Geoscience Reference
In-Depth Information
Fig. 5.9 Splitting of the desorption isotherm of dimefuron in the presence of 0.01 M CaCl
2
into
two other isotherms, corresponding to the two-compartment (linear, exponential) model of
desorption isotherms (Barriuso et al.
1992b
)
Desorption isotherm may differ from adsorption isotherms for systems that are
not at equilibrium, because the desorption rate is lower than the adsorption rate.
Theoretical treatments by Ponec et al. (
1974
), for gas adsorption, and Giles et al.
(
1974
), for solute adsorption, indicate that the activation energy for adsorption is
zero or near zero. Under these conditions, these authors showed that the activation
energy for desorption is greater than that for adsorption; consequently, the rate of
desorption is lower than adsorption. This behavior pattern also is valid when
adsorption is accompanied by dissociation of the adsorbed molecules (Ponec et al.
1974
). Adsorbed molecules may be classified according to two categories: mole-
cules retained through physical interactions and able to desorb and molecules that
interact strongly with the solid matrix and therefore are released slowly or not at
all (Barriuso et al.
1992a
,
b
). Because different mechanisms are involved in the
adsorption-desorption process, different types of desorption isotherms can be
observed. In one case, desorption is described by a linear isotherm; in another case,
the release is described by an exponential function for equilibrium concentrations
in solution. Figure
5.9
gives an example of a combination of such correlations for
the release of the herbicide dimefuron adsorbed on a clay loam soil.
Several other explanations have been put advanced to explain retention hys-
teresis, including (1) surface precipitation of metallic cations whose hydroxides,
phosphates, or carbonates are sparingly soluble; (2) chemical reactions with solid
surfaces, including organic surfaces, which form complexes with metallic cations;
and (3) incorporation into the subsurface organic matter through chemical reac-
tions and biochemical transformation. For the case described by Fig.
5.9
or
