Abstract
Chlorinated ethenes are prevalent groundwater contaminants. To better constrain (bio)chemical reaction mechanisms of reductive dechlorination, the position-specificity of reductive trichloroethene (TCE) dehalogenation was investigated. Selective biotransformation reactions (i) of tetrachloroethene (PCE) to TCE in cultures of Desulfitobacterium sp. strain Viet1; and (ii) of TCE to cis-1,2-dichloroethene (cis-DCE) in cultures of Geobacter lovleyi strain SZ were investigated. Compound-average carbon isotope effects were −19.0‰ ± 0.9‰ (PCE) and −12.2‰ ± 1.0‰ (TCE) (95% confidence intervals). Using instrumental advances in chlorine isotope analysis by continuous flow isotope ratio mass spectrometry, compound-average chorine isotope effects were measured for PCE (−5.0‰ ± 0.1‰) and TCE (−3.6‰ ± 0.2‰). In addition, position-specific kinetic chlorine isotope effects were determined from fits of reactant and product isotope ratios. In PCE biodegradation, primary chlorine isotope effects were substantially larger (by −16.3‰ ± 1.4‰ (standard error)) than secondary. In TCE biodegradation, in contrast, the product cis-DCE reflected an average isotope effect of −2.4‰ ± 0.3‰ and the product chloride an isotope effect of −6.5‰ ± 2.5‰, in the original positions of TCE from which the products were formed (95% confidence intervals). A greater difference would be expected for a position-specific reaction (chloride would exclusively reflect a primary isotope effect). These results therefore suggest that both vicinal chlorine substituents of TCE were reactive (intramolecular competition). This finding puts new constraints on mechanistic scenarios and favours either nucleophilic addition by Co(I) or single electron transfer as reductive dehalogenation mechanisms.
Highlights
Chlorinated organic compounds have natural and anthropogenic sources and are represented in nearly every organic chemical class [1]
Selective reductive dechlorination of PCE to TCE was accomplished in anaerobic biodegradation of PCE with the Firmicute Desulfitobacterium sp. strain Viet1, and selective transformation of TCE to cis-DCE was facilitated by the Deltaproteobacterium Geobacter lovleyi strain SZ
Selective reductive dechlorination of PCE to TCE and chloride was performed with the microorganism Desulfitobacterium sp. strain Viet1, whereas Geobacter lovleyi strain SZ converted
Summary
Chlorinated organic compounds have natural and anthropogenic sources and are represented in nearly every organic chemical class [1]. Step from Co(I) to the organohalide (Scheme 2) [12,13,14,15,16,17,18] It remains unclear whether the transformation of TCE to cis-DCE is stereoselective at the geminal chlorine substituent in E-position—as one would presume for a nucleophilic substitution mechanism [19]—or whether both geminal E- and Z-positions are involved—as brought forward for SET by computational results from Nonnenberg et al resulting in the formation of radical intermediates [20]. Mathematical equations were derived for reactant and product isotope ratios to model chlorine isotope trends and to extract primary and secondary chlorine isotope effects This approach provided a first benchmark how chlorine isotope data can be interpreted in typical scenarios of reductive dechlorination of chlorinated ethenes.
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