Page 174 - Soil and water contamination, 2nd edition

9

Organic pollutants

9.1 INTRODUCTION

In Section 4.3 we saw that organic substances consist of a variety of compounds made up
of carbon, oxygen , hydrogen, and small amounts of nitrogen , phosphorus , sulphur, chlorine
and several other elements. The chemical bonds between the carbon atoms and between
these atoms and hydrogen or other elements are established by covalent bonding, which
implies that a carbon atom shares one or more electrons with its neighbouring atoms. In
the case of one shared electron, a single bond occurs, indicated by a single line (e.g. ethane:
CH – CH ). If two atoms share two electrons, a double bond is formed (e.g. ethene: CH
3 3 2
= CH ), and, logically, a triple bond is formed when two atoms share three electrons (e.g.
2
ethyne: CH ≡ CH). A chain of carbon molecules can thus have single, double, or triple
bonds at any arbitrary location within the chain. If a chain with two or more substituents
(i.e. atoms or groups other than hydrogen) consists of four or more carbon atoms, the
atoms forming a substance can be structured spatially in more than one manner, resulting
in so-called isomers . Obviously, if organic matter molecules consist of many carbon atoms,
multiple branches of short or long chains may occur. Although isomers are built up from
the same elements, their physico-chemical properties (e.g. melting point, vapour pressure ,
aqueous solubility , and chemical reactivity ) may differ substantially.
Instead of chains, carbon atoms may also form ring structures that sometimes have
double bonds. Such rings are usually composed predominantly of carbon atoms, but they
may also contain elements other than carbon, such as oxygen or nitrogen . Organic molecules
and their isomers are named according to the systematic nomenclature that has been briefly
summarised in Box 9.I, which also gives some examples of the structure of organic molecules
and isomers. Further details about the nomenclature can be found in any organic chemistry
textbook. Apart from the strictly structural approach adopted in this nomenclature, organic
chemicals are commonly classified on the basis of their source (e.g. petroleum products) or
use (e.g. pesticides (insecticides, herbicides, fungicides), plasticisers, solvents) or some of their
physico-chemical properties (e.g. volatile organic compounds (VOCs ), persistent organic
pollutants (POPs), or absorbable organic halogens (AOX)). The latter classification often
reflects the analytical procedure applied for determining the compounds.
Most organic substances are potential pollutants as they enter groundwater or surface
water, since they may cause a depletion of dissolved oxygen . However, some organic
compounds are directly toxic, thereby potentially causing direct harmful effects on
living organisms and ecosystems. These organic compounds include petroleum and its
derivatives, polycyclic aromatic hydrocarbons (PAHs), and chlorinated hydrocarbons. The
possible effects of organic pollutants on human health include cancer, allergies, disruption
of the immune system, damage to the nervous system, and reproductive disorders. Most
polluting organic compounds are man-made and industrially produced, although a number
of these chemicals also occur naturally, produced by biochemical synthesis, incomplete
decomposition of organic matter, volcanic eruptions, forest fires, or lightning. The uses made

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