Letters▼

342 A ■ ENVIRONMENTAL SCIENCE & TECHNOLOGY / OCTOBER 1, 2003 © 2003 American Chemical Society

Letters▼

Results misinterpreted

In Ori Schipper’s news story, entitled“Phytoremediation releases TCE tothe atmosphere” (1), he describes therecent publication by Ma and Burken(2) in which diffusion from the trunkwas found to be a significant lossroute in the phytoremediation oftrichloroethylene (TCE). He alsomentioned our work (3).

Schipper misinterpreted our re-sults in the 1999 paper, saying that wereported that mineralization of TCEin the soil was primarily by bacteriaresiding in the rhizosphere aroundplant roots and that metabolic degra-dation in tree tissue appeared to be aminor process. In fact, we measuredthe microbial degradation of TCE insoil samples from the root zones andwere unable to show any increase inmicrobial degradation of TCE inplanted soils compared to controlsoils in which little TCE was lost. Thespecific rates of TCE degradation inthese soils were much lower than therates of TCE removal in the phytore-mediation field tests.

While root zone microbial activi-ties appear to be important in thedegradation of some pollutants, suchas polycyclic aromatic hydrocarbons,there has been no evidence for en-hanced microbial degradation of chlo-rinated hydrocarbons in aerobic rootzone soils, other than the reports oflimited mineralization of TCE fromWalton and Anderson (4, 5). Sincethen, other studies have not support-ed the hypothesis that aerobic micro-bial degradation of TCE is enhancedin plant root zones. For example, TCE-degrading methanotrophic bacteriawere not enhanced in the root zone ofalfalfa exposed to TCE (6). If the rootzones of trees are anaerobic, the pro-duction of organic matter by rootsmay promote the reductive dechlori-nation of TCE to cis-dichloroethylene,vinyl chloride, and perhaps ethylene(7). However, this process is very lim-

ited under aerobic conditions, such asthose in our experiments and in mostphytoremediation sites.

In our 1999 experiments, we re-covered a significant portion of theTCE chlorine as chloride ion in thesoil, although the methods used toobtain our mass balance admitted asizable error. Although an excess ofchloride ion was recovered in the soilrather than in tree tissues, this shouldnot be construed as demonstratingmicrobial soil activity as the chloridesource. Chloride ion is an essentialnutrient for plants and an importantco-ion. Channels for transport ofchloride ion into and out of plantcells and vacuoles are well known(8), and chloride effluxes from plantshave been observed (9). Thus, metab-olism of TCE in the roots of poplar, aswe have reported in axenic laboratoryexperiments (10), could have producedthe excess chloride observed in the1999 field experiments.

Ma and Burken’s observation thatTCE is lost from the trunks of poplaris important and not in disagreementwith our observations. They note thatthey did not attempt mass balances,so the percentage of TCE that was lostfrom the stems cannot be comparedto the total loss of TCE. However,their results suggest that plant metab-olism of TCE is confined to the roots.We caution that the relative impor-tance of the various loss mechanismsfor phytoremediation of TCE cannotbe evaluated until stem loss measure-ments are included in mass balanceexperiments.

STUART E. STRANDCollege of Forest Resources and

Department of Civil andEnvironmental Engineering

University of WashingtonSeattle, Wash.

XIAOPING WANGHancock Biological Station

Murray State UniversityMurray, Ky.

MILTON P. GORDONDepartment of Biochemistry

University of WashingtonSeattle, Wash.

References(1) Schipper, O. Environ. Sci. Technol. 2003,

37, 206 A.(2) Ma, X.; Burken J. G. Environ. Sci. Technol.

2003, 37, 2534–2539.(3) Newman, L.; et al. Environ. Sci. Technol.

1999, 33, 2257–2265.(4) Walton, B. T.; Anderson, T. A. Appl.

Environ. Microbiol. 1990, 56, 1012–1016.(5) Anderson, T. A.; Walton, B. T. Environ.

Toxicol. Chem. 1995, 14, 2041–2047.(6) Brigmon, R. L.; et al. Int. J. Phytoremediat.

1999, 1, 241–253.(7) Godsy, E. M.; Warren, E.; Paganelli, V. V.

Int. J. Phytoremediat. 2003, 5, 73–87.(8) Barbier-Brygoo, H.; et al. Biochim. Biophys.

Acta-Biomembr. 2000, 1465, 199–218.(9) Beffagna, N.; et al. Plant Cell Physiol.

1997, 38, 503–510.(10) Newman, L. A.; et al. Environ. Sci.

Technol. 1997, 31, 1062–1067.

Response to “Resultsmisinterpreted”Newman et al. have shown in theirpaper that chloride accumulated inTCE-treated, poplar-planted soilwithout providing conclusive evi-dence, however, of whether TCE wasdegraded by the trees or by soil mi-croorganisms. In either case, TCEdegradation in the soil was not thefocus of my report.

I cited Strand et al.’s work in mynews article because the mass bal-ance analysis suggested that evapora-tion of TCE out of leaves accountedfor less than 6% (1.15 mole out of19.2 mole total recovered chlorine) ofmeasured TCE loss. I agree that Burkenand Ma’s new findings do not contra-dict, but complement, the study under-taken by Strand et al., which reported70% recovery of chlorine. In my eyes,the assumption that TCE volatilizationout of the tree stems made up themissing 30% in Newman et al.’s massbalance is very intriguing.

ORI SCHIPPER