PCE Dechlorination Research Articles

Biotransformation of chlorinated aliphatic solvents in the presence of aromatic compounds under methanogenic conditions

Transformation of carbon tetrachloride (CT) and tetrachloroethylene (PCE) was studied under methanogenic conditions, in the presence or absence of toluene, ethylbenzene, phenol, and benzoate. Microbial inocula for the experiments were derived from three groundwater aquifers contaminated by jet fuel or creosote. CT and PCE were reductively dechlorinated in all the examined cases (CT to chloroform [CF]; PCE to trichloroethylene [TCE], trans-l,2-dichloroethylene [DCE], and vinyl chloride [VC]). In the aquifer microcosms, the electron donors used for the reductive transformations were most likely the unidentified organic compounds present on aquifer solids, or storage materials in microorganisms. Alternatively, molecular hydrogen from the anaerobic incubator atmosphere could have been used. The addition of benzoate caused a decrease in rates of dechlorination if benzoate was transformed. Phenol and ethylbenzene were not degraded and did not influence the transformation of CT or PCE. Toluene, in most of the studied cases, had no influence on reductive dechlorination of either CT or PCE. Only in microcosms derived from a JP-4 jet fuel-contaminated aquifer did the anaerobic degradation of toluene occur simultaneously with reductive dechlorination of PCE, suggesting that toluene might possibly have been used as an electron donor for reductive transformation of chlorinated solvents.

Reductive dechlorination of chloroalkenes in microcosms developed with a field contaminated soil

Static microcosms were constructed with a low organic carbon content, field-contaminated soil containing (in μmol/kg wet soil) tetrachloroethylene (PCE; 69.1), trichloroethylene (TCE; 18.1) and cis-1,2-dichloroethylene (cDCE; 2.3). Incubation at 20°C resulted in reductive dechlorination of PCE and TCE to cDCE under both sulfate-reducing and fermentative/methanogenic conditions. Low levels of vinyl chloride (VC) were also produced. Addition of electron donors (such as ethanol, acetate and lactate) was necessary for reductive dechlorination to occur. However, less than 0.25% of the electron equivalents potentially made available by the electron donor(s) were actually used for the dechlorination of the target chlorinated alkenes. More than 99% of the soil PCE was dechlorinated in less than 200 days of incubation under either sulfate-reducing or methanogenic conditions when external electron donors were added to these microcosms. Nitrate reduction conditions led to a much lower rate and extent of PCE dechlorination.

Biotransformation of tetrachloroethylene by anaerobic attached-films at low temperatures

This study was conducted to examine the feasibility of using an anaerobic attached-film expanded-bed (AAFEB) process for the treatment of tetrachloroethylene (PCE) at 15°C. A laboratory-scale continuous-flow reactor, with an expanded-bed volume of 900 ml, was operated at hydraulic retention times of 1.8-4 h and influent PCE concentrations of 8–12 mg/l. Small samples (50 ml) of attached film media were used for batch testing 3–7 mg/l of PCE in a separate 300 ml AAFEB reactor. The attached films were a mixed anaerobic consortium grown on diatomaceous earth support particles under methanogenic conditions. Sucrose was used as an external electron donor and growth substrate. Reductive dechlorination of PCE to trichloroethylene (TCE), cis-1,2-dichloroethylene (DCE), vinyl chloride (VC) and ethylene (ETH) was observed. The conversion efficiency of PCE and TCE to lesser chlorinated compounds and ETH was above 98% during continuous-flow testing. VC accumulated as the major dechlorination product and ETH was produced at very low rates. The maximum PCE dechlorination rate, q max, was 5.33 mg PCE/g volatile solids-day (32.1 μmol/g VS-d) and the one-half velocity coefficient, K s, was 0.009 mg PCE/l (0.054 μM) under continuous-flow conditions. Since the AAFEB carried more than 20 g volatile solids per liter of bed, low temperature conversion rates would be expected to exceed 60 mg PCE/l bed-day. This indicates removal efficiencies greater than 99% could be obtained at hydraulic retention times of less than 1 h at ambient groundwater temperatures with this process.

