Experiments have shown that ultrasound could be an effective means of enhancing the removal of chlorinated hydrocarbon contaminants from groundwater by the zero-valent-metal treatment process. This process, which has been a subject of research in recent years, is attractive because it does not involve above-ground treatment or the use of pumps, and because the materials needed to effect treatment are safe and relatively inexpensive.
The process involves the use of a permeable wall that contains a metal (iron or zinc) in zero-valent condition and in porous form (e.g., dust or filings) and is buried in the ground. The position and orientation of the wall are chosen to take advantage of the natural groundwater gradient to carry the groundwater through the wall. As the contaminated groundwater flows through the wall, chlorine is removed from chlorinated hydrocarbons and from breakdown byproducts. The dechlorination reaction mechanism, which is not known precisely, appears to involve the simultaneous oxidation of metal and dechlorination of chlorinated hydrocarbons. For example, the removal of trichloroethane by treatment with iron appears to include the reactions:
Fe0 --> Fe2+ + 2e- and C2HCl3 + 3H+ + 6e- --> C2H4 + 3Cl-.
Thus, the destruction of one mole of trichloroethane yields three moles of chloride and results in an increase in pH because of the consumption of protons. In addition, corrosion of iron is inherently part of the electron-transfer process that drives the dechlorination. The rate of dechlorination by a treatment wall decreases with time, apparently because the precipitation of corrosion products reduces the iron surface area accessible for dechlorination. In some cases, the rate of dechlorination could be reduced further because accumulation of corrosion products in pores could reduce the permeability of the wall.
The foregoing observations lead to the concept of using ultrasound to enhance treatment. It has been postulated that ultrasound can be used to remove corrosion products from iron surfaces and therefore can be used to restore and maintain rates of dechlorination. Results of batch and column-flow experiments confirm that exposure to ultrasound increases dechlorination rates significantly (see figure). Further experimental studies would be needed to develop a full-scale, practical ultrasonic system to enhance the function of a full-scale treatment wall.
This work was done by Jacqueline Quinn of Kennedy Space Center and Nancy E. Ruiz, Debra R. Reinhart, Cherie L. Geiger, and Christian A. Clausen III of the University of Central Florida. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Materials category.
This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to
the Technology Programs and Commercialization Office,
Kennedy Space Center,
Refer to KSC-11959.