Treatment of Ion-Exchange Processes

In all quantitative treatments of ion exchange, the quantities characterizing the process mostly depend on the molar fraction of the ions on the surface. This fact is treated in different ways in the models describing the ion exchange. 

The models applying the law of mass actions (Gaines and Thomas 1953 Howery and Thomas 1965 Argersinger et al. 1950 Ekedahl et al. 1950 Hogfeldt 1950) introduce the term of surface activity coefficient. 

The models in which the ion-exchange processes are included in the surface complexation models take into account the surface electric work (Section 1.3.2.1 Table 1.7). 

Those that use competitive ion-exchange isotherm equations consider a heterogeneous surface characterized by an energy distribution function (Sposito 1981 Kinniburgh 1983 de Wit et al. 1990 Borkovec and Koper 1994). 

All models, however, have the same deficiency, that is, the concentrations in the solid and solution can be measured experimentally however, the surface activity coefficients, the surface electric work, as well as the energy distribution function can only be estimated. It means that the models are adapted to the experimental data and the best-fitted model is used, and therefore the selected model has no thermodynamically significant meaning (Cemik et al. 1995). In this chapter, these problems will be illustrated and discussed. 

---

In the quantitative treatment of ion-exchange processes, several authors used the law of mass action. The main difference among these approaches is how the activities and surface concentration of the ions are treated. The first such approach was the Kerr equation, which uses the concentration of the ions on the solid and liquid as well but totally neglected the activity coefficients (Kerr 1928). The Vanselow (1932) equation applied activities in the solution and expressed the concentration of the ions on the solid phase in mole fraction, and in this way, it defined the selectivity coefficient (Equation 1.79). 

---