Dynamics of reductive TCE dechlorination in two distinct H2 supply scenarios and at various temperatures

Abstract

Anaerobic microbial dechlorination of trichloroethene (TCE) by a mixed, Dehalococcoides containing culture was investigated at different temperatures (4-60 degrees C) using propionate and lactate as a slow- and fast-releasing hydrogen (H(2)) source, respectively. Distinct temperature-dependent dynamics of substrate fermentation and H(2) levels could explain observed patterns of dechlorination. While varying the temperature caused changes in rate, the overall pattern of dechlorination was characteristic of the supplied electron donor. Feeding cultures with a rapidly fermentable substrate such as lactate generally resulted in high H(2) concentrations and fast and complete dechlorination accompanied by rapid methanogenesis. In contrast, low H(2) release rates resulting from fermentation of propionate were associated with 2 to 3-fold longer time frames necessary for complete dechlorination at intermediate temperatures (15-30 degrees C). A lag-phase prior to dechlorination of cis-dichloroethene (cDCE), together with a characteristic build-up of H(2) and methane, was consistently observed at slow H(2) supply. At temperatures of 10 degrees C and lower, the system remained in this lag phase and no dechlorination past cDCE was observed within the experimental time frame. However, when lactate was the substrate, complete dechlorination of TCE occurred within 74 days at 10 degrees C, accompanied by methane production. The choice of fermentable substrate decisively influenced the rate and degree of dechlorination at an electron donor/TCE ratio as high as 666:1. Temperature-dependent H(2) levels resulting from fermentation of different substrates could be satisfactorily explained through thermodynamic calculations of the Gibbs free energy yield assuming a constant metabolic energy threshold of -20 kJ/(mol reaction).

Reductive Dechlorination of Trichloroethylene (TCE) in Competition with Fe and Mn Oxides—Observed Dynamics in H2-dependent Terminal Electron Accepting Processes

“…and the H 2 consumption by dechlorinators and/or iron and manganese reducers present in the system. The H 2 concentrations exceed 1000 nM and suggest that the dechlorination process is not limited due to a too low H 2 level (Heimann et al, 2007;Lee et al, 2007). This H 2 level also suggests the possibility of additional H 2 -consuming processes such as methanogenesis and acetogenesis along with the observed Fe(III) and Mn(IV) reduction in the experimental treatments.…”

“…Here, we did not observe distinct H 2 levels corresponding to the TCE dechlorination and Fe(III) succession. The first dechlorination step in all systems was characterized by H 2 concentrations far beyond any metabolic threshold or related to partial equilibrium due to rapid formate fermentation producing an excess of reducing equivalents (Heimann et al, 2007). This ample availability of electron donor in all batches is a likely reason for the observed similar dechlorination rates and the lack of a larger time lag in the Fe(III) rich systems.…”

“…1f and j, 2e), probably accompanied by active methanogenesis. Methanogens are commonly found to be growing concomitantly with Dehalococcoides within chloroethene-degrading communities (Heimann et al, 2007). The H 2 concentrations observed in biotic control are also in the range observed for methanogenic conditions.…”

“…Chlorinated ethenes (TCE, DCE isomers and vinyl chloride) were analyzed by gas chromatography (Agilent 6890N, Mass Spectrometry: Agilent 5973) as described by Heimann et al (2007). A 200 µL headspace sample is injected into a 21 ml sample vial containing 0.5 mL acidified internal standard (10 ppmv aqueous solution of chloroform) and 9.5 mL of distilled water.…”

“…Samples for organic anions (formate, acetate and lactate) and inorganic anions (sulfate and chloride) were analyzed by suppressed ion chromatography on a Dionex ICE-AS1 ion exclusion column and sulfate samples by Dionex Ion Pac AS14 column (Heimann et al, 2007).…”