Dechlorination of PCE by Mixed Methanogenic Cultures Using Hollow-Fiber Membranes

The study investigated the use of hollow-fiber membranes for hydrogen (H₂) delivery to support the biological reductive dechlorination of tetrachloroethene (PCE) Two experiments were performed in which H₂ was supplied through membranes placed in stirred batch reactors containing a mixed methanogenic/dechlorinating culture and PCE (≤10?µM. Reductive dechlorination of PCE to cis-dichloroethene was sustained in the reactors receiving H₂ (1% H₂ and 50% H₂), while negligible dechlorination was observed in control reactors (100% N₂). The 1%-H₂-fed reactor outperformed the 50%-H₂-fed reactor in the first experiment. However, the dechlorinating performance in the two reactors was similar in the second experiment. Despite relatively high H₂ concentrations (4.6 to 178?µM) that led to H₂ consumption (and CH₄ production) by methanogens, dechlorination was effectively maintained for the duration of the experiments (35 to 62 days). The results of this study are significant in that dechlorination was sustained in a minimal medium by membrane-delivered H₂. Dechlorination was also maintained at aqueous H₂ concentrations that exceeded the thermodynamic thresholds for not only dechlorination (<0.1 to 2?nM, but also methanogenesis (～10?n_M) and homoacetogenesis (94 to 400?n_M. The results of these experiments suggest that membranes are a promising H₂ delivery technology for stimulating the bioremediation of chlorinated ethene-contaminated aquifers.     

Determination of intrinsic monod kinetic parameters for two heterotrophic tetrachloroethene (PCE)‐respiring strains and insight into their application

A complete set of mathematically identifiable and meaningful kinetic parameters estimates is needed to accurately describe the activity of individual populations that dehalorespire tetrachloroethene (PCE) and other chlorinated ethenes. These data may be difficult to extract from the literature because kinetic parameter estimates obtained using mixed cultures may reflect the activity of multiple dehalorespiring populations, while those obtained at low initial substrate‐to‐biomass ratios (S₀/X₀) are influenced by culture history and are generally not relevant to other systems. This study focused on estimation of electron donor and acceptor utilization kinetic parameters for the heterotrophic dehalorespirers Desulfuromonas michiganensis strain BB1 and Desulfitobacterium sp. strain PCE1. Electron acceptor utilization kinetic parameters that are identifiable and independent of culture history, i.e., intrinsic, could be estimated at S₀/X₀ ≥ 10, with both concentrations expressed as chemical oxygen demand (COD). However, the parameter estimates did not accurately describe dechlorination kinetics at lower S₀/X₀ ratios. The maximum specific substrate utilization rates (q_max) and half‐saturation constants (K_S) for PCE and trichloroethene (TCE) estimated for the two heterotrophic strains are higher than the values reported for Dehalococcoides cultures. These results suggest that the natural niche of Dehalococcoides strains that can metabolize a range of chlorinated ethenes may be to respire dichloroethene and vinyl chloride produced by Desulfuromonas and Desulfitobacterium strains or other populations that dechlorinate PCE and TCE at faster rates. Few data exist on the electron donor utilization kinetics of heterotrophic dehalorespirers. The results of this study suggest that Desulfuromonas and Desulfitobacterium strains should be able to compete for organic electron donors with other heterotrophic populations in the subsurface. Biotechnol. Bioeng. 2009; 104: 301–311 © 2009 Wiley Periodicals, Inc.     

Impact of Chlorofluorocarbon 113 on Chlorinated Ethene Biodegradation

The inhibition of tetrachloroethene (PCE) degradation in anaerobic, ethanol-fed PCE-enrichment cultures by chlorofluorocarbon 113 (CFC113) was a function of the initial CFC113 concentration. Typically, aqueous CFC113 concentrations up to 1 mg/L slowed, but did not stop PCE-degradation, but cis-1,2-dichloroethene (cDCE) degradation was inhibited by 0.2 mg/L CFC113. In some cultures, however, PCE degradation was stopped by as little as 0.15 mg/L CFC113. CFC113 also slowed the consumption of hydrogen and the concurrent methane production. CFC113 slowly degraded in PCE-enrichment cultures to hydrochlorofluorocarbon 123a (HCFC123a). Chlorotrifluoroethene was also detected. Although relatively non-toxic, CFC113 may nevertheless pose remediation challenges when present at sites that also contain PCE.     

Effective Bead Preparation of Coimmobilized Methanogenic and Methanotrophic Bacteria for Tetrachloroethene Degradation

Three types of coimmobilized methanogenic and methanotrophic bacterial beads – Ca-alginate, Ba-alginate, and Ca-alginate chitosan – were used for tetrachloroethene (PCE) degradation. For the purpose of effective preparation of coimmobilized bacterial beads, the diameter and broken-loading of beads were measured. The activity tests to find the optimal bacteria concentration in the bead were performed. It was found that Ba-alginate beads had superiority in bacterial growth and the degree of strength of beads from the diameter and broken-loading tests. Also, it was shown that it is most effective to add 200 mL of methanogens into 500 mL of 2% alginate solution and 20 mL of methanotrophs into 500 mL to 2% alginate solution. When methanogens and methanotrophs were applied with the Ba-alginate bead in the actual dechlorination of PCE, the biological PCE dechlorination rate was 92%, and there was highly effective degradation of PCE based on the coimmobilized bead. Additionally, relation to the diameter (X) and broken-loading (Y) of the Ba-alginate bead was derived following equation, Y = 438.02 exp(–1.4815 X).     

