Ion Exchange as an Adsorption Reaction

Prototypical ion exchange reactions can be expressed by the chemical equations  

The reactions in Eqs. 5.1 and 5.2, taken in the broadest geochemical sense, refer only to the replacement of one ion by another existing in a solid structure. Thus, for example, Eq. 5.1 could be applied to provide a description of solid solution composition alternate to that in Section 3.3. Instead of basing the composition of Al-goethiteon the solid components diaspore and goethite, as in Eq. 3.41, one could describe a continuum of possible compositions by combining Eqs. 3.39a and 3.39b into the cation exchange reaction 

The correspondence between Eq. 5.6 and Eqs. 5.1 and 5.2 is apparent. Special cases of Eq. 5.6 appear in the fifth and tenth rows of Table 4.1. 

Examples of selectively adsorbed anions include F and the oxyanions SeOg, PO4, C2O4, BO3, and MoO, along with their protonated forms. 

Exchange isotherms can be used to calculate an exchange selectivity coefficient, the conventional terminology for the conditional equilibrium constant associated with Eq. 5.1 or 5.2. For example, in the case of the cation exchange reaction in Eq. 5.1, 

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