Carbon and Chlorine Isotope Fractionation during Aerobic Oxidation and Reductive Dechlorination of Vinyl Chloride and cis-1,2-Dichloroethene Y. ABE, † R. ARAVENA, § J. ZOPFI, ‡ O. SHOUAKAR-STASH, § E. COX, | J. D. ROBERTS, | AND D. HUNKELER* ,† Centre for Hydrogeology, University of Neucha ˆtel, Rue Emile Argand 11, CH-2009 Neucha ˆtel, Switzerland, Laboratory for Microbiology, University of Neucha ˆtel, Rue Emile Argand 11, CH-2009 Neucha ˆtel, Switzerland, Department of Earth and Environmental Sciences, University of Waterloo, 200 University Avenue West, Waterloo ON N2L 3G1, Canada, and Geosyntec Consultants, 130 Research Lane, Guelph ON N1G 5G3, Canada Received June 25, 2008. Revised manuscript received September 18, 2008. Accepted October 1, 2008. The study investigated carbon and chlorine isotope fractionation during aerobic oxidation and reductive dechlorination of vinyl chloride (VC) and cis-1,2-dichloroethene (cDCE). The experimental data followed a Rayleigh trend. For aerobic oxidation, the average carbon isotope enrichment factors were -7.2‰ and -8.5‰ for VC and cDCE, respectively, while average chlorine isotope enrichment factors were only -0.3‰ for both compounds. These values are consistent with an initial transformation by epoxidation for which a significant primary carbon isotope effect and only a small secondary chlorine isotope effect is expected. For reductive dechlorination, larger carbon isotope enrichment factors of -25.2‰ for VC and -18.5‰ for cDCE were observed consistent with previous studies. Although the average chlorine isotope enrichment factors were larger than those of aerobic oxidation ( -1.8‰ for VC, -1.5‰ for cDCE), they were not as large as typically expected for a primary chlorine isotope effect suggesting that no cleavage of C-Cl bonds takes place during the initial rate- limiting step. The ratio of isotope enrichment factors for chlorine and carbon were substantially different for the two reaction mechanisms suggesting that the reaction mechanisms can be differentiated at the field scale using a dual isotope approach. Introduction The use of stable isotope analysis has become an important tool to assess degradation of organic compounds at con- taminated sites (1). The approach relies on isotope frac- tionation caused by a more rapid transformation of molecules with light isotopes compared to those with one or several heavy isotopes. As a result, the remaining contaminant pool becomes progressively enriched with the heavy isotopes. The decreasing contaminant concentration and increasing iso- tope ratios can be mathematically related by the Rayleigh equation with the isotope enrichment factor ǫ as the key parameter (2). Since the magnitude of the isotope enrichment factor depends on the initial transformation step, the reaction mechanism needs to be known for a quantitative interpreta- tion of field isotope data. Although different mechanisms are generally associated with a different ǫ, it is usually not possible to identify the reaction mechanism based on isotope data for a single element only (1). Contaminant concentra- tions are usually also affected by physical processes in addition to transformation, so the calculation of supposed field-based isotope enrichment factors is not a reliable practice (3, 4). In addition, if a compound is degraded by different pathways simultaneously at a site, the observed isotope fractionation reflects the mixed effect of individual isotope fractionations caused by these pathways. Since different reaction mechanisms frequently involve different bonds during the initial step, the measurement of multiple isotopes has been proposed as a method to identify reaction mechanisms of organic contaminants and/or quantify the relative contribution of two pathways (5-11). This approach is well established for evaluating the origin and fate of inorganic compounds (NO 3 - , SO 4 2- ) and is usually denoted as the dual isotope approach (2). For organic compounds, the term two-dimensional isotope approach has been used as well. The premise of the dual isotope approach is that by measuring the isotope ratios of two elements of a compound simultaneously (such as 13 C/ 12 C and 2 H/ 1 H or 13 C/ 12 C and 37 Cl/ 35 Cl), a correlation between isotope fractionation of two elements is observed that is characteristic of the reaction mechanism. Unlike a field-based calculation of ǫ-values based on the Rayleigh fractionation model (1), such a correlation is independent of contaminant concentrations and therefore can assist in elucidating the responsible reaction mechanism. This dual isotope approach was successfully employed to identify the mechanism of methyl tert-butyl ether transfor- mation at the field scale (10, 12). Furthermore, laboratory studies indicate that it may also be useful to identify the mechanism of benzene degradation based on combined carbon and hydrogen isotope analysis (13). However, it has to be taken into account that even for given redox conditions, compounds may be degraded by a different mechanism leading to different ratios of isotope fractionation of two elements (13, 14). Once the reaction mechanism at a contaminated site is identified, typical Rayleigh-type inter- pretations of field isotope data can be carried out. The quantification has to take into account potential variations in isotope fractionation for a given pathway. The present study aims at evaluating the potential of the dual isotope approach for analyzing the fate of vinyl chloride (VC) and cis-dichloroethene (cDCE) as they are degraded or transformed by a number of different pathways. VC and cDCE can be reductively dechlorinated by microorganisms under strongly reducing conditions or oxidized under aerobic and anaerobic conditions (15). Furthermore, abiotic reduction of cDCE and VC by iron-bearing minerals has been reported (16). This study compares the effect of reductive dechlori- nation and aerobic oxidation of cDCE and VC on carbon and chlorine isotope fractionation. Significant carbon isotope fractionation was previously reported for aerobic oxidation of VC (-3.2 to -8.2‰) (17, 18), for reductive dechlorination of VC (-22.4 to -31.1‰) (19-21), and for reductive dechlo- rination of cDCE (-14.1 to -20.4‰) (19, 21) while no data are available so far for aerobic cDCE degradation. Aerobic * Corresponding author phone: ++41 32 718 25 60; e-mail: Daniel.Hunkeler@unine.ch. † Centre for Hydrogeology, University of Neucha ˆtel. § University of Waterloo. ‡ Laboratory for Microbiology, University of Neucha ˆtel. | Geosyntec Consultants. Environ. Sci. Technol. 2009, 43, 101–107 10.1021/es801759k CCC: $40.75  2009 American Chemical Society VOL. 43, NO. 1, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 101 Published on Web 11/26/2008