Tetrachloroethylene (PCE) and trichloroethylene (TCE), common industrial solvents, are among the most frequent contaminants found in groundwater supplies. Due to the potential toxicity and carcinogenicity of chlorinated ethylenes, knowledge about their transformation potential is important in evaluating their environmental fate. The results of this study confirm that PCE can be transformed by reductive dehalogenation to TCE, dichloroethylene, and vinyl chloride (VC) under anaerobic conditions. In addition, [14C]PCE was at least partially mineralized to CO2. Mineralization of 24% of the PCE occurred in a continuous-flow fixed-film methanogenic column with a liquid detention time of 4 days. TCE was the major intermediate formed, but traces of dichloroethylene isomers and VC were also found. In other column studies under a different set of methanogenic conditions, nearly quantitative conversion of PCE to VC was found. These studies clearly demonstrate that TCE and VC are major intermediates in PCE biotransformation under anaerobic conditions and suggest that potential exists for the complete mineralization of PCE to CO2 in soil and aquifer systems and in biological treatment processes. * Corresponding author. land). VC, 0.1 mg per ml of methanol (Supelco Inc., Bellefonte, Pa.), was used. [14C]PCE (Amersham Corp., Arlington Heights, Ill.) was diluted initially in methanol (absolute; J. T. Baker Chemical Co.) to 5.8 x 104 dpm/p.l. Analytical methods. The halogenated aliphatic reactants and products were determined with a lower detection limit of 0.1 ,ug/liter by gas chromatography (GC). Samples were sealed in 10or 60-ml vials and extracted with 1 or 2 ml of pentane (6), respectively, using 1,2-dibromoethane (10 Fg/liter) as an internal standard. A 2-dl sample of the extract was injected into a packed-column GC (10% squalane on Chromosorb W/AW) equipped with a linearized 63Ni detector. Quantitation was achieved by injecting standards, treated like samples, and comparing relative areas with a Spectra
The aromatic hydrocarbons toluene and benzene were anaerobically transformed by mixed methanogenic cultures derived from ferulic acid-degrading sewage sludge enrichments. In most experiments, toluene or benzene was the only semicontinuously supplied carbon and energy source in the defined mineral medium. No exogenous electron acceptors other than CO2 were present. The cultures were fed 1.5 to 30 mM unlabeled or 14C-labeled aromatic substrates (ring-labeled toluene and benzene or methyl-labeled toluene). Gas production from unlabeled substrates and 14C activity distribution in products from the labeled substrates were monitored over a period of 60 days. At least 50% of the substrates were converted to CO2 and methane (>60%). A high percentage of 14CO2 was recovered from the methyl group-labeled toluene, suggesting nearly complete conversion of the methyl group to CO2 and not to methane. However, a low percentage of 14Co2 was produced from ring-labeled toluene or from benzene, indicating incomplete conversion of the ring carbon to CO2. Anaerobic transformation pathways for unlabeled toluene and benzene were studied with the help of gas chromatography-mass spectrometry. The intermediates detected are consistent with both toluene and benzene degradation via initial oxidation by ring hydroxylation or methyl oxidation (toluene), which would result in the production of phenol, cresols, or aromatic alcohol. Additional reactions, such as demethylation and ring reduction, are also possible. Tentative transformation sequences based upon the intermediates detected are discussed.
Toluene and benzene were anaerobically transformed and eventually mineralized in mixed methanogenic cultures. However, the source of oxygen for the initial oxidation step had been unknown, owing to the presence of both methanol and water. No exogenous electron acceptors other than carbon dioxide, toluene, and benzene were present in the defined mineral medium. Through the use of 180-labeled water, the oxygen incorporated into the monoaromatic compounds was shown to come from water. The cresol from the toluene and the phenol from the benzene contained up to 8% 180 label after incubation in 9% 180-labeled medium. Gas chromatography-mass spectrometry was used to detect the 180-labeled aromatic metabolites.
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