United States
Environmental Protection
Agency
EPA/540/S5-90/004
August 1990
SUPERFUND INNOVATIVE
TECHNOLOGY EVALUATION
Emerging Technology
Summary
Development of Electro-Acoustic
Soil Decontamination (ESD)
Process for In Situ Applications
Laboratory studies were con-
ducted on clayey soils contaminated
with decane (organic), zinc chloride
(inorganic), and a mixture of zinc and
cadmium chlorides to evaluate the
effect of electro-acoustics to decon-
taminate these soils. The objectives
of the study were to develop an
electro-acoustic leaching process
that has the potential to :
• Decontaminate soils containing
hazardous organics in situ by the
application of d.c. electrical and
acoustic fields.
• Decontaminate soils containing
heavy metals by the application of
d.c. electric and acoustic fields.
Using the electro-acoustic leaching
process it was demonstrated that:
• Removal of decane from clayey
soils was not feasible.
• Up to 90 percent removal/
concentration of zinc and cad-
mium from soils was achieved.
This report represents the first
phase of the ESD process develop-
ment aimed at decontamination of
soils contaminated with organics
such as decane and inorganics such
as zinc and cadmium in fulfillment of
a SITE cooperative agreement by
Battelle under the sponsorship of the
U.S. Environmental Protection
Agency. This report covers the period
September 1988 to December 1989.
This Summary was developed by
EPA's Risk Reduction Engineering
Laboratory, Cincinnati, OH, to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
Many sites in the U.S. are contami-
nated with nonaqueous phase liquids
(NAPL) and heavy metals. The U.S. EPA
has estimated that 189,000 underground
storage tanks are leaking at retail fuel
outlets alone. NAPL contamination in the
form of coal tars and petroleum sludges
from above-ground tanks is also a
significant problem. Following an NAPL
spill or release, the liquid typically
migrates to the water table where it
spreads out and floats, since it is lighter
than water.
Moreover, improper disposal of
industrial wastes containing heavy metals
has created a serious problem in a
number of locations. Because of
increasing proliferation of these wastes,
contamination of the ground and ground-
-------
water at a number of locations is causing
a serious threat to the environment.
The current state-of-the-art in
remediating these sites is to recover all
pumpable separate phase organic liquids
and then treat the residuals either in situ
(via bioreclamation, soil venting, or soil
washing or flushing) to pump and treat or
to excavate. The initial recovery of
pumpable product, depending upon the
site, is typically limited to 20-25 percent
recovery and in many cases even less.
The electro-acoustic soil decontami-
nation process is based on the syn-
ergistic application of a d.c. electric field
and an acoustic field to contaminated
soils to increase the transport of
leachants through the soils.
Acoustics, when properly applied in
conjunction with electroseparation and
waterflow, enhances dewatering or
leaching. The phenomena that augment
dewatering when using the combined
technique are not understood fully.
However, we have developed some
hypotheses about possible mechanisms.
It is theorized that, in the presence of
a liquid phase continuum, the acoustic
phenomena (e.g., inertial and cavitational
forces) that separate the liquid from the
solid into the continuum are facilitated by
the electric field and a pressure
differential to enhance dewatering by
means of one or more of the electro-
separation phenomena. There is also
evidence of synergistic effects of the
combined approach. In addition, as the
cake densities (by sequestration and
electroosmosis), the liquid continuum
normally would be lost, but it is believed
that by channelling, on a microscale,
acoustic energy delays the loss of the
continuum, making additional leaching
possible.
Besides electroosmosis, passage of
d.c. current through a wet soil also
produces effects such as ion exchange,
development of pH gradients,
electrolysis, gas generation, oxidation
and reduction, and heat generation. It is
conceivable that the heavy metals
present in contaminated soils can be
precipitated out of solution by
electrolysis, oxidation and reduction
reactions, or ionic migration. The
contaminants in the soil may be cations,
such as Cd + + , Cr + + + , and
Pb+ + + +, or anions, such as CN",
CrO4-, andCr2O72~.
