&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

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