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Vermicomposting for the bioremediation of PCB
congeners in SUPERUND site media
By J. Tharakan, A. Addagada, D. Tomlinson, and A. Shafagati
Department of Chemical Engineering, Howard University, Washington, DC,
USA
Abstract
In this study we investigated the potential
of using earthworms (E.foetida) to effect the biotransformation of
polychlorinated biphenyls (PCB’s) in sludge and sediment from the Ralston
Street Lagoon (RSL) SUPERFUND site in Gary, IN. Vermiomposting bioreactors
(VB’s) were established with mass fractions of contaminated sludge ranging from
10% to 75%. The VB’s comprised a drainage layer overlaid with a mesh screen and
then the earthworm bedding mixed with contaminated sludge. The VB’s were each inoculated
with equal biomass of earthworms. Suitable negative controls to monitor
reproduction and biomass increase without contaminated sludge were also
established. The VB’s were kept moist by water spray and earthworms were
periodically fed cornmeal. At set times, VB’s were sampled and analysed by GC-ECD
for PCB levels. Earthworm biomass and number were also measured. Results
demonstrate that earthworms survived and reproduced in the presence of
contaminated media; however, biomass increase decreased rapidly with increasing
mass fraction of sludge. Biomass increased ranged from 103% in the negative
control to biomass reduction of 54% with 75% sludge. Gas chromatography results
demonstrated an 80% reduction in PCB level in all VBs, although the time
required for this level of reduction increased with increasing sludge mass
fraction. Analysis of earthworms showed elevated PCB levels in the worm. These
results suggest vermicomposting may be a viable option for on-site in situ
bioremediation and site clean-up.
Keywords:
vermicomposting, earthworms, polychlorinated biphenyls, bioremediation,
SUPERFUND.
1 Introduction
Polychlorinated Biphenyl’s (PCB’s) are
chlorinated aromatic organics that were extensively used in industry until they
were banned in 1976 by the United States Congress’s Toxic Substances Control Act
(TSCA) due to their adverse impact on health and the environment. This limited the distribution and future
usage of PCB products, as described by Chawla et al [1]. PCB’s were manufactured as Aroclors (USA), Kanechlors (Japan),
and Sovols (Soviet Union)[2]. The Aroclors vast range of utility as a key
ingredient in various adhesives, transformer dielectric fluids and machine oils
due to its capacity for heat resistance led to its global distribution and
subsequent contamination of multiple environmental matrices such as sediments,
soils, and inland water-bodies [2]. Lax
PCB disposal practices have lead to almost ubiquitous PCB environmental
distribution. Approved treatment
technologies, such as incineration, are expensive and can generate
harmful by-products. Biotransformation of the PCB contaminants is
a potential alternative method to be developed as a possible process
for site
cleanup.
2 Earthworms and Chemical
Toxicity
Earthworms are the key organisms in the
breakdown of plant organic matter; their populations expand in relation to the
availability of organic matter. Due to their widespread distribution and
importance in soil systems, they are considered as very useful organisms for
evaluating contamination of the soil environment. Residues of chemicals can bio-accumulate
in the earthworms and may be distributed by them to the tissues of animals in higher trophic levels within the food web, as
discussed in Edwards [3]. There has also been concern over the fate of
long-lived contaminants such as dioxin [4] and PCBs [5]. Earthworms have
a role in bio-monitoring because they can bio-accumulate or bio-concentrate
xeno-biotic chemicals. They have been used to measure the level of heavy metal
and persistent organic contamination of soil media.
3 Materials and Methods
3.1
Earthworms
E.foetida
earthworms were reared in our laboratory and these worms were used in all our
experiments. This is the species most commonly used for degrading organic
wastes. The rationale for choosing this species is that it tends to be ubiquitous
and many organic wastes become naturally colonized by this species as described by Edwards and Neuhauser [6]. It also has a wide temperature tolerance and
can live in wastes with a wide range of moisture content. It is a tough worm that is readily handled
and, in mixed cultures, usually becomes dominant so that even when field
systems begin with other species they end up with a large proportion of E.
foetida.
