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How does deionization remove dissolved ions from water?

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chemical structure of cation exchanger — ion-exchange reaction Figure 1: Chemical structure of cation exchanger. The exchangeable ions are marked ⁺. The whole structure is permeated by solvent molecules, usually water (not shown).

Deionization removes dissolved ions from water by passing it through two distinct types of ion-exchange resins that act as chemical filters.1

The process typically involves the following steps:

Cation Exchange: Water first passes through a bed of cation-exchange resin loaded with hydrogen ions. This resin exchanges the hydrogen ions for the dissolved metal ions in the water, effectively converting the dissolved salts into their corresponding free acids.1
Anion Exchange: The water then flows through a second bed containing an anion-exchange resin loaded with hydroxyl ions. This resin captures the anions from the previously formed acids and releases hydroxyl ions, which neutralize the hydrogen ions to form water. This leaves only nonionic impurities behind.1

Alternatively, "mixed-bed" deionization combines both resin types into a single column, which is often used in small-scale laboratory units. Once the resins become saturated with the removed ions, they must be regenerated using strong acid and alkali solutions to restore their original exchange capacity.1