946 PCBs AND ASSOCIATED AROMATICS
Bioremediation In-situ biological remediation is an attrac-
tive technology because it represents a permanent cleanup
remedy which, although slow compared with an in-tank
reaction, can be allowed to proceed in-situ for as long as
necessary. The disadvantage of the method is usually associ-
ated with the need to maintain environmental factors such as
moisture, temperature, pH and microbial nutrients. A pos-
sible result of PCB degradation by microbial treatment is the
formation of PCDFs.Photolysis Photodecomposition of PCBs at soil surfaces
has been reported in the presence of triethylamine as a proton
donor. Methanol has been used as a proton donor for the photo-
decomposition of PCDDs. Some difficulty has been observed
at the very low concentrations of concern in distinguishing
photolysis from evaporation. Photodegradation is limited to
the surface of contaminated soil.Treatment Techniques Applicable to Excavated SoilsIn contrast with in-situ soils a relatively wide range of tech-
nologies is applicable to the treatment of excavated soils:- incineration,
- biological treatment,
- matrix isolation
- vitrification,
- chemical dechlorination,
- electrocoagulation
- solvent extraction,
- physicochemical adsorption.
Incineration Incineration methods are required to achieve
a destruction and removal efficiency of 99.9999%. This is
described as the six 9’s rule in 40 CFR 761.70.
There is a wide variety of equipment available for the
incineration of PCBs which ranges from large fixed site
equipment to more mobile units. Rotary kilns, for example,
are usually large and expensive to operate but can fulfill the
six 9’s destruction efficiency requirement.
A kiln consists of a refractory lined cylinder with its axis
slightly inclined to produce a tumbling action which mixes
the wastes. Residence times of PCB contaminated material
in the hot zone can be controlled to the extent necessary for
complete combustion. Combustion temperatures range from
870°C to 1650°C.
The following high energy destruction methods are either
being used on a large scale or are in the development stages.- Circulating Bed Incineration
- High temperature fluid wall reactor
- Rotary kiln
- Plasma arc
- Penberthy glass-melting furnace
- Molten salt
- Wet oxidation
- Supercritical water oxidation
Biological Treatment Biological treatments for industrial
waste systems have been in use for many years but it is only
recently that new microbial strains have been developed to
degrade PCBs by either enhancing conditions for existing
microbes or the application of proprietary strains.
Fundamental biochemical degradation mechanisms are
not well understood. Chemical pathways are therefore dif-
ficult to predict and the potential exists for the release of haz-
ardous compounds into the environment through incomplete
degradation or the formation of more toxic products.
The EPA considers biological processes which involve
the excavation of contaminated material for treatment to
be eligible for a treatability variance when the process can
be shown to achieve a residual concentration of 2 ppm PCB
or less. The treatment residuals can only be re-deposited
on-site without long-term management controls when the
treatment by-products do not present a threat to the envi-
ronment.
Matrix Isolation Solidification is a term which is applied
to the encapsulation of a waste to prevent its leaching into
the environment. For example, PCB contaminated soils
have been encapsulated into concrete without the forma-
tion of chemical bonding between the PCB and the encap-
sulant. On the other hand, solidification implies that some
degree of chemical interaction is taking place to inhibit
leaching. For example, pozzolanic chemistry has been
used to chemically fix PCBs in a rock-like product which
has good load bearing properties and which is hydrolyti-
cally stable.
Vitrification Vitrification utilizes an electrical process
which uses about 4 MW of power to produce an in-situ
pyrolyzed mass between large electrodes placed in the soil.
Volatilized organics have to be captured and treated. The
process results in complete pyrolysis of a localised area.
The perimeter of the treated area needs to be defined by an
appropriate analytical procedure to determine the need for
on-going management controls.
Chemical Dechlorination Once PCBs have been
extracted from the soil into a non-aqueous solvent the con-
centrated extract can be treated with organometallic reagents
such as those invented by the author. Alternatively, alkali
metal polyethylene glycol reagents, (APEG), can be used
which are not directly reactive with water.
Both sodium and potassium PEG derivatives have been
used with limited success. The reagent efficiency varies
with the type of Aroclor and the amount of dilution the
reagent receives from water contained in the soil. Recovery
of excess reagent by water washing tends to reduce the
cost-effectiveness of the process although PCB reduction to
below 2 ppm PCB has been achieved.
Electrocoagulation Coagulation is the process by which
particles in water are destabilized so that they can be removed
by solid/liquid separation processes such as sedimentation
and filtration.C016_003_r03.indd 946C016_003_r03.indd 946 11/18/2005 1:12:45 PM11/18/2005 1:12:45 PM