3.2
Soil Matrix
PCB contaminated sludge was obtained from
the SUPERFUND site at the Ralston Street Lagoon in Gary, Indiana. The Gary Sanitary District (GSD) was
responsible for the excavation, removal and transport of the sludge to our
laboratory. Three five-gallon buckets of
sludge were delivered to the Howard University Biochemical Engineering Lab
(HUBEL) containing 3 different levels of PCB contamination. The samples were from different locations
within the RSL: northwest, mid-west, and southwest, with approximate
concentrations of 1000ppm, 780ppm, and 220ppm respectively, as reported to us
by GSD. The sludge from the mid-west
sample was chosen for our experiments.
3.3
Experimental Setup
Two sets of VB’s were established, each
containing different mass fractions of sludge and hence different starting PCB
concentrations. Each set of VBs
contained five reactors with 0%, 10%, 25%, 50%, and 75% sludge mass fractions,
respectively. The VBs were established
with a gravel drainage layer on which was overlaid a mesh screen. The rock layer permitted excellent drainage
so earthworms did not get flooded during the periodic VB wetting. The mesh
screen, placed on the single layer of rock ensured that the rock would not mix
with the soil above. All the VBs were covered with mesh on top so earthworms
would not escape. One set of the
different sludge mass fraction containing VBs was inoculated with 9 gms of live
earthworms. The VBs were also wrapped in
aluminium foil so that external light would not keep the earthworms away from
the reactor walls.
4 Experimental Protocol
All VB’s were periodically sprayed with water
to maintain the moisture content at around 50%. At given time intervals, each
set of VBs was emptied, the earthworms were separated from the sludge using
light and the sludge-earthworm bedding mixture was mixed and sampled for
analysis by GC-ECD.
4.1
PCB Measurement
The VB samples were placed in a laboratory
fume hood to air dry. After drying,
samples were extracted with acetonitrile. One gram of the dried sample was
extracted in 15ml acetonitrile. The solvent was then filtered through a 0.4-mm pore-size nylon filter. 1 ml of this filtered sample was injected
into the gas chromatograph (GC) and run using an electron capture detector
(ECD). The instrument used was a HP 5890 Series II GC utilizing a 0.32 mm
internal diameter, 30 m fused silica column with a 0.5mm
film and ECD. Calibration curves were run prior to each set of samples; Aroclor
1248 was used as the calibration standard, since the initial report from the GSD
RSL demonstrated that the main PCB congener grouping in the RSL sludge was Arolcor
1248.
4.2
Total PCB Mass balance
After 180 days, the experiments were
terminated for 10% and 25% sludge mass fractions, and the total biomass of the
earthworms in the different VB’s was measured. The final sludge in the VB’s was
weighed and dried for extraction and analysis. The complete sludge-bedding
mixture for each VB was dried and extracted which provided the information to
complete the mass balance on the fate and distribution of the PCBs. The
earthworms were also crushed and dried until a constant weight was recorded.
The dried earthworm samples were extracted with methylene chloride in a Soxhlet
extraction apparatus for 18 hrs with a reflux rate of 3-4 cycles per hour. The extract was then concentrated to 2 ml and
the solvent exchanged with hexane [7] before being brought to a final volume of
10 ml. The 10 ml sample was washed with
equivalent volume of concentrated sulfuric acid until the sample became clear;
the was hed and cleaned samples were then run on the GC with ECD.
5 Results and Discussions
5.1
PCB Concentration
The total final reduction of PCB’s in the
different sludge concentrations was fairly similar in all VBs although the time
required to achieve this level of reduction increased significantly with
increase in the contaminated sludge mass fraction in the VBs. All sludge
concentrations demonstrated around an 80% reduction in total PCB concentration by
the time of termination of the experiment. Table 1 show the reduction rate of
PCB’s in different sludge concentrations.
Table 1: PCB concentrations in different
sludge concentrations
|
Sludge
Concentration (%)
|
PCB concentration
at tinitial (ppm)
|
PCB concentration
at tfinal (ppm)[days]
|
Net % reduction
in PCB concentration
|
|
10
|
223.65
|
40.07 [150]
|
82.08
|
|
25
|
476.28
|
87.46 [150]
|
81.63
|
|
50
|
719.57
|
73.8 [185]
|
89.67
|
|
75
|
942.17
|
160.83 [200]
|
82.93
|
Figures 1, 2, 3, and 4 show the PCB
reduction in the VBs with RSL sludge mass fractions of 10%, 25%, 50% and 75%,
respectively. The increase in PCB concentration that appears after particular
periods in some of the VBs suggests de-chlorination of higher chlorinated PCB
congeners and generation of greater amounts of lower chlorinated compounds,
hence resulting in a net increase in integrated area from the GC chromatograms.
