Just as with membrane filtration and reverse osmosis in particular, ion exchange is a technique used to soften and desalinate or demineralise water.
Important applications for ion exchangers are softening, decarbonation and demineralisation. A distinction is made between cation and anion exchangers, making it possible to remove salts from a liquid. In demineralisation, total desalination takes place, making use of cation and anion exchangers.
Ion exchangers may be liquid or solid. The solid form is used in water treatment. In the solid form, this forms small synthetic resin beads of polystyrene, with a diameter of 0.3 – 1.2 mm. A functional group carrying a positive (H+ or Na+) or a negative (OH-) charge becomes chemically attached to the matrix of the polystyrene.
Ion exchange reactions are stoichiometric, which means that for one unit of an ion removed from the solution, one equivalent unit of another ion is released from the resin and returns to the solution.
If water containing CaCl2 comes into contact with Na+ active ion exchanger, the calcium (Ca2+) is exchanged for two units of Na+ and a different form of salt is created: NaCl. This is how the process works in desalination, for example.
Water from which all ions are to be removed (desalination or demineralisation) is forced through a strong-acid cation exchanger in the H+ form, so as to all cations to be exchanged for an equivalent number of H+ ions. The anions are subsequently exchanged for OH- ions using a strong-base anion exchanger in the OH- form.
The H+ and OH- ions react together to form water. In this way, all ions are removed from the water and replaced by pure water.
Once there are no reactive groups remaining on the ion exchanger, the resin is exhausted and must be regenerated. This regeneration takes place using abundant saline solution (NaCl) in the case of water softening, and for demineralisation, using hydrochloric acid (HCl) and caustic soda (NaOH) solution respectively for the cation and anion exchangers. The ion changers are returned to their original state by the regeneration step, so that once the excess chemicals have been flushed from the resin, the process can be repeated from the start.
The applicability of ion exchange is limited by the salt content in the water to be treated. If the water to be treated has a higher salt content, the costs of chemicals can rapidly accumulate to the extent that a membrane system becomes more attractive. Nevertheless, over the past 40 years, ion exchangers have proven to be a reliable method for obtaining a guaranteed water quality.