Environmental Engineering Reference
In-Depth Information
replacing power or the strength of attraction of cations the soil particle surfaces is given by
the lytropic series. An example of some typical cations and the replacing power is given
as follows:
4
+
3
+
3
+
2
+
2
+
2
+
2
+
+
+
+
+
+
Th
>
Fe
>
Al
>
Cu
>
Ba
>
Ca
= >>=
Mg
Cs
KNH
> Li
>
Na
4
Exchange-equilibrium equations can be used to determine the proportion of each
exchangeable cation to the total CEC as the outside ion concentration varies. The simplest
of these is the Gapon relationship:
1
+
m
+
m
M
m
M
N
o
e
=
K
,
(9.1)
+
n
1
e
+
n
N
n
o
where
m
and
n
refer to the valence of the cations and the subscripts
e
and
o
refer to the
exchangeable and bulk solution ions, respectively. The constant
K
is dependent on the
effects of speciic cation adsorption and the nature of the clay surface.
K
decreases in value
as the surface density of charges increases.
The adsorption of ions due to the mechanism of electrostatic bonding is called
physical
adsorption
or
nonspeciic adsorption
. The ions involved in this type of process are identi-
ied as
indifferent ions
. The other mechanism of ion adsorption is a chemical reaction that
involves covalent bonds and activation energy in the process of adsorption. This type of
adsorption process is generally identiied as
chemisorption
or
speciic adsorption
and occurs
at speciic sites.
Speciic cation adsorption
refers to the situation where the ions penetrate
the coordination shell of the structural atom and are bonded by covalent bonds via the O
and OH groups to the structural cations. It is useful to note that when the energy barrier
is overcome by the activation energy (in the chemisorption process), desorption of the
ions will not be easily accomplished since desorption energy requirements may be pro-
hibitively large. This has considerable signiicance in the evaluation of contaminant-soil
interaction, especially with respect to the environmental mobility of sorbed contaminants.
Chemisorption and adsorption on soil particle surfaces involving siloxane cavities are gen-
erally conined to the surface layer and the monolayer next to the electriied interface. To
obtain a better picture of the adsorption processes at the surface monolayer and beyond,
we need to look more closely into the various energies of interaction developed. In short,
the net energy of interaction due to adsorption of a solute ion or molecule onto the surfaces
of the soil fractions is the result of both short range chemical forces such as covalent bond-
ing, and long range forces such as electrostatic forces. Furthermore, sorption of inorganic
contaminant cations is related to their valences, crystallinities, and hydrated radii.
9.4.3 Elements of Abiotic Reactions between Organic Chemicals and Soil Fractions
Abiotic adsorption reactions or processes involving organic chemicals and soil fractions
are governed by (a) the surface properties of the soil fractions, (b) the chemistry of the pore-
water, and (c) the chemistry and physical-chemistry of the contaminants. Mechanisms
pertaining to ion exchange involving organic ions are essentially similar to those between
inorganic contaminants and soil fractions. The adsorption of the organic cations is related
to the molecular weight of the organic cations. Large organic cations are adsorbed more
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