What Is Present - Field Sampling: Principles and Practices in Environmental Analysis - page 238

abbreviations and acronyms also are in common use. (See Chapter 1 and Appendix A.)
9.3.4. Humus
As stated above, during the process of microbial decomposition new organic matter is
synthesized. This results in the formation of humus, which is a
FIGURE 9.5 Examples of compounds containing a variety of alkyl, benzene,
and functional groups. Hydrogen atoms are omitted for simplicity.
complex compound composed of a large number of diverse groups. Some have said that
humus is a polymer. No monomer or mer * had ever been identified, however, and so it
cannot really be said to be a true polymer. It is a large, complex molecule, that is dark in
color and resistant to decomposition.
Soil scientists and others routinely separate humus into a number of different
components: humic acid is extracted with alkali, fulvic acid is soluble in acid, and humin
is not soluble in any solvents. Being able to separate humus into different components
has not generally led to an illumination of its structure. Knowing the structure of these
separate parts also does not shed much light on the chemical characteristics of humus,
and humus as a whole has characteristics that are different from those of any one of these
components. In spite of not knowing its structure it is known that humus has a
pronounced effect on water and soil and their characteristics. This is particularly true
when trying to describe humus and soil interaction with environmental pollutants.
Because of its complex organic structure and numerous functional groups, humus has a
high affinity for many organic compounds and metals. (See chelation below.) It does not
act like it has a surface, but as if it has organic compounds dissolved in it. In some cases
it may be hard to tell when the dissolved contaminant ends and the humus begins. Humus
will hold many different types of organic compounds containing many different types of
functional groups. Once adsorbed by humus, compounds are generally slower to move in
soil.
Colloidal organic matter and humus may remain suspended in water, sorb components,
and move through the environment. This may result in movement of components not
expected to move and result in their being present in higher than expected concentrations.
Humus has a strong affinity for metals because of its cation exchange sites, and so it will
attract and hold cations by cation exchange mechanisms. The two groups most often sited
as being responsible for cation exchange in humus are acid and phenolic groups. Phenolic
* A mer is the individual unit or units of which a polymer is made.