Ion exchange is the displacement of one ion by another. The displaced ion is originally a part of an insoluble material, and the displacing ion is originally in solution. At the completion of the process, the two ions are in reversed places: the displaced ion moves into solution and the displacing ion becomes a part of the insoluble material.
Two types of ion exchange materials are used: the cation exchange material and the anion exchange material. The cation exchange material exchanges cations, while the anion exchange material exchanges anions. The insoluble part of the exchange material is called the host. If R- represents the host part and C+m the exchangeable cation, the cation exchange material may be represented by (Rdn)r(Cm)rn/m, where r is the number of active sites in the insoluble material, rn/m is the number of charged exchangeable particles attached to the host material, -n is the charge of the host, and +m is the charge of the exchangeable cation. On the other hand, if R+o represents the host part of the anion exchange material and A- its exchangeable anion, the exchange material may be represented by (R +o)r(A~P)rolp, where the subscripts and superscripts are similarly defined as those for the cation exchange material. Letting C+q be the displacing cation from solution, the cation exchange reaction is
(R -n)r ( C+m)rn/m + ^ C+q ^ (R ^)r ( C+q)rnlq + ^ C (16.1)
Also, letting A-' be the displacing anion from solution, the anion exchange reaction may be represented by
(R+o)r(A-p)rolp + y A- ^ (R+o)r(A-')rolt + P A~P (16.2)
As shown by the previous equations, ion exchange reactions are governed by equilibrium. For this reason, effluents from ion exchange processes never yield pure water.
TABLE 16.1 Displacement Series for Ion Exchange
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