Evaluation and Separation of Interfacial Processes

In the geological and soil science literature, ion exchange and precipitation are frequently considered as adsorption and thermodynamically described by adsorption equations, or isotherms. This is not correct because, as shown previously, the processes are principally different adsorption is directed by the decrease of surface energy, and it takes place on the free surface sites ion exchange is just a competitive process on an already covered surface, determined by the ionic composition of the liquid phase. Precipitation, including colloid formation, is governed by the composition of the liquid phase, the crystal structure (coprecipitation), or primary chemical forces. 

Therefore, when studying interfacial reactions on rocks and soils, it must always be determined what the mechanism of the interfacial reaction is, and what kind of processes take place. Also, it must identify the dominant processes responsible for surface excess concentration. If this is not done, and the resultant process is evaluated without knowing it in conventional ways, incorrect thermodynamic data are obtained. The concepts of adsorption, ion exchange, and surface precipitation have to be clearly differentiated, as done previously. When the character of the process can be neglected, only surface accumulation is considered, and we can speak about sorption, including all of the surface processes. In this case, only aphenomenological description can be given, and no thermodynamics can be applied. 

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