Nanoparticles for Treatment of Chlorinated Organic Contaminants - Nanotechnologies for Water Environment Applications - page 98

dioxygenation of 4,4'-CB (Erickson and Mondello 1993) are available in other
microbes. These alternative pathways, however, appear to be less significant compared
to the 2,3-dixoygenation pathway.
Anaerobic Degradation. At least six different pathways have been reported for
PCBs dechlorination (Bedard and III 1995). The reductive dechlorination of chiral PCBs
depends on the chlorine substitution pattern of the PCB congeners as well as the nature
of the microbial population (Pakdeesusuk et al., 2003).
The reductive dehalogenation of PCBs under anaerobic conditions makes them
more bioavailable for the subsequent aerobic degradation and reduces the toxicity of
PCBs (Furukawa 2000). Hexachloro CB congeners including 2,2',4,4',5,5'- and
2,2',3,4,4',5'-CBs were previously found to transform into tetrachloro CBs of 2,2',4,4'-;
2,2',4,5- and 2,2',4,5'-CBs in methanogenic and sulfidogenic cultures. No mono- or
dichloro CBs were found in the degradation products and chlorine atoms were removed
mainly from meta position (Ofjord et al., 1994). In another study, however,
dechlorination of tri-, tetra- and pentachlorobiphenyls to mono- and dichlorobiphenyls
had been reported by methanogens (Alder et al., 1993).
Six trichloro-CBs including 2,3,4-, 2,3,5-, 2,3,6-, 2,4,5-, 2,4,6-, and 3,4,5-CB
were reduced to 2,4-, 2,5-, 2,6-, 2,5-, 2,6-, and 3,5-CBs, respectively. Those 2,4- and
2,5-CBs were continuously reduced to 2-CB while 3,5-CB was reduced further to 3-CB
and ultimately to biphenyls. Chlorine atoms were again mainly removed at meta and
para positions. As 3,4,5-CB does not contain any ortho chlorine atom, all three chlorine
atoms were removed to produce biphenyls (Williams 1994).
Although many studies have demonstrated anaerobic degradation of PCBs in
aquatic sediments by a consortium of microbes, little is known about the
microorganisms responsible for the dehalogenation of PCBs. Attempts to isolate the
pure culture of microorganisms responsible for PCB dehalogenation have not yet been
successful (Furukawa 2000). Sequences related to members of genus Clostridium were
found abundant in PCB degrading microbial consortia (Hou and Dutta 2000). Some
Clostridium species can harbor enzymes to catalyze dechlorination of perchloroethylene
(Okeke et al., 2001), suggesting that Clostridium might be an important microorganism
in PCB degradation (Abraham et al., 2002).
4.3.2.2 Rate of PCB Degradation
As PCBs are known to be very persistent, it is important to understand the
kinetics of PCBs degradation for field applications. However, unlike pure chemical
processes, biological processes, especially those in soils, vary greatly and usually do not
provide consistent kinetic rates in laboratory studies because of the complex nature of