Acetate versus hydrogen as direct electron donors to stimulate the microbial reductive dechlorination process at chloroethene-contaminated sites

A study to evaluate the dechlorination end points and the most promising electron donors to stimulate the reductive dechlorination process at the chloroethene-contaminated Bachman Road site in Oscoda, MI, was conducted. Aquifer materials were collected from inside the plume and used to establish microcosms under a variety of electron donor conditions using chlorinated ethenes as electron acceptors. All microcosms that received an electron donor showed dechlorination activity, but the end points depended on the sampling location, indicating a heterogeneous distribution of the dechlorinating populations in the aquifer. Interestingly, several microcosms that received acetate as the only electron donor completely dechlorinated PCE to ethene. All acetate-amended microcosms rapidly converted PCE to cis-DCE, whereas PCE dechlorination in H-2-fed microcosms only occurred after a pronounced lag time and after acetate had accumulated by H-2/CO2 acetogenic activity. The microcosm experiments were corroborated by defined co-culture experiments, which demonstrated that H-2 sustained PCE to cis-DCE dechlorination by acetotrophic populations in the presence of H-2/CO2 acetogens. In sediment-free nonmethanogenic enrichment cultures derived from ethene-producing microcosms, acetate alone supported complete reductive dechlorination of chloroethenes to ethene, although the addition of H-2 resulted in higher cisDCE and VC dechlorination rates. Measurements of H-2 production and consumption suggested that syntrophic acetate-oxiclizing population(s) were active in the enrichment cultures. These findings demonstrated that either acetate or H-2 alone can be sufficient to promote complete reductive dechlorination to ethene, provided syntrophic acetate-oxidizing population(s) and H-2/CO2 acetogenic population(s) are present, respectively. Approaches that result in increased fluxes of both electron donors (e.g., by addition of fermentable substrates) seem most promising to sustain complete high rate reductive dechlorination to ethene in the contaminated zone of the Bachman aquifer, although acetate or H-2 alone may be sufficient to drive the dechlorination process to completion.