Department of Civil and Environmental Engineering, The University of Iowa, Iowa City, lA, 52246, 319-335-5053
Several recent studies have shown that chlorinated aliphatic hydrocarbons (CAHs) may be reduced by metallic iron. These studies have focused on abiotic processes, while limited attention has been given to combined microbial and abiotic dechlorination. Methanogenic bacteria can use metallic iron as an energy source by coupling the anodic dissolution of iron, which is an otherwise unfavorable reaction, with the consumption of water-derived H2, which is a very thermodynamically favorable reaction.
The microbial transformation of CAHs has been observed in pure and mixed methanogenic cultures. Hence, under methanogenic conditions in the presence of zero-valent iron, two degradation mechanisms may be important: Fe(0) may reduce CAHs abiotically, and Fe(0) may indirectly reduce CAHs via biodehalogenation. In light of this, we investigated the transformation of carbon tetrachloride (CT), chloroform (CF) and dichloromethane (DCM) in methanogenic incubations amended with iron metal.
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 anaerobically in duplicate at 20ºC using 25 ml liquid volume in 38 ml serum bottles. Resting (unfed) cells were used in incubations containing cell suspension. Experiments examined the transformation of CT, CF and DCM in bottles containing (1) iron and cell suspension, (2) cell suspension only, and (3) iron in cell-free supernatant. For CT and CF, the value of the pseudo-first order rate constant, k, for the iron-cell (IC) treatments was significantly greater than k for the iron-supernatant (IS) and resting cell (RC) treatments, respectively. DCM was not transformed. Analysis of the rate coefficients also revealed that the interaction between cells and iron was synergistic with regard to CT and CF degradation. The increased CT and CF transformation kinetics in treatment IC may be due to cometabolism by hydrogen-oxidizing methanogens. Methane production was negligible in bottles containing cell suspension only, or in bottles containing iron and cell-free supernatant. Methane production was observed, however, in CT- and CF-free bottles containing iron and cell suspension.
These experiments indicate that methanogens coupled the biocorrosion of iron metal and biodehalogenation of CT and CF via cometabolism, with water-derived hydrogen acting as energy source. Further work is required to investigate if the results observed here are sustainable in flow-through environments.
iron, cometabolism, hydrogen, methanogens, biocorrosion
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.