Product distribution during transformation of multiple contaminants by a high-rate, tetrachlorethene-dechlorinating enrichment culture

Radiolabeled tetrachloroethene (PCE) and carbon tetrachloride (CT) were added to batch systems containing a lactate-enrichment culture displaying apparent dehalorespiration abilities to analyze the influence of mixtures on product distribution. Both CT and PCE were readily dechlorinated, although significant carbon disulfide (CS₂) formation was observed during CT transformation. Calculated 1,2-¹⁴C-PCE recoveries for biotic treatments were between 91 and 104%, but an inability to recover products such as CS₂ led to lower recoveries of ¹⁴C-CT (55 to 62%). While the majority of activity in ¹⁴C-CT-spiked treatments was recovered in the volatile fraction, ¹⁴CO₂ increased significantly over time. 1,2-¹⁴C-PCE was primarily recovered in volatile and non-strippable fractions, but a significant increase in ¹⁴CO₂ relative to cell-free controls suggested that the presence of a non-specific dechlorination pathway complementing dehalorespiration. The addition of both CT and PCE inhibited the transformation of the individual compounds and reduced the percentages recovered as ¹⁴CO₂. However, the magnitude of these reductions was not severe and appeared to be the result of slower overall transformation rather than a complete inhibition of mineralization pathways.     

Anaerobic Microbial Reductive Dehalogenation of Chlorinated Ethenes

The current knowledge on microbial reductive dechlorination of chlorinated ethenes (CEs) and its application are discussed. Physiological studies on CEs dechlorinating microorganisms indicate that a distinction can be made between cometabolic dechlorination and halorespiration. Whereas cometabolic dechlorination is a coincidental and nonspecific side reaction, catalyzed by several methanogenic and acetogenic bacteria, halorespiration is a specific enzymatic reaction from which metabolic energy can be gained. In contrast to the well-studied biological dechlorination of PCE to cis-DCE, little is known about the biology of the further dechlorination from cis-DCE to ethene. Bacteria performing the latter reaction have not yet been isolated. Microbial reductive dechlorination can be applied to the in situ bioremediation of CEs contaminated sites. From laboratory and field studies, it has become clear that the dechlorination of tetrachloroethene (PCE) to cis-clichloroethene (cis-DCE) occurs rapidly and can be stimulated relatively easily. However, complete reduction to ethene appears to be a slower process that is more difficult to achieve.     

Complete dechlorination of tetrachloroethene to ethene in presence of methanogenesis and acetogenesis by an anaerobic sediment microcosm

An anaerobic consortium taken from brackish sediments, enriched byPCE/CH₃OH sequential feeding, was capable of completely dechlorinating tetrachloroethene(PCE) to ethene (ETH). In batch experiments, PCE (0.5 mM) was dechlorinated to ethene (ETH) in approximately 75 h with either CH₃OH or H₂ as the electron donor. When VC (0.5 mM) was added instead of PCE it was dechlorinated without any initial lag by the PCE/CH₃OHenriched consortium, although at a lower dechlorination rate. In batch tests H₂ could readilyreplace CH₃OH for supporting PCE dechlorination, with a similar PCE dechlorination rate andproduct distribution with respect to those observed with methanol. This indicates that H₂ productionduring CH₃OH fermentation was not the rate-limiting step of PCE or VC dechlorination.Acetogenesis was the predominant activity when methanol was present. A remarkable homoacetogenicactivity was also observed when hydrogen was supplied instead of methanol.     

Trichloroacetic acid of different origin in Norway spruce needles and chloroplasts

Trichloroacetic acid (TCA), a secondary atmospheric pollutant, is also formed in forest soil and thus ranked among natural organohalogens. The observed biooxidation of atmospheric tetrachloroethene (PER) to TCA in chloroplasts has led to the investigation of the mode of action of TCA in spruce needles, since TCA is also accumulated in the needles after its rapid uptake from soil by roots. Being phytotoxic, TCA considerably influences conifers by affecting their photosynthetic apparatus. We examined the transport of TCA from soil into chloroplasts in order to compare the effects of TCA on conifers from both sources, i.e. endogenously produced within chloroplasts or taken up by roots. The influence of TCA formed in chloroplasts was found to be much more adverse than that of “soil” TCA.     

The Impact of H2 Addition on Dechlorinating Microbial Communities

Laboratory-scale column experiments were performed to investigate the effects of membrane-supplied H₂ on tetrachloroethene (PCE) dechlorination and microbial community composition. Columns were filled with aquifer material from one of two TCE-contaminated sites and fed a PCE-spiked anaerobic minimal medium for approximately 1 year. For each experiment, one or more experimental columns were supplied with H₂ via gas-permeable hollow-fiber membranes with one control column not receiving any H₂. After approximately 1 year of operation, aquifer material samples were collected along the length of the columns. Bacterial communities in the samples were analyzed by amplifying the highly variable V3 region of the 16S rRNA gene and separating amplicons using denaturing gradient gel electrophoresis. Microbial community profiles in H₂-fed (continuous or pulsed delivery) columns were compared with those in untreated control columns and microbial community profiles were also compared with dechlorination profiles. Selected bands were sequenced for identification. Supply of the simple electron donor H₂, changed the microbial community composition, but did not decrease overall diversity. Continuous H₂ addition via hollow-fiber membranes enriched for Dehalococcoides-like species, whose relative abundance correlated with enhanced dechlorination activity. PCE was completely dechlorinated to ethene in columns packed with aquifer material from Cape Canaveral, Florida; PCE was dechlorinated to only cis-dichloroethene, however, in columns packed with aquifer material from a TCE-contaminated wetland near Minneapolis, Minnesota. Unexpectedly, Dehalococcoides-like populations were detected in samples from both sets of column experiments. These results suggest that the mere detection of Dehalococcoides-like species in a sample of aquifer material is not a sufficient indicator of the potential to dechlorinate PCE to ethene via biostimulation by H₂.