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.

/pdf/biological-reductive-dechlorination-of-tetrachloroethylene-3cdz2gyfeo.pdf

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

Tetrachloroethene, also known as perchloroethylene (PCE), is a common groundwater contaminant throughout the United States. The incomplete reductive dechlorination of PCE--resulting in accumulations of trichloroethene, dichloroethene isomers, and/or vinyl chloride--has been observed by many investigators in a wide variety of methanogenic environments. Previous mixed-culture studies have demonstrated that complete dechlorination to ethene is possible, although the final dechlorination step from vinyl chloride to ethene is rate limiting, with significant levels of vinyl chloride typically persisting. In this study, anaerobic methanol-PCE enrichment cultures which proved capable of dechlorinating high concentrations PCE to ethene were developed. Added concentrations of PCE as high as 550 microM (91-mg/liter nominal concentration; approximately 55-mg/liter actual aqueous concentration) were routinely dechlorinated to 80% ethene and 20% vinyl chloride within 2 days at 35 degrees C. The methanol level used was approximately twice that needed for complete dechlorination of PCE to ethene. The observed transformations occurred in the absence of methanogenesis, which was apparently inhibited by the high concentrations of PCE. When incubation was allowed to proceed for as long as 4 days, virtually complete conversion of PCE to ethene resulted, with less than 1% persisting as vinyl chloride. An electron balance demonstrated that methanol consumption was completely accounted for by dechlorination (31%) and acetate production (69%). The high volumetric rates of PCE dechlorination (up to 275 mumol/liter/day) and the relatively large fraction (ca. one-third) of the supplied electron donor used for dechlorination suggest that reductive dechlorination could be exploited for bioremediation of PCE-contaminated sites.

https://aem.asm.org/content/aem/57/8/2287.full.pdf

Reductive dechlorination of high concentrations of tetrachloroethene to ethene by an anaerobic enrichment culture in the absence of methanogenesis.

Hydrogen served as an electron donor in the reductive dechlorination of tetrachloroethene to vinyl chloride and ethene over periods of 14 to 40 days in anaerobic enrichment cultures; however, sustained dechlorination for more extended periods required the addition of filtered supernatant from a methanol-fed culture This result suggests a nutritional dependency of hydrogen-utilizing dechlorinators on the metabolic products of other organisms in the more diverse, methanol-fed system Vancomycin, an inhibitor of cell wall synthesis in eubacteria, was found to inhibit acetogenesis when added at 100 mg/liter to both methanol-fed and hydrogen-fed cultures The effect of vancomycin on dechlorination was more complex Methanol could not sustain dechlorination when vancomycin inhibited acetogenesis, while hydrogen could These results are consistent with a model in which hydrogen is the electron donor directly used for dechlorination by organisms resistant to vancomycin and with the hypothesis that the role of acetogens in methanol-fed cultures is to metabolize a portion of the methanol to hydrogen Methanol and other substrates shown to support dechlorination in pure and mixed cultures may merely serve as precursors for the formation of an intermediate hydrogen pool This hypothesis suggests that, for bioremediation of high levels of tetrachloroethene, electron donors that cause the production of a large hydrogen pool should be selected or methods that directly use H2 should be devised

Hydrogen as an electron donor for dechlorination of tetrachloroethene by an anaerobic mixed culture.

Degradation of the BTEX (benzene, toluene, ethylbenzene, and o-, m-, and p-xylenes) group of organopollutants by the white-rot fungus Phanerochaete chrysosporium was studied. Our results show that the organism efficiently degrades all the BTEX components when these compounds are added either individually or as a composite mixture. Degradation was favored under nonligninolytic culture conditions in malt extract medium, in which extracellular lignin peroxidases (LIPs) and manganese-dependent peroxidases (MNPs) are not produced. The noninvolvement of LIPs and MNPs in BTEX degradation was also evident from in vitro studies using concentrated extracellular fluid containing LIPs and MNPs and from a comparison of the extents of BTEX degradation by the wild type and the per mutant, which lacks LIPs and MNPs. A substantially greater extent of degradation of all the BTEX compounds was observed in static than in shaken liquid cultures. Furthermore, the level of degradation was relatively higher at 25 than at 37 degrees C, but pH variations between 4.5 and 7.0 had little effect on the extent of degradation. Studies with uniformly ring-labeled [14C]benzene and [14C]toluene showed substantial mineralization of these compounds to 14CO2.

Degradation of benzene, toluene, ethylbenzene, and xylenes (BTEX) by the lignin-degrading basidiomycete Phanerochaete chrysosporium.

Toluene degradation occurred concomitantly with sulfate reduction in anaerobic microcosms inoculated with contaminated subsurface soil from an aviation fuel storage facility near the Patuxent River (Md.). Similar results were obtained for enrichment cultures in which toluene was the sole carbon source. Several lines of evidence suggest that toluene degradation was directly coupled to sulfate reduction in Patuxent River microcosms and enrichment cultures: (i) the two processes were synchronous and highly correlated, (ii) the observed stoichiometric ratios of moles of sulfate consumed per mole of toluene consumed were consistent with the theoretical ratio for the oxidation of toluene to CO2 coupled with the reduction of sulfate to hydrogen sulfide, and (iii) toluene degradation ceased when sulfate was depleted, and conversely, sulfate reduction ceased when toluene was depleted. Mineralization of toluene was confirmed in experiments with [ring-U-14C]toluene. The addition of millimolar concentrations of amorphous Fe(OH)3 to Patuxent River microcosms and enrichment cultures either greatly facilitated the onset of toluene degradation or accelerated the rate once degradation had begun. In iron-amended microcosms and enrichment cultures, ferric iron reduction proceeded concurrently with toluene degradation and sulfate reduction. Stoichiometric data and other observations indicate that ferric iron reduction was not directly coupled to toluene oxidation but was a secondary, presumably abiotic, reaction between ferric iron and biogenic hydrogen sulfide.

Microbial degradation of toluene under sulfate-reducing conditions and the influence of iron on the process

: This research investigated the potential for anaerobic, biological degradation of five chlorinated C1 and C2 hydrocarbons: tetrachloroethylene (PCE), trichloroethylene (TCE), 1,1,1-trichloroethane (1,1,1-TCA), chloroform (CF), and dichloromethane (DCM). Attempts were made to delineate the importance of microbial -- as opposed to purely chemical -- mechanisms of degradation; products formed; and pathways involved. The five compounds were studied separately in batch, anaerobic systems employing a diverse community of microflora from a municipal waste treatment plant. The added substrate concentrations ranged from 2-25 mg/L for the C1 compounds, and from 1-2 mg/L for the C2 compounds. Radiotracer studies were conducted with 14C-chloroform and 14C-dichloromethane to investigate their fate, as well as to delineate degradative pathways. PCE was completely converted to TCE, but the product was not further degraded to any significant degree. Results suggest that the microbially mediated reductive dechlorination of PCE is an inducible process. Anaerobic conversion of 1,1,1-trichloroethane did not require acclimation. Anaerobic, microbial utilization of chloroform (CF) proceeded without acclimation. Microbial degradation of DCM occurred readily after an acclimation period of variable length. CF inhibits utilization of DCM in a manner that is slowly reversible. Inhibition persists long after levels of CF in the solution are depleted. Keywords: Biological Treatment; Hazardous Wastes; Solvents; Microbial Metabolism.

James M Gossett

Papers

Anaerobic Degradation of C1 and C2 Chlorinated Hydrocarbons