Abstract
A stall of cis-1,2-DCE and vinyl chloride (VC) is frequently observed during bioremediation of groundwater chloroethenes via reductive dechlorination. These chloroethenes may be oxidised by aerobic methanotrophs or ethenotrophs co-metabolically and/or metabolically. We assessed the potential for such oxidation at 12 sites (49 groundwater samples) using hydrochemical and molecular biological tools. Both ethenotroph (etnC and etnE) and methanotroph (mmoX and pmoA) functional genes were identified in 90% of samples, while reductive dehalogenase functional genes (vcrA and bvcA) were identified in 82%. All functional genes were simultaneously detected in 78% of samples, in actively biostimulated sites in 88% of samples. Correlation analysis revealed that cis-1,2-DCE concentration was positively correlated with vcrA, etnC and etnE, while VC concentration was correlated with etnC, etnE, vcrA and bvcA. However, feature selection based on random forest classification indicated a significant relationship for the vcrA in relation to cis-1,2-DCE, and vcrA, bvcA and etnE for VC and no prove of relationship between cis-1,2-DCE or VC and the methanotroph functional genes. Analysis of hydrochemical parameters indicated that aerobic oxidation of chloroethenes by ethenotrophs may take place under a range of redox conditions of aquifers and coincide with high ethene and VC concentrations.
Highlights
Chloroethene tetrachloroethene (PCE) and trichloroethene (TCE) are amongst the most abundant pollutants of groundwater and soil due to their frequent use in industrial applications. These pollutants can be biodegraded through natural or enhanced anaerobic reductive dechlorination, where chloroethenes serve as electron acceptors and molecular hydrogen and acetate, both released as by-products of organic substrate fermentation reactions, are used by the dechlorinating bacteria as electron donors and as carbon sources, respectively [1]
Despite the presence of anaerobic dechlorinators, cis-1,2-DCE and vinyl chloride (VC) often accumulate in groundwater as the sequential steps of the reductive dechlorination process are less and less favourable thermodynamically and kinetically [15], and/or the conditions for complete dechlorination are not always optimal
All functional genes together were detected in 38 of 49 (78%) samples analysed, indicating that both aerobic oxidation and reductive dechlorination of chloroethenes may take place simultaneously at the same place or in close microenvironments. This finding is consistent with the study of Liang et al [20], which detected functional genes from all three bacterial guilds in 99% of groundwater samples collected at six contaminated sites
Summary
Chloroethene tetrachloroethene (PCE) and trichloroethene (TCE) are amongst the most abundant pollutants of groundwater and soil due to their frequent use in industrial applications These pollutants can be biodegraded through natural or enhanced anaerobic reductive dechlorination, where chloroethenes serve as electron acceptors and molecular hydrogen and acetate, both released as by-products of organic substrate fermentation reactions, are used by the dechlorinating bacteria as electron donors and as carbon sources, respectively [1]. During this process, PCE is converted stepwise to TCE by removing one chlorine atom and replacing it with a hydrogen atom; likewise, trichloroethene (TCE), is primarily converted to cis-1,2-DCE, to vinyl chloride (VC), and to ethene [2]. Despite the presence of anaerobic dechlorinators, cis-1,2-DCE and VC often accumulate in groundwater as the sequential steps of the reductive dechlorination process are less and less favourable thermodynamically and kinetically [15], and/or the conditions for complete dechlorination are not always optimal
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