to allow co-ions to exchange with internally located counterions is desired because it increases the ability per unit weight (capacity) of an exchanger to exchange co-ions and leads to more effective functioning.

Ion exchange resins have become very important in a large number of areas of industrial processing, chemical analysis, and chemical research. Most of these chemicals are organic polymers made in two main steps. First, small molecules, called monomers,  undergo polymerization reactions. This polymerization forms the giant, cross-linked, lattice-like polymers that become the backbones of the ion exchangers. Second, various chemical treatments are used to introduce the desired fixed ions that convert a polymer into a particular ion exchange resin. For example, reaction with fuming sulfuric acid will add sulfonic acid groups (-SO₃H) to various polymers. Sulfonic acid groups become anionic sulfonate groups (–SO₃–), useful for cation exchange, when treated appropriately.

Many different ion exchange resins have been produced by numerous manufacturers.

These resins are generally used after they are packed into (allowed to fill) tanks or many other types of containers, through which the sample to be exchanged and the electrolyte solution used for exchange are passed. The choice of an appropriate ion exchange resin for a given application requires experience and careful consideration of several factors, based on the type of chemical to be exchanged. Among these factors is the chemical nature of the polymer portion of the resin; inappropriate choices here will lead to unwanted adsorption of chemicals unrelated to the desired ion exchange process and may also foul the resin, making it inoperable. Another factor is the extent of polymer cross-linking, which determines both the mechanical properties of the ion exchange resin and its capacity (the number of co-ions exchanged per unit weight of resin).

Increased cross-linking increases resin selectivity but generally decreases capacity and flow rate; this may make a desired process occur too slowly to be practical. A third factor is whether the fixed ion produces an anion exchange or a cation exchange resin, because one of the two resin types will be most appropriate for a desired ion exchange process.

Two other factors that must be considered are related to the fact that a given ion exchange resin is produced as essentially spherical beads. The resin beads may be purchased in various size ranges from a few micrometers up to about 1 millimeter. The bead size chosen is important because, although smaller resin beads give faster ion exchange, they do not allow the solution being exchanged to flow through a resin container as rapidly as do larger beads. This slowing of the flow-through rate of an ion exchange process may make it unfeasible.

Furthermore, resin beads may be microporous (microreticular) or macroporous (macroreticular) in nature. The difference here is made apparent by their microscopic examination; a microporous bead appears to be a smooth and transparent sphere, while macroporous beads are rough and opaque. These apparent differences reflect structural differences in the makeup of the beads that affect their use ranges. The macroporous beads, for example, are tougher and more rigid than microporous beads, and they are used most often in chemical processes that require several solvent changes and the use of solvents other than water.

The selectivity of ion exchangers is very important because of the great number of processes for which they are used. Part of this selectivity is imposed by the nature of the completely synthetic ion exchange resins. In addition, a wide variety of ion exchangers exists that are based on the natural polysaccharide (carbohydrate) polymers, such as cellulose, to which fixed ions have been chemically attached, and on inorganic ion exchangers.

Applications

Ion exchange has applications that are related to virtually every aspect of human life.

One well-known, wide use of ion exchange is the treatment of the water from lakes, rivers, and other natural water sources. This results from the fact that such water contains dissolved electrolytes that produce many undesired ions. Dissolved calcium and magnesium ions (Ca²⁺ and Mg²⁺) are of very special relevance because their presence makes water hard. Hard water, when mixed with soap, forms curdy material that lowers effective cleansing and stains clothing washed in it.

Furthermore, hard water in pipes, water heaters, and industrial boilers produces hard crusts that may block pipes and damage both the heaters and boilers.

Hard water is most often treated to remove calcium and magnesium ions via water softening. Water softening is an ion exchange that replaces these ions with sodium ions, which do not cause hardness. The process uses a tank containing a cation exchanger, through which hard water is passed. Water flowing through the resin in the tank—a resin bed—constantly exchanges with sodium counterions, and calcium and magnesium ions bind to exchange fixed ions. The water leaving the resin bed is soft, free of the Ca²⁺ and Mg²⁺ ions. It contains two sodium ions (Na⁺) for every “hardness” ion removed; however, because exchange must obey the law of electrical neutrality.

