Resins are polymers that are capable of exchanging particular ions within the polymer with ions in a solution that is passed through them. This ability is also seen in various natural systems such as soils and living cells. The synthetic resins are used primarily for purifying water, but also for various other applications including separating out some elements.
In water purification the aim is usually either to soften the water or to remove the mineral content altogether. The water is softened by using a resin containing Na+ cations but which binds Ca2+ and Mg2+ more strongly than Na+ . As the water passes through the resin the resin takes up Ca2+ and Mg2+ and releases Na+ making for a ‘softer’ water. If the water needs to have the mineral content entirely removed it is passed through a resin containing H+ (which replaces all the cations) and then through a second resin containing OH- (which replaces all the anions). The H+ and OH- then react together to give more water.
The process has some disadvantages in that there are substances occuring in some water (such as organic matter or Fe3+ ions) which can foul the resin, but in general the advantages of the process (long life of resins, cheap maintainance etc.) outweigh the disadvantages. In addition, the process is very environmentally friendly because it deals only with substances already occuring in water.