CSIA of carbon (δ13C) is routinely used to evaluate reductive dechlorination of PCE, TCE and associated daughter products. As described below however, carbon isotope fractionation can also be used to assess aerobic biodegradation, abiotic degradation and chemical oxidation of chlorinated ethenes. CSIA results have also been used to help distinguish between biotic and abiotic degradation (Elsner et al. 2008; Liang et al. 2007).
- Anaerobic Biodegradation: Reductive dechlorination of PCE, TCE, DCE isomers and vinyl chloride results in significant carbon isotope fractionation (Hunkeler et al. 1999) making assessment of anaerobic biodegradation of chlorinated ethenes one of the most common applications for CSIA in environmental remediation.
CSIA can be performed in conjunction with qPCR or QuantArray analyses to quantify known organohalide respiring bacteria and functional genes to provide multiple lines of evidence in evaluating anaerobic biodegradation of chlorinated ethenes (Courbet et al. 2011; Damgaard et al. 2013).
- Aerobic Biodegradation: Carbon isotope fractionation has been observed during aerobic biodegradation of chlorinated ethenes and has been used as a line of evidence supporting assessment of aerobic biodegradation of TCE and cDCE in a fractured bedrock aquifer (Pooley et al. 2009).
Isotopic enrichment factors (δ) during aerobic biodegradation can differ based on the enzymes involved. For example, an enrichment factor of -18.2‰ was reported for aerobic cometabolism of TCE by a toluene degrader (Barth et al. 2002) while low fractionation (ε= -1.1‰) was observed during cometabolic TCE degradation by a methanotroph (Chu et al. 2004).
CSIA can be performed in conjunction with qPCR or QuantArray analyses to quantify organisms and functional genes involved in the aerobic biodegradation of chlorinated ethenes. Given the differences in the reported isotopic enrichment factors for the various aerobic biodegradation mechanisms, characterization of the microbial community in conjunction with CSIA will be particularly valuable.
- Abiotic Degradation: Abiotic degradation of chlorinated ethenes mediated by zero valent iron (ZVI) and iron bearing minerals (e.g. FeS, pyrite) results in carbon isotope fractionation (Elsner et al. 2008; Liang et al. 2007; Liang et al. 2009; Slater et al. 2002; VanStone et al. 2004).
Enrichment factors vary in the literature potentially due to differences in experimental methods. However, Elsner et al. (2008) report carbon isotope enrichment factors of -20.9 to -26.5‰ for TCE, -21.7‰ for cDCE, and -19.4‰ for vinyl chloride during abiotic degradation catalyzed by nanoscale ZVI. Bulk enrichment factors of -23.0 and -21.7‰ have been determined for TCE transformation by chloride green rust and pyrite, respectively (Liang et al. 2009).
- Chemical Oxidation: Chemical oxidation of chlorinated ethenes can also cause carbon isotope fractionation. Permanganate oxidation of PCE, TCE and cDCE have reported enrichment factors of -17.0, -21.4, and -21.1‰, respectively (Poulson and Naraoka 2002). Although lower, a carbon isotope enrichment factor has also been reported for TCE oxidation by a Fenton’s-like chemical oxidation product (Liu et al. 2014).
When to consider 2D-CSIA: Two dimensional compound specific isotope analysis (2D-CSIA) or multi-dimensional CSIA is simply the analysis of the isotope ratios of multiple elements (e.g. 13C/12C, 2H/1H, or 37Cl/35Cl). For chlorinated hydrocarbons, 2D-CSIA typically combines δ13C with δ37Cl although 3D-CSIA with δ13C, δ37Cl, and δ2H has also been described (Kuder et al. 2013). For the 2D-CSIA approach, δ37Cl vs δ13C plots are constructed. The slope of the line is Λ which is often characteristic of specific degradation mechanisms.
2D-CSIA is more sensitive than single-element CSIA and provides improved identification of the bonds involved in the isotope-sensitive degradation step (Braeckevelt et al. 2012). Therefore, 2D-CSIA should be considered for certain applications:
- Multi-dimensional CSIA is often performed in forensic studies to identify contaminant sources in legal liability cases (Braeckevelt et al. 2012).
- Multi-dimensional CSIA is particularly helpful at sites where geochemical monitoring is ambiguous, subsurface conditions are heterogeneous or several degradation pathways may play a role.
- Multi-dimensional CSIA results can aid in selecting the most appropriate isotopic enrichment factor (ε), a critical constant used in calculations of fraction remaining (f) or percent of contaminant biodegraded (B=(1-f) x 100).
