In the field of water purification, ion exchangers are used in water treatment to produce very pure process water and also in wastewater treatment.
A distinction is made between anion exchangers and cation exchangers. The former bind anions such as sulphate or chloride, the latter bind cations such as calcium or magnesium. A further distinction is made between weak and strong ion exchangers. In general, the ions that are to be removed from the water are exchanged at the ion exchanger for other anions or cations. In this way, the neutrality of the charges is maintained. The exchanged ions are less strongly bound to the ion exchanger than the ions to be removed from the water. In addition, they should not introduce any new impurities into the purified water. Therefore, the exchanged ions are mostly hydroxide ions (OH-) as anions and hydrogen ions (H+) as cations.
Thus, drinking water can be obtained from salty seawater by desalination with ion exchangers. Strong ion exchangers can also be used to remove low concentrations of pollutants from water, e.g. in the treatment of boiler feed water in water softening.
In order to be able to work economically, most ion exchangers are regenerable. For this purpose, anion exchangers are reloaded with hydroxide ions by strong alkalis, whereby the bound pollutants are displaced by the high concentration of hydroxide ions in the strong alkali. Similarly, cation exchangers are regenerated with strong acids.


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