Environmental Engineering Reference
In-Depth Information
22.5.3 Natural Attenuation Potential of Tar Oil
The most important Natural Attenuation process for organic contaminants is
biodegradation. It is known that a lot of different organic contaminants such as
BTEX or hydrocarbons, etc., are well degradable even under anaerobic in situ
conditions by autochthonous microorganisms. The biodegradability of PAH is also
investigated intensively. Most of these studies were performed using single contam-
inants and/or pure cultures in batch experiments. These results are not transferable
to tar oil contaminated sites as the situation there differs considerably. The multi-
component mixture tar oil may have a huge impact on the biological degradability
of individual components. In some cases co-metabolic effects may enhance degra-
dation of recalcitrant contaminants. But, it is also possible that biodegradation may
be inhibited due to, for example, toxic effects.
Several microorganisms are able to consume 2- to 4-ring PAHs as a carbon
source and gain energy from this process. Additionally co-metabolic degradation
of some PAHs was observed. Because molecules with lower water solubility are
less bioavailable, they are less biodegradable and the degradation rates are low.
Therefore degradation of these high molecular weight PAHs is difficult to monitor.
Besides bioavailability, the degradability depends also on the electron accep-
tors available. Aerobic biodegradation of PAHs is the most effective and the best
understood degradation process. The biodegradability of low molecular PAHs using
nitrate, ferric iron, manganese(IV), or sulphate is also observed. Nevertheless
there is less information available. Only for some contaminants (e.g., naphthalene)
the degradation pathways are investigated in detail (e.g., Annweiler et al. 2002 ;
Safinowski and Meckenstock 2006 ). In general, the degradation rates for PAH are
low compared to, for example, mono aromatic contaminants.
The biodegradation of NSO-HET is not yet well investigated. It is known that N-,
and O-heterocyclic contaminants better degrade compared to S-heterocyclic con-
taminants. In some cases an inhibition of aerobic PAH degradation was identified
when NSO-HET are available and vice versa (Meyer and Steinhart 2000 ; Werner
et al. 2008a ). Similar to PAH the degradability is influenced by the kind of avail-
able electron acceptors. The contaminants are likely to be degradable under aerobic
conditions. Table 22.4 gives an overview of the degradability under different redox
conditions (Werner et al. 2008a ).
Substituents and their position in the molecule have a substantial influence on the
degradability. If an alkyl group (e.g., methyl group) is located in the direct neigh-
bourhood or is directly bounded to the hetero atom, it will reduce the biodegradation
considerably.
Some recalcitrant NSO-HET contaminants are dimethylbenzofuran, methyl-
benzo(b)furan and benzothiophene (Werner et al. 2008a , b ). The length of the
plume of these contaminants is comparable or may even be longer than e.g. an
acenaphthene plume.
It has been recognized that polycyclic aromatic hydrocarbons may form non-
extractable residues (neR) in soil and that this process may be stimulated by
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