Multicomponent Equilibrium Thermodynamics - Elements of Environmental Engineering: Thermodynamics and Kinetics - page 97

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1, 2-Dichloroethane

1, 1, 1-Trichloroethane

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C i / μg / L C i / μg / L

FIGURE 3.16 Linear adsorption isotherms for two organic compounds on a typical
soil. (Data from Chiou, C.T., Peters, L.J., and Fread,V.H. 1979. Science 206, 831-832.)

earlier section), it is no surprise that it has a large adsorption constant. For a majority
of hydrophobic organic compounds, the linear adsorption isotherms on both soils and
sediments afford a good representation of adsorption data.

It is well documented in both the surface chemistry and environmental chemistry
literaturethatvariousformsofadsorptionisothermsarepossible.Mostoftheisotherm
shapes can be explained starting from what is known as the Brauner-Emmett-Teller
(BET) isotherm . The Langmuir isotherm limits the adsorption capacity to a single
molecular layer on the surface. The BET approach relaxes the assumption in the
Langmuir model and suggests that adsorption need not be restricted to a single
monolayer and that any given layer need not be complete before the subsequent
layers are formed. The first-layer adsorption on the surface occurs with an energy
of adsorption equivalent to the heat of adsorption of a monolayer just as in the case
of the Langmuir isotherm. The adsorption of subsequent layers on the monolayer
occurs via vapor condensation. Figure 3.17 is a schematic of multi-layer formation.
A multi-layer can be formed in the case of adsorption of solutes from the gas phase
and of solutes from the liquid phase onto both solid and liquid surfaces. The the-
ory underlying the BET equation is well founded and can be derived either from
a purely kinetic point of view or from a statistical thermodynamic point of view
(Adamson, 1990). Without elaborating on the derivation, we shall accept the final
form of the BET equation. If it is assumed that (i) the number of layers on the
surface is n = 64, and (ii) the energy of adsorption for any molecule in all layers