&EPA
United States ,
Environmental Protection
Agency
EPA/540/MR-97/509
November 1997
SUPERFUND INNOVATIVE
TECHNOLOGY EVALUATION
Demonstration Bulletin
In situ Electrokinetic Extraction System
Sandia National Laboratories
Technology Description
Sandia National Laboratories (SNL) has developed an in situ soil
remediation system that uses electrokinetic principles to remediate
hexavalent chromium-contaminated unsaturated or partially satu-
rated soils. The technology involves the in situ application of direct
current to the soil, which results in dissolved chromate ions migrat-
ing through water in soil pores to the oppositely charged electrode,
a phenomenon known as electromigration. In addition to
electromigration, a bulk flow of pore water moves toward the
cathode in a phenomenon known as electroosmosis. Chromate
ions are extracted in the anode effluent. The In situ Electrokinetic
Extraction (ISEE) technology was demonstrated at SNL's Unlined
Chromic Acid Pit (UCAP) from May 15 to November 24,1996. The
target contaminant was hexavalent chromium in the form of chro-
mate ions. This demonstration was funded by the U.S. Depart-
ment of Energy's Office of Science and Technology through the
Subsurface Contamination Focus Area. The ISEE technology was
independently evaluated under the U.S. Environmental Protection
Agency's (EPA) Superfund Innovative Technology Evaluation (SITE)
Program.
The SNL ISEE system consists of the following: an electrode
system, a vacuum control system, a .water control system, a
power application system, a monitoring system, and ancillary
equipment. Figure 1 shows the ISEE system's main components.
The electrode system consists of anodes and cathodes. The
layout of the system used for performance testing at the UCAP
site consisted of a central row of four anodes and two rows of four
cold finger cathodes each placed 3 feet from either side of the
anode row.
The vacuum control system maintains a vacuum in the anode.
This vacuum creates the pressure gradient between the anode
porous ceramic casing and the surrounding soil necessary to
hydraulically control water movement between the anode casing
and the soil. Hydrogen gas, produced by electrolysis reactions of
applying direct current, is purged from the electrode casing to
eliminate the danger of explosion.
The water control system consists of a water circulation and pH
control systems. The anode water circulation system mixes the
Cathode water control system
Power supply system
Chiller
Compressed air
Vacuum control system
Ground surface
j Temperature probe
to monitoring system
Conductivity and pH probe
to monitoring system
.. f
0)
0) J.
"-
N;
CD
0,75-inch-diameter
PVC pipe
1-inch-diameter
copper pipe
Cold finger cathode
• 3 feet
3-inch-diameter
PVC casing
3.5-inch-diameter
ceramic casing
Iridium-coated
titanium pipe
Anode
Bladder pump
Electrode system
Figure 1. ISEE system schematic diagram. (Notes: NOT TO SCALE; PVC = Polyvinyl chloride)
Printed on Recycled Paper
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anolyte In the electrode, removes and samples anolyte, monitors
ihe chemical condition of the anolyte, and maintains the anolyte
temperature at about 12°C. Circulation is maintained in a recycle
How loop by a bladder pump. Because the bladder pump works
under vacuum conditions, an additional vacuum source is neces-
sary for Hs operation. The pH control system maintains the pH of
the anoiyte at about 6 using a 10 percent sodium hydroxide
solution. The cathode water control system consists of chiller
water to cool the cathodes with no electrode fluid recirculation
systems.
The power application system energizes the electrodes. The dem-
onstration system consisted of four 10-kilowatt power supply units.
Each unit was operated independently under constant voltage
conditions. The current applied to each electrode was monitored
and limited to 15 amperes.
The monitoring system records water control system information
such as anode casing water level, recycle flow rate, influent and
effluent rates, recycle flow temperature, conductivity, and pH. The
monitoring system also records air purge rates, vacuum in the
anodes, and ancillary equipment information such as soil tem-
perature and voltage profiles of subsurface soil.
Waste Applicability
The ISEE technology is applicable for treating unsaturated soil
contaminated with chromate ions. According to SNL, this technol-
ogy can be modified to treat saturated contaminated soils and to
remove contaminants besides chromate dissolved in the pore
waler. Because other ions will compete with contaminant ions to
be removed, it is necessary to determine the electrical conductiv-
ity of the soil pore water and the contaminant ion concentration to
determine the applicability of the technology.
Demonstration Approach
During the SITE demonstration, 13 tests were performed during
six phases. The test areas ranged from 36 to 72 square feet over
& zone of contaminated spHfrom 8 to 14 feet below ground
surface. The first 12 tests were conducted so that SNL could
determine the ISEE system's preferred operating conditions for
test 13 and to facilitate the migration of hexavalent chromium
toward the central portion of the test area. Test 13 consisted of
system performance testing.
Test 13 targeted a central portion of the demonstration area
measuring 12 by 6 feet. Four anodes and eight cold finger cath-
odes were configured based on efficiency data obtained during
the previous tests. This system was operated for over 700 hours
from October 21 to November 24, 1996, a period long enough to
determine the system performance and treatment cost data.
In general, anolyte samples were collected daily during the system's
period of operation. The samples were analyzed in-house for
chromate by SNL, and confirmation was obtained by having split
samples analyzed by an outside laboratory. The in-house anolyte
results constitute a significant basis for EPA's evaluation of sys-
tem performance.
|n addition, pre- and post-demonstration soil samples were col-
lected and analyzed for (1) total and hexavalent chromium and (2)
total chromium in leachate using the toxicity characteristic leach-
ing procedure (TCLP).
Demonstration Results
Because of the demonstration site logistics, the demonstration
was terminated before chromium extraction rates declined and the
soil was completely remediated. From the SITE evaluation of the
ISEE system, the following preliminary findings were made:
• At the system's preferred operating conditions determined by
SNL (test 13), (1) approximately 200 grams (g) of hexavalent
chromium were removed during about 700 hours of operation
(0.29 g per hour), and (2) the overall removal efficiency for the
system was approximately 0.13 g of hexavalent chromium per
kilowatt hour.
• Within the test areas, 18 out of 43 pre-demonstration soil
samples exceeded the TCLP limit of 5 milligrams per liter (mg/L)
of total chromium. The samples that exceeded the TCLP limit
contained chromium at concentrations of 5.6 to 103 mg/L, with a
median concentration of 15.4 mg/L. Post-demonstration results
indicate that 18 out of 35 soil samples exceeded the TCLP
regulatory criterion for chromium. The samples that exceeded
the TCLP limit contained chromium at concentrations ranging
from 6 to 67 mg/L, with a median concentration of 20.4 mg/L.
Key findings of the SITE demonstration, including sample analyti-
cal results, will be discussed in detail in the innovative technology
evaluation report and the technology evaluation report. The re-
sults will also be summarized in a demonstration capsule.
For Further Information:
EPA Project Manager: Randy Parker
U.S. Environmental Protection Agency
Office of Research and Development
26 West Martin Luther King Drive
Cincinnati, OH 45268
Telephone No.: (513) 569-7271
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
Official Business
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