CSIA of carbon (δ13C) and chlorine (δ37Cl) can also be used to evaluate degradation of chlorinated ethanes, provide insight into the active degradation pathways, and help distinguish biotic and abiotic degradation.
- Anaerobic Biodegradation: While less extensively studied than chlorinated ethenes, a few research groups have reported carbon and chlorine isotope enrichment factors for reductive dechlorination of chlorinated ethanes including trichloroethanes (TCA) and dichloroethanes (DCA). In a Dehalobacter-containing mixed culture, carbon isotope enrichment factors (ε) of -1.8‰ and -10.5‰ during reductive dechlorination of 1,1,1-TCA and 1,1-DCA, respectively (Lollar et al. 2010). Similarly, Hunkeler et al. (2002) reported carbon isotope enrichment factors of -2.0‰ and -32.1‰ during anaerobic biodegradation (dichloroelimination) of 1,1,2-TCA and 1,2-DCA.
In addition to CSIA, CENSUS qPCR or QuantArray-Chlor can be used to quantify Dehalogenimonas, Dehalobacter, and other halorespiring bacteria as a supporting line of evidence in assessing biodegradation of chlorinated ethanes.
- Aerobic Biodegradation: Carbon isotope fractionation during aerobic biodegradation depends on the enzymes involved in the pathway. Large fractionation (ε = -27 to -33‰) is associated with the hydrolytic dehalogenase pathway (Hirschorn et al. 2004; Hunkeler and Aravena 2000) while a smaller fractionation factor (ε = -3.0‰) was measured for a strain where aerobic 1,2-DCA biodegradation is initiated by a monooxygenase (Hirschorn et al. 2004).
- Abiotic Degradation: In groundwater, 1,1,1-TCA can degrade abiotically by hydrolysis and dehydrohalogenation with a measured carbon isotope enrichment factor of -1.6‰ and a chlorine isotope enrichment factor of -4.7‰ (Palau et al. 2014).
Abiotic degradation of 1,1,1-TCA mediated by zero valent iron (ZVI) and ferrous sulfide (FeS) also results in carbon isotope fractionation (Broholm et al. 2014; Elsner et al. 2007; Palau et al. 2014). For ZVI, carbon isotope enrichment factors range from -7.8 to -13.6‰. Similar carbon isotope enrichment factors have also been suggested for FeS (-10 to -14‰). Palau et al. (2014) reported a chlorine isotope enrichment factor of -4.7‰ for 1,1,1-TCA degradation by ZVI.
- Chemical Oxidation: Chemical oxidation of 1,1,1-TCA by persulfate has been reported to result in significant carbon isotope fractionation (Marchesi et al. 2013; Palau et al. 2014) but not chlorine isotope fractionation.
Also Consider 2D-CSIA: Two dimensional compound specific isotope analysis (2D-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 often constructed and may aid in identifying and distinguishing active biotic and abiotic degradation mechanisms (Broholm et al. 2014; Palau et al. 2016; Palau et al. 2014).
Broholm, M.M., D. Hunkeler, N. Tuxen, S. Jeannottat, and C. Scheutz. 2014. Stable carbon isotope analysis to distinguish biotic and abiotic degradation of 1,1,1-trichloroethane in groundwater sediments. Chemosphere 108: 265-273.
Elsner, M., D.M. Cwiertny, A.L. Roberts, and B. Sherwood Lollar. 2007. 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. Environmental Science & Technology 41 no. 11: 4111-4117.
Hirschorn, S.K., M.J. Dinglasan, M. Elsner, S.A. Mancini, G. Lacrampe-Couloume, E.A. Edwards, and B. Sherwood Lollar. 2004. Pathway Dependent Isotopic Fractionation during Aerobic Biodegradation of 1,2-Dichloroethane. Environmental Science & Technology 38 no. 18: 4775-4781.
Hunkeler, D., and R. Aravena. 2000. Evidence of Substantial Carbon Isotope Fractionation among Substrate, Inorganic Carbon, and Biomass during Aerobic Mineralization of 1,2-Dichloroethane by Xanthobacter autotrophicus. Applied and Environmental Microbiology 66 no. 11: 4870-4876.
Hunkeler, D., R. Aravena, and E. Cox. 2002. Carbon Isotopes as a Tool To Evaluate the Origin and Fate of Vinyl Chloride: Laboratory Experiments and Modeling of Isotope Evolution. Environmental Science & Technology 36 no. 15: 3378-3384.
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.
Lollar, B.S., S. Hirschorn, S.O.C. Mundle, A. Grostern, E.A. Edwards, and G. Lacrampe-Couloume. 2010. Insights into Enzyme Kinetics of Chloroethane Biodegradation Using Compound Specific Stable Isotopes. Environmental Science & Technology 44 no. 19: 7498-7503.
Marchesi, M., N.R. Thomson, R. Aravena, K.S. Sra, N. Otero, and A. Soler. 2013. Carbon isotope fractionation of 1,1,1-trichloroethane during base-catalyzed persulfate treatment. Journal of Hazardous Materials 260: 61-66.
Palau, J., P. Jamin, A. Badin, N. Vanhecke, B. Haerens, S. Brouyère, and D. Hunkeler. 2016. Use of dual carbon–chlorine isotope analysis to assess the degradation pathways of 1,1,1-trichloroethane in groundwater. Water Research 92: 235-243.
Palau, J., O. Shouakar-Stash, and D. Hunkeler. 2014. Carbon and Chlorine Isotope Analysis to Identify Abiotic Degradation Pathways of 1,1,1-Trichloroethane. Environmental Science & Technology 48 no. 24: 14400-14408.