Contrasting dual (C, Cl) isotope fractionation offers potential to distinguish reductive chloroethene transformation from breakdown by permanganate

Abstract

cis-1,2-Dichloroethene (cis-DCE) and trichloroethene (TCE) are persistent, toxic and mobile pollutants in groundwater systems. They are both conducive to reductive dehalogenation and to oxidation by permanganate. In this study, the potential of dual element (C, Cl) compound specific isotope analyses (CSIA) for distinguishing between chemical oxidation and anaerobic reductive dechlorination of cis-DCE and TCE was investigated. Well-controlled cis-DCE degradation batch tests gave similar carbon isotope enrichment factors ε (‰), but starkly contrasting dual element isotope slopes ΔδC/ΔδCl for permanganate oxidation (ε=-26‰±6‰, ΔδC/ΔδCl≈-125±47) compared to reductive dechlorination (ε=-18‰±4‰, ΔδC/ΔδCl≈4.5±3.4). The difference can be tracked down to distinctly different chlorine isotope fractionation: an inverse isotope effect during chemical oxidation (ε=+0.2‰±0.1‰) compared to a large normal isotope effect in reductive dechlorination (ε=-3.3‰±0.9‰) (p≪0.05). A similar trend was observed for TCE. The dual isotope approach was evaluated in the field before and up to 443days after a pilot scale permanganate injection in the subsurface. Our study indicates, for the first time, the potential of the dual element isotope approach for distinguishing cis-DCE (and TCE) concentration drops caused by dilution, oxidation by permanganate and reductive dechlorination both at laboratory and field scale.

δ13C and δ37Cl Isotope Fractionation To Characterize Aerobic vs Anaerobic Degradation of Trichloroethylene

“…Our results show that for strain F1 as a model organism, a distinction between aerobic and anaerobic degradation may indeed be possible if both δ 13 C and δ 37 Cl values are measured ( Figure 2). The observed ε 13 C vs. ε 37 Cl trend for TCE oxidation by F1 strain is similar to permanganate oxidation 33 and it differs significantly from reported trends for biotic 25,33,34 and abiotic 25,68 reduction. This observation meets the intuitive assumption that chlorine isotope enrichment along TCE oxidation should be negligible, facilitating the distinction between both pathways 33,63 .…”

“…This is also in agreement with previously reported values along VC and cis-DCE oxidation (ɛ 37 Cl=~-0.3‰) 63 . Likewise, negligible chlorine isotope enrichment values were reported for TCE oxidation by permanganate (ɛ 37 Cl=0.1±0.1‰) 33 . Carbon isotope enrichment along TCE oxidation by strain OB3b was significantly different from strain F1, in consistence with previous studies 36,37 .…”

“…Green: Methylosinus trichosporium OB3b expressing sMMO; Orange: Methylosinus trichosporium OB3b expressing pMMO; Blue: Pseudomonas putida F1 expressing TOD. Black: abiotic permanganate oxidation 33 . Red: biotic reductive dechlorination by Geobacter lovleyi 25 , Desulfitobacterium hafniense Y51 25 and Dehalococcoides 34 , abiotic reductive dechlorination by zero valent iron 68 and enzymatic cofactor cobalamin 25 .…”

“…Carbon isotope enrichment factors have been reported for TCE oxidation, ranging from εC = -1.1 to -20.7‰ [35][36][37] and overlapping with the range of its reductive dechlorination (εC =-2.5 to -15.3‰ 25,33,34,[38][39][40][41] ). While mechanistic details of TCE oxidative biodegradation have been investigated [42][43][44] (Scheme 1), reasons for the differences in carbon isotope enrichment factors are still not understood.…”

Mechanistic Dichotomy in Bacterial Trichloroethene Dechlorination Revealed by Carbon and Chlorine Isotope Effects

“…Previously observed stable isotope fractionation is consistent with the mechanistic dichotomy observed in this study. Figure 5 shows our data in the context of previously reported dual element isotope trends ΛC/Cl in reductive dehalogenation by bacteria 38,39,[45][46][47][48][49][50] , in enzyme extracts 44 , by pure cofactors 38,44 or model systems 14,38 . These smaller dual element isotope slopes potentially do not reflect the chemical bond conversion but rather a preceding step (e.g., mass transfer into the cell, substrate-enzyme binding, etc.)…”