Sorption Kinetics - Transport and Fate of Chemicals in Soils: Principles and Applications - page 51

the area under an adsorption isotherm curve for concentration from C = 0
to some solution concentration C = C , and A d represents the area under a
desorption isotherm at the same concentration range, we define the para-
meter λ as:

AA
A

−

×

d a

λ= 100

(2.11)

a

Upon further substitution, we obtain λ as:

×

N
N

+
+

1
1

a

λ= − 1 100

(2.12)

d

Based on the above formulations, one can derive values for λ as well as ω.
Selim and Zhu (2005) found that λ decreased as C i increased for Sharkey soil,
but no such relationship was observed for Commerce. Similar trends were
observed for ω, whereas the opposite was observed for H . Ma et al. (1993)
calculated ω for atrazine on Sharkey soil and indicated that ω increased lin-
early with incubation time, which is the time interval between the end of
adsorption and the beginning of the desorption process. However, they did
not observe an effect of C i on ω. Seybold and Mersie (1996) calculated ω for
metolachlor in two soils and found that ω is C i dependent for Cullen soil
that contained 31% clay and 1.3% of organic carbon, but this phenomenon
was not apparent in Emporia soil, which contains less clay and less organic
carbon. This dependence of desorption on C i has been reported for other
herbicides (e.g., Bowman and San, 1985; Graham and Conn, 1992, among oth-
ers). It was postulated that λ increases with desorption time, indicative of
dependency on the desorption history. Such behavior might be explained by
the existence of irreversible reactions, which cause a decrease in desorbed
herbicide amounts as desorption time increased.

2.3 Measuring Sorption
There are several experimental methods, which are described in detail in
the literature, to measure sorption or affinity of a solute species to the soil
matrix. Methods for measuring kinetic sorption and adsorption are avail-
able in soil chemistry and environmental soil chemistry textbooks as well
as numerous scientific journals. It should be emphasized that most methods
are laboratory measurements and as such suffer various shortcomings and
do not depict the fate of a solute under field or in situ conditions of varying