THE ION EXCHANGE PROCESS

THE ION EXCHANGE PROCESS

It is important to keep in mind that the ion exchange process only works with ions. Substances that do not ionize in water are not removed by ion exchange. Each type of ion exchange resin exhibits an order of preference for various ions. This can be stated quantitatively through selectivity coefficients. These are similar to the equilibrium constants in ionic equilibria. In a similar manner, the equilibrium concentrations of ions in the resin phase and in the water phase can be calculated from the selectivity coefficient. Each ion pair has a unique selectivity value for each ion exchange resin. The higher the selectivity coefficient, the higher the relative affinity of the ion for the resin. The higher the affinity, the easier it is to load the ion, and conversely the more difficult it is to remove during regeneration. The operating performance, capacity, and leakage data for ion exchange resins for the common ions found in water are usually provided by the resin supplier.

In general, the ion exchange resin is used in and regenerated to an ionic form, which will exchange ions that are acceptable in the treated water. As the untreated water passes through the resin, the undesirable ion or ions exchange for the unobjectionable ion on the resin. For example, a cation exchange resin is regenerated with sodium chloride and operated in the sodium cycle. The resin exchanges sodium ions for all the positively charged ions (cations), including hardness-causing calcium and magnesium ions,...