Nic Korte and Liyuan Liang, both group leaders in the Earth and Engineering Sciences Section of ORNL's Environmental Sciences Division, and their associates have studied the effectiveness of iron filings in removing chlorine from organic solvents in contaminated groundwater. They evaluated whether iron filings could be incorporated in a porous wall placed in front of an advancing contaminant plume in groundwater to treat water at two DOE sites. The wall technology was initially developed and patented by a group at the University of Waterloo in Canada.
Groundwater at DOE's uranium enrichment facilities is contaminated with trichloroethylene (TCE), a chlorine-containing organic solvent once commonly used to remove dirt and grease from metals. Because the toxic material is present in concentrations exceeding Environmental Protection Agency (EPA) limits, DOE's Office of Environmental Management is interested in the possibility of using iron wall technology for in situ treatment of groundwater. Korte and Liang were asked to evaluate the effectiveness of this technology for use at the Paducah, Ky., and Portsmouth, Ohio, gaseous diffusion plants.
In the ORNL studies, it was found that dechlorination of TCE occurs on the surface of the iron filings. Also, the researchers observed that the byproducts of the reaction include partially dechlorinated compounds, such as dichloroethene and vinyl chloride, and completely dechlorinated hydrocarbons, such as ethene and ethane, which are not of great concern at trace levels.
"We found that in a batch reactor, about 20 percent of the byproducts contain chlorine, indicating that complete dechlorination had not occurred with elemental iron alone," Liang says. "One of these byproducts-about 8 percent of the total-is vinyl chloride, which must be kept at trace levels-1 part per billion or below-to meet EPA or state regulations."
To achieve complete rather than partial dechlorination, the ORNL and University of Arizona researchers jointly designed and tested an iron-based material containing a trace amount of palladium. The presence of the palladium, a catalyst, increases the rate of dechlorination 10 to 100 times and minimizes the production of undesirable vinyl chloride (reducing it from 8 percent to less than 1 percent of total byproducts).
"Because of the effectiveness of this bimetallic system," Liang says, "we can achieve total dechlorination in less time."
Liang and Korte say that palladized iron also shows promise for separating organic solvents from radionuclides in mixed waste and in degrading dichloromethane, a hazardous product of the breakdown of carbon tetrachloride. In addition, iron is more efficient than a conventionally used synthetic resin for removing radioactive technetium from water; technetium is a waste product of some nuclear processes.
To determine the exact proportions of byproducts formed in the reaction between TCE and iron, the researchers used zero-headspace extractors and a purge-and-trap concentrator with a gas chromatograph (GC) in the laboratory. The extractor is a closed cylinder that contains the sample solution but excludes air space (head space), preventing the organic compounds from volatilizing from the water. Lack of control in keeping organic compounds from escaping from water could cause inaccuracies in the measurements of the amounts and proportions of TCE byproducts in the treated water.
A water sample for GC analysis is obtained from the top of the apparatus by forcing the internal piston up, using pressurized gas introduced at the bottom. "Because the batch reactor is a closed system," Liang says, "all byproducts and their concentrations in water during treatment can be precisely measured and a good mass balance can be achieved."
The researchers also studied the rates of the dechlorination reactions using fine iron filings and iron filings coated with palladium. "One flowthrough column study," Liang said, "showed that removal of half of the TCE in a pure iron system occurred in about 10 minutes. With the addition of the palladium in the amount of about 0.05 percent of the iron, the removal time is reduced to seconds."
The detailed pathways and mechanisms for the dechlorination reactions involving the metals are still unknown. In the dechlorination reaction, iron atoms donate electrons, breaking the bonds between the chlorine atoms and the carbon atoms in the organic compounds. Most chlorine is released as a harmless chloride ion or combines with ferrous iron to precipitate out as green rust.
The research was sponsored by DOE's Office of Environmental Management. Korte and Liang's collaborators in the research include John D. Goodlaxson and Abinash Agrawal, both of ORNL's Environmental Sciences Division; Rosy Muftikian, Carina Grittini, and Quintus Fernando, all of the University of Arizona in Tucson; and Jay Clausen of DOE's Paducah Gaseous Diffusion Plant.
ORNL, one of the Department of Energy's multiprogram research laboratories, is managed by Lockheed Martin Energy Research Corp.