Microbial Mineralization of Dichloroethene and Vinyl Chloride under Hypoxic Conditions

Abstract

Mineralization of 14C‐radiolabled vinyl chloride ([1,2‐14C] VC) and cis‐dichloroethene ([1,2‐14C] cis‐DCE) under hypoxic (initial dissolved oxygen (DO) concentrations about 0.1 mg/L) and nominally anoxic (DO minimum detection limit = 0.01 mg/L) was examined in chloroethene‐exposed sediments from two groundwater and two surface water sites. The results show significant VC and dichloroethene (DCE) mineralization under hypoxic conditions. All the sample treatments exhibited pseudo‐first‐order kinetics for DCE and VC mineralization over an extended range of substrate concentrations. First‐order rates for VC mineralization were approximately 1 to 2 orders of magnitude higher in hypoxic groundwater sediment treatments and at least three times higher in hypoxic surface water sediment treatments than in the respective anoxic treatments. For VC, oxygen‐linked processes accounted for 65 to 85% of mineralization at DO concentrations below 0.1 mg/L, and 14CO2 was the only degradation product observed in VC treatments under hypoxic conditions. Because the lower detection limit for DO concentrations measured in the field is typically 0.1 to 0.5 mg/L, these results indicate that oxygen‐linked VC and DCE biodegradation can be significant under field conditions that appear anoxic. Furthermore, because rates of VC mineralization exceeded rates of DCE mineralization under hypoxic conditions, DCE accumulation without concomitant accumulation of VC may not be evidence of a DCE degradative “stall” in chloroethene plumes. Significantly, mineralization of VC above the level that could reasonably be attributed to residual DO contamination was also observed in several nominally anoxic (DO minimum detection limit = 0.01 mg/L) microcosm treatments.

Reinterpreting the Importance of Oxygen‐Based Biodegradation in Chloroethene‐Contaminated Groundwater

“…DCE and VC biodegradation at low concentrations of dissolved oxygen may be a key contaminant attenuation mechanism (Bradley and Chapelle 2011; Gossett 2010), and failure to recognize this process can have important consequences. Existing protocols for assessing in situ biodegradation of chloroethene contaminants focus on chlororespiration.…”

A universal design approach for in situ bioremediation developed from multiple project sites

“…At sulfate concentrations as high as 500 mg/L, ethene generation was observed, albeit more slowly than at sites where sulfate concentrations were more depressed. Abiotic or possibly hypoxic aerobic degradation was observed (Bradley, 2011;Bradley & Chapelle, 2011) since ethene generation did not match observed reductions in VOC concentrations. It is possible that the abiotic reactions were mediated by reduced iron sulfides.…”

Perils of categorical thinking: “Oxic/anoxic” conceptual model in environmental remediation

“…The 0.1-0.5 mg/L field standard does not indicate an absence of oxygen-linked biodegradation. Oxygen-linked mineralization of reduced daughter products like DCE and VC can be quite efficient at dissolved oxygen concentrations well below 0.1 mg/L (Bradley, 2011;Bradley & Chapelle, 2011;Gossett, 2010). Failure to acknowledge this biodegradation potential increases the risk of…”

“…For a number of reasons (reviewed in detail in Bradley, 2011), dissolved oxygen concentrations in the 0.1-0.5 mg/L range are routinely employed as the diagnostic threshold for distinguishing those conditions (oxic) under which oxygen-linked metabolism is expected to substantially impact contaminant biodegradation from those conditions under which the impact of oxygen is considered negligible. Recent publications, however, have demonstrated substantial oxygen-linked biodegradation of chloroethene contaminants (dichloroethene [DCE] and vinyl chloride [VC]) at dissolved oxygen concentrations circa 0.01 mg/L (Bradley & Chapelle, 2011;Gossett, 2010).…”

“…The significance of oxygen-linked contaminant biodegradation at dissolved oxygen concentrations well below the accepted field standard for anoxic conditions (Bradley, 2011;Bradley & Chapelle, 2011;Gossett, 2010) argues for a reexamination of the oxic/anoxic model of in situ oxygen condition. A simple refinement of the field standard for anoxic conditions is not adequate, however.…”