Determination of carbon isotope enrichment factors of cis-dichloroethene after precursor amendment

Bacterial reductive dechlorination of the groundwater contaminant tetrachloroethene (PCE) involves the formation of lower chlorinated metabolites. Metabolites can be instantaneously formed and consumed in this sequential process; quantification and validation of their isotopic effects conventionally rely on separate laboratory microcosm studies. Here, we present an evaluation method enabling the determination of the carbon isotope enrichment factor (ε) for the intermediate cis-dichloroethene (cis-DCE) by a single laboratory microcosm study initially amending the precursor PCE only.

Environmental samples harboring organohalide-respiring bacteria were incubated under anaerobic conditions and then successively and repeatedly amended with PCE and cis-DCE in two separate laboratory microcosm studies. Reductive dechlorination was monitored by analyzing liquid samples using Purge-and-Trap gas chromatography isotope ratio mass spectrometry GC/MS-C/IRMS. The prerequisites of the presented evaluation method are mass and δ-value balancing. The evaluation method was validated by agglomerative hierarchical classification of Rayleigh plot data points.

The sample-sensitive range of ε_cis-DCE extended from -10.6 ± 0.2‰ to -26.8 ± 0.6‰ (R² ≥98%). The maximum standard deviations of ε_cis-DCE were ±1.8‰ for single microcosms, ±1.8‰ for replicates and ±1.0‰ for the compiled replicate data of PCE and cis-DCE amendments. A linear regression of the ε_cis-DCE for replicates obtained by each amendment study showed a slope of 95% (5 of the 7 data points are within a 95% confidence interval), demonstrating factor congruency and the practicability of the evaluation method.

We found metabolite degradation and formation to be sequential but also stepwise during bacterial reductive dechlorination. The stepwise phases of the degradation of the intermediate eliminate the impact of instantaneous precursor degradation. These stepwise sections were used to determine ε_cis-DCE -values. Our results showed the validity of ε_cis-DCE -values over a wide range at initial precursor degradation (PCE). The presented evaluation method could substantially decrease lab costs for microcosm studies designed for ε_cis-DCE determinations. Moreover, the results indicated that the evaluation method can be applied to other PCE-metabolites.