Environmental Engineering Reference
In-Depth Information
4.3.2 Biodegradation of PCB
Treatment of PCBs by chemical and physical methods is costly and labor-
intensive. The PCB decontamination project of Hudson River Basin in upstate New
York has cost EPA hundreds of millions of dollars (Ross 2004). Biodegradation has
significant potential to remove PCBs inexpensively. Similar to TCE and PCE, PCBs can
be degraded aerobically or dehalogenated by the reduction process under anaerobic
conditions. The biodegradability of PCBs is highly dependent on chlorine substitution,
i.e., the number and position of chlorine (Furukawa 2000). A brief review of the
biochemical pathways and the degradation rates of PCBs are discussed below.
4.3.2.1 Degradation Pathways
Aerobic Degradation. One mechanism of the aerobic degradation of PCBs is
through the use of a microbial enzyme biphenyl dioxygenases (BP Dox) to attack PCBs,
resulting in the formation of chlorinated 2,3-dihydro-2,3-dihydroxydiphenyls, which is
further degraded to chlorinated 2,3-dihydroxydiphenyls and ultimately to chlorobenzoic
acid (CBA) (Figure 4.1). This biodegradation pathway has been confirmed from a
investigation on sediment samples in the Hudson River Basin which showed that
chlorobenzoic acids were found only in samples containing PCBs and were correlated
with PCBs distribution pattern (Flanagan and May 1993).
OH
OH
OH
H
O
H
H
O
O
H 2
C
O
OH
+O 2
+O 2
OH
-2H
+H 2 O
+
OH
H
Cl
Cl
Cl
Cl
Cl
O
A B C D E
Figure 4.1 Biodegradation pathway of PCBs activated by dioxygenases: A:
Polychlorinated byphenyls (PCBs); B: Chlorinated 2,3-dihydro-2,3-dihydroxy-
biphenyls; C: Chlorinated 2,3-dihydroxibiphenyls; D: Chlorinated 2-hydroxy-6-oxo-6-
phenylhexa-2,4-dilenoic acid; E: Chlorinated benzoic acid (CBA) and a five-carbon
fragment (Flanagan and May 1993).
Recent studies have confirmed that the biodegradation pathway of PCBs by 2,3-
dioxygenation with BP Dox is the dominant pathway (Furukawa 2000). The widest
range of PCBs congeners with numbers of chlorine atoms up to six that can be
biodegraded was achieved with this pathway by the strain Burkholderia cepacia LB400
(Pieper 2005). Alternative biodegradation pathways of other PCBs congeners such as the
3,4-dioxygenation of 2,5,4'- and 2,5,2',5'-CBs (Suenaga et al., 1999) and the 5,6-
 
 
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