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Response to Comment on “1,1,2,2-Tetrachloroethane Reactions with OH-, Cr(II), Granular Iron, and a Copper-Iron Bimetal: Insights from Product Formation and Associated Carbon Isotope Fractionation” Dr. Noubactep takes exception to our use of batch reactors (1, 2). We concur that they do not directly reflect conditions in field-scale granular iron permeable reactive barriers (PRBs), and that column reactors, oft-employed in our previous work (3-8), more accurately simulate field-scale hydrodynamic regimes. Even conditions in long-term column studies are not, however, invariably representative of PRBs, as each field site possesses unique hydrologic and geochemical attributes. Such complexities are intrinsic to environmental science. A well-established methodology to deal with complexities are reductionistic approaches. Experiments are conducted under idealized conditions (e.g., in batch reactors) so that individual factors that may influence a PRB’s success can be investigated separately. Contrary to Dr. Noubactep’s view we believe that such studies are an important component of a suite of complementary approaches that each possess both merits and limitations. The control of mass transport afforded by rotating disk electrodes, for example, renders them particularly well-suited to exploring the relative contributions of mass transfer and surface reactions to organohalide reduction rates (9). The relative ease with which kinetic data can be obtained as conditions are systematically varied makes batch studies particularly useful in exploring the relationship between surface composition and reactivity of bimetallic reductants (10), or the phenomena responsible for the inhibitory affect of silica (11). The good mass balances that batch reactors can provide also enhances their suitability for studies of competing reduction and dehydrohalogenation (12). On the other hand, the spatial and temporal changes in surface composition afforded by long-term column reactors contributes to their utility for exploring the influence of cosolutes or interspecies competition on contaminant removal (4, 7). Moreover, solution pH may stabilize in column reactors (or in PRBs) affording better control than in batch studies. In turn, column experiments typically suffer from difficulties in establishing mass balances, and it may not be possible to obtain kinetic data of the quality that batch reactors provide. Noubactep voices three specific concerns with our study. First, he suggests that organohalide removal in our system may not involve direct transfer of electrons from Fe0 (apparently in the mistaken belief that we make such a claim). He cites previous evidence that granular iron can serve as a sorbent for non-reducible heavy metals and viruses, while noting that ferrous iron or hydrogen may represent alternative electron donors. In fact, no specific claims were made in our study, although we did suggest that atomic hydrogen could represent the active agent in Cu/Fe systems, based upon our prior work (13). For granular iron, the identity of the redox active surface species remains the subject of debate, although our results indicate that it has little impact on the isotopic fractionation resulting from alkyl polyhalide reductive dehalogenation. Noubactep maintains that the existence of “reductive processes...should have been carefully demonstrated.” We are puzzled by this assertion; in both the present study, as well as in scores of prior column and field-scale investigations 10.1021/es072046z CCC: $37.00 Published on Web 10/19/2007
2007 American Chemical Society
of organohalide reduction, the observation of the relevant products provides clear evidence of reductive dehalogenation. In fact, for the iron-based reductants investigated in our study, product studies demonstrated that reductive dehalogenation was the major reaction in these systems. Our study did include a non-reductive transformation (dehydrohalogenation of 1,1,2,2-TeCA); we showed that nonreductive and reductive transformation pathways could be clearly differentiated either by product analysis or by isotope measurements (1). The second concern of Noubactep relates to whether an oxide film is present on our granular iron particles. Specifically, he asserts that studies with granular iron should be conducted under conditions favoring the formation of an oxide surface film, and that shear forces from rapid mixing could alter the reactivity of such a film in a manner not representative of field-scale systems. In our investigation, we did not assume that our surfaces were free of an oxide coating. In fact, our prior work with Fisher electrolytic iron explicitly acknowledges the likely importance of such oxide coatings (14). We note that the study of Cwiertny and Roberts (14) cited by Noubactep (2) revealed no change in the reactivity of these same iron particles over a broad range of mixing speeds (45-250 RPM). If shear forces did indeed influence particle reactivity, we would anticipate a relationship between the rate of organohalide reduction and the degree of particle agitationscontrary to observations (14). We note that insufficient mixing may lead to situations where mass transfer to the surface becomes slower than the actual surface reaction. In such cases, isotopic fractionation will be masked, as every molecule that reaches the surface will be transformed, regardless of whether it contains light or heavy isotopes (15). If the goal is to develop a step-by-step description of a chemical transformation at the molecular level, providing information about the location of all nuclei and electrons as bonds are broken or formedsin brief, to elucidate the mechanism of the reactionsit is imperative to employ the tool of isotopic fractionation in a situation in which mass