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Electrokinetic Extraction at the Unlined Chromic Acid Pit, Sandia National Laboratories, New Mexico

Site Name:

Sandia National Laboratories


New Mexico

Period of

May 15 to November 24, 1996


Field demonstration


Dr. Eric R Lindgren
Sandia National Laboratories
P.O. Box 5800
Albuquerque, NM 87185-0719
Telephone: (505) 844-3820

- SNL's patented electrode - constructed of a porous, ceramic outer casing and an inner, iridium-coated titanium electrode; extracts contaminants by moving them into water held under tension (a partial vacuum) inside the outer casing
- Three rows of electrodes in 144 ft2 area; center row - five anodes; outer two rows - each five cathodes
- Voltage applied between electrodes - 1,572 kW hrs total; current applied to each electrode <15 amps
- Additional components included a liquid control system, a vacuum control system, a power application system, and a monitoring system

Cleanup Authority:
Not identified

Site Contact:
Not identified
Regulatory Authority:
Not Identified

Heavy metals (chromium)
- Total chromium concentrations were measured in soil as high as 200 mg/kg, up to 17 ft bgs
- TCLP chromium concentrations were measured in soil as high as 28 mg/L

Waste Source:
Waste pit

Type/Quantity of Media Treated:
- Near surface geology consists of alluvial fan deposits with some eolian deposits
- Sediments consist of intercalated fine-to-coarse grained, well-sorted to poorly-sorted sands, gravels, and cobbles
- Water table located 485 ft bgs
- Soil moisture content about 10 weight percent; conductivity is <10 mS/m

Purpose/Significance of Application:
The first field demonstration of electrokinetics for removal of contaminant ions from arid soil

Regulatory Requirements/Cleanup Goals:
- Demonstrate extraction of chromate from unsaturated soils without addition of significant amounts of water

- 13 tests were performed in the demonstration (12 operating conditions; 1 system performance testing)
- A total of approximately 600 grams of hexavalent chromium were removed from the soil after 2700 hours of operation (0.22 g/hr)
- At the system's preferred operating conditions, approximately 200 grams of hexavalent chromium were removed during 700 hours of operation (0.29 g per hour)
- After treatment, soil samples adjacent to the cathodes had total chromium concentrations of 72 ppm and TCLP concentrations less than 5 mg/L
- Addition of significant amounts of water not required

Cost Factors:
Not provided

Sandia National Laboratories is located southeast of Albuquerque, New Mexico, within the boundaries of Kirtland Air Force Base. The Unlined Chromic Acid Pit is located in the Chemical Waste Landfill at SNL, which is located in Technical Area III. The chromium disposed of in the Unlined Chromic Acid Pit was in the form of chromic sulfuric acids. A chromium plume resides in the vadose (unsaturated) zone beneath the pit, with the most contaminated horizon beneath the pit containing concentrations of chromium higher than 200 mg/kg.

A field demonstration of in situ electrokinetic extraction technology was conducted at the Unlined Chromic Acid Pit to show that chromate could be extracted from unsaturated soils on a field scale without the addition of significant amounts of water. The field demonstration targeted the floor of the former pit at a horizon 8 to 14 feet below the surface, with three rows of electrodes placed in a 12-foot by 12-foot area. Test results met the goal, with the soil samples adjacent to the cathodes showing total chromium concentrations of 72 ppm and TCLP concentrations less than 5 mg/L. In addition, the electrokinetic process was found to be stable over long periods of time. While SNL's electrokinetic extraction system was successful in removing chromium from unsaturated sandy soil, SNL noted that the electrode system was a research prototype and was not specifically engineered for commercialization. After the 1996 field demonstration, SNL began developing a passive system, where the system is operated at a lower power, thereby avoiding the expense of actively cooling the electrokinetic electrode system. The new system uses a solid matrix capture system, eliminating the need for the liquid control and vacuum systems.