Nanoparticles for Treatment of Chlorinated Organic Contaminants - Nanotechnologies for Water Environment Applications - page 100

with a very low reaction rate. In order to be degraded, PCBs need to be used as an
electron acceptor by microorganisms, while other natural electron acceptors for bacteria
such as nitrate, sulfate and ferric ions are commonly present in soils. These natural
acceptors are superior to PCBs in receiving electrons. They support the growth of
anaerobic microbial consortia and increase microbial activities that eventually contribute
to enhanced reductive dechlorination of PCBs. However, mixed results on the impact of
sulfate on the dechlorination of PCBs have been reported (Zwiernik et al., 1998).
Therefore, there is still uncertainty in improving PCB dechlorination by the addition of
inorganic electron acceptors.

In dechlorination processes, highly chlorinated CBs are reduced to lowly
chlorinated compounds that are unlikely to be used as substrates for microbial growth.
The reduction of PCBs is therefore often viewed as a cometabolic process. The rate of
microbial cometabolic degradation can be increased by feeding the microorganisms with
organic compounds having similar molecular configurations (Vaccari et al., 2006) such
as biphenyls (Erickson and Mondello 1993) and naphthalene (Wilson and Madsen
1996). To facilitate PCB degradation by cometabolism, these organic compounds should
contain at least one benzene ring easily degradable and have low toxicity when being
applied in-situ.

To increase the dechlorination rate, adding a reductive chemical agent might be
another way to yield a better removal of PCBs in groundwater. Recently, scientific
communities have extensively studied the removal rates of chlorinated organic
compounds including TCE, PCE and PCBs catalyzed by zero valence iron. The detailed
findings and results of such studies are presented in the following section.

4.4 Nanoscale Zero-Valence Iron (NZVI)

With the development of emerging nanotechnology, NZVI represents a new
generation of remediation technologies to remove the persistent chlorinated organic
compounds in the environment. Compared to anaerobic or aerobic biological methods
discussed earlier, abiotic methods using NZVI have great potential in reducing and
removing chlorinated compounds more rapidly and effectively, thereby providing cost-
effective solutions to removal chlorinated organic solvents, such as PCE, TCE, PCBs,
from the environment.

4.4.1 Synthesis, Commercial Sources and Environmental Applications

Several approaches of fabricating iron nanoparticles are available. Among them
the solution method is most widely used because of its relatively low cost and easy