Ion Exchangers

(also called ion-exchange sorbents), solid, virtually insoluble materials capable of ion exchange. Ion exchangers are capable of absorbing positive or negative ions (cations or anions) from solutions of electrolytes (salts, acids, and bases), releasing into the solution in exchange an equivalent quantity of other ions of the same charge. The molecular structure of ion exchangers may be represented as a three-dimensional network, or lattice carrying fixed ions, whose charges are compensated by oppositely charged mobile ions, so-called counter-ions, which participate in ion exchange with the solution.

Ion exchangers are divided into cation and anion exchangers, depending on the sign of the charge of ions being exchanged. Cation exchangers have acidic properties, and anion exchangers have basic properties. Ion exchangers capable of exchanging both cations and anions are called amphoteric. Ion exchangers may be inorganic (mineral) or organic in nature and of natural or artificial origin. Ion exchangers are divided into types and groups according to their specific properties, structural features, and purpose. In particular, ion exchangers with sufficiently dense structural networks and “windows” of a certain size, which selectively absorb only ions that are capable of passing through the windows, are called ion-exchange sieves.

Inorganic ion exchangers of practical importance include natural and synthetic aluminosilicates (certain clay minerals, zeolites, and permutites) and hydroxides and salts of polyvalent metals, such as zirconium hydroxide and phosphate. Ion exchangers prepared by chemical treatment of carbon, cellulose, and lignin are being used. The leading role, however, belongs to the synthetic organic ion exchangers, the ion-exchange resins.

The most important property of ion exchangers is their absorption capability, or the so-called ion-exchange capacity. This quantity is expressed in terms of the maximum number of mil-liequivalents (meq) of ions absorbed per unit mass or volume of the ion exchanger under conditions of equilibrium with the electrolyte solution (total capacity) or under conditions of filtration of the solution through an ion-exchange bed to the point of “breakthrough” of the ions into the filtrate (dynamic, or operating, capacity). The capacity values of most ion exchangers are in the range from 2 to 10 meq/g. Measurements of ion-exchange capacity have been standardized. The dynamic capacity is always less than the static capacity.

In addition to high exchange capacities, ion exchangers must also have mechanical strength (mainly abrasion resistance), as well as thermal and chemical stability. Ion exchangers are usually capable of prolonged service and can be easily regenerated repeatedly.

Depending on the production method and on the intended application, ion exchangers are produced in various forms, including powders, irregular grains, spherical granules, fibrous material, sheets, or films (ion-exchange membranes). Ion exchangers are available on the international markets under the following brand names: Amberlite (USA and Japan), Duolite (USA, France), Dowex (USA), Zerolite (Great Britain), Lewatit (Federal Republic of Germany), and Wofatit (German Democratic Republic). The main industrial brands of Soviet ion exchangers include the cation exchangers KU-1, KU-2, SG-1, KB-2, and KB-4 and the anion exchangers AV-16, AV-17, AN-1, AN-2F, AN-18, AN-31, and EDE-10P.

The most important area of use of ion exchangers has been, and still is, water treatment. The use of ion-exchange filters yields demineralized (desalinized) water for steam generators, many modern industrial processes, and domestic use. Ion-exchange filters and electrodialysis units with ion-exchange membranes are being used for the desalinization of seawater or groundwater with a high salt content. In hydrometallurgy, ion exchange is used for beneficiation of raw materials, as well as in the separation and purification of rare elements. Ion exchangers make possible the extraction of gold, platinum, silver, copper, chromium, uranium, and other metals from solutions. Ion exchangers are being used successfully in processing radioactive wastes, as well as in the removal of numerous harmful impurities from waste waters.

Ion exchangers are used in the chemical industry for the purification or separation of the products of organic and inorganic synthesis, as catalysts, and as a means of analytical monitoring of industrial processes. In the food industry they are used in sugar refining, to improve the quality of wines and juices, and in the production of vitamins and pharmaceuticals. They are used to extract valuable biochemicals from vegetable and animal raw materials, for the preservation of blood plasma, and for treatment of some diseases. Ion exchangers are being increasingly used in industrial production, science, and everyday life.