Sorption Isotherms - Transport and Fate of Chemicals in Soils: Principles and Applications - page 1

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Sorption Iso therms

Adsorption isotherms or, more accurately, sorption isotherms are conve-
nient ways to graphically represent the amount of an adsorbed compound,
or adsorbate, in relation to its concentration in the equilibrium solution or
adsorbent. In other words an adsorption isotherm is a relationship relating
the concentration of a solute on the surface of an adsorbent to the concentra-
tion of the solute in the liquid with which it is in contact at a constant tem-
perature. Freundlich and Langmuir sorption isotherms are extensively used
to describe sorption isotherms for a wide range of chemicals. Knowledge of
sorption isotherms and adsorption phenomena is essential for understand-
ing heavy solute retention and transport in soils and geological media. It
is crucial for assessing the environmental risk of contamination and pollu-
tion provoked by these elements. Studies on solutes adsorption in soils are
often conducted as one-component systems, where the ions or molecules are
treated individually, or they can be conducted as a multicomponent system,
where the ions are subjected to competition among them.

1.1 Freundlich Isotherm
The Freundlich isotherm is one of the simplest approaches for quantifying
the behavior of retention of reactive solute with the matrix surfaces. It is
certainly one of the oldest nonlinear sorption equations and was based on
quantifying the gas adsorbed by a unit mass of solid with pressure, which
was described as

S= K C

b

(1.1)

f

where S is the amount of solute retained by the soil in μg g -1 or mg kg -1 , C is
the solute concentration in solution in mg L -1 or μg ml -1 , K f is the partitioning
coefficient in L kg -1 or ml g -1 , and b is often considered a dimensionless para-
meter. A major disadvantage of the Freundlich approach is that it is incapable
of describing sorption at or near saturation or when the adsorption maximum
is reached. The parameter K f represents the partitioning of a solute species
between solid and liquid phases over the concentration range of interest and

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