Introduction to Soil Chemistry

extract cleanup 141
Table 7.3. Some Common Extraction Cleanup Methods
Method Based on
Sorbent Used Advantages Disadvantages
Alumina Diffferent pH and activity May decompose or
ranges available for irreversibly sorb
different cleanup needs compounds
Florisil Cleanup of chlorinated Has basic properties and
hydrocarbons, nitrogen, may not be compatible
and aromatic compounds with acids
Silica gel Separation on the basis Components with the
of differing polarity same polarity will not
be separated
Gel permeation Components separated Different components of
on the basis of size the same size will not
be separated
procedures; however, the former has the advantage of both separating the
component of interest from other materials and concentrating it.
In either case materials commonly used as sorbants are stationary phases
used in chromatography. Because of their common chromatographic use,
these sorbants are well described in the literature and their characteristics and
sorbtive capacities known. Thus all three aluminas—acid, neutral, and basic—
have been used, along with silica gel, Florisil, and gel permeation for different
compounds and environmental samples.
Solid-phase cleanup separates and concentrates components of interest.
However, care must be taken to ensure that the capacity of the column is not
exceeded. If it is, some of the component of interest will not be retained on
the column and thus lost, resulting in analytical results lower that the true
amounts present.
7.5.3. Water Removal
Water is always present in soil; even air dry soil contains a significant amount
of water. The temptation is to remove all water before analysis, but to accomplish
this, soil must be dried at a temperature above 100°C. This procedure
works but causes irreversible changes in soil characteristics such that its extraction
and subsequent analytical results are inaccurate, at least when compared
with the results obtained on “fresh” unheated soil samples. The issue then
becomes how to circumvent these problems.
The most common and standard method is to allow soil to dry naturally at
room temperature or sometimes a little above room temperature (35°C
maximum) before extraction or analysis. This will result in a soil sample con