To understand how the resin works, it helps to start from the reaction itself: ion exchange is a reversible interchange of charged particles, or ions, with those of like charge. Ions held on an insoluble resin matrix swap places with ions of similar charge present in the surrounding solution.
This happens because functional groups are fixed ions permanently bound within the polymer matrix. These charges readily bond with ions of opposing charge delivered by the solution. Exchange continues until equilibrium is reached, at which point the bed is exhausted for practical purposes.
In operation, the stream to be treated flows through the resin bed. As the solution passes the beads, functional groups attract counterions from the solution. When the available sites are loaded, the resin is regenerated with a concentrated solution that reverses the reaction, restoring the original ionic form and releasing captured species in a small, manageable waste volume. This cycle can be repeated hundreds to thousands of times.