Eventually, the resin bed becomes saturated—all of its Na+ ions are replaced—and it must be regenerated if softening is to continue. Regeneration is accomplished by passing brine, a strong sodium chloride solution, through the resin bed (at a time when it is not in use) to drive off all the bound Ca²⁺ and Mg²⁺ ions. After regeneration is complete, the resin bed is ready to soften more water.

Often, it is valuable to remove all the ions present in water. When the water contains relatively small amounts of ions (for example, softened tap water), this can be done by an ion exchange process called deionization. Deionization of water uses two ion exchange resins, an anion exchanger and a cation exchanger. The process is similar to the softening methodology; however, the deionization process removes all the ions that were originally present in the water.

In many cases, deionization uses separate tanks of anion and cation exchangers. In some cases, however, both exchangers are placed in one tank. These mixed-bed resins give water of higher purity than the two-tank systems. They are difficult to regenerate, however, because the resins must be removed from the tank and separated prior to regeneration. Both the two-tank and mixed-bed deionization methods are too expensive for use in the desalination of seawater.

Another aspect of water treatment that is carried out by ion exchange is the removal of toxic wastes from industrial effluents, the coolant used in nuclear reactors, and mine drainage.

Such ion exchange prevents pollution, facilitates handling of dangerous radioactive wastes for controlled storage or disposal in specialized facilities, and in some instances allows the recovery of valuable metals in concentrated, usable forms. Special, highly selective cation exchange resins are often used here.

Laboratory-scale ion exchange is very important. It is done by a process called ion exchange chromatography. This procedure is used to identify the types (qualitative analysis) and quantities (quantitative analysis) of chemicals present in industrial, sewage, pharmaceutical, and other samples. In addition, the method is widely used by life scientists to purify biomolecules such as proteins and nucleic acids.

Ion exchange chromatography is carried out in a cylindrical glass, plastic, or stainless-steel tube with a valve at its bottom and an inlet at its top. Usual column dimensions range from cigarette size (about a teaspoon) to the height of an average person and a 0.3-meter diameter (about 18.9 liters). The main kinds of ion exchange chromatography are conventional exchange, using polysaccharide or plastic exchangers, and high-performance liquid chromatography with small particle matrices (often silicic acid or glass beads). Both high-performance liquid chromatography and conventional ion exchange chromatography have important places in chemical analysis and separations.

High-performance liquid chromatography is most useful for rapid analysis of many small samples (for example, in medicine) because the very small ion exchange particles used (8 to 20 micrometers) provide higher surface area and improved separation efficiency, and small columns, which run (elute) quickly, can be used. Also, because of the form of the exchange matrices, high-performance liquid chromatography columns can be eluted at high pressures, accelerating their speed. Conventional matrices are much slower and do not withstand attempts to speed them up via high pressure.

Such efforts will collapse their beads and stop columns from running at all. Conventional exchangers are most valuable where needed use of large columns removes the speed difference usually obtained through high-performance liquid chromatography.

In both cases, chemicals studied (test chemicals) are applied to a column and bind to the exchanger by displacing counterions. Often, consecutive solutions containing increasing amounts of an electrolyte that produces appropriate ions (eluant solutions) are then passed through the column. The ions and the test chemicals compete for binding sites. Depending on relative binding strength (affinity) for the exchanger, different test chemicals leave the column with eluant solutions of different electrolyte content (for example, those with low affinities leave first, and those with the highest affinities leave last).

Another elution method uses the fact that chemicals bind to exchangers only if their charges are opposite to the fixed ions. Uncharged chemicals and those with the same charge as fixed ions pass through without binding. Test chemicals are added to a column under conditions in which they all bind; then, column pH is changed by eluting with solutions of different pH.

This changes bound test chemicals to inappropriate charge forms at pHs related to their properties, which vary. Therefore, test chemicals can often be separated by a pH change elution.

Ion exchange is also a promising technique for the separation of rare earths from secondary resources. Rare earths occur as mixtures in nature and can be difficult to separate into single elements. Some rare earths elements are found in minerals such as bastnäsite, monazite, and xenotime.

Context

All chemicals in the world are broadly classified into the two groups: millions of organic chemicals, carbon compounds first seen as materials isolated from living organisms; and a similar number of inorganic chemicals, which contain any of the other elements, but no carbon. Both organic and inorganic chemicals can function in ion exchange. It was first identified, however, as a chemical phenomenon of inorganic chemicals because the initial research in the field, in the 1850s, was done by chemists interested in studying ion exchange between fertilizers and inorganic soil and sand.

