Nanoparticles for Treatment of Chlorinated Organic Contaminants - Nanotechnologies for Water Environment Applications - page 94

chlorinated compounds are common groundwater contaminants because of spillage,
overfilling, sewer leakage, or the illegal disposal at many industrial facilities.

PCBs are a family of chemically related compounds (or congeners) consisting of
two phenyls radicals and at least 1 to 10 chlorine atoms. PCBs were widely used as
insulation fluids in electrical transformers and electronic capacitors, heat-exchange fluid,
coolant, and are present in a wide variety of consumer products such as plastics,
adhesives, and paints. Before being banned in 1979 by the US Environmental Protection
Agency (EPA), about 1.1 billion pounds of PCBs were synthesized between 1929 and
1977 in the United States (Ross 2004).

4.2.2 Public Health Concerns

As with any anesthetic gas, both TCE and PCE are depressants of the central
nervous system. PCBs are easily accumulated in the food chain due to their high
persistency in the environment (Ross 2004). Starting from vegetative leaves, PCBs can
eventually build up inside the human body through the food chain (e.g., through the
consumption of fish, EPA 2008a). Since their ban in 1979, the levels of PCBs in the
environment have been diminished rapidly (Ross 2004). Laboratory experiments have
proved that PCBs affect the growth of testing animals and kill the organisms within a
short period of time. Although there is still inadequate evidence, the US EPA has
determined that PCBs are probably carcinogenic to humans (EPA 2008b).

4.2.3 Current Regulatory Status

PCBs are ruled by the federal code 40 CFR 761 under the 1976 Toxic Substance
Control Act. In the most recently updated version (2007), PCBs concentrations are
limited to 50 ppm for human use and all media containing PCBs concentrations 50
ppm must be treated (EPA 2007). The US EPA has set the maximum contaminant level
(MCL) of both TCE and PCE at 5 ppb in the drinking water standards (EPA 2008c).

4.3 Biodegradation of Chlorinated Organic Solvents

4.3.1 Biodegradation of PCE and TCE

Both TCE and PCE (or called tetrachloroethylene) are widely present in
groundwater to potentially impact human health. Aerobic biodegradation is one of the
proposed approaches for treatment of TCE in groundwater. Unlike anaerobic reductive
methods, aerobic biodegradation will not produce toxic intermediates such as trans-1,2-
dichloroethylene (trans-DCE), cis-1,2-dichloroethylene (cis-DCE), 1,1-dichloroethylene