Introduction to Soil Chemistry

extraction 129
having cation exchange capacity (CEC). There are, however, some soils that
have significant anion exchange capacity, and this possibility must never be
overlooked.
In cation exchange reactions typically both the charge and concentration
of the cation in the extracting solution are important in determining its ability
to exchange with cations already on the exchange sites. Generally, the larger
the charge on the cation, the more effective it is in replacing other cations,
especially those with less charge. Cations with less charge but at high concentration
will replace cations with higher charge. This latter approach is generally
the one taken in carrying out analysis involving cation exchange; thus an
extracting solution containing a high concentration of lower charged cation is
used in the extraction [11,12].
In the case of compounds not attracted or attached to cation or anion
exchange sites in soil, an aqueous solution containing a ligand or a mixture of
ligands and auxiliary compounds that form an attraction for the compound of
interest and result in the formation of a highly soluble compound, complex,
or species is chosen. A solution of this ligand in distilled water is prepared and
used to extract the soil [13–15].
Another approach that has been used is to add a surfactant to aqueous
extractants, particularly when insoluble components are to be extracted or are
present. Typically soaps and both ionic and nonionic surfactants have been
used. Because of the complex nature of some surfactants and their effect on
viscosity, care must be taken to make sure that they do not adversely affect
subsequent analytical procedures [16–18].
In a similar fashion organic compounds are typically extracted using organic
solvents or mixtures of organic solvents. In this case, because of the constant
occurrence of water in soil, the solubility of water or the mutual solubility of
the compound of interest and water in the extractant will be an important
consideration.
An attempt to circumvent these problems is to use mixed solvents, which
are soluble in each other as well as in water. Thus a mixture of acetone, which
is miscible with water, and hexane, which is a hydrophobic hydrocarbon insoluble
in water, could be used as a soil extractant. The idea is to have a solvent
that will dissolve in water and yet also dissolve hydrophobic contaminants in
soil (see Section 7.2.1).
Nonaqueous or hydrophobic extractants can also be used, although typically
their usefulness is limited. Examples of these types of extractants are
halogenated solvents such as the freons and chloroethanes. A problem with
all these types of extractants is that water in nondraining pores is not accessible
to them and thus any contaminant surrounded by water will not or will
only partially be extracted.
In the case of all of the above mentioned extractants, there are three important
questions:
1. Will the extractant extract the component or contaminant of interest?
2. Is the extractant compatible with the analytical procedures to be used?