Geoscience Reference
In-Depth Information
isotherms for different organic contaminants, adsorbed from either water or hexane
solution on kaolinite, attapulgite, montmorillonite, and a red Mediterranean soil
(Yaron et al.
1996
). These isotherms may be used to deduce the adsorption
mechanism.
Weber and Miller (
1989
) summarized published data of 230 adsorption iso-
therms in which organic compounds were adsorbed from aqueous solutions onto
various soils. They found the following distribution of behaviors using the clas-
sification defined by Giles et al. (
1960
): S = 16 %, L = 64 %, H = 12 %,
C = 8 %. Based on this result, it can be concluded that the adsorbing material is
the most important controlling factor in defining the pattern of the adsorption
isotherm. This fact is confirmed by the work of Weber et al. (
1992
) for the
herbicide fluoridone, which exhibited the S-type sorption isotherm on soil with low
organic matter and high montmorillonite content, and the L-type sorption isotherm
on a soil with moderate organic matter content and mixed mineralogy.
In addition to these characterizations of adsorption curves, mathematical
descriptions of adsorption isotherms, based on physical models, often are used to
study solid interactions with contaminants. The main adsorption isotherms include
those of Langmuir, Freundlich, and Brunauer-Emmet-Teller (BET); they are
depicted in Fig.
5.2
.
The Langmuir equation (Eq.
5.1
), derived originally to describe the adsorption
of gases on solids, assumes that the adsorbed entity is attached to the surface at
specific, homogeneous, localized sites, forming a monolayer. It is also assumed
that the heat of adsorption is constant over the entire monolayer, that there is no
lateral interaction between adsorbed species, that equilibrium is reached, and that
the energy of adsorption is independent of temperature:
x
m
¼
KCb
1
þ
KC
¼
Kb
C
þ
K
;
ð
5
:
1
Þ
1
where x is the amount of adsorbed chemical, m is the mass of adsorbent, C is the
equilibrium concentration, K is a constant related to the bonding strength, and b is
the maximum amount of adsorbate that can be adsorbed.
The best way to determine the parameter values is to plot the distribution
coefficient (K
d
), which is the ratio between the amount adsorbed per unit mass of
adsorbent (x/m) and the concentration in solution (C):
K
d
¼
x
=
m
C
:
ð
5
:
2
Þ
Multiplying Eq. (
5.1
)by1/C + K and substituting into Eq. (
5.2
) gives a linear
equation for K
d
, expressed as
x
m
:
K
d
¼ Kb
K
ð
5
:
3
Þ
