INACTIVATION OF METHANOGENS RESULTING FROM THE BIOTRANSFORMATION OF CARBON TETRACHLORIDE AND CHLOROFORM

L.J. Weathers and G.F. Parkin

Department of Civil and Environmental Engineering, The University of Iowa, Iowa City, IA, 52246, 319-335-5053


ABSTRACT

Carbon tetrachloride (CT) and chloroform (CF) are common ground water contaminants. Products of CT and CF biotransformation by mixed, methanogenic cultures include the less-chlorinated homologues, CF and dichloromethane (DCM), respectively, and carbon dioxide. The microbial transformation of CT and CF may involve trichloromethyl and dichloromethyl radicals, respectively. The binding of these radicals to cellular lipids and proteins accounts, in part, for the toxicity of CT and CF in mammalian systems. It is possible that CT- or CF-biotransforming microorganisms may be harmed by these radicals in a similar manner. Parallel reasoning has been offered for the inactivation of methanotrophic cells by TCE transformation: TCE epoxide, a toxic, reactive, known TCE metabolite in mammals may also be produced by methanotrophs, resulting in the loss of methanotrophic activity following TCE transformation. The objective of this work was to determine the toxicity associated with the transformation of CT and CF by a methanogenic consortium.

An acetate-enriched, mixed, methanogenic culture having a volatile suspended solids (VSS) concentration of 220 mg/L was used as a source of organisms. Experiments were conducted in 38 ml serum bottles using 25 ml cell suspension. Following the biotransformation of CT or CF, bottles were stripped with N2/carbon dioxide (80/20 v/v) gas to remove residual volatile compounds. The methanogenic activity of the consortium decreased 11% and 24% per 100 Ámol of CT or CF transformed, respectively, corresponding to the inactivation of 5 and 14 mg VSS of methanogenic biomass per Ámol CT or CF transformed, respectively. Control experiments determined that the inactivation of methanogenic bacteria did not result from inhibition by a nonvolatile CT- or CF-metabolite, nor to exposure to DCM. Data analysis showed that the observed inactivation was independent of the maximum CT or CF concentration over the ranges studied and the time-integrated CT or CF dose.

KEY WORDS

inactivation, methanogens, carbon tetrachloride, chloroform

This paper is from the Proceedings of the 10th Annual Conference on Hazardous Waste Research 1995, published in hard copy and on the Web by the Great Plains/Rocky Mountain Hazardous Substance Research Center.