Geoenvironment Impact Mitigation and Management - Sustainable Practices in Geoenvironmental Engineering - page 377

By deinition, biodegradation refers to the decomposition of organic matter by microor-
ganisms. The end result of the metabolic and enzymatic processes is seen in terms of
smaller compounds, and in respect to organic contaminant remediation goals, ultimately
as CO 2, CH 4 , and H 2 O. The organic wastes and chemicals of interest in this topic are con-
taminants such as organic compounds that contain carbon, and organic waste matter.
Strictly speaking, biodegradation is a particular form of biotransformation, since biotrans-
formation (or biological transformation) means the conversion of a chemical substance into
another chemical substance (generally a metabolite) by enzymatic action or other biologi-
cal processes. The use of the term biodegradation implies that the biological transformation
process reduces the original organic chemical compound into smaller fragments, with the
presumed conclusion that these smaller fragments are less toxic or less threatening to the
receptors. To some extent, this is probably valid. However, the well-documented and well-
reported example of anaerobic “degradation” of C 2 Cl 4 (perchloroethylene, PCE) to C 2 H 3 Cl
(vinyl chloride) shows that the latter “degraded” compound is more of a threat than the
original, because C 2 H 3 Cl is more toxic and volatile than PCE, and does not partition well
in the soil. In any event, the transformation of original organic chemical compounds in the
subsurface soil into smaller units occurs by oxidation and reduction mechanisms (redox
reactions) resulting from the metabolic activities of the microorganisms in the soil. In the
next few subsections, we will show examples from Yong and Mulligan (2004) of biotrans-
formation and biodegradation of some organic chemical compounds. Greater details of the
many kinds of transformations and conversions can be found in specialized texts dealing
with bioremediation.
10.6.2.1 Petroleum Hydrocarbons: Alkanes, Alkenes, and Cycloalkanes
Petroleum hydrocarbon consists of various compounds such as alkanes, cycloalkanes, aro-
matics, polycyclic aromatic hydrocarbons, asphaltenes, and resins. Their biodegradability
in the subsoil range from very biodegradable to recalcitrant. This is because of the varying
degrees of branching, chain lengths, molecular sizes, substitution with nitrogen, oxygen,
or sulfur atoms. Many of the alkanes found in petroleum are branched.
Alkanes (C 2 H 2n+2 ) are aliphatic compounds. Low molecular weight alkanes are the
most easily degraded by microorganisms. However, as the chain length increases from
C 20 to C 40 , hydrophobicity increases, whereas solubility and biodegradation rates decrease.
Conversion of alkanes leads to the formation of an alcohol using a monooxygenase
enzyme. This is followed by oxidation to an aldehyde and then to a fatty acid (Pitter and
Chudoba, 1990). Further oxidation (β-oxidation) of the fatty acid yields products less vola-
tile than the original contaminants. Anaerobic bacteria such as sulfate-reducing bacteria
are capable of degrading fatty acids via this step (Widdel, 1988). Although bacteria that
are capable of degrading n -alkanes cannot degrade branched ones (Higgins and Gilbert,
1978), Brevibacterium ethrogenes , Corynebacteria sp., Mycobacterium fortuitum , Mycobacterium
smegmatis , and Nocardia sp. have been shown to grow on branched alkanes. The irst deg-
radation step is the same as for the unbranched alkanes. However, the β-oxidation is more
dificult and less eficient (Pirnik, 1977). In addition, in the presence of n -alkanes, the
metabolism of the branched alkanes will be repressed, which will cause dificulties dur-
ing the degradation of mixtures such as petroleum.
Alkenes with a double bond between carbons and have not been extensively studied for
biodegradation. Those containing the double bond on the irst carbon may be more easily
degradable than those alkenes with the double bond at other positions (Pitter and Chudoba,
1990). The products of oxidation of 1-alkenes can be either diols or the methyl group.