Sorption Isotherms - Transport and Fate of Chemicals in Soils: Principles and Applications - page 7

heterogeneous nature of soils, extensive laboratory studies may be needed to
determine these reactions. Thus, predictions from transport models based
on the surface-complexation approach may not describe heavy metal sorp-
tion by a complex soil system. As a result, the need for direct measurements
of the sorption and desorption/release behavior of heavy metals in soils is
necessary. Consequently, retention, or the commonly used term sorption,
should be used when the mechanism of heavy metals removal from soil solu-
tion is not known, and the term adsorption should be reserved for describing
the formation of solute-surface site complexes.

1.5 Nonlinearity and Heterogeneity
The description of sorption isotherms in soils remains empirical where the
Freundlich and Langmuir models are commonly used. For several decades,
however, it has been recognized that isotherm patterns or the shape of an
adsorption isotherm is a reflection of the heterogeneity of the soil matrix.
The fact that a soil is made up of numerous constituents with distinctly dif-
ferent properties lends credence to this general concept. One may view a soil
as a complex mixture of numerous constituents, thus forming a highly het-
erogeneous system. Consequently, the following general isotherm equation
that describes the affinity of solutes to different sorption sites on surfaces of
soils was proposed:

∫ = Γζ

Sg ()(, ) Cd ζ

(1.6)

where g (ζ) represents the affinity distribution of a soil for a specific chemical
or may be referred to as a probability distribution function (PDF; Kinnebergh,
1986). The function Γ(ζ, C ) represents the sorption isotherm function used to
represent each constituent. For a discrete number of constituents ( n ), each hav-
ing different affinities, the general sorption isotherm equation is reduced to

∑

S = F Γ ζ (,) C

(1.7)

i i

n

where F i is the fraction relative to the total as discussed above. Conceptually,
the adsorption site associated with each constituent provides an isotherm
having its own affinity (ζ) and capacity. Consequently, a complete isotherm
may be regarded as the sum of all individual isotherms. The function g(ζ)
has been referred to as a ''weighting function,” a ''site affinity distribution
function,” or a ''frequency distribution of the affinity coefficient ξ for each
constituent'' (Limousin et al. 2007).