Environmental▼News Phytoremediation releases TCE to the atmosphere Researchers at the University of Missouri–Rolla have shown that trichloroethene (TCE) volatilizes from plant stems into the atmosphere (Environ. Sci. Technol. 2003, 37, 2534–2539), identifying for the first time diffusion as an important mechanism in phytoremediation to remove this and other volatile organic compounds (VOCs) from contaminated soil and groundwater.
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TCE is an organic solvent mainly used in dry cleaning and metal-degreasing operations. It is one of the most prevalent groundwater contaminants and degrades to more toxic compounds under anaerobic conditions. However, in air, TCE is highly reactive and is quickly destroyed by photo-oxidation, leading to complete mineralization within one to two weeks. Thus, trees can remove TCE from groundwater and “cut down the persistence [of the contaminant] by several orders of magnitude,” says Joel Burken, associate professor of civil engineering and coauthor of the study. The relative importance of VOCs’ possible fates following uptake by
the plants is under debate. In an earlier study, it was reported that mineralization of TCE in soil, primarily by bacteria residing in the rhizosphere around plant roots, was the key mechanism, and metabolic degradation or accumulation in tree tissue appeared to be minor processes (Environ. Sci. Technol. 1999, 33, 2257–2265). Moreover, because only a little TCE was measured diffusing out of leaves, volatilization was believed to be not very important. The new study by graduate research assistant Xingmao Ma and Burken challenges these ideas. “The most significant finding is that TCE diffusion was shown to be a primary loss mechanism in phytoremediation applications,” says Don Vroblesky with the U.S. Geological Survey in Columbia, S.C. Vroblesky had earlier hypothesized that diffusion could be an important mechanism after analyzing the concentrations of chlorinated ethenes in tree cores in the forested floodplain at a Savannah River site and finding that the TCE concentrations were declining by 30–70% with trunk height (Environ. Sci. Technol. 1999, 33, 510–515). Ma and Burken’s results now confirm this hypothesis. In laboratory experiments, they placed “diffusion traps”, sealed glass tubes, at different heights around the stems of hybrid poplar whip cuts. Two syringe needles inserted into the
tubes collected gaseous discharges from the stems. Consistently, more TCE was found in the lower diffusion traps, indicating that it is lost while climbing up the tree. Earlier reports failed to find TCE diffusing out of leaves, essentially “because the compound is gone before it reaches the canopy,” says Burken. He also emphasizes that this lowers the exposure risk for animals feeding on the leaves, because TCE does not accumulate there. Ma and Burken also measured the distribution of TCE in poplar trees at the U.S. Army’s Aberdeen Proving Grounds field site in Maryland. The ongoing phytoremediation project was set up in 1996. TCE concentrations were found to decrease not only along the tree’s height, but also when moving outward from the core to the bark. This radial concentration gradient is consistent with diffusion being an important mechanism in the field and not just in the laboratory. As a result, trees can “act as a pump to pull the contaminated water out of the soil” and then release the pollutant into the atmosphere, says Burken. The length of the roots, which can be up to 10 meters, determines the depth to which contaminated groundwater can be remediated. Contaminated water in deeper aquifers could be pumped up to the roots. However, he adds, “hydrogeology defines the rate-limiting step; the rate of TCE removal depends on how much water is reached and transpired by the trees.” —ORI SCHIPPER
“Cracking” the structure of petroleum Independent research groups at the National High Field Magnet Laboratory (NHFML) at Florida State University and Schlumberger-Doll Research (SDR), a global research and technology company, have found that the actual molecular weights of petroleum constituents are up to 10 times lower than those reported a decade ago. The groups demonstrated that earlier studies were flawed because they relied on methods that require highly con-
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centrated samples, which lead to components aggregating into larger structures. The new studies relied on high-resolution techniques that analyzed these components in dilute samples. Knowing these details about petroleum’s complex composition could help limit its environmental harm. Establishing the true chemical composition of crude oil is essential to predicting its properties and behavior and could minimize some
