Ion exchange in water treatment removes monovalent ions from water such as Na+, K+, and Cl−; divalent ions such as Ca2+ and Mg2+; and other polyvalent inorganic ions such as SO42− and PO43−. These ions contribute to the conductivity of water and the removal of these ions is necessary to produce the resistivity necessary for ASTM type II and type I water.
Ion exchange resins are tiny synthetic beads which have hydrogen ions (H+) and hydroxyl ions (OH−) on their surfaces. As the water passes across these beads the H+ is exchanged with the positive ions in the feed water and the OH− is exchanged with the negative ions in the feed water. The replaced OH− and H+ combine to produce H2O while the larger ions are bound to the resin beads. Cation exchange resins remove the positively charged ions and anion exchange resins remove the negatively charged ions. Eventually all the binding sites on the resin are filled and the resin has to be regenerated or replaced. In the case of a home water softening system the cation exchange resin can be regenerated by passing a concentrated salt solution through the resin. Laboratory water purification systems use a mixed bed ion exchange cartridge where both cation and anion exchange resins are present so regeneration is not practical and the cartridge must be replaced.
Mixed bed ion exchange is very efficient at removing dissolved inorganic ions and produces water with >18 MΩ⋅cm resistivity. To produce Type II or type I water ion exchange must be used with other water purification technologies. Ion exchange does not remove particles or colloids (filtration), it does not remove organics (distillation/reverse osmosis), and pyrogens or bacteria (reverse osmosis/ultrafiltration).