Isotopic Fractionation and Masking Effects during Biotransformation of Chlorinated Ethanes

Chlorinated ethanes, such as 1,1,1-trichloroethane (1,1,1-TCA) and 1,1-dichloroethane (1,1-DCA), are common groundwater contaminants that undergo abiotic and biotic transformation, although the specific mechanisms remain unresolved. Compound-specific isotope analysis (CSIA) can provide insight into transformation mechanisms, but its application to chlorinated ethanes has been limited compared to the extensive literature on chlorinated ethenes. This study presents the first dual-element stable isotope (carbon and chlorine) analysis of 1,1,1-TCA and 1,1-DCA biotransformation by an enriched microbial culture commonly used in field remediation. The isotopic fractionation (ε) values for carbon and chlorine are -5.8 ± 0.8‰ and -2.8 ± 0.3‰, respectively, for 1,1,1-TCA and are -8.9 ± 1.0‰ and -4.3 ± 0.5‰, respectively, for 1,1-DCA. Dual-isotope plots produced slopes (Λ_Cl/C) of 2.1 ± 0.1 for 1,1,1-TCA and 2.1 ± 0.2 for 1,1-DCA, consistent with previously reported Λ_Cl/C values for abiotic transformation, despite significant differences in ε values, suggesting a masking effect that suppresses carbon and chlorine isotopic fractionation. Significantly, chlorine isotope values in the formed products are more enriched than the substrates, which may be diagnostic for masking effects in chlorinated organic compounds.