Letters▼ Results misinterpreted In Ori Schipper’s news story, entitled “Phytoremediation releases TCE to the atmosphere” (1), he describes the recent publication by Ma and Burken (2) in which diffusion from the trunk was found to be a significant loss route in the phytoremediation of trichloroethylene (TCE). He also mentioned our work (3). Schipper misinterpreted our results in the 1999 paper, saying that we reported that mineralization of TCE in the soil was primarily by bacteria residing in the rhizosphere around plant roots and that metabolic degradation in tree tissue appeared to be a minor process. In fact, we measured the microbial degradation of TCE in soil samples from the root zones and were unable to show any increase in microbial degradation of TCE in planted soils compared to control soils in which little TCE was lost. The specific rates of TCE degradation in these soils were much lower than the rates of TCE removal in the phytoremediation field tests. While root zone microbial activities appear to be important in the degradation of some pollutants, such as polycyclic aromatic hydrocarbons, there has been no evidence for enhanced microbial degradation of chlorinated hydrocarbons in aerobic root zone soils, other than the reports of limited mineralization of TCE from Walton and Anderson (4, 5). Since then, other studies have not supported the hypothesis that aerobic microbial degradation of TCE is enhanced in plant root zones. For example, TCEdegrading methanotrophic bacteria were not enhanced in the root zone of alfalfa exposed to TCE (6). If the root zones of trees are anaerobic, the production of organic matter by roots may promote the reductive dechlorination of TCE to cis-dichloroethylene, vinyl chloride, and perhaps ethylene (7). However, this process is very lim-
ited under aerobic conditions, such as those in our experiments and in most phytoremediation sites. In our 1999 experiments, we recovered a significant portion of the TCE chlorine as chloride ion in the soil, although the methods used to obtain our mass balance admitted a sizable error. Although an excess of chloride ion was recovered in the soil rather than in tree tissues, this should not be construed as demonstrating microbial soil activity as the chloride source. Chloride ion is an essential nutrient for plants and an important co-ion. Channels for transport of chloride ion into and out of plant cells and vacuoles are well known (8), and chloride effluxes from plants have been observed (9). Thus, metabolism of TCE in the roots of poplar, as we have reported in axenic laboratory experiments (10), could have produced the excess chloride observed in the 1999 field experiments. Ma and Burken’s observation that TCE is lost from the trunks of poplar is important and not in disagreement with our observations. They note that they did not attempt mass balances, so the percentage of TCE that was lost from the stems cannot be compared to the total loss of TCE. However, their results suggest that plant metabolism of TCE is confined to the roots. We caution that the relative importance of the various loss mechanisms for phytoremediation of TCE cannot be evaluated until stem loss measurements are included in mass balance experiments. STUART E. STRAND College of Forest Resources and Department of Civil and Environmental Engineering University of Washington Seattle, Wash. XIAOPING WANG Hancock Biological Station Murray State University Murray, Ky.
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MILTON P. GORDON Department of Biochemistry University of Washington Seattle, 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 “Results misinterpreted” Newman et al. have shown in their paper that chloride accumulated in TCE-treated, poplar-planted soil without providing conclusive evidence, however, of whether TCE was degraded by the trees or by soil microorganisms. In either case, TCE degradation in the soil was not the focus of my report. I cited Strand et al.’s work in my news article because the mass balance analysis suggested that evaporation of TCE out of leaves accounted for less than 6% (1.15 mole out of 19.2 mole total recovered chlorine) of measured TCE loss. I agree that Burken and Ma’s new findings do not contradict, but complement, the study undertaken by Strand et al., which reported 70% recovery of chlorine. In my eyes, the assumption that TCE volatilization out of the tree stems made up the missing 30% in Newman et al.’s mass balance is very intriguing. ORI SCHIPPER
© 2003 American Chemical Society
