Geoscience Reference
In-Depth Information
The degradation pathway involves independent oxidation to fatty acids, followed
by b-oxidation.
Anaerobic degradation proceeds with nitrate, Fe
3+
, or sulfate as the terminal
electron acceptor, without any intermediate, such as alcohols. Sulfate reducers
apparently show specificity toward utilization of short chain alkanes. Susceptibility
to n-alkane degradation is an inverse function of chain length. Branched alkanes
are less susceptible than straight-chain n-alkanes, and the most resilient saturated
components are the pristine and phytane isoprenoids (Wang et al.
1998
).
As with aliphatic hydrocarbons, oxidative biodegradation of aromatic com-
pounds requires insertion of oxygen into the molecule to form catechol. The
susceptibility to biodegradation increases with decreasing molecular weight and
degree of alkylation. The most easily degradable polyaromatic hydrocarbons
mentioned are the alkyl homologues of dibenzothiophene, fluorine, phenanthrene,
and chrysene (Wang et al.
1998
).
Microbially induced degradation is recognized as the major mechanism in the
transformation of PAHs in the aquatic environment (NRC
2000
). In general, PAH
degradation rates are a factor of 2-5 slower than degradation rates of monoaro-
matic hydrocarbons and of similar magnitude as for high-molecular-weight n-
alkanes (C
15
-C
36
). Under similar aerobic conditions, the most rapid biodegrada-
tion of PAHs occurs at the water-sediment interface. Prokaryotic microorganisms
metabolize PAHs primarily by an initial dioxygenase attack to yield cis-dihyd-
rodiols and finally catechol. General aerobic biodegradation pathways for aromatic
hydrocarbons are shown in Fig.
13.8
.
Higher-molecular-weight PAHs, such as pyrene, benzo(a)pyrene, and ben-
zo(e)pyrene, exhibit a high resistance to biodegradation. PAHs with three or more
condensed rings tend not to act as a sole substrate for microbial growth and require
cometabolic transformations. Neilson and Allard (
1998
) report a cometabolic
reaction of pyrene, 1,2-benzanthracene, 3,4-benzopyrene, and phenanthrene in the
presence of either naphthalene or phenanthrene. However, the co metabolic
reactions are very slow in natural ecosystems.
Natural attenuation processes occurring in groundwater, which include dilution,
sorption, volatilization, and biodegradation, may affect the downward migration of
hydrocarbon plumes over time. Cozzarelli et al. (
1999
) report such behavior in the
configuration of a petroleum hydrocarbon plume in groundwater, due to rupture of
an oil pipeline that occurred in Minnesota in 1979. The oil spill induced formation
of an oil lens floating on the water table. The behavior of this lens was studied
between 1980 and 1990. In 1980, the dissolved hydrocarbon plume was composed
mainly of soluble benzene, toluene, ethylbenzene, and xylene (BTEX), and
developed a downward gradient. From groundwater samples collected in 1985, it
was observed that the BTEX plume stopped spreading. The dynamic steady state
of the plume reflected a balance between the rate at which soluble hydrocarbons
spread
into
the
groundwater
and
the
rate
at
which
the
hydrocarbons
were
