Environmental Engineering Reference
In-Depth Information
PCE
TCE
cDCE
Anaerobic Oxidation
(Fe-III, Mn-IV)
CO
2
Aerobic Oxidation (O
2
)
CO
2
VC
Ethene
Fig. 22.4
Schematic overview of biological degradation pathways of chlorinated ethenes
required for the degradation is supplied during the degradation of organic non-
chlorinated contaminants originating from natural organic matter (NOM) or from
additional anthropogenic contaminations, like e.g. BTEX.
Apart from this major degradation pathway, low-chlorinated contaminants like
cDCE and VC may also be degraded under aerobic conditions (Hanson and
Brusseau
1994
), either co-metabolically with e.g. methane as primary substrate
(inducer) or productively (i.e. these substances serve as carbon and energy source)
which has been observed in the lab and field at least for VC (e.g. Davis and
Carpenter
1990
). Furthermore, cDCE and/or VC seem to be subject to anaerobic
oxidative degradation to CO
2
in various biogeochemical environments (manganese
reduction, iron reduction, sulphate reduction) (Bradley and Chapelle
1996
,
1997
;
Hata et al.
2003
) (Fig.
22.4
). It should be noted that this degradation pathway has
been observed so far only in lab experiments. Hence, the relevance of anaerobic
oxidative CVOC degradation for MNA is considered to be small, as well as the
aerobic co-metabolic (but not productive) CVOC degradation.
Considering suitable conditions for biodegradation of CVOCs, the geochemi-
cal situation at CVOC contaminated sites often shows a redox environment which
changes to more oxidizing conditions with increasing distance from the source due
to impoverishment of electron donors originating from point sources. However,
one crucial requirement - besides the presence of microorganisms that are capa-
ble to degrade CVOCs - for complete anaerobic CVOC degradation is the opposite:
with increasing distance from the source the portion of low chlorinated metabolites
increases more and more and their reductive dechlorination to ethene and ethane
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