Use of an appropriate hydrogen level is necessary to favor dehalogenation of chlorinated solvents, such as tetrachloroethene (PCE) and trichloroethene (TCE), over other hydrogen using processes. This study examined the competition between dehalogenators and other microorganisms occurring in a benzoate-acclimated dehalogenating methanogenic mixed culture. Results show that the dehalogenators competed best against methanogens and homoacetogens when the hydrogen level was maintained between 2 and 11 nM. The 2 nM hydrogen concentration represents the lower threshold value found here for cis-1,2-dichloroethene (cis-DCE) dehalogenation. The usefulness of this hydrogen range was further confirmed with both batch-fed and continuously-fed reactors. In batch studies, three times more ethene was produced from dehalogenation of cis-DCE using propionate than benzoate as electron donor, while benzoate produced three times more methane than propionate. A three times greater hydrogen utilization efficiency for dehalogenation was obtained with a CSTR than with batch reactors when benzoate was used as substrate because a constant hydrogen concentration in the appropriate range could be maintained with the CSTR. These results suggest different approaches that might be used to favor dehalogenators in competition with other microorganisms.

One major problem with tetrachloroethene (PCE) contamination of aquifers is its ability to form dense, nonaqueous-phase liquids (DNAPL), which can act as a persistent contamination source for decades. Batch studies were performed to determine the potential for biological reductive PCE dehalogenation at high concentration and the effect on competing microorganisms, including methanogens and homoacetogens. Results show that PCE dehalogenation can be obtained at saturation concentration (>0.9 mM). Also, trichloroethene was dehalogenated up to 2.26 mM, and no apparent inhibitory effect on dehalogenation was found with cis-1,2-dichloroethene (cDCE) and ethene at the highest tested levels of 0.66 and 1.05 mM, respectively. However, such high concentrations of PCE, cDCE, and ethene were inhibitory to methanogens, and high concentrations of PCE were inhibitory to homoacetogens. Such inhibition is highly beneficial as it greatly diminished the competition by methanogens and homoacetogens for added electron donors, including hydrogen, resulting in highly efficient substrate utilization for dehalogenation. PCE DNAPL dehalogenation in a column study required less than 1 g of the electron donor pentanol to dehalogenate 1 g of PCE to cDCE (<2 mol of pentanol/mol of PCE). Additionally, DNAPL dissolution rate was significantly enhanced when directly coupled with biological dehalogenation.

Tetrachloroethene (PCE) dense nonaqueous-phase liquid (DNAPL) can act as a persistent groundwater contamination source for decades. Biologically enhanced dissolution of pure PCE DNAPL has potential for reducing DNAPL longevity as indicated previously (Environ. Sci. Technol. 2000, 34, 2979). Reported here are expanded studies to evaluate donor substrates that offer different remediation strategies for bioenhanced DNAPL dissolution, including pentanol (soluble substrate, fed continuously), calcium oleate (insoluble substrate, placed in column initially by alternate pumping of sodium oleate and calcium chloride), and olive oil (mixed with PCE and placed in column initially). Compared with a no-substrate column control, the DNAPL dissolution rate was enhanced about three times when directly coupled with biological transformation. The major degradation product formed was cDCE, but significant amounts of VC and ethene were also found with some columns. Extensive methanogenesis, which reduced PCE transformation, occurred in both the pentanol-fed and oleate-amended columns, but not in the olive-oil-amended column, suggesting that methanogens managed to colonize column niches where PCE DNAPL was not present. Detrimental methane production in the pentanol-fed column was nearly eliminated by presaturating the feed solution with PCE. These results suggest potential DNAPL remediation strategies to enhance dehalogenation while controlling competitive methanogenic utilization of donor substrates.

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