Nanoparticles for Treatment of Chlorinated Organic Contaminants - Nanotechnologies for Water Environment Applications - page 99

microbial communities involved in these studies. A diverse family of PCBs with 209
congeners adds more to this complexity. As a result, a wide range of PCBs degradation
rates were reported. For example, up to 65% meta and para chlorines were removed by a
methanogenic culture in 2 months in one PCB reductive dechlorination study (Alder et
al., 1993). While in another study, only 23-25% removal of meta and para chlorines
from PCBs by methanogens after 12 months was reported (Ofjord et al., 1994). Hence,
only some notable degradation rates of PCBs are mentioned below.
At laboratory scale, a dechlorination rate of 11-23% after 13 weeks was
achieved with a recycling-upflow fixed-bed (R-UFB) reactor (Pagano et al., 1995). At
low temperature of 4 o C, biodegradation of PCBs still occurred with > 50% removal of
mono- and dichloro CBs after 5 months (Williams and May 1997). Sequential
anaerobic-aerobic processes were capable of reducing total PCBs content from 59 μg/g
soil to 20 μg/g soil after 5 months, which is equal to 34% removal of total PCBs (Master
et al., 2002).
In situ field PCB degradation likely yields lower rates compared to that of PCB
biodegradation from laboratory experiments. A field study on Hudson River sediment
showed that initial degradation rates of PCBs by indigenous microorganisms were 2-3
times lower than that of laboratory experiments (Harkness et al., 1993). This study also
reported an enhanced PCB degradation rate on site when adding biphenyls, nutrients and
oxygen to the sediment and improving mixing conditions, resulting in 37-50% removal
of mono- and dichloro-CBs in surface soils after 73 days.
To some extent, the above results suggest that PCBs having one or two chlorine
atoms may be degraded rapidly in months. For PCBs containing 3 chlorine atoms or
more, the degradation process is much slower. A recent long-term field study of 13 years
statistically predicted that the half-lives for tri- and tetrachloro-CBs in soil are 10.9
and11.3 years, respectively (Doick et al., 2005).
Oxygen may play an important role in PCB degradation. Because of the
limitation of oxygen transfer in soils, removal of PCBs in soil is only noticeable at the
surface layer of 10-30 cm. To increase the oxygen transfer rate, mechanical methods can
be applied (Harkness et al., 1993). Plantation also increases the oxygen transfer into
soils via plant root systems. Root systems not only create an oxic rhizosphere but also
provide shelter and nutrients for symbiotic microorganisms. Excretions of mulberry
( Morus sp. ) were reported to support the growth of strain LB400 (Leigh et al., 2002),
one of the aforementioned PCB degraders.
Both mechanical mixing and plantation cannot reach deep soils, especially the
saturation zone that exhibits anoxic conditions and is considered to have higher risk of
leaching PCBs to groundwater. In such cases the reductive dechlorination occurs, but