The existence of these ions in their
respective states depends upon the local
pH and concentration gradients existing
in the soil systems. Application of an
electric field is expected to increase the
leaching rate and precipitate the
respective heavy metals out of solution
by establishing appropriate pH and
osmotic gradients. For example, CdCI2
in solution can be precipitated out as
Cd(OH)2 at the cathode due to the
generation of (OH) ion at the cathode and
flushed out of the ground and separated
by conventional techniques.
Tesft Equipment and
Procedures
The test unit design was developed
primarily to accommodate the intro-
duction and characterization of the
acoustical energy. The test unit is shown
in Figure 1. The intent was to reasonably
simulate the field conditions under which
the acoustics would be applied. That is,
the design was to simulate the earth as
much as could be expected in a
laboratory apparatus.
Relatively low frequencies were
chosen to penetrate the earth an
appreciable distance. The unit was
designed to generate plane-wave
acoustics in which points of constant
phase form a plane. The direction of
propagation is normal to the plane.
This approach reduces the acoustics
system to a one-directional case and the
acoustic field can be accurately
characterized with a few point measure-
ments. This is an equivalent situation to
the electric field formed by the two
parallel-plate electrodes.
The acoustic instrumentation includes
an acoustic shaker, a load cell, an
accelerometer, and two hydrophones.
The acoustic source is an Unholtz-Dickie
Model 1 electro-magnetic shaker*. This
shaker is the source of the acoustic
excitation. Acoustic data were acquired
during testing with the four channel
analyzer. This was under computer
control to automate acoustic data
collection and storage. Two test cells, 3-
inch internal diameter, 4 and 6 inches
high made of acrylic tubing were used to
hold the contaminated soil. The cell
consisted of two electrodes, an anode at
the top and a cathode at the bottom. The
anode is a 3-inch diameter, 100-mesh
stainless steel screen, whereas the
cathode is a perforated s.s. supporting
plate. The cathode is supported by four
s.s. rods. A leachate collecting chamber
was placed under the cathode. Leachate
from the soil was drained through pipes
to the leachate collecting pans. Distance
between two electrodes represented the
sample cake thickness, which was varied
from 2 inches to 4.5 inches depending on
the contaminated soil type used during
the experiment. Leachate from the soil
was drained through pipes to the
leachate collecting pans.
Zinc Tests
The soil sample was prepared in the
lab by spiking 2,000 mg/kg of zinc
(ZnCI2) on a dry weight basis. Experi-
mental results showed substantial
removal of zinc by the ESD process. In
one extended test, more than 90 percent
zinc removal from approximately 3/4 of
the soil sample was obtained in 100
hours of operation. The voltage gradient
increased from 0.3 volt/inch at the start of
this test to 20 volts/inch at the end to
maintain a constant current at 50
milliamps. The average electrical power
consumed during the test was 1.423
watts. Zinc accumulated in 1/4 of the soil
sample next to the cathode. Accumu-
lation of zinc was caused by precipitation
of zinc hydroxide from reaction of zinc
ions with hydroxyl ions generated at the
cathode. By neutralizing the hydroxyl
ions, precipitation of zinc hydroxide can
be prevented and zinc can be flushed out
from the cathode.
Zinc and Cadmium Tests
A soil sample was prepared in the
laboratory by spiking 11,000 mg/kg zinc
(ZnCI2) and 1,000 mg/kg cadmium
(CdCI2) on a dry basis. One test was
conducted to demonstrate that a mixture
of ion contaminants in the soil can be
transported in the presence of electric
field. The treated cake (4.5 inches) was
divided into five layers to monitor the
ions removal from each layer. Layer
thickness and gradient are shown in
Table 1. Test results confirm that ESD is
effective in moving both zinc and
cadmium ions from the layer in contact
with the anode to the layer in contact with
the cathode. For example, cake gradient
1 (layer in contact with anode) shows a
removal of 97.05 percent cadmium and
85.09 percent zinc. Also, there was zinc
and cadmium removal from the rest of all
layers except the layer in contact with the
cathode (0.4 inch thick). This analysis
indicates that both zinc and cadmium
removal occurred in more than 90
percent of the treated cake.
Mention of trade names or commercial
products does not constitute endorsement or
recommendation for use.
