Comparison of butyric kid, ethanol, lactic acid, and propionic acid as hydrogen donors for the reductive dechlorination of tetrachloroethene

Previous studies indicated that dechlorinators can utilize H-2 at lower concentrations than can methanogens. This suggests a strategy for selective enhancement of dechlorination-managing Hz delivery so as to impart a competitive advantage to dechlorinators. Four H-2 donors-butyric and propionic acids, which can only he fermented when the Hp partial pressure is lower than 10(-3.5) or 10(-4.4) arm, respectively, and ethanol and lactic acid, which are readily fermented at H-2 partial pressures 2-3 orders of magnitude higher - were 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, H-2 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 H-2 levels. Ethanol did not support complete dechlorination during shortterm 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.