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Electrokinetic Remediation - CASE STUDY 1: U.S Department of Energy Paducah Gaseous Diffusion Plant, Paducah, Kentucky

  

CASE STUDY 1: U.S Department of Energy Paducah Gaseous Diffusion Plant, Paducah, Kentucky 

 

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Figure 4: Paducah, KY (U.S Department of Energy) 

 

Site History

The Paducah plant is located 3 miles south of Ohio River and 12 miles west of Paducah, KY. The plant was involved with the first nuclear weapons program in the United States. In 1952 they began enriching of Uranium for commercial power plants in the nation. The plant ran continuously and remained the only gaseous diffusion uranium enrichment plant in the U.S (DeGarady and Halbrook., 2003; Sharma and Reddy., 2004). The plant was owned by the U.S Department of energy (DOE) and managed mostly by the Kentucky Department of Fish and Wildlife. Their total area encompassed 3,556 acres of reservation and about 750 acres were used for the plant facilities. DOE leased the Paducah processing facilities for Uranium Enrichment to the United States Enrichment Corporation. The plant supported more than 50 years of uranium enrichment leading to a huge amount of hazardous, radioactive and non-chemical sanitary waste in the site. The soil, ground and surface water got contaminated.

 

Site Missions at Present

DOE has cleanup missions including environmental cleanup, waste disposition, decontamination and decommissioning (D&D) of the site, conversion of depleted uranium and long-term monitoring. DOE leases the plant to USEC which has been privatized and now the Nuclear Regulatory Commission regulates the site.

Electrokinetic remediation along with other in-situ methods is employed at this site. The soil at this site has very low permeability and the contaminants involved were TCE (Trichloroethylene). The field test work started in January 1995 and ended only in May 1995. The field experiment served as a pilot study to see the electrokinetic effects.

 

Remediation

The site consisted of 4 feet gravel and clay layers over 40 feet of sandy clay loam with interbedded sand layers. The hydraulic conductivity of the sandy clay was about 10-7 cm/s. The test area consisted of 150 sq ft area for a depth of 15 ft. Average contaminant concentration was 83.2 ppm. In this site, steel panels were used as electrodes and granular activated carbon was used to capture the contaminants as they moved towards the electrodes. The area was split into 4 treatment zones with 2 electrodes 10 ft apart. The treatment zones consisted of granular activated carbon in a staggered arrangement as shown in Figure 4. The electrodes had eight steel panels installed next to each other with a gap of half-inch. Each panel was 16 ft long and 18 inches wide. Fluid was circulated with tubes and pumps. A collection system was designed above the soil to collect vapors containing TCE. The test area was subjected to 40A and an initial power of 138 V for the first month. Then the voltage was reduced to 105 V. The average elecroosmotic flow was 4 to 5 L/hour. The temperature of the soil started increasing from 15oC to 30oC due to the applied electric field. The voltage gradient varied between 0.35 - 0.45 V/cm. the removal efficiency was promising since it was around 98%. The level of contamination fell to 1.2 ppm. The TCE concentration in the carbon treatment zones were high and were replaced. The cost for complete remediation in this test was $1310 per cubic yard. The electroosmosis flushes TCE from clay since this clay did not absorb much TCE and allowed the contaminant to move with the flow (Sharma & Reddy., 2004).

 

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Figure 4: Schematic Representation of the Electrodes and the Carbon treatment zones (Sharma and Reddy., 2004)

 

Following the success of the small field test, a large scale remediation was carried out from August 1996 to March 1997. The soil type was the same as the small test field site. The test area had a larger size of 620 sq. ft. and a depth of 45 feet. The groundwater table started at 30 to 40 ft below ground surface. The electrodes were 1.5 in thick consisting of equal volume of iron filings and Loresco coke and were inserted to a depth of 45 feet. The electrodes were 21 ft apart. The iron filings help in promoting iron corrosion thereby reducing the hydrogen ion formation which occurs due to electrolysis.  The area was split into 3 treatment zones and was a mix of Kaolin clay and iron filings. This setup was run with a constant voltage of 150 V. As high concentrations of TCE started showing up, the voltage was increased to a 200 V but this led to heating up of the soils and the voltage had to be brought down to 120 V. 

 

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Figure 5: Contaminant and Pore Water Transport in Remediation process (Terran Corporation; Sharma & Reddy., 2004)

 

Results

The voltage gradient was 0.25 V/cm and it moved the water at a speed of 0.43 cm/day. The overall efficiency of the technique was 99.7%. TCE which existed in free phase in the site was believed to be removed in the same phase. The emissions were insignificant and the complete cost for this remediation was about $80 per cubic yard for a quarter of an acre at a depth of 45 feet.

 

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