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Former
Chemical Manufacturing Facility
W.R. Grace
Charleston, South Carolina, USA
Project
Bioremediation of Soil
Containing Toxaphene and DDT
Summary
The application of DARAMEND® served to
reductively dechlorinate and subsequently
mineralize chlorinated herbicides and pesticides
present in soil. This project was the third in a
series of three pilot-scale projects aimed at
validating DARAMEND® bioremediation for use
on various chlorinated organic pesticides.
The Challenge
Treatment of about 250 tons of soil impacted with
various chlorinated pesticides including
Toxaphene and DDT was conducted in a covered
in situ treatment cell. Toxaphene, the
principle target contaminant, is known to be
extremely recalcitrant and is generally
considered to resist biodegradation. Due to the
in situ nature of this application, soil
contaminant concentrations were highly variable
and spatially distributed. For example, Toxaphene
concentrations ranged from 1,600 mg/kg closest to
the former production facility, to 12 mg/kg at
the furthest point. The higher pesticide
concentrations presented a particularly
significant challenge for effective
remediation.
The Solution
The treatment process involved the application of
DARAMEND® to a 60 cm thick layer of soil
within a treatment area. To prevent unwanted
precipitation from impacting treatment, the
treatment area was covered with a polyethylene -
clad greenhouse. Soil undergoing treatment was
subjected to several anaerobic/aerobic treatment
cycles. Anaerobic conditions were generated by
the application of DARAMEND® organic
amendments and zero valent powdered iron. Soil
was then irrigated to increase the soil moisture
content to 90% of the soil water holding capacity
to minimize the diffusion of oxygen into the
soil. Aerobic conditions were generated through
the repeated introduction of oxygen into the soil
via mechanical tillage. Eight separate treatment
cycles were required to achieve the remedial goal
in some areas of the treatment cell (total
DARAMEND loading was <3.5% by weight).
The Result
The mean concentrations of the key target
contaminants, Toxaphene and DDT, were reduced
from 239 mg/kg and 89.7 mg/kg to 5.1 mg/kg and
16.5 mg/kg, respectively. This corresponded to
98% and 82% reductions in contaminant
concentrations.
In heavily impacted areas of the treatment cell,
the Toxaphene concentration was reduced from
1,600 mg/kg to 11 mg/kg after eight treatment
cycles. The application of four additional
treatment cycles in this area resulted in a
further reduction of Toxaphene concentration to
5.1 mg/kg, which corresponds to a 99.7%
reduction.
The Cost
Estimates to apply this technology full scale at
a similar site are approximately at $95/ton,
including the treatment cell cover, and about
$75/ton if the cover was not required.
The Timeline
Treatment of this material
required application of eight treatment cycles.
Each treatment cycle required approximately one
month, for a total active treatment period of
approximately eight months. Following a thorough
analysis of data from the site, an additional
four treatment cycles (for a total of 12 cycles)
were applied to heavily impacted areas of the
treatment cell. |
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