Any of a wide variety of synthetic polymers containing positively or negatively charged sites that can interact with or bind to an ion of opposite charge from a surrounding solution. Light, porous solids in granules, beads, or sheets, they absorb the solution and swell as they attract the target ions; when exhausted, they are removed from use and regenerated by an inexpensive brine or carbonate solution. A solid support of styrene-divinylbenzene copolymer to which are attached sulfonic or carboxylic acid groups is often used to attract and exchange cations (e.g., ions of hydrogen or metals). Quaternary ammonium groups on the solid matrix are used to attract anions (e.g., ions of chlorine). Industrially, the resins are used to soften hard water, purify sugar, and concentrate valuable elements (gold, silver, uranium) from their ores. In the laboratory they are used to separate and concentrate substances and sometimes as catalysts. Zeolites are minerals with ion-exchange properties.
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