Anaerobic Degradation of Vinyl Chloride in Aquifer Microcosms

Abstract

The anaerobic degradation potential at a chloroethene-contaminated site was investigated by operating two anoxic column aquifer microcosms enriched in iron(III). One column was fed with vinyl chloride (VC) only (column A) and one with VC and acetate (column B). In column A, after about 600 pore volume exchanges (PVEs), VC started to disappear and reached almost zero VC recovery in the effluent after 1000 PVEs. No formation of ethene was observed. In column B, effluent VC was almost always only a fraction of influent VC. Formation of ethene was observed after 800 PVEs and started to become an important degradation product after 1550 PVEs. However, ethene was never observed in stoichiometric amounts compared with disappeared VC. The average stable isotope enrichment factor for VC disappearance in column A was determined to be -4.3‰. In column B, the isotope enrichment factor shifted from -10.7 to -18.5‰ concurrent with an increase in ethene production. Batch microcosms inoculated with column material showed similar isotope enrichment factors as the column microcosms. These results indicated that two degradation processes occurred, one in column A and two in parallel in column B with increasing importance of reductive dechlorination with time. This study suggests that in addition to reductive dechlorination, other degradation processes such as anaerobic oxidation should be taken into account when evaluating natural attenuation of VC and that isotope analysis can help to differentiate between different pathways of VC removal.

Identification of abiotic and biotic reductive dechlorination in a chlorinated ethene plume after thermal source remediation by means of isotopic and molecular biology tools

“…sulfate-reducing, Chapelle, 1996). Finally, while anaerobic oxidation was suggested as a possible degradation pathway (Smits et al, 2011), the likeliness of such a process to occur remains unclear. Gossett et al previously suggested that what was in microcosms thought to be anaerobic oxidation might actually be aerobic oxidation with O 2 concentrations so low that O 2 is quickly consumed and not measurable (Gossett, 2010).…”

Chlorinated ethene plume evolution after source thermal remediation: Determination of degradation rates and mechanisms

“…Non-detection of the VC reductase genes does not necessarily indicate that VC reduction does not occur, as lower chlorinated ethene concentrations in total would provide fewer electron acceptors for this bacterial guild, and thus these bacteria would be present in smaller quantities and are possibly below detection. It is also possible that VC respiration in this part of the plume is facilitated by currently unidentified species or genes or by microaerophilic or anaerobic oxidation (Bradley et al, 1998;Liang et al, 2017;Smits et al, 2011).…”

“…This discovery has shifted the paradigm that complete biotic degradation of chlorinated ethenes is only possible when Dehalococcoides is a main player in a groundwater microbial community. Further, it is also possible for cDCE and VC to be degraded by microbial oxidation, including microaerophilic oxidation, cometabolic oxidation by methanotrophs and etheneotrophs, or anaerobic oxidation (Bradley et al, 1998;Bradley and Chapelle, 2000;Liang et al, 2017;Smits et al, 2011). Chlorinated ethene degradation may also proceed abiotically in ironand sulfate-reducing conditions when mediated by reactive iron minerals such as iron sulfides, magnetite, and green rust (He et al, 2015).…”