Transport and Sorption - Environmental Modeling - page 109

Fig. 6.1 Schematic illustration of ad- and desorption processes

one phase are usually connected with high or low concentrations in the other phase.
Such phenomenon can be formulated by a mathematical relationship. In the case of
fast sorption, the relationship is mostly stated in the functional form

c s ðcÞ (6.1)

in which the solid phase concentration is given in dependency of the fluid phase
concentration. This is called a (sorption-) isotherm 1 and can be understood as
an equilibrium, very much like the equilibrium in chemical reactions. The
concentrations in one phase are adjusted if, for whatever reason, the concentration
in the other phase is changing.
The simplest example is the linear isotherm

c s ¼ K d c (6.2)

where the distribution coefficient K d determines the ratio between solid phase and
fluid phase concentrations. The physical unit [volume/mass] can be attributed to the
fact that the concentrations in the fluid and solid phase are usually not measured
in the same physical units. Strongly sorbing components have a high K d , while it
is low for weakly sorbing components. Non-sorbing components do not interact

1 The notation 'isotherm' stems from the fact that such measurements are mostly performed
for constant temperatures, i.e. isothermal conditions. In general the isotherm changes with
temperature.