SPECIATION - Introduction to Soil Chemistry - page 195

CHAPTER
10
SPECIATION
The term speciation for many people brings to mind oxidation forms of metals
and nonmetals; however, this is much too restrictive an idea of speciation,
especially when applied to soil. Speciation can broadly refer to the form of an
element or molecule present under a set of environmental conditions as illus-
trated in Figure 10.1. This would include metal cation and oxyanion oxidation
states, cation and anion associations, ionization states of organic compounds,
and organic and inorganic associations. It will also include association of
metals, nonmetals, organic ions, and compounds with both inorganic and
organic components of soil, especially colloids.
Several different types of species commonly discussed in soil science are
illustrated in Figure 10.1. The potassium cation (K + ) at the top is separated
from the soil surface by a water molecule and would thus be considered an
outer-sphere species. At the bottom the potassium cation is directly connected
to the soil particle by an ionic charge and thus would be an inner-sphere
species. Above this is an inner-sphere phosphate directly bonded to a soil
surface aluminum. Also shown are potassium cations attached (inner sphere)
to colloidal clay (CC) and colloidal soil organic matter (COM). Each of these
is a different species.
Colloidal associations are particularly important in soil because they allow
for movement of otherwise immobile or slowly mobile species. They can also
lead to confusing analytical results in that atoms, ions, or molecules may
appear to be soluble above their solubility limit when in reality they actually
represent a different species.
Organic compounds might also be regarded as having species, although they
are seldom discussed in this manner. Organic compounds are capable of exist-
ing in various conformations, some of which are easily changed, while others
are more or less fixed by steric, hydrogen bonding, or other atomic and mol-
ecular interactions. In addition, compounds containing double bonds and
chiral centers can exist as different optical isomers. For instance, fatty acids
contain double bonds, which can be either cis or trans . Amino acids can be
either S or R (D or L) optical isomers. These various “species” are illustrated
in Figure 10.2. It is well documented that the various conformations and
optical isomers have dramatically differing biological activities. An especially