Ion Exchange:

The ion exchange process is reversal interchange of ions between a solid in exchange medium and a solution. Since this method works on principle of Base Exchange, it is also known as Base Exchange process.  In the process, ions of a given species are displaced from insoluble exchange materials by ions of species from solution. The most commonly used exchange materials are zeolites, which are complex sodium alumino-silicates and green sands. When used for water softening, the hard water is passed through the bed of natural zeolites, which exchange their sodium ions for the calcium and magnesium ions in water, giving complete removal of hardness:

Ca⁺⁺ + Na₂Z → CaZ + 2Na⁺

Mg⁺⁺ + Na₂Z → MgZ + 2Na⁺

When the capacity for the exchanging ions has been depleted, a regenerating solution with a high concentration of the original ions is pumped through the bed. The process displaces the contaminant ions and rejuvenates the exchangers as given below:

CaZ     +          2NaCl             →        Na₂Z   +          CaCl₂

Used               Sodium Chloride     Regenerated Zeolite

Zeolites          Solution

MgZ + 2NaCl  →  Na₂Z + MgCl₂

The use of ion exchangers is not limited to water softening. Ion exchangers can be used for the removal of other ions such as ammonia, nitrate, chloride, fluoride etc. from water. A wide variety of ion exchangers are commercially available. The widest application of ion exchangers is in industrial water supply treatment rather than in domestic water supply treatment. The ion-exchange materials used is known as zeolites and are mainly processed green sand and synthetic zeolites made from sulphonated coals and condensation polymers.