Geology Reference
In-Depth Information
where c can be measured and, knowing the initial concentration and
volume of the solution containing the chemisorbed ion, x can be
calculated. By plotting c/x against c, a straight line will result (if the
Langmuir equation is a suitable model for the process), which will have
a slope of 1/x
m
and an intercept of 1/Kx
m
, allowing the theoretical
maximum chemisorption of the ion to be calculated. In practice,
application of the Langmuir equation is often not strictly valid, as a
straight line relationship is not found. But it is commonly used in order
to obtain a value for x
m
.
5.5.4 Complexation/Chelation
Humic compounds can react with metal cations to form organometallic
complexes:
M
x1
þ
OM-LHy
OM-LM
(x
y)1
þ
yH
1
"
(5.43)
These are particularly stable if more than one bond is formed (chelation)
and are important for heavy metals such as Cu
21
and Pb
21
, and also for
Fe
31
and Al
31
for certain soil-forming processes. Complexation by low
molecular weight organic compounds, such as those in root exudates,
can act as a means of bringing metals into solution, making them more
mobile and bioavailable, but reaction with high molecular weight humic
material is an immobilizing process.
The stability of the metal complex can be expressed as the equilibrium
constant (see also Chapter 4). For example,
if Equation (5.43) is
simplified to
M
þ
L
"
ML
(5.44)
(where M is the metal ion, L the organic ligand, and ML the metal
complex)
K
¼
[ML]/[M][L]
(5.45)
While experimentally [M] can be measured and [ML] calculated, it is
dicult to quantify [L], given the polymeric and chemically diverse
nature of soil organic matter.
5.5.5 Precipitation/Dissolution
The concentration of some ions in soil solution is controlled mainly by
the presence of poorly soluble solid phase components. Good examples
of this are iron and aluminium, both of which are highly insoluble at the
pH of most soils, and which are controlled by the solubility of the
