Reductive Outer-Sphere Single Electron Transfer Is an Exception Rather than the Rule in Natural and Engineered Chlorinated Ethene Dehalogenation

Abstract

Chlorinated ethenes (CEs) such as perchloroethylene, trichloroethylene and dichloroethylene are notorious groundwater contaminants. Although reductive dehalogenation is key to their environmental and engineered degradation, underlying reaction mechanisms remain elusive. Outer-sphere reductive single electron transfer (OS-SET) has been proposed for such different processes as Vitamin B-dependent biodegradation and zerovalent metal-mediated dehalogenation. Compound-specific isotope effect (C/C, Cl/Cl) analysis offers a new opportunity to test these hypotheses. Defined OS-SET model reactants (CO radical anions, S-doped graphene oxide in water) caused strong carbon (ε = -7.9‰ to -11.9‰), but negligible chlorine isotope effects (ε = -0.12‰ to 0.04‰) in CEs. Greater chlorine isotope effects were observed in CHCl (ε = -7.7‰, ε = -2.6‰), and in CEs when the exergonicity of C-Cl bond cleavage was reduced in an organic solvent (reaction with arene radical anions in glyme). Together, this points to dissociative OS-SET (SET to a σ* orbital concerted with C-Cl breakage) in alkanes compared to stepwise OS-SET (SET to a π* orbital followed by C-Cl cleavage) in ethenes. The nonexistent chlorine isotope effects of chlorinated ethenes in all aqueous OS-SET experiments contrast strongly with pronounced Cl isotope fractionation in all natural and engineered reductive dehalogenations reported to date suggesting that OS-SET is an exception rather than the rule in environmental transformations of chlorinated ethenes.

Unravelling long-term source removal effects and chlorinated methanes natural attenuation processes by C and Cl stable isotopic patterns at a complex field site

“…4, Λ = 5.8 ± 0.4, Rodríguez-Fernández et al, 2018b). The obtained Λ value for S3 is also consistent i) with an OS-SET mechanism (Heckel et al, 2017a) (6.7 ± 0.4); ii) with biodegradation obtained in microcosms experiments using sediment slurry from this well (6.1 ± 0.5, Rodríguez-Fernández et al, 2018a), and iii) with a reductive pathway involving Pseudomonas stutzeri, obtained when vitamin B 12 was added in the slurry (5 ± 1, Rodríguez-Fernández et al, 2018a). This result confirmed in situ CT anaerobic reduction in S3, consistently with the steady Eh controlled by the Fe-system during the studied period (Fig.…”

“…These studies are mainly based on carbon isotope measurements to confirm CT degradation in the non-saturated zone (Kirtland et al, 2003), to characterize the very depleted δ 13 C of CT and CF industrially produced from methane (Nijenhuis et al, 2013), to confirm CF degradation, along with concentration data, in a multi-contaminant polluted aquifer (Hunkeler et al, 2005) and/or to evaluate CF remediation treatment by alkaline hydrolysis in Òdena site (NE Spain) . New methods for chlorine isotope analyses of CMs and reference Λ values have only became available recently (Heckel et al, 2017a(Heckel et al, , 2017bTorrentó et al, 2017;Rodríguez-Fernández et al, 2018a, 2018b. Thus, there are not previous field site studies testing 2D-CSIA approach potential to identify CMs degradation pathways in complex fractured systems with multi-contaminant spills like that of Òdena.…”

Subtle changes in the active site architecture untangled overlapping substrate ranges and mechanistic differences of two reductive dehalogenases

“…In contrast, NpRdhA attacks the substrate at the halogen substituent and forms a cobalt-halogen bond, which is allowed by an appropriate positioning of the organohalide toward the cobalt ion [5]. Results obtained in a very recent study by Elsner et al [46], who applied dual-element isotope analysis, pointed out that an intimate interaction between both reaction partners might be a common theme in enzymatic reductive dehalogenation of chlorinated ethenes. strain Viet1 and Geobacter lovleyi strain SZ indicated a direct nucleophilic attack by [Co I ] onto the carbon atom, even though a dissociative single electron transfer could not be excluded [45].…”

“…The latter would require an intimate interaction of both reaction partners, but the actual positioning of the substrates at the active site of the enzymes involved is not known so far. Results obtained in a very recent study by Elsner et al [46], who applied dual-element isotope analysis, pointed out that an intimate interaction between both reaction partners might be a common theme in enzymatic reductive dehalogenation of chlorinated ethenes. However, data from electron paramagnetic resonance spectroscopy of PceA Smul equipped with various substrates did not support this assumption [28].…”

Reductive Dehalogenation of Trichloromethane by Two Different Dehalobacter restrictus Strains Reveal Opposing Dual Element Isotope Effects

“…Different enzymatic reaction mechanisms can lead to the same products and would, therefore not be recognized with conventional analysis. If they are associated with different isotope effects, however, compound-specific isotope effect analysis (CSIA) offers the opportunity to make differences in reaction mechanisms visible, and can at best case even elucidate the underlying reaction chemistry, as was recently shown for chlorinated ethene dehalogenation by Vitamin B12 2,[16][17][18][19][20][21] . Here, carbon ( 13 C/ 12 C) and chlorine isotope ratios ( 37 Cl/ 35…”