PCBs AND ASSOCIATED AROMATICS 945
phases is linked to the relative affinity of the components of
concern for each phase and are represented as ‘Distribution
Coefficients’. A distribution coefficient is calculated as the
ratio of the concentration of a chemical in one phase to its
concentration in a second phase. PCBs, for example, are
hydrophobic and so are largely retained by the non-aqueous
phase liquid, usually referred to as NAPL, and the organic
matter in the soil. Hence the distribution coefficient for PCBs
in oil is very large.
The mobility of a component of concern through a soil
bed is measured in terms of a retardation factor. The retar-
dation factor describes the relative velocity of a constituent
compared to the rate of movement of water through the sub-
surface. A retardation factor which is greater than unity indi-
cates that a component is moving more slowly than water
through a system. The retardation factor for PCBs is very
large.
The combination of large distribution coefficients for
NAPL and soil organic matter and a large retardation factor
means that PCBs tend to remain at the surface of a spill
site and migrate either by erosion or by dust emission. As
a consequence, spill sites or landfills tend to be long term
sources of PCBs.
Remedial response levels for PCBs are as follows:soil 10 mg/Kg (EPA/NYSDEC soil
removal criteria)
air 1.67 mg/m3 (NY State Ambient Air
Level)
ground water 1.00 mg/L (NYSDOH advisory
level)
surface water 7.9 10 −5 mg/L (Clean Water Act)The surface water criterion of 7.9 10 ^5 μ g/L i.e., less
than about 1.0 parts per trillion, for a lifetime cancer risk of
10 −6 is below the method detection limit for EPA method 608
and even below the MDL for the method.
The extent and type of information obtained from mod-
eling the migration and persistence of contamination is used
to select treatment approaches for further evaluation with
respect to their feasibility and cost effectiveness. Remediation
is accomplished using one or more of three types of systems:
(1) in-situ; (2) prepared bed, and (3) in-tank reactor.
There are five major categories of in-place treatment
techniques:- Extraction,
- Immobilization,
- Degradation,
- Attenuation, and
- Reduction of voltalization.
ExtractionExtraction involves washing contaminants from the soil with
a suitable solvent. The solvent must be non-polluting and
must not alter the soil properties in an unacceptable way. Anefficient method is needed to capture the elutriate and while
the method has shown application in the mining industry it
is not likely to be suitable for PCBs.ImmobilizationImmobilization methods are intended to reduce the rate of
release of contaminants from the soil so that the concentra-
tions along pathways of exposure are restricted to within
acceptable limits. Activated carbon is a strong adsorbent for
PCBs and can be readily incorporated into soil. The amount
of carbon which may be required is strongly affected by the
amount of other organic material is the soil. Since the PCBs
are retained at the site, re-application of carbon may be nec-
essary as the original material erodes.DegradationOxidation, reduction and polymerization reactions have
been carried out in attempts to transform soil contaminants
into less toxic or less mobile products.Oxidation Chemical oxidation of compounds of concern
with reagents such as ozone or hydrogen peroxide can lead
to oxidation products which are more toxic than the original
contaminants. For example, the formation of hydroxylated
products would tend to increase the water solubility of PCBs
and may even lead to the generation of polychlorinated
dibenzofurans (PCDFs) or polychlorinated dibenzodioxins
(PCDDs).Reduction Reducing agents such as iron powder or sodium
borohydride have been used to degrade toxic organics. An
electrochemical method which uses iron/copper pellets
has been applied to the treatment of PCBs in a tank reac-
tor but has not been demonstrated as an in-place treatment
method.
Reductive dehalogenation of PCBs can be applied
using organometallic reagents and sodium based reagents
similar to those developed by the author. The reagents
are typically very reactive with water and were originally
developed for the PCB decontamination of oils. A two-step
procedure has been used for soil remediation in which the
soil is first extracted and then solvent exchanged with a
water immiscible solvent. The relatively dry, water immis-
cible solution is then reacted with a water sensitive, PCB
reactive reagent.
Problems associated with water reactivity can be cir-
cumvented by using sodium polyethylene glycol. The alkali
metal (APEG) reagent is water miscible and has been used
with some limited success for the treatment of both PCB
contaminated oils as well as soils (148,162).Polymerization In-situ polymerization reactions have been
initiated to react polymerizable organics into polymers which
are less toxic and less mobile than the monomer compounds.
This type of reaction is not applicable to PCBs.C016_003_r03.indd 945C016_003_r03.indd 945 11/18/2005 1:12:45 PM11/18/2005 1:12:45 PM