Page 95 - Soil and water contamination, 2nd edition

82 Soil and Water Contamination

constitute an essential part of organic matter. Under natural conditions, the mineralisation of
organic matter is a major source of nutrients in soil and water. The degree to which detritus
is broken down depends on environmental factors such as the pH and redox potential and
the accompanying microorganisms.
The composition of organic matter is extremely heterogeneous and consists of numerous
organic compounds. For this reason there is no general chemical formula for organic matter.
Nonetheless, all organic matter consists of between 45 and 55 percent carbon (C), with
smaller amounts of oxygen (O) and hydrogen (H) plus small quantities of nitrogen (N),
phosphorus (P), sulphur (S), chlorine (Cl), and several other elements. The carbon in
organic matter is referred to as organic carbon . The basic skeleton of organic compounds
consists of carbon atoms bonded together into unbranched and branched chains or rings
of various sizes with bonds to hydrogen atoms. Together with the other elements attached,
they make up a wide range of organic-matter substances, such as lignins, phenols,
carbohydrates (cellulose and sugars), proteins, lipids (fats), oils, and waxes. Because of their
complex composition, humic substances are often classified according a classical method
involving treatment with a sodium hydroxide solution. The portion of humus that is
insoluble in the dilute sodium hydroxide is called humin. The part that dissolves in the
solution consists of humic acids and fulvic acids , but humic acids precipitate when the
solution is made acidic. Fulvic acids are distinguished from humic acids by their lower
molecular weight.
In surface waters, a general distinction is made between dissolved organic matter
1

(DOM ) and particulate organic matter (POM). Naturally occurring POM consists
largely of recalcitrant remains of woody terrestrial and aquatic plants; the POM
-1
concentration generally ranges between 10 and 20 mg l . DOM is usually assumed to
be approximately similar to the soluble organic matter present in soils. In general, the
-1
DOM concentration in rivers varies from about 5 mg l in temperate, arid, and semi-
-1
-1
arid regions, via about 10 mg l in tropical regions, to about 40 mg l in subarctic
regions (Hem, 1989). Obviously, there is a wide range in DOM concentrations in any
given stream, as well as substantial differences amongst streams. DOM concentrations in
groundwater are usually smaller than those in surface waters. The presence of coloured
DOM in surface water and groundwater may tinge the water from pale yellow to dark
brown, although the colour has no direct link with the actual concentration of DOM.
Coloured DOM consists largely of colloidal fulvic and humic acids and may significantly
affect the water clarity. Among the environments in which intensively coloured water
occurs are swamps and bogs.
Similar to clay minerals , the particulate organic matter in soil and water also has a

negatively charged surface. Because of the small size and large specific surface area of the
humic substances , organic matter is very reactive and is able to interact intensively with

both the liquid phase and other solid phase constituents. Before discussing these interaction
mechanisms, it is useful to make a few comments on the common chemical structure
of organic compounds in order to better understand the nature of these interactions. As
mentioned above, the carbon atoms are bonded together to make carbon skeletons consisting
of chains and/or rings. The spatial structure of atoms in the carbon skeletons of organic
molecules will be further elaborated upon in Chapter 7. In the context of interaction of
organic matter with the liquid phase and other solid matter, the so-called functional groups
bonded to the skeleton are important. These chemical subunits display a more or less similar
behaviour in the variety of carbon skeletons to which they are attached. Figure 4.7 shows
some common functional groups present in environmental organic compounds. The next

1
Some researchers in carbon cycling use DOM as an abbreviation for dead organic matter in or on top of the
mineral soil.

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