Sorption Isotherms - Transport and Fate of Chemicals in Soils: Principles and Applications - page 4

With strict plateau

Without strict plateau

C C

(a) e “C” isotherm (b) e “L” isotherm

Point of inection

C C

(c) e “H” isotherm (d) e “S” isotherm

FIGURE 1.1
The four main types of isotherms. (After Giles, D'Silva, and Easton, 1974.)

isotherm is characterized by decreasing slopes as the vacant sites become
occupied by the sorbed ion or molecule. Freundlich and Langmuir isotherms
are commonly referred to as L-curve isotherms. In L-type isotherms, at low
solution concentrations, high-energy sites are occupied first. Subsequently,
as the concentration in solution increases, sites of moderate and low affinities
become occupied. The H-type isotherms are best characterized by extremely
high sorption possibly due to irreversible reactions. The S-type isotherms
indicate low affinity for sorption at low solution concentrations followed by
a gradual sorption increase. At higher concentration, sorption decreases and
sorption maximum is perhaps attained.
In the literature, L-type isotherms are frequently encountered for most
trace elements and for heavy metals. Specifically, Freundlich and Langmuir
isotherms are adopted for a wide range of solutes. C-type isotherms are often
observed for pesticides (herbicides and insecticides) where linear isotherms
are often observed.

1.4 Empirical versus Mechanistic Models
Fontes (2012) argued that the adsorption phenomenon can be represented
by two main conceptual models: empirical models initially derived from
experiments, and semiempirical or mechanistic models, based on reaction
mechanisms. The main difference between these two types of models is the