Deionization

Deionization, or demineralization, is a water treatment process that removes dissolved ions from water. It commonly uses ion-exchange resins or ion-exchange membranes that replace dissolved ions with ions from the resin, producing water with very low mineral content.

In a typical ion-exchange deionization system, a cation-exchange resin exchanges cations such as calcium, magnesium, and sodium for hydrogen ions, while a separate anion-exchange resin exchanges anions such as chloride and sulfate for hydroxide ions. The released H+ and OH- recombine to form water, yielding deionized water. Systems can be configured as twin-bed units (separate cation and anion beds) or mixed-bed units (both resin types in one vessel).

Resins have finite capacity and must be regenerated periodically. Cation resins are regenerated with acid, and anion resins with caustic. Regeneration produces chemical waste streams, and effluent must be managed. Pretreatment to remove organics and particulates helps extend resin life. An alternative approach, electrodeionization (EDI), uses ion-exchange media, ion-selective membranes, and electrical current to continuously remove ions without batch chemical regeneration.

Applications include pharmaceutical-grade water, laboratory-grade solvents, power plant boiler feedwater, and microelectronics manufacturing where high-purity water is required. Deionization is often combined with pretreatment and other purification steps. Limitations include incomplete removal of non-ionic contaminants, resin fouling, chemical handling, and waste disposal from regeneration.