TVA
NE W S OF NEWS OF T THE H E W EEK EEK
ENVIRONMENT: Arsenic lingers downstream of 2008 accident site in Tennessee N DEC. 22, 2008, 978 million gal of wet coal ash spilled into the Emory River and its tribu taries near Kingston, Tenn. The spill polluted downstream sediments with a particularly toxic form of arsenic, according to a new report from Duke Uni versity (Environ. Sci. Technol., DOI: 10.1021/es1026739). The spill occurred when a holding pond ruptured, releasing its waste from the Tennessee Valley Author ity’s Kingston Fossil Plant, a coal-fired power plant. Ash produced by burning coal isn’t regulated as hazard ous waste by EPA because the agency’s testing protocol assumes that coal ash contaminants do not seep from municipal landfills into nearby water. The TVA spill provided a useful—if tragic—opportunity to test this assumption, according to Duke geochemist Avner Vengosh. He and his colleagues measured concentrations of five coal ash contaminants from more than 220 surfacewater and sediment samples they collected during
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NEW RUBBER BEATS
HEAT AND COLD
MATERIALS: Nanotube rubber
maintains its viscoelasticity in extreme temperatures
ARBON NANOTUBES are taking rubbery be havior to new extremes. A novel rubberlike ma terial made from long, tangled strands of single-, double-, and triple-walled carbon nanotubes (CNTs) maintains its viscoelasticity at temperatures as low as –196 °C and as high as 1,000 °C in an oxygen-free envi ronment (Science, DOI: 10.1126/science.1194865). Most rubbery materials, in contrast, turn brittle in the cold and degrade when things heat up. Because of its temperature-invariant viscoelasticity, the CNT-based material could find use in vehicles that travel to the cold reaches of interstellar space. It could also be used inside high-vacuum furnaces, where it could take the heat without running the risk of reacting with oxygen. A team led by Don N. Futaba, Kenji Hata, and Ming Xu of the Nanotube Research Center at Japan’s Na tional Institute of Advanced Industrial Science & Tech nology (AIST) created the CNT-based material using a
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an 18-month period of TVA’s site cleanup. The scientists found that anaerobic bacteria in the sediments produce conditions that reduce arsenic from the common pentava lent form to the more toxic trivalent form. EPA’s testing protocol does not predict these findings because it does not consider redox chemistry, according to Heileen (Helen) HsuKim, a coauthor of the report. The team also examined the ef fects of TVA’s cleanup methods, which some experts feared would increase surfacewater contamination. The researchers saw no evidence of such a spike. But buried ash still contaminates the river: Arsenic levels in sediment samples reach concen trations greater than 2,000 ppb. In comparison, EPA’s maximum contaminant level for arsenic in drinking water is 10 ppb. The Duke team’s data are sure to influence EPA’s decision whether to designate coal ash as hazardous waste, says James C. Hower, a senior scientist at the University of Kentucky’s Center for Applied Energy Research. But designating coal ash as hazardous waste might hamper use of coal ash as filler in concrete pro duction. “There’s no doubt these findings are damaging to the coal industry,” Hower says.—CHARLES SCHMIDT, special to C&EN
combination of water-assisted chemical vapor deposi tion, reactive ion etching of the catalyst film used to grow the nanotubes, and compression. When the researchers characterized the material, they observed that the tubes are tangled in such a way that they make numerous short contacts with one an other. The scientists believe that the material’s thermal stability arises from the fact that the tubes can zip and unzip at those contact points. In polymeric rubbers, viscoelas ticity is typically governed by the arrangement of polymer chains. High temperature breaks these ar rangements, and the materials de grade. The researchers believe that in the CNT-based material, the energy from heat goes into over coming the large van der Waals at traction between the CNTs, result ing in an unzipping of the contact points. Virtually no energy, however, is required for zipping, so this process acts like a heat pump. Yury Gogotsi, an engineering professor at Drexel University, calls the results “exciting.” Although carbon-based materials possess many extreme properties, he says, “this work uncovers another example of ex treme performance of a carbon material that no other solid has shown so far.”—BETHANY HALFORD
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The Emory River near Kingston, Tenn., after a coal ash spill in 2008.
SCIENCE
COAL ASH SPILL
STILL A PROBLEM
A length of the new carbon-nanotube based rubber.
