ecause of its ability to remove grease and dirt, trichloroethylene (TCE) was once used by nearly every metal machining shop, automotive repair shop, and dry cleaner in the United States. Since 1976 environmental regulations have greatly raised the costs of TCE disposal, so its use has been discontinued. However, because some of the TCE used widely in the past was released to the environment, it is now one of the most commonly found contaminants in groundwater. Among its resting places are most Department of Energy sites.
In hopes of cleaning up groundwater and process streams contaminated with TCE and other volatile organic compounds, Oak Ridge K-25 Site scientists are testing the use of special inorganic membranes illuminated by near-ultraviolet (near-uv) light. In this scheme, the contaminated water is circulated through membranes in a cleanup module. The membranes both filter the water and destroy TCE molecules through a photocatalytic interaction on the surface. Specifically, when photons of near-uv light interact with a titanium oxide (TiO2) photocatalyst on the wet membrane surface, active sites are created that break down TCE into benign molecules.
The membranes were based on technology developed for separating uranium isotopes. The K-25 Site has been a center of inorganic membrane research for many years. The technology can be adapted to fabricate new classes of membranes that are of no value for uranium enrichment but have enormous commercial value for a variety of uses.
"A key advantage of the photocatalytic membrane," says John Stockdale, a developer, "is that the contaminant molecules are brought into close contact with the active surface as the stream flows through the membrane. Other photocatalytic remediation schemes have attempted to overcome this 'contact' problem but are less successful. One method disperses the photocatalyst into the process stream, but the photocatalyst must be added and then filtered out. Another method coats the photocatalyst onto surfaces surrounding the process stream, but ensuring good contact between contaminants and the active surface is difficult."
The researchers have proposed building a cleanup module and lowering it into a contaminated underground aquifer through a vertical borehole at a DOE site. A mercury arc lamp at the surface will supply near-uv light to an optical fiber. The fiber will carry the light down to the cylindrical membrane where it would emit a cone of near-uv light into the interior of the membrane, illuminating it fairly evenly along a 25-centimeter (10-inch) length.
"A practical groundwater remediation system will contain a bundle of membrane tubes," Stockdale says. "Each tube will be illuminated by a fiber split out from a fiber bundle coming down from the surface. Contaminated water will be pumped into the membrane by a small submersible pump, and clean water will be returned to the aquifer."
Such an in situ remediation system will be designed for long-term unattended operation. It could be used to purify industrial process and waste streams to protect the environment.
The K-25 Site researchers have been testing membranes under various flow and illumination conditions. Their goal is to improve the photocatalytic process so that visible light will be effective.
"If visible light could be used," Stockdale says, "it might be possible to develop a solar-powered device. Sunlight would be used by solar cells to power the water pump and by the membranes to break apart pollutants. Currently, only 1% of sunlight falls within a wavelength band suitable for photocatalysis. However, a field of implanted solar-powered devices might enable the use of natural sunlight to decontaminate groundwater."
This work is partly supported by DOE's Environmental Restoration and Waste Management Program. Developers of the system are Stockdale, Douglas Fain, and Brian Bischoff, all of the Membrane Technology Department of the Technical Division, K-25 Site, which is managed by Lockheed Martin Energy Systems .
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