The x-axis in each figure shows the end-time for each VB experiments and this was
different for the different sludge mass fractions. Hence, the rates of
biotransformation decreased as the concentration of PCB contaminants increased.
Figure 1.
PCB concentration in 10% sludge
from T= 0days to T=150days
Figure 2.
PCB concentration in 25% sludge
from T= 0days to T=150days
Figure 3.
PCB concentration in 50% sludge
from T= 0days to T=185days
Figure 4.
PCB concentration in 75% sludge
from T= 0days to T=200days
5.2
Earthworms
The total biomass of earthworms in all the
VBs was also monitored. Table 2 shows the earthworm biomass from inoculation to
time of termination. The negative control, containing earthworms with no
sludge, demonstrated an earthworm biomass increase of 103%.
Table 2: Earthworm biomass in different
sludge concentrations
|
Sludge Concentration (%)
|
Initial worm weight (grams)
|
Final worm weight (grams)
|
Net % gain/loss in weight
|
|
10
|
9.026
|
12.2
|
35.16
|
|
25
|
9.057
|
5.563
|
-38.58
|
|
50
|
9.09
|
7.173
|
-21.089
|
|
75
|
8.997
|
4.11
|
-54.318
|
Table 3 shows the PCB concentration in the
earthworms from each of the VBs. Analysis of dried and extracted earthworms from
the control VB with no PCB contaminated sludge demonstrated no peaks and hence no
PCBs. By the end of the experiment, the earthworms in the different sludge mass
fraction VBs had bio-accumulated PCBs in their bodies, and the data shows that
the higher the surrounding PCB levels, more PCBs were accumulated in the worms.
Table 3: PCB concentrations in the
earthworm present in different sludge concentrations
|
Sludge Concentration (%)
|
PCB concentration in earth worm (ppm)
|
|
10
|
148.02
|
|
25
|
212.9
|
|
50
|
188.35
|
|
75
|
313.08
|
6 Conclusion
This study shows that the earthworms (E.foetida)
were able to bio-accumulate the PCBs from the sludge and reduce the amount of
PCB congeners left in the sludge. Some
biotransformation of PCBs is suggested by the data, especially a close
examination of the various congener peaks. However, the bulk of the removal of
PCBs from the VB matrices appeared to be by transport into the earthworm
biomass. There is a decrease in the PCB concentrations in the sludge after
exposure to, and working over by, the earthworms. Approximately 20% of the PCBs
remain in the sludge-earthworm bedding mixture at the time of termination of
each of the experimental studies. The
fate of this remaining 20% and the reasons for its recalcitrance to further
biotransformation requires more study and investigation in order to elucidate what
might be occurring. Several possibilities make themselves apparent. Of course,
it is entirely possible that a certain percentage of PCB contamination is hard
to access and hence recalcitrant to any transformative activity. Bioavailability
of the PCB in the sludge may be a large hindering factor, especially given the
nature of the sludge which was extremely dense and clay-like. Another
possibility is that the earthworms did not have sufficient time and sufficient
additional feeding; hence it may be that with sufficient additional resources
and longer experiments, more complete biotransformation may occur. Much of the
PCBs are not biotransformed but simply bioaccumulated into the earthworms. The higher the surrounding PCB
concentrations, the larger the amount of PCBs found in the earthworm biomass,
as shown in Table 3. A high degree of bioaccumulation
is occurring, and it is likely that bioaccumulation and biotransformation may take
place together.
Nevertheless,
these results do suggest that vermicomposting may be a potentially viable
alternative for removal of PCB congeners from contaminated sludge or soils. Some
further processing, however, may be required for complete elimination of the
PCB.
Acknowledgements
The authors gratefully acknowledge the
financial support of the Gary Sanitary District, City of Gary, Indiana, USA, for this investigation.
References
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Tharakan J.P., E. Sada, R. Liou
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[7]
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