A number of natural materials such as zeolites, which are complex sodium-aluminium silicates, have the ability to exchange ions in their structure for other ions with which they come into contact. Synthetic ion-exchange resins have been developed to provide higher exchange capacities or for specialized applications. Ion Exchange resins provide either cation exchange (positive ions) or anion exchange (negative ions) and the commonest application in water treatment is as an alternative to precipitation methods in softening. Ion-exchange softening uses a sodium-cycle cation exchange which takes up calcium and magnesium ions in the water passing through the bed and releases sodium ions back into the water. This gives complete removal of hardness, since sodium does not contribute to hardness. The presence of high concentrations of sodium in water is undesirable for drinking water.
The ion exchange reaction can be depicted as:
Ca2+ CO32- Na2CO3 + CaX
+ Na2X →
Mg2+ SO42- Na2SO4 + MgX
When all the available sodium has been released, no further softening can take place but the ion exchange material can then be regenerated by bringing it into contact with a strong solution of sodium, for which common salt solution is the most readily available form:
+ 2NaCl → Na2X +
The calcium and magnesium ions appear as a waste stream of chlorides which must be disposed of in a suitable manner; the ion-exchange material can be used again and again. The performance of an ion exchanger is expressed in terms of its exchange capacity (g equivalents removed/m3) and its regenerant requirement (equivalents/equivalent exchanged). For a natural zeolite these factors would be of the order of 200 g/m3 and 5 equiv./equiv., respectively.
Ion exchange can also provide a means for removing nitrates from water using a cation-exchange system, although at the present time nitrate-specific resins are not fully developed. Ion exchange methods can also be used to treat certain industrial wastewaters, particularly those containing metals in solution. The recovery for re-use of silver from photographic processing wastes is a good example of the use of an ion-exchange technique for both pollution control and conservation of raw materials. It must be noted that, in common with most treatment processes, ion exchange methods do not destroy the contaminants but merely make it possible to remove and concentrate them.