Barth, J.A.C., G. Slater, C. Schüth, M. Bill, A. Downey, M. Larkin, and R.M. Kalin. 2002. Carbon Isotope Fractionation during Aerobic Biodegradation of Trichloroethene by Burkholderia cepacia G4: a Tool To Map Degradation Mechanisms. Applied and Environmental Microbiology 68 no. 4: 1728-1734.
Braeckevelt, M., A. Fischer, and M. Kästner. 2012. Field applicability of Compound-Specific Isotope Analysis (CSIA) for characterization and quantification of in situ contaminant degradation in aquifers. Applied Microbiology and Biotechnology 94 no. 6: 1401-1421.
Chu, K.-H., S. Mahendra, D.L. Song, M.E. Conrad, and L. Alvarez-Cohen. 2004. Stable Carbon Isotope Fractionation during Aerobic Biodegradation of Chlorinated Ethenes. Environmental Science & Technology 38 no. 11: 3126-3130.
Courbet, C., A. Rivière, S. Jeannottat, S. Rinaldi, D. Hunkeler, H. Bendjoudi, and G. de Marsily. 2011. Complementing approaches to demonstrate chlorinated solvent biodegradation in a complex pollution plume: Mass balance, PCR and compound-specific stable isotope analysis. Journal of Contaminant Hydrology 126 no. 3–4: 315-329.
Damgaard, I., P.L. Bjerg, J. Bælum, C. Scheutz, D. Hunkeler, C.S. Jacobsen, N. Tuxen, and M.M. Broholm. 2013. Identification of chlorinated solvents degradation zones in clay till by high resolution chemical, microbial and compound specific isotope analysis. Journal of Contaminant Hydrology 146: 37-50.
Elsner, M., M. Chartrand, N. VanStone, G. Lacrampe Couloume, and B. Sherwood Lollar. 2008. Identifying Abiotic Chlorinated Ethene Degradation: Characteristic Isotope Patterns in Reaction Products with Nanoscale Zero-Valent Iron. Environmental Science & Technology 42 no. 16: 5963-5970.
Hunkeler, D., R. Aravena, and B.J. Butler. 1999. Monitoring Microbial Dechlorination of Tetrachloroethene (PCE) in Groundwater Using Compound-Specific Stable Carbon Isotope Ratios: Microcosm and Field Studies. Environmental Science & Technology 33 no. 16: 2733-2738.
Kuder, T., B.M. van Breukelen, M. Vanderford, and P. Philp. 2013. 3D-CSIA: Carbon, Chlorine, and Hydrogen Isotope Fractionation in Transformation of TCE to Ethene by a Dehalococcoides Culture. Environmental Science & Technology 47 no. 17: 9668-9677.
Liang, X., Y. Dong, T. Kuder, L.R. Krumholz, R.P. Philp, and E.C. Butler. 2007. Distinguishing Abiotic and Biotic Transformation of Tetrachloroethylene and Trichloroethylene by Stable Carbon Isotope Fractionation. Environmental Science & Technology 41 no. 20: 7094-7100.
Liang, X., R. Paul Philp, and E.C. Butler. 2009. Kinetic and isotope analyses of tetrachloroethylene and trichloroethylene degradation by model Fe(II)-bearing minerals. Chemosphere 75 no. 1: 63-69.
Liu, Y., Y. Gan, A. Zhou, C. Liu, X. Li, and T. Yu. 2014. Carbon and chlorine isotope fractionation during Fenton-like degradation of trichloroethene. Chemosphere 107: 94-100.
Pooley, K.E., M. Blessing, T.C. Schmidt, S.B. Haderlein, K.T.B. MacQuarrie, and H. Prommer. 2009. Aerobic Biodegradation of Chlorinated Ethenes in a Fractured Bedrock Aquifer: Quantitative Assessment by Compound-Specific Isotope Analysis (CSIA) and Reactive Transport Modeling. Environmental Science & Technology 43 no. 19: 7458-7464.
Poulson, S.R., and H. Naraoka. 2002. Carbon Isotope Fractionation during Permanganate Oxidation of Chlorinated Ethylenes (cDCE, TCE, PCE). Environmental Science & Technology 36 no. 15: 3270-3274.
Slater, G.F., B. Sherwood Lollar, R. Allen King, and S. O’Hannesin. 2002. Isotopic fractionation during reductive dechlorination of trichloroethene by zero-valent iron: influence of surface treatment. Chemosphere 49 no. 6: 587-596.
VanStone, N.A., R.M. Focht, S.A. Mabury, and B.S. Lollar. 2004. Effect of Iron Type on Kinetics and Carbon Isotopic Enrichment of Chlorinated Ethylenes During Abiotic Reduction on Fe(0). Ground Water 42 no. 2: 268-276.