Environmental Engineering Reference
In-Depth Information
As the result of manufacturing processes and spills, several hundred
million pounds of PCBs have been released into the environment (Hutzinger
and Veerkamp, 1981), and the same properties that made them so industrially
useful make them environmentally persistent. Because they are sparingly
soluble in water, they have a limited potential for migration through soil,
and even the bulk of PCBs deposited in sediments may remain in place for
decades. They are also lipid soluble and therefore bioaccumulate, increasing
risks associated with exposure through the food chain. A variety of adverse
biological effects have been ascribed to them. Perhaps the most notable
ecotoxicological effect of PCBs concerns poor reproductive success and
deformities in some fish and fish-eating birds (Ludwig et al., 1993). Also,
PCBs are suspected carcinogens, and there is epidemiological evidence that
they can cause abnormal neurological development in infants and children
and alter immunological responses (ATSDR, 2000). Thus, they are recognized
as one of the most problematic and persistent environmental contaminants.
The remediation of PCB-contaminated soils and sediments typically
involves excavation of the contaminated material followed by landfill dis-
posal or incineration. The high costs, long-term liability, and regulatory
issues associated with this approach have reduced the attractiveness of exca-
vation as an ultimate remediation option. In addition, excavation and off-site
transport of PCB-contaminated wastes may actually increase the potential
for human exposure. Recognition of the potential economic and health impli-
cations associated with traditional PCB treatment methods has led to a
renewed interest in the development of
in situ
and on-site treatment tech-
nologies, including enhanced bioremediation processes. Due to their low
solubility in water (or hydrophobicity) and low vapor pressures, PCB con-
geners are not effectively removed from soil/sediment systems by conven-
tional abiotic remediation technologies such as soil vapor extraction or sol-
vent flushing. Thus, the current state-of-the-art for PCB remediation typically
involves the excavation of PCB-contaminated soil/sediment, followed by
incineration. Estimated costs for incineration are on the order of $300 to
$600/ton of soil, including transportation and excavation costs. As noted
above, this remediation method frequently involves increased risk of human
exposure, due to the excavation and transport of PCB-contaminated soils.
The primary routes of exposure for this scenario are inhalation and dermal
contact with soil particles containing sorbed-phase PCBs.
A competing
in situ
technology that is currently under development
involves thermal desorption and oxidation of PCB-contaminated surface
soils (Iben et al., 1996). The technique involves the use of a thermal blanket
containing resistive tubular heaters spaced at 8-cm intervals. The thermal
blanket is placed over the soil surface and covered with a layer of insulation
(vermiculite or ceramic fiber) and an impermeable sheet of fiberglass-rein-
forced silicon rubber. Off-gases are extracted through a central tube and
passed through a thermal oxidizer operated at about 900˚C. A pilot-scale
study has been conducted at an abandoned racetrack where PCB-containing
oil was applied to the soil surface to reduce dust. PCB concentrations
