Ion exchange is a Reverseible process in which ions are released from an insoluble permanent material (resin) in exchange for other ions in a surrounding solution; the direction of the exchange depends upon the affinities of the ion exchanger for the ions present, and the concentration of the ions in the solution.
The ion exchange process: In water treatment this process adds and removes ions from water. In general the resin is used as the exchange site.
Resins are basically classified as either cationic or anionic. The choice of resin for a particular application will depend on the ion or ions to be removed and recovered, and the chemistry of the process.
In (demineralization, deionisation), for example, bicarbonate [HCO3-], chloride [CL-] and sulphate [SO4–] anions are removed from the solution in exchange for a chemically equivalent number of hydroxide anions [OH-]from the anion exchange resin.
Further in demineralization , for example, calcium [Ca++], magnesium [ Mg++] and potassium [K+] anions are removed and exchanged for a chemically equivalent number of hydrogen cations [H+] from the cation exchange resin.
In softening, calcium [Ca++] and magnesium [ Mg++] are removed from a solution in exchange for a chemically equivalent number of sodium anions[ Na+] from the anion exchange resin.
Mixed bed ion exchanger is used for total demineralization of water. Here both a cation exchanger and an anion exchanger are incorporated into the same.