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Why are separate cation-exchange and anion-exchange beds used to deionize 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).

Separate cation-exchange and anion-exchange beds are used to deionize water to ensure the complete removal of dissolved ions. This process functions through a two-step chemical reaction:

Cation-exchange bed: The water first passes through a resin bed containing hydrogen ions. This resin exchanges the cations in the water for hydrogen ions, effectively converting the dissolved salts into their corresponding free acids.1
Anion-exchange bed: The water then flows through a second bed containing an anion-exchange resin loaded with hydroxyl ions. This resin neutralizes the acids produced in the first step by holding back the anions and releasing hydroxyl ions, which combine with the hydrogen ions to form water. This leaves only nonionic impurities in the solution.1

While "mixed-bed" deionization exists—where both resins are combined in a single column—separate beds are often preferred because the resins must be separated for regeneration using strong acid and strong alkali, respectively.1