Introduction to Soil Chemistry

202 speciation
atomic absorption is applicable, it requires specialized heating of the sample
such as a graphite furnace and other specialized sample handling [14,15].
10.1.2.5 Aluminum
Aluminum deserves special attention because, although it is present in only
one oxidation state, it is commonly associated with both oxygen and hydroxy
groups and is an extremely important ion, particularly in acidic soils. Although
it is not toxic to most animals, it is toxic to most plants and is regarded as
being present only in the Al 3+ oxidation state. However, Al 3+ reacts with water,
releasing protons into the soil solution. Under acid conditions aluminum is
more soluble and thus some of the following reactions lead to additional
acidity. Reactions of aluminum ions in soil solution with the release of protons
are shown in equations below:
Al 3+ +H2O AlOH 2+
+ H +
AlOH 2+
Al(OH) 2 +
+ +
+ H2O Al(OH) 2 + H
+ H2O Al(OH) 3 + H +
(10.1a)
(10.1b)
(10.1c)
Other reactions of aluminum lead to the formation of other species. In the
solid inorganic compartment it is most commonly found octahedrally bonded
to a combination of oxygen atoms and hydroxy groups. In solution it may be
in any one of the species shown in reactions (10.1). It may also be bonded or
associated with the colloidal inorganic and organic particles and the surfaces
of other soil components.
Aluminum is always present in soil as it is a constituent of soil minerals,
particularly clay minerals. As the pH of soil decreases, aluminum from various
sources is brought into solution. At very low pH levels, the very fabric of soil
begins to erode, which causes two things to happen:
1. The soil has very high buffering capacity at this point because the added
acid is decomposing the inorganic components in soil. This means that a
large amount of acid is needed to decrease the pH of soil to a point
where metals are solubilized and can be leached out.
2. The soil itself is destroyed and hence at the end of the extraction process
soil is no longer present, and what is left is a mixture of highly acid salts,
which must be disposed of. It is for this reason that extraction or remediation
methods that depend on the acidification of soil to low pH fail
and should never be undertaken [4,16,17].
10.2. ANIONS
Simple anions are those that exist in only one oxidation state in soil and generally
are associated only with water. Complex anions are typically oxyanions
of nonmetals, although molybdenum occurs as an oxyanion [4].