TABLE OF CONTENTS Ion exchange involves removing unwanted ions from water and replacing them with other ions, changing the composition of the water. This is done by passing water through a bed of insoluble material made of a synthetic resin. The resins used are designed such that they will remove either cations or anions from the water passing through the bed, replacing the ions removed from the water with ions of the same charge from within the resin. The main uses of ion exchange in potable-water treatment are to remove nitrate or to soften water.
This section will touch on three aspects of ion exchange:
classification of resin used;
the factors affecting ion removal; and
uses of ion exchange in water treatment.
The basic classification of resins is into cation-exchange resins (which contain exchangeable cations) and anion-exchange resins (which contain exchangeable anions). These can then be sub-divided further depending on their affinity for cations or anions. In water treatment the resins of most interest are strong-acid exchange resins, which are the cation-exchange resins used in softening, and selective strong-base resins, which are the anion-exchange resins used in nitrate removal. In water treatment both of these are regenerated with sodium chloride solution.
In softening, sodium ions within the resin are released into the water and replaced by cations from the water being treated. Regeneration involves introducing sodium ions into the resin, displacing the ions removed from the water. In nitrate removal, chloride ions within the resin are...
