In a finding that could just as likely lead to a new, cheap method of chemical waste cleanup as it could predict disaster at the nation's worst nuclear waste site, researchers at Northwestern University and the University of Notre Dame have shown that common soil minerals can work with ionizing radiation to break down toxic wastes into simpler compounds. The finding, however, also serves as a warning that breakdown products in enclosed storage tanks may build up pressure and explode.
In laboratory experiments, the researchers showed that alumina, an oxide of aluminum that is found in many soils, can greatly accelerate chemical reactions in which gamma rays break down toxic chlorinated chemicals. Silica and other oxides -- clay -- in soil may behave similarly. Gamma rays are high energy X-rays given off by many of the highly radioactive wastes produced in weapons manufacture, such as cobalt-60.
The good news is that gamma irradiation thus may be an effective means of degrading some highly toxic pollutants, such as dioxin or PCBs, in contaminated soil. The bad news is that 177 huge underground tanks on the Hanford Nuclear Reservation in eastern Washington, which hold 54 million gallons of high-level radioactive and chemical waste, may face an increasing risk of rupture or explosion as volatile gases, including hydrogen and perhaps methane, are generated as the chemicals are broken down by minerals in the tanks.
"They're big cauldrons of radioactive soup," says Kimberly A. Gray, associate professor of civil engineering at Northwestern's Robert R. McCormick School of Engineering and Applied Science. Gray conducted the new study with physical chemist Prashant V. Kamat of Notre Dame's Radiation Laboratory and Northwestern graduate student George A. Zacheis. The results are reported in the April 8 issue of the Journal of Physical Chemistry.
The Hanford tanks, Gray said, contain radioactive metals and nonradioactive metal oxides and organic chemicals that were byproducts of nuclear weapons production. Metal oxides are driving reactions in which the radiation breaks down the chemicals, she said.
"By storing radioactive liquid with solid material, they are degrading components of the mixture and producing gases," Gray said. The rate of gas production can not currently be predicted, she said, so engineers and chemists want to learn how they are generated.
Last year, Gray and her colleagues were looking for ways to use radiation to cleanse excavated soil when they observed that some soils were more easily cleaned than others. "We showed that this is a robust technology that seems to work on a wide variety of soils," Gray said, "but we realized that when the soils were high in minerals, the process worked really, really well."
In the new study, the researchers showed that the chemical hexachlorobenzene, or HCB, can be easily stripped of its chlorine atoms and degraded by gamma rays when the HCB is coating alumina particles. The same reaction does not occur in the presence of potassium bromide, a simple salt-like compound not found in soil. HCB, which is a common pollutant in its own right, also serves as a model compound for its less prevalent but more toxic chemical cousins, the PCBs and dioxins.
When mixed, HCB and similar chemicals tend to coat particles of minerals or salts. The study shows that when the particle is an oxide, like alumina, gamma rays interact not with the chemical but with the oxide, activating it so that it can catalyze the reaction that breaks down the chemical.
Gray says the finding suggests radiation-induced breakdown, or radiolysis, may be useful for detoxification in both environmental and industrial settings. It has never been employed for either.
In the environment, radiation can penetrate soil and act at a distance, making it unnecessary to wash pollutants off the soil for treatment. In industry, adding minerals to the chemical waste-stream and zapping the mixture with gamma rays may be an effective way to detoxify the wastes or even generate useful feedstock chemicals that could be recovered.
"This research helps us understand the risks associated with stored radioactive wastes in places like Hanford," Gray said. "I think this research also helps us develop treatment technologies for soil contamination. And I think it shows the potential for us to develop new kinds of catalysts that we could adapt for either selectively breaking bonds or making new chemicals in treatment for waste-stream reduction."
The 560-square-mile Hanford Reservation is where the government produced plutonium from World War II through the end of the Cold War. Of the 177 tanks on the site, 70 have already leaked about one million gallons of waste into the soil and groundwater, threatening the Columbia River 12 miles away. Hanford, whose only activity now is storage and cleanup, is administered by the U.S. Department of Energy.
The research reported in the Journal of Physical Chemistry was funded by the National Science Foundation, the Department of Energy and the Occidental Petroleum Corporation.
Materials provided by Northwestern University. Note: Content may be edited for style and length.
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