The ion exchange resin works in this way because of its functional groups, which are essentially fixed ions permanently bound within the polymer matrix of the resin. These charged ions will readily bond to ions of opposite charge which are delivered through the solution to be treated. These counter ions will continue to bond to the functional group until equilibrium is reached.

During an ion-exchange cycle, the solution to be treated would be added to the ion-exchange resin bed and allowed to flow through the beads. As the solution moves through the ion-exchange resin, the functional groups of the resin attract any counterions present in the solution.  If the functional groups have a greater affinity for the new counterions than those already present, then the ions in solution will dislodge the existing ions and take their place, bonding with the functional groups through shared electrostatic attraction. In general, the greater the size and/or valency of an ion, the greater affinity it will have with ions of an opposite charge.

Let’s apply these concepts to a typical ion-exchange water softening system. In this example, Calcium (Ca2+) and magnesium (Mg2+) ions that cause water hardness can be removed fairly easily by using an ion exchange procedure.

 
The softening mechanism consists of a cation exchange resin where sulphonate anion (SO3–) functional groups are fixed to the ion-exchange resin matrix. A counterion solution containing sodium cations (Na+) is then applied to the resin. The Na+ are held to the fixed SO3– anions by electrostatic attraction, resulting in a net neutral charge in the resin. As hard water passes through a softener, the Calcium (Ca2+) and magnesium (Mg2+) trade places with sodium cations. During this process, “free” sodium ions are released into the water. The hardness ions (Ca2+ or Mg2+), on the other hand, are retained by the ion-exchange resin.

After softening a large quantity of hard water, the exchange medium becomes coated with calcium and magnesium ions. When this occurs, the exchange resin must be recharged or regenerated.