Sorption Kinetics - Transport and Fate of Chemicals in Soils: Principles and Applications - page 46

400

Hysteresis
Adsorption-Desorption
Isotherms

300

Adsorption
Isotherm

200

100

Desorption Isotherms

0 0

5 10 15 20

Atrazine in Solution(mg/L)

FIGURE 2.11
Traditional desorption isotherms of atrazine by sugarcane mulch residue. The solid line is the
adsorption isotherm for 504 hours of reaction. Dashed curves are predictions using (a) the
multireaction model (upper), and (b) the Freundlich model (lower).

The extent of hysteresis is further illustrated by the results for Zn desorp-
tion versus time for soils with distinctly different sorption affinity for Zn
(see Figure 2.12). In Webster soil, desorption or release of Zn appears to be
slow, indicative of strong sorption. In contrast, rapid release was observed
in Windsor soil. The respective isotherms for the two soils are given in
Figure 2.12 and clearly illustrate extensive hysteresis for Webster sorption
consistent with strong kinetic behavior and possible irreversible reactions.
The hysteresis phenomenon has been reported for several decades in
numerous studies published on colloids and colloidal chemistry. In fact, the
term hysteresis is not restricted to solute sorption isotherms but is used in
other disciplines, such as soil physics and hydrology. In water-unsaturated
porous media, hysteresis was observed in soil-moisture content and applied
suction. Discrepancies between wetting and drying curves and subsequent
scanning curves give rise to the term hysteresis.
Reasons for the observed hysteresis have been discussed and various
explanations have been advanced in the literature. Most center on irreversible
reactions, change of phase, and formation of other ions during desorption.
Selim, Davidson, and Mansell (1976) developed a mathematical proof that
sorption kinetics can explain in part the discrepancies between adsorption
and desorption isotherms. In fact, the explanation explicitly show that when
kinetics is absent, that is, when instantaneous or equilibrium is dominant,
identical adsorption and desorption isotherms are obtained. In other words,
both isotherms coalesce and nonsingularity or hysteresis is not observed.