Multisite and Multilayer Adsorption

A number of recent studies involving the adsorption of solutes from solution by mineral surfaces have resulted in data suggesting multiple-site adsorption. That is, several different arrays of sites are postulated, each of which fulfills the requirements of the Langmuir model. For example, Fig. 9.10 shows data for phosphate adsorption on 

FIGURE 9.10. Phosphorus adsorption data plotted according to the Langmuir equation. (After J. K. Syers, M. G. Browman, G. W. Smillie, and R. B. Corey. 1973. Soil Set. Soc. Am. Proc. 37 358-363.) 

In addition to multisite adsorption, many gases and vapors adsorbed by solids do not produce a typical monolayer-type adsorption isotherm (Fig. 9.9a), but rather produce an isotherm indicating multilayer adsorption (Fig. 9.9c). An equation that treats multilayer adsorption is the BET equation, named after developers Brunauer, Emmett, and Teller. Multilayer adsorption is characteristic of physical or van der Waals attraction. It often proceeds with no apparent limit, since multilayer adsorption merges directly into capillary condensation as the vapor pressure of the adsorbate approaches its saturation value. 

The BET equation has been used to determine the surface area of solids from gas adsotption data. The equation not only predicts the shape of the adsorption isotherm, but also gives the volume of gas Vm required to form a monolayer. The BET equation has the form 

The BET equation has been applied to ion adsorption from soil solutions, although the extended Langmuir equation (Eq. 9.11) would seem to apply as well. The BET equation has also been used to study the adsorption of pesticides having relatively high vapor pressures. 

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