transport does not control the reaction rate. Noubactep’s third concern is that our study did not resolve an apparent contradiction between two studies of uranium (VI) reduction by granular iron (16, 17). We are mystified by this; the topic of U(VI) reduction is clearly remote from that of organohalide reduction. After reading these two studies carefully, however, we find that both provide evidence for the reduction of U(VI) by zerovalent iron. We, therefore, fail to see how the results of these studies conflict, as suggested by Noubactep. In addition, as pointed out by the authors, the absence of observable isotope fractionation reported during abiotic U(VI) reduction in one of the cited studies (17) may be attributable to precisely the effect of masking by mass transfer discussed above. In other words, there may be a kinetic isotope effect associated with the abiotic surface reaction that may not have been observed under the experimental conditions chosen. In brief, we maintain that science is best served by bringing as wide an array as possible of different tools to bear on complex problems. To abandon batch experiments altogether would be, in our opinion, to “throw out the baby with the bathwater”. It is only through harnessing the combined data afforded by a variety of different experimental approachess notwithstanding the limitations of eachsthat we may hope to better understand the multiple roles that granular iron may play in affecting contaminant attenuation. VOL. 41, NO. 22, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Literature Cited (1) Elsner, M.; Cwiertny, D. M.; Roberts, A. L.; Sherwood Lollar, B. 1,1,2,2-Tetrachloroethane Reactions with OH-, Cr(II), Granular Iron, and a Copper-Iron Bimetal: Insights from Product Formation and Associated Carbon Isotope Fractionation. Environ. Sci. Technol. 2007, 41, 4111-4117. (2) Noubactep, C. Comment on “1,1,2,2-Tetrachloroethane Reactions with OH-, Cr(II), Granular Iron, and a Copper-Iron Bimetal: Insights from Product Formation and Associated Carbon Isotope Fractionation.” Environ. Sci. Technol. 2007, 41, 7947-7948. (3) Klausen, J.; Vikesland, P. J.; Kohn, T.; Burris, D. R.; Ball, W. P.; Roberts, A. L. Longevity of Granular Iron in Groundwater Treatment Processes: Solution Composition Effects on Reduction of Organohalides and Nitroaromatic Compounds. Environ. Sci. Technol. 2003, 37, 1208-1218. (4) Vikesland, P. J.; Klausen, J.; Zimmermann, H.; Roberts, A. L.; Ball, W. P. Longevity of Granular Iron in Groundwater Treatment Processes: Changes in Solute Transport Properties over Time. J. Contam. Hydrol. 2003, 64, 3-33. (5) VanStone, N. A.; Focht, R. M.; Mabury, S. A.; Sherwood Lollar, B. Effect of Iron Type on Kinetics and Carbon Isotopic Enrichment of Chlorinated Ethylenes during Abiotic Reduction on Fe(0). Ground Water 2004, 42, 268-276. (6) Kohn, T.; Livi, K. J. T.; Roberts, A. L.; Vikesland, P. J. Longevity of Granular Iron in Groundwater Treatment Processes: Corrosion Product Development. Environ. Sci. Technol. 2005, 39, 2867-2879. (7) Kohn, T.; Roberts, A. L. Interspecies Competitive Effects in Reduction of Organohalides in Connelly Iron Columns. Environ. Eng. Sci. 2006, 23, 874-885. (8) Kohn, T.; Roberts, A. L. The Effect of Silica on the Degradation of Organohalides in Granular Iron Columns. J. Contam. Hydrol. 2006, 83, 70-88. (9) Scherer, M. M.; Westall, J. C.; ZiomekMoroz, M.; Tratnyek, P. G. Kinetics of Carbon Tetrachloride Reduction at an Oxidefree Iron Electrode. Environ. Sci. Technol. 1997, 31, 23852391. (10) Bransfield, S. J.; Cwiertny, D. M.; Roberts, A. L.; Fairbrother, D. H. Influence of Copper Loading and Surface Coverage on the Reactivity of Granular Iron toward 1,1,1-Trichloroethane. Environ. Sci. Technol. 2006, 40, 1485-1490. (11) Kohn, T.; Kane, S. R.; Fairbrother, D. H.; Roberts, A. L. Investigation of the Inhibitory Effect of Silica on the Degradation of 1,1,1-Trichloroethane by Granular Iron. Environ. Sci. Technol. 2003, 37, 5806-5812. (12) Arnold, W. A.; Winget, P.; Cramer, C. J. Reductive Dechlorination of 1,1,2,2-Tetrachloroethane. Environ. Sci. Technol. 2002, 36, 3536-3541. (13) Cwiertny, D. M.; Bransfield, S. J.; Livi, K. J. T.; Fairbrother, D. H.; Roberts, A. L. Exploring the Influence of Granular Iron
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(14) (15)
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Additives on 1,1,1-Trichloroethane Reduction. Environ. Sci. Technol. 2006, 40, 6837-6843. Cwiertny, D. M.; Roberts, A. L. On the Nonlinear Relationship between kobs and Reductant Mass Loading in Iron Batch Systems. Environ. Sci. Technol. 2005, 39, 8948-8957. Elsner, M.; Zwank, L.; Hunkeler, D.; Schwarzenbach, R. P. A New Concept Linking Observable Stable Isotope Fractionation to Transformation Pathways of Organic Pollutants. Environ. Sci. Technol. 2005, 39, 6896-6916. Gu, B.; Liang, L.; Dickey, M. J.; Yin, X.; Dai, S. Reductive Precipitation of Uranium(VI) by Zero-Valent Iron. Environ. Sci. Technol. 1998, 32, 3366-3373. Rademacher, L. K.; Lundstrom, C. C.; Johnson, T. M.; Sanford, R. A.; Zhao, J. Z.; Zhang, Z. F. Experimentally Determined Uranium Isotope Fractionation during Reduction of Hexavalent U by Bacteria and Zero Valent Iron. Environ. Sci. Technol. 2006, 40, 6943-6948.
Martin Elsner* Institute of Groundwater Ecology GSF-National Research Center for Environment and Health Ingolsta¨dter Landstrasse 1, D-85764 Neuherberg, Germany
David M. Cwiertny Department of Chemical & Environmental Engineering A242 Bourns Hall University of California, Riverside Riverside, California 92521
A. Lynn Roberts Department of Geography and Environmental Engineering Johns-Hopkins University 3400 North Charles Street Baltimore, Maryland, 21218
Barbara Sherwood Lollar Stable Isotope Laboratory University of Toronto 22 Russell Street Toronto, ON M5S 3B1, Canada ES072046Z
