Electrodeionization theory, mechanism and environmental applications. A review

Increasing pollution of ecosystems is calling for sustainable methods to remove pollutants. In particular, electrodeionization has recently emerged as an efficient technique to remove ionic compounds from contaminated waters. Electrodeionization involves a continuous process of electrochemical water deionization using ion-specific membranes, mixed-bed resins and a direct current voltage. Electrodeionization is thus safer than classical treatments with acids, bases and other chemicals. Electrodeionization produces highly pure water, allows efficient ion removal and does not require chemicals for resin regeneration. Electrodeionization for water purification overcomes limitations of resin beds for ion exchange, particularly the release of ions as the beds exhaust. Applications include the removal of toxic chemicals, radioactive pollutants, heavy metal ions and corrosive anions; the regeneration of valuable metals; the deionization of water and the removal of nitrates. Continuous electrodeionization requires performance optimization and modification of the stack configuration. This article reviews principles and applications of ion removal, transport and reaction mechanisms, and factors controlling the efficiency.