Ion Exchange ? Basic Principles

Ion exchange is a chemical process in which ions of a certain charge contained in a solution (e.g. cations) are removed from this solution by adsorption onto a solid material (the ion exchanger), to be replaced by an equivalent quantity of other ions of the same charge emitted by the solid. Ions of opposite charge are not affected. This technology was developed on an industrial scale after the Second World War, mainly between 1950 and 1970.

An ion exchanger is a salt, acid or base, solid and insoluble in water, but hydrated, i.e. swollen with water like a sponge. The water content of an apparently dry material can be more than 50% of its total mass, and the exchange reactions take place in this water, known as swelling or hydration water, inside the exchanger.

Ion exchange forms the basis of a large number of industrial and laboratory processes, which can be divided into three main categories: substitution, separation and elimination:

Substitution: a noble ion (e.g. copper) is removed from the solution and replaced by a harmless one. Similarly, a harmful ion (e.g. cyanide) is removed from solution and replaced by a harmless one.

Separation: a solution containing a number of different ions (e.g. Li ⁺ , Na ⁺ , K ⁺ , Rb ⁺ , Cs ⁺ , all in chloride form) is passed through a column of resin beads. The ions are separated and emerge in ascending order of affinity for the resin.

Removal: using a combination of cation exchange resin (in H ⁺ form) and anion exchange resin (in OH form ^– ), all ions are removed and replaced by water (H ⁺ OH ^– ). The solution is thus demineralized. This is actually a particular variant of substitution.

Although substitution and elimination can be carried out batchwise without stirring, the compact separation column process is the most frequently adopted for all three types of application, due to the greater efficiency of the two phases of the exchange cycle, namely fixation and elution.

Ion exchange is a technology particularly suited to the treatment of low-concentration solutions, usually below 1 g/L. However, there are exceptions.

This article presents the chemical principles governing the manufacture of ion exchange resins, their physicochemical properties and the exchange reactions most commonly...