Environmental Engineering Reference
In-Depth Information
9.3.1. Hydrocarbons
The backbone of all organic compounds is carbon atoms bonded together. The most
highly reduced compounds are alkanes, which contain only carbon and hydrogen and are
part of a group of compounds called hydrocarbons. As the name implies, these
compounds contain only carbon and hydrogen. This group is composed of three families
of compounds that are used as fuels, are common in the environment, and are common
pollutants. The three families are the alkanes, alkenes, and alkynes. Alkenes have at least
one double bond, and alkynes have at least one triple bond in addition to carbon,
hydrogen and single bonds.
The hydrocarbons are less dense than water and are insoluble in water. Because of
these characteristics they move with water through the environment by floating on it.
When analyzing soil for hydrocarbon contamination, it is common to treat any
combination of hydrocarbons as a single unit or entity. In analytical results they are
reported as the total amount of all of these types of compounds, and thus they are referred
to as total petroleum hydrocarbons (TPH). This analysis thus gives no indication of the
types of compounds present or their relative concentration. Indeed, a gas
chromatographic analysis of hydrocarbon fuels gives a very complex and poorly resolved
chromatogram with a large number of peaks showing the presence of a large number of
compounds.
Groups that contain only carbon and hydrogen are called alkyl groups and are
represented by the letter R. This represents a group of any configuration that contains
only carbon and hydrogen and is attached to another, usually larger, component. In some
cases an R group may be used to represent other constituents in a molecule. If this is the
case, however, this usage must be expressly and distinctly shown.
In addition to compounds that contain only carbon and hydrogen are numerous
environmentally sensitive compounds that contain carbon, hydrogen, and one or more
halogen atoms. The halogens are substituted for hydrogens in these compounds and are
generally called halocarbons. They are similar to hydrocarbons in that they are nonpolar,
insoluble in water, sometimes denser than water, and generally slowly biodegradable.
When denser than water halocarbons will be at the bottom rather than at the top of
aqueous environments and aqueous extracts of soil.
Most halocarbons are man-made or the result of man's activities, some examples of
which are given in Table 9.4. There are, however, a large number (over 1,500), of natural
halogenated organic compounds occurring in the environment. A small amount of
chlorinated organic compounds are produced by such common soil microorganisms as
the bacteria Pseudomo-
TABLE 9.4 Common Environmentally Important Halogen-Containing Organic
Compounds
Name
Structure
C1CH 3
Chloromethane
 
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