206 A ■ ENVIRONMENTAL SCIENCE & TECHNOLOGY / JUNE 1, 2003

Environmental▼News

Researchers at the University ofMissouri–Rolla have shown thattrichloroethene (TCE) volatilizesfrom plant stems into the atmos-phere (Environ. Sci. Technol. 2003,37, 2534–2539), identifying for thefirst time diffusion as an importantmechanism in phytoremediation toremove this and other volatile or-ganic compounds (VOCs) fromcontaminated soil and ground-water.

TCE is an organic solvent mainlyused in dry cleaning and metal-de-greasing operations. It is one of themost prevalent groundwater conta-minants and degrades to more toxiccompounds under anaerobic con-ditions. However, in air, TCE ishighly reactive and is quickly de-stroyed by photo-oxidation, leadingto complete mineralization withinone to two weeks. Thus, trees canremove TCE from groundwater and“cut down the persistence [of thecontaminant] by several orders ofmagnitude,” says Joel Burken, asso-ciate professor of civil engineeringand coauthor of the study.

The relative importance of VOCs’possible fates following uptake by

the plants is under debate. In anearlier study, it was reported thatmineralization of TCE in soil, pri-marily by bacteria residing in therhizosphere around plant roots, wasthe key mechanism, and metabolicdegradation or accumulation intree tissue appeared to be minorprocesses (Environ. Sci. Technol.1999, 33, 2257–2265). Moreover, be-cause only a little TCE wasmeasured diffusing out of leaves,volatilization was believed to be notvery important.

The new study by graduate re-search assistant Xingmao Ma andBurken challenges these ideas. “Themost significant finding is that TCEdiffusion was shown to be a prima-ry loss mechanism in phytoremedi-ation applications,” says DonVroblesky with the U.S. GeologicalSurvey in Columbia, S.C. Vrobleskyhad earlier hypothesized that diffu-sion could be an important mecha-nism after analyzing theconcentrations of chlorinatedethenes in tree cores in the forestedfloodplain at a Savannah River siteand finding that the TCE concen-trations were declining by 30–70%with trunk height (Environ. Sci.Technol. 1999, 33, 510–515).

Ma and Burken’s results nowconfirm this hypothesis. In labora-tory experiments, they placed “dif-fusion traps”, sealed glass tubes, atdifferent heights around the stemsof hybrid poplar whip cuts. Twosyringe needles inserted into the

tubes collected gaseous dischargesfrom the stems. Consistently, moreTCE was found in the lower diffu-sion traps, indicating that it is lostwhile climbing up the tree. Earlierreports failed to find TCE diffusingout of leaves, essentially “becausethe compound is gone before itreaches the canopy,” says Burken.He also emphasizes that this lowersthe exposure risk for animals feed-ing on the leaves, because TCE doesnot accumulate there.

Ma and Burken also measuredthe distribution of TCE in poplartrees at the U.S. Army’s AberdeenProving Grounds field site inMaryland. The ongoing phytoreme-diation project was set up in 1996.TCE concentrations were found todecrease not only along the tree’sheight, but also when moving out-ward from the core to the bark. Thisradial concentration gradient isconsistent with diffusion being animportant mechanism in the fieldand not just in the laboratory.

As a result, trees can “act as apump to pull the contaminatedwater out of the soil” and then re-lease the pollutant into the atmos-phere, says Burken. The length ofthe roots, which can be up to 10meters, determines the depth towhich contaminated groundwatercan be remediated. Contaminatedwater in deeper aquifers could bepumped up to the roots. However,he adds, “hydrogeology defines therate-limiting step; the rate of TCEremoval depends on how muchwater is reached and transpired bythe trees.” —ORI SCHIPPER

Phytoremediation releases TCE to the atmosphere

“Cracking” the structure of petroleumIndependent research groups at theNational High Field Magnet Lab-oratory (NHFML) at Florida StateUniversity and Schlumberger-DollResearch (SDR), a global researchand technology company, havefound that the actual molecularweights of petroleum constituentsare up to 10 times lower than thosereported a decade ago. The groupsdemonstrated that earlier studieswere flawed because they relied onmethods that require highly con-

centrated samples, which lead tocomponents aggregating into largerstructures. The new studies reliedon high-resolution techniques thatanalyzed these components in di-lute samples. Knowing these detailsabout petroleum’s complex compo-sition could help limit its environ-mental harm.

Establishing the true chemicalcomposition of crude oil is essen-tial to predicting its properties andbehavior and could minimize some