Ion exchange is a naturally occurring chemical process by which one ion in solution (typically aqueous but not exclusively) is exchanged for another. A very common commercial application of this principle is found in many homes with a water softener.
In industrial applications such as de-ionized water systems used in the production of ultrapure water (UPW) free naturally occurring cations (Ca, Mg, Fe, etc.) are removed from water and replaced with H+ and free anions (SO4, Cl, F, etc.) are replaced with OH-.
The capacity of a resin to take up target cations or anions is limited and when exhausted the resin must be regenerated. Ion exchange reactions are generally reversible and the regeneration process reverses the reactions freeing up the ion exchange sites previously "consumed" during the take up of the target species.
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