Comparison of Butyric Acid, Ethanol, Lactic Acid, and Propionic Acid as Hydrogen Donors for the Reductive Dechlorination of Tetrachloroethene

Abstract

Previous studies indicated that dechlorinators can utilize H2 at lower concentrations than can methanogens. This suggests a strategy for selective enhancement of dechlorinationmanaging H2 delivery so as to impart a competitive advantage to dechlorinators. Four H2 donorsbutyric and propionic acids, which can only be fermented when the H2 partial pressure is lower than 10-3.5 or 10-4.4 atm, respectively, and ethanol and lactic acid, which are readily fermented at H2 partial pressures 2−3 orders of magnitude higherwere administered to anaerobic mixed cultures. Comparison of the resulting enrichment cultures during time-intensive, short-term tests showed significant differences in patterns of donor degradation, H2 production and use, and distribution of reduction equivalents between dechlorination and competing methanogenesis. Amendment with butyric and propionic acids resulted in less methanogenesis than did amendment with ethanol or lactic acid, which generated much higher H2 levels. Ethanol did not support complete dechlorination during short-term tests, but it was a viable donor over long-term testing because a portion was converted to a pool of slowly degraded propionic acid and because during long-term tests, cultures were routinely co-amended with pre-fermented yeast extract, a source of slowly fermented volatile fatty acids. Understanding the fate of electron donors and their fermentation products is an important component in understanding dechlorinating communities.