Intellectually interesting but a curiosity, ion exchange was unused industrially until 1905, when a German chemist, R. Gans, found that aluminosilicate zeolite minerals softened water and aided sugar refining by exchanging calcium and magnesium ions. At first, these exchangers were expected to have many uses. Their actions, however, were found to be largely limited to the removal of hardness ions, and their use is limited mostly to water softening and to refining sugar.

In the 1930s, the synthesis of polymeric, organic chemical ion exchange resins began in England and spread worldwide. Industrial chemists found that changing the polymers used, their cross-linking, and fixed ions attached to them produced great functional versatility. This led to resins tailored for diverse industrial functions, such as isolation of specific antibiotics and of metals from dilute solutions. Modern ion exchange is so widespread in homes and industries that no one is unaffected by its use. Advancements include the development of next-generation ion exchange membranes with sub-nanometer channels, improving selectivity and efficiency for applications such as energy storage, hydrogen production, and lithium recovery.

Industrial recovery of metals from water solution, hydrometallurgy, began to flower in the 1950s. It has many uses, including isolation of rare earth metals for industrial catalysts and electronic components. Intense study led to the discovery that variants of simple fixed ions, chelating agents, could be attached to polymer molecules and used for even more specific recovery of metals. Some newer applications include uranium recovery from low-grade ore and concentration of radioactive wastes from nuclear power plants. This last use simplifies waste storage in controlled long-term facilities for the thousands of years that must pass before it becomes harmless. A need for more durable exchangers has led to the development of new inorganic materials that have become valuable in other areas.

Ion exchangers have also helped to revolutionize biochemistry and the pharmaceutical industry, becoming essential parts of the isolation and identification of drugs, hormones, and other types of medicinals. For example, they contributed heavily to the isolation of genetically engineered human insulin. In medicine, their uses include identification of the presence of chemical evidence for many disease states; treatment of cardiovascular problems; and removal of toxic chemicals at medical treatment settings such as hospitals. Ion exchange materials have been widely used to remove persistent pollutants such as “forever chemicals” and toxic metals from water, with newer materials offering much higher removal efficiency. It is also hoped that the use of ion exchangers will lead to a better understanding of nutrient passage across cell membranes and other life processes.

Another continuing aspect of ion exchange has been the production of exchange membranes for special electrochemical cells, as well as in nuclear submarines, the space program, and the beginnings of seawater desalination. It is hoped that they will become more useful in desalination as the world population climbs and per capita fresh water resources decrease. Developments include ultrahigh charge-density ion exchange membranes that significantly improve electrodialysis efficiency and enable better concentration and reuse of brine in desalination processes.

Understanding ion exchange has produced a great many practical applications and advanced understanding of the world in ways that would not have been possible otherwise.

Principal terms

ANION EXCHANGER: an ion exchanger that contains positively charged (cationic) fixed ions; therefore, it can exchange negatively charged counterions (anions) from the solutions with which it interacts

CAPACITY: simply stated, the number of co-ions exchanged per unit weight of an ion exchanger

CATION EXCHANGER: an ion exchanger that contains negatively charged (anionic) fixed ions; therefore, it can exchange positively charged counterions (cations) from the solutions with which it interacts

CO-ION: an ion in solution that has the same charge as the fixed ion and is not exchanged during the process

COUNTERION: an ion bound loosely to an oppositely charged fixed ion in an ion exchanger; it leaves the exchanger when another counterion replaces it during ion exchange

ELECTROLYTE: a substance that produces aqueous solutions that conduct electricity by breaking apart into charged particles (ions) when it dissolves

FIXED ION: a chemically attached group that determines whether an ion exchanger is a cation exchanger or an anion exchanger

GROUP: an anionic or cationic portion attached chemically to ion exchangers

ION: an electrically charged atom or group of atoms, produced when an electrolyte dissolves in water; ions are positively charged cations or negatively charged anions

ION EXCHANGER: a solid capable of exchanging counterions for other counterions of the same charge in solutions with which it interacts; synthetic ion exchangers are called ion exchange resins

POLYMER: a giant molecule formed by chemical combination (polymerization) of many smaller molecules, called monomers; it may be chainlike or cross-linked into a lattice-like structure

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