Reductive dechlorination of Tri- and tetrachloroethylenes depends on transition from aerobic to anaerobic conditions

Aerobic enrichment cultures from contaminated groundwaters dechlorinated trichloroethylene (TCE) (14.6 mg/liter; 111 mumol/liter) and tetrachloroethylene (PCE) (16.2 mg/liter; 98 mumol/liter) reductively within 4 days after the transition from aerobic to anaerobic conditions. The transformation products were equimolar amounts of cis-1,2-dichloroethylene and traces of 1,1-dichloroethylene. No other chlorinated product and no methane were detected. The change was accompanied by the release of sulfide, which caused a decrease in the redox potential from 0 to -150 mV. In sterile control experiments, sulfide led to the abiotic formation of traces of 1,1-dichloroethylene without cis-1,2-dichloroethylene production. The reductive dechlorination of PCE via TCE depended on these specific transition conditions after consumption of the electron acceptor oxygen or nitrate. Repeated feeding of TCE or PCE to cultures after the change to anaerobic conditions yielded no further dechlorination. Only aerobic subcultures with an air/liquid ratio of 1:4 maintained dechlorination activities; anaerobic subcultures showed no transformation. Bacteria from noncontaminated sites showed no reduction under the same conditions.

Stimulation of the reductive dechlorination of tetrachloroethene in anaerobic aquifer microcosms by the addition of toluene

Chlorinated solvents are among the most common industrial contaminants of groundwater. Tetrachloroethene (PCE) is resistant to aerobic biodegradation; however, it is occasionally reductively dechlorinated in anoxic contaminated aquifers (2, 3). This is particularly true if the subsurface also contains other organic compounds that can serve as electron donors and whose utilization by subsurface bacteria will deplete the available oxygen. Certain alkylbenzenes, such as toluene, could potentially serve as electron donors for the reductive dechlorination of PCE. Field studies conducted by this laboratory have suggested that this may indeed be occurring at field sites exposed to both alkylbenzenes and chlorinated solvents. In this study, the biologically mediated interactions of toluene and PCE under anaerobic conditions were investigated by using microcosms constructed with aquifer solids from an area that was exposed to both alkylbenzenes and chlorinated ethenes at the US Coast Guard Air Station, in Traverse City, Michigan. The results of this study indicate that toluene can act as an initial source of reducing potential for the reductive dechlorination of chloroethenes under anaerobic conditions. Also, the cultivation of this consortium from a site impacted by alkylbenzenes and chloroethenes is an indication that this process may occur in contaminated aquifers.

Biological reductive dechlorination of tetrachloroethylene and trichloroethylene to ethylene under methanogenic conditions

A biological process for remediation of groundwater contaminated with tetrachloroethylene (PCE) and trichloroethylene (TCE) can only be applied if the transformation products are environmentally acceptable. Studies with enrichment cultures of PCE- and TCE-degrading microorganisms provide evidence that, under methanogenic conditions, mixed cultures are able to completely dechlorinate PCE and TCE to ethylene, a product which is environmentally acceptable. Radiotracer studies with [14C]PCE indicated that [14C]ethylene was the terminal product; significant conversion to 14CO2 or 14CH4 was not observed. The rate-limiting step in the pathway appeared to be conversion of vinyl chloride to ethylene. To sustain reductive dechlorination of PCE and TCE, it was necessary to supply an electron donor; methanol was the most effective, although hydrogen, formate, acetate, and glucose also served. Studies with the inhibitor 2-bromoethanesulfonate suggested that methanogens played a key role in the observed biotransformations of PCE and TCE.

Dependence of tetrachloroethylene dechlorination on methanogenic substrate consumption by Methanosarcina sp. strain DCM.

Tetrachloroethylene (perchloroethylene, PCE) is a suspected carcinogen and a common groundwater contaminant. Although PCE is highly resistant to aerobic biodegradation, it is subject to reductive dechlorination reactions in a variety of anaerobic habitats. The data presented here clearly establish that axenic cultures of Methanosarcina sp. strain DCM dechlorinate PCE to trichloroethylene and that this is a biological reaction. Growth on methanol, acetate, methylamine, and trimethylamine resulted in PCE dechlorination. The reductive dechlorination of PCE occurred only during methanogenesis, and no dechlorination was noted when CH4 production ceased. There was a clear dependence of the extent of PCE dechlorination on the amount of methanogenic substrate (methanol) consumed. The amount of trichloroethylene formed per millimole of CH4 formed remained essentially constant for a 20-fold range of methanol concentrations and for growth on acetate, methylamine, and trimethylamine. These results suggest that the reducing equivalents for PCE dechlorination are derived from CH4 biosynthesis and that the extent of chloroethylene dechlorination can be enhanced by stimulating methanogenesis. It is proposed that electrons transferred during methanogenesis are diverted to PCE by a reduced electron carrier involved in methane formation.