Ion exchange is a process used to remove dissolved ions from a solution by electrostatic sorption into ion exchange materials (most commonly into ion exchange resins). The removed ions are replaced with equivalent amounts of other ions of the same charge. Ion exchange is most commonly used for purification purposes, but is also widely implemented in the separation and extraction of valuable substances. Deionisation of water and water softening can be cited as the most common application. However, the spectrum of applications varies from large scale extraction of metals in hydrometallurgical processes to laboratory purification of highly valuable proteins.
Metal ions initially contained in an aqueous solution are exchanged with ions initially contained in a solid material (most of them in an organic ion exchange resin). Such a process is called cation exchange and can be illustrated by reaction
where R = ion exchanger; A+ = positively charged metal ion; bars indicate phase of the ion exchanger. A similar process involving anions is called anion exchange:
where B- and Y- = anions or negatively charged metal ion complexes. Chemical selectivity of reactions (1) and (2) is desirable but is not a requirement. Contrary to many other chemical separations, reactions (1) and (2) can be successfully used even if they are shifted to the "wrong" direction. To achieve an efficient separation, column techniques are applied.
Ion exchange materials
There is a wide variety of organic and inorganic ion exchange materials. The organic materials (for example ion exchange resins) can be both cation- and anion exchangers. Only cation exchange inorganic materials (for example zeolites) are known. Organic resins consist of functional groups bound to different polymeric frameworks (most commonly to crosslinked polystyrene). Inorganic materials are negatively charged porous structures with exchangeable cations located in internal voids. Similarly to conventional acids, cation exchangers are classified into strong and weak cation exchangers depending on the type of functional group attached to the polymer.
- Most typical strong acid exchangers contain sulfonic groups (-SO3-). Such materials are active over the entire pH range.
- Most weak acid exchangers have carboxylic groups (-COOH). The weak acid exchangers are not active at pH values below 4-6 (this value significantly differs for different materials). However, they often have higher ion exchange capacities than the strong acid exchangers and have other specific advantages as well.
Anion exchangers are classified in a similar way into strong base anion exchangers and weak base anion exchangers.
- Strong base exchangers have quaternary ammonium groups (-NR3+). They are active over the entire pH range.
- Weak base exchangers have primary (-NH2), secondary (-NRH), and/or tertiary (-NR2) amine groups. The weak base exchangers are not active at alkaline pH. However, they are advantageous in many practical cases.
Conventional ion exchange operations
Most commonly the ion exchange is performed in cyclic operations. Each cycle is divided into the following main sub-processes: sorption, elution and, eventually, regeneration. In most techniques, solutions are consequently pumped through a column loaded with the ion exchange resin.
- Sorption: The solution containing the targeted ions is passed slowly through the column. The ions bind into the resin. Ions initially contained in the exchanger are released.
- Elution (stripping): The target ions are subsequently stripped from the loaded resin with a small volume of an eluent. The eluent replaces and hence also releases the target ions from the resin into the solution phase.
- Regeneration: Depending on the type of the ion exchanger and the stripping agent, the ion exchanger sometimes has to be regenerated. For example, if the sorption step uses a cation exchanger loaded with H+ ions but the elution leaves Na+ ions in the exchanger phase, the material has to be protonated. A strong acid could be applied in order to convert (regenerate) the exchanger in the initial state.
The great success of ion exchange in the area of water purification and water softening "backfired" in a certain sense. The technique is commonly perceived as suitable almost only for water purification. However, it can be successfully applied for almost any separation of ionic, ionisable, or locally chargeable substances. The technique is robust, allowing successful separations even in non-optimised chemical systems. This is one of main advantages in comparison with competing techniques, such as solvent extraction.
The main limitation of the method is economical. The separation process is inexpensive if operated with a low concentration of ions. Increase of the concentration results in a cost increase. Thus, competing techniques (membrane separations, solvent extraction, etc) could be considered (but not necessarily preferred) for treating concentrated solutions. As a rule of thumb, treatment of solutions with metal ion concentrations below 10 ppm is most efficient with ion exchange. However, there are economically successful applications to solutions and even slurries containing more than 3M of target ions.
- F. Helfferich, Ion Exchange, McGraw Hill, New York, 1962 (Bible of the subject).
- Ion Exchangers (K. Dorfner, ed.), Walter de Gruyter, Berlin, 1991.
- C. E. Harland, Ion exchange: Theory and Practice, The Royal Society of Chemistry, Cambridge, 1994.
- Ion exchange (D. Muraviev, V. Gorshkov, A. Warshawsky), M. Dekker, New York, 2000.
- A. A. Zagorodni, Ion Exchange Materials: Properties and Applications, Elsevier, Amsterdam, 2006.