CENSUS – Chlorinated Ethanes

Chlorinated ethanes including trichloroethane (TCA) and dichloroethane (DCA) isomers were widely used as solvents, metal degreasers, and chemical intermediates in industrial processes and now are common groundwater contaminants. With potential adverse health effects including increased cancer risk, drinking water maximum contaminant levels (MCLs) have been established for 1,1,1-TCA, 1,1,2-TCA, and 1,2-DCA. In addition, chlorinated ethanes are also common co-contaminants at tetrachloroethene (PCE) and trichloroethene (TCE) impacted sites due to their similar commercial applications. The presence of 1,1,1-TCA can be especially problematic at PCE/TCE sites due to inhibition of reductive dechlorination chlorinated ethenes, particularly vinyl chloride.

CENSUS Targets for Reductive Dechlorination

Under anaerobic conditions, chlorinated ethanes are susceptible to reductive dechlorination by several groups of halorespiring bacteria including Dehalobacter, Dehalogenimonas, Dehalococcoides, and Desulfitobacterium spp. While the reported range of chlorinated ethanes utilized varies by genus, species, and sometimes at the strain level, several general observations can be made regarding biodegradation pathways and daughter product formation.

  • Dehalobacter spp. have been isolated that are capable of sequential reductive dechlorination of 1,1,1-TCA through 1,1-DCA to chloroethane.
  • Biodegradation of 1,1,2-TCA by several halorespiring bacteria proceeds via dichloroelimination producing vinyl chloride.
  • Similarly, 1,2-DCA biodegradation often occurs via dichloroelimination producing ethene.

The following table describes the individual CENSUS targets and their importance in evaluating reductive dechlorination as a treatment mechanism.

Target

MI Code 

  Relevance / Data Interpretation

Dehalobacter DHB Dehalobacter spp. have been implicated in the biodegradation of chlorinated ethanes ranging from tetrachloroethanes (TeCA) to dichloroethanes (DCA) and are therefore particularly important in assessing the potential for reductive dechlorination of chlorinated ethanes. Dehalobacter sp. and Dehalobacter-containing cultures have been shown to be responsible for sequential reductive dechlorination of 1,1,1-TCA through 1,1-DCA to chloroethane. Moreover, Dehalobacter spp. mediate the dichloroelimination of 1,1,2-TCA and 1,2-DCA to vinyl chloride and ethene, respectively.
Dehalogenimonas DHG Dehalogenimonas spp. are a relatively recently described bacterial genus of the phylum Chloroflexi that are probably best known for reductive dechlorination of chloropropanes (1,2,3-TCP and 1,2-DCP). However, Dehalogenimonas isolates utilize several important chlorinated ethanes including 1,1,2-TCA and 1,2-DCA as growth supporting electron acceptors.
Dehalococcoides DHC Perhaps the most important reason to perform CENSUS® quantification of Dehalococcoides when evaluating a site impacted by chlorinated ethanes is to assess the potential for the reductive dechlorination of vinyl chloride produced by biodegradation of 1,1,2-TCA by Dehalobacter and Dehalogenimonas spp. In addition however, several Dehalococcoides are capable of reductive dechlorination of 1,2-DCA via dichloroelimination.
Desulfitobacterium DSB The range of electron acceptors utilized varies considerably between Desulfitobacterium isolates. For example, Desulfitobacterium dichloroeliminans strain DCA1 is capable of utilizing 1,1,2-TCA and 1,2-DCA as well as vicinal dichloropropanes and –butanes as growth supporting electron acceptors. Conversely, Desulfitobacterium hafniense Y51 cannot utilize or even transform TCA and DCA. Finally, Desulfitobacterium spp., unlike the organisms described above, are not obligate halorespiring bacteria.
1,2 Dichloroethane Reductive Dehalogenase  DCAR Targets the 1,2 dichloroethane reductive dehalogenase gene from members of Desulfitobacterium and Dehalobacter  which dechlorinate 1,2 DCA to ethene.
1,1 Dichloroethane Reductive Dehalogenase DCA Targets the 1,1 dichloroethane reductive dehalogenase gene found in some strains of Dehalobacter.
Methanogens MGN Methanogens utilize hydrogen and carbon dioxide to produce methane. While common in the anaerobic environments conducive to reductive dechlorination, methanogens can compete with dechlorinating bacteria for available hydrogen. However, cometabolic dechlorination of 1,2-DCA by methanogens has also been reported.

CENSUS Targets for Cometabolism

While 1,2-DCA can be utilized as a growth supporting substrate, biodegradation of chlorinated ethanes under aerobic conditions is generally through cometabolism. The cometabolism or co-oxidation of chlorinated ethanes is mediated by monooxygenase enzymes with “relaxed” specificity that oxidize a primary (growth supporting) substrate and co-oxidize the chlorinated compound. To date, co-oxidation of chlorinated ethanes has been observed with a wide variety of primary substrates including methane, ethane, propane, butane, and ammonia.

The following table describes the individual CENSUS targets, their importance in evaluating cometabolism as a treatment mechanism.

Target

MI Code 

  Relevance / Data Interpretation

Soluble Methane Monooxygenase sMMO Targets the soluble methane monooxygenase gene encoding the enzyme generally believed to support faster rates of cometabolism of TCE and co-oxidation of TCA and DCA isomers.
Propane Monooxygenase PPO Propane can be added as a primary substrate to promote growth of propane utilizing bacteria capable of cometabolism of TCE and chlorinated ethanes including 1,1,2-TCA.
Butane Monooxygenase BMO Like propane, butane can be added as a primary substrate to support cometabolism of chlorinated ethenes and chlorinated ethanes including 1,1,1-TCA.
Total Bacteria EBAC Gives an estimate of total bacterial biomass.