-------
c
CO
IT
c
8
CO
10
(polymetric material)
3.5 in.
Hydrophone
Acoustic Head
Acrylic Tubing
3-3/8" O.D., 3" I. D.
Liquid Sampling Ports
Electrodes
S.S. Screen (100 mesh)
3/4" S.S.
Soil
c
in
1
« Wood Box
Soil
Figure 1. Schematic of laboratory test cell.
Decane Tests
Batch ESD tests were conducted on a
decane-contaminated soil sample. The soil
sample was spiked with 8 percent decane
(dry weight). Tap water was added to
moisten the soil to 45 percent moisture
(net). Decane analysis on the ESD-treated
soil was performed by both EPA and Zande
Laboratories. Results from the decane
experiments were inconclusive because of
substantial experimental uncertainty in
decane analysis from the two labs and also
possibly in experimental procedures.
However, based on a few tests in which the
decane values from the two labs were
relatively close, the data indicated between
10-25 percent decane removal.
Conclusions
(1) Electro-acoustic decontamination
of soil in a laboratory mode was
proven technically feasible for
inorganic contaminants.
(2) Zinc removal/concentration (80-90
percent) was observed in the
presence of the electric field.
(3) There appears to be a combined
electric and acoustics effect during
zinc removal. However, further
testing is required to accurately
determine the magnitude of the
effect.
(4) Longer leaching times yielded
higher zinc removal efficiencies.
(5) Higher power levels yielded higher
zinc removal rates.
(6) Cadmium/zinc removal/con-
centration (90-95 percent) was
observed in the presence of the
electric field.
(7) Since a large variability in
analytical determination of decane
in the soil was observed, no
definitive con-elusions can be
drawn on the effect of electro-
acoustics on decane removal from
soils.
Recommendations
Based on Phase I laboratory
experimental results for decon-
tamination of heavy metals in clayey
soil, a study is recommended and
should be conducted to further
evaluate the ESD process in field
conditions. Such.a study would validate
the Phase I results and would provide
the basis for developing design and
operational changes for successful field
applications.
We also recommend no additional
work on the decane contaminated soil
until the analytical and experimental
problem can be solved. The results
from the decane experiments were
inconclusive because of substantial
experimental uncertainty in the decane
analysis and also possibly in
experimental procedures.
The full report was submitted in
fulfillment of Cooperative Agreement
No. CR-815324-01-0 by Battelle under
the sponsorship of the U.S. Environ-
mental Protection Agency.
•&U.S. GOVERNMENT PRINTING OFFICE: 1990/748-012/20084
-------
Table 1. Performance of BAD Process on Zinc/Cadmium Soil
Cake
Gradient
0 ANode ( + )
1
2
3
3.5
.t
4 Cathode (-)
Layer
Thickness
(in.) pH
0
1
1
1
0.6
0.4
3.65
3.55
3.64
4.12
7.66-9.2
Zinc Concentration
(mg/kg) dry soil
Zande
0
167
182
207
409
7755
EPA
0
158
167
197
344
7180
Ave
0
163
175
202
377
7468
Percent
Zinc
Removed
100
85.09
83.99
81.52
65.51
Cadmium Concentration
(mg/kg) dry soil
Zande
0
29.2
26.0
53.5
207
6187
EPA
0
25
22
51
208
6310
Ave
0
27.1
24.0
52.3
207.5
6249
Percent
Cadmium
Removed
100
97.05
97.39
94.32
77.45
Initial Sample Solids % = 56.73%
Initial Zinc Concentration = 1093 mg/kg dry soil (see Table 7)
Initial Cadmium Concentration = 920 mg/kg dry soil (see Table 7)
H. S. Muralidhara, B. F. Jirjis, F. B. Stulen, G. B. Wickramanayake, A. Gill, and
R. E. Hinchee are with Battelle, Columbus, OH 43201.
Diana Guzman is the EPA Project Officer (see below).
The complete report, entitled "Development of Electro-Acoustic Soil
Decontamination (ESD) Process for In Situ Applications" (Order No. PB
90-204 728/AS; Cost: $23.00, subject to change) will be available only
from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Risk Reduction Engineering Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/540/S5-90/004
------- |