Dechlorination Of Perchloroethene Research Articles

Mechanistic insight into co-metabolic dechlorination of hexachloro-1,3-butadiene in Dehalococcoides

Hexachloro-1,3-butadiene (HCBD) as one of emerging persistent organic pollutants (POPs) poses potential risk to human health and ecosystems. Organohalide-respiring bacteria (OHRB)-mediated reductive dehalogenation represents a promising strategy to remediate HCBD-contaminated sites. Nonetheless, information on the HCBD-dechlorinating OHRB and their dechlorination pathways remain unknown. In this study, both in vivo and in vitro experiments, as well as quantum chemical calculation, were employed to successfully identify and characterize the reductive dechlorination of HCBD by Dehalococcoides. Results showed that some Dehalococcoides extensively dechlorinated HCBD to (E)-1,2,3-tri-CBD via (E)-1,1,2,3,4-penta-CBD and (Z,E)-1,2,3,4-tetra-CBD in a co-metabolic way. Both qPCR and 16S rRNA gene amplicon sequencing analyses suggested that the HCBD-dechlorinating Dehalococcoides coupled their cell growth with dechlorination of perchloroethene (PCE), rather than HCBD. The in vivo and in vitro ATPase assays indicated ≥78.89% decrease in ATPase activity upon HCBD addition, which suggested HCBD inhibition on ATPase-mediated energy harvest and provided rationality on the Dehalococcoides-mediated co-metabolic dechlorination of HCBD. Interestingly, dehalogenation screening of organohalides with the HCBD-dechlorinating enrichment cultures showed that debromination of bromodichloromethane (BDCM) was active in the in vitro RDase assays but non-active in the in vivo experiments. Further in vitro assays of hydrogenase activity suggested that significant inhibition of BDCM on the hydrogenase activity could block electron derivation from H2 for consequent reduction of organohalides in the in vivo experiments. Therefore, our results provided unprecedented insight into metabolic, co-metabolic and RDase-active-only dehalogenation of varied organohalides by specific OHRB, which could guide future screening of OHRB for remediation of sites contaminated by HCBD and other POPs.

Enhancement of perchloroethene dechlorination by a mixed dechlorinating culture via magnetic nanoparticle-mediated isolation method

Highly enriched active dechlorinating cultures are important in advancing microbial remediation technology. This study attempted to enrich a rapid perchloroethene (PCE) dechlorinating culture via magnetic nanoparticle-mediated isolation (MMI). MMI is a novel method that can separate the fast-growing and slow-growing population in a microbial community without labelling. In the MMI process, PCE dechlorination was enhanced but the subsequent trichloroethene (TCE) dechlorination was inhibited, with TCE cumulative rate reached up to 80.6% within 70 days. Meanwhile, the microbial community was also changed, with fast-growing genera like Dehalobacterium and Petrimonas enriched, and slow-growing Methanosarcina almost ruled out. Relative abundances of several major genera including Petrimonas and Methanosarcina were positively related to TCE dechlorination rate and the relative abundance of Dehalococcoides. On the other hand, Dehalobacterium was negatively related to TCE dechlorination rate and Dehalococcoides abundance, suggesting potential competition between Dehalobacterium and Dehalococcoides. The regrowth of Methanosarcina coupled well with the recovery of TCE dechlorination capacity, which implied the important role of methanogens in TCE dechlorination. Via MMI method, a simpler but more active microbial consortium could be established to enhance PCE remediation efficiency. Methanogens may act as the indicators or biomarkers for TCE dechlorination, suggesting that methanogenic activity should also be monitored when enriching dechlorination cultures and remediating PCE contaminated sites. CapsuleA rapid perchloroethene dechlorinator was gotten via magnetic nanoparticles and dechlorination of trichloroethene coupled well with growth of Methanosarcina.

Synergistic effects of microbial anaerobic dechlorination of perchloroethene and nano zero-valent iron (nZVI) – A lysimeter experiment

Perchloroethene (PCE) is a hazardous and persistent groundwater pollutant. Both treatment with nanoscaled zero-valent iron (nZVI) and biological degradation by bacteria have downsides. Distribution of nZVI underground is difficult and a high percentage of injected nZVI is consumed by anaerobic corrosion, forming H2 rather than being available for PCE dechlorination. On the other hand, microbial PCE degradation can suffer from the absence of H2. This can cause the accumulation of the hazardous metabolites cis-1,2-dichloroethene (DCE) or vinylchloride (VC). The combination of chemical and biological PCE degradation is a promising approach to overcome the disadvantages of each method alone. In this lysimeter study, artificial aquifers were created to test the influence of nZVI on anaerobic microbial PCE dechlorination by a commercially available culture containing Dehalococcoides spp. under field-like conditions. The effect of the combined treatment was investigated with molasses as an additional electron source and after cessation of molasses addition. The combination of nZVI and the Dehalococcoides spp. containing culture led to a PCE discharge in the lysimeter outflow that was 4.7 times smaller than that with nZVI and 1.6 times smaller than with bacterial treatment. Moreover, fully dechlorinated end-products showed an 11-fold increase compared to nZVI and a 4.2-fold increase compared to the microbial culture. The addition of nZVI to the microbial culture also decreased the accumulation of hazardous metabolites by 1.7 (cis-DCE) and 1.2 fold (VC). The stimulatory effect of nZVI on microbial degradation was most obvious after the addition of molasses was stopped.

Inhibitory effects of metal ions on reductive dechlorination of polychlorinated biphenyls and perchloroethene in distinct organohalide-respiring bacteria

Bioremediation of sites co-contaminated with organohalides and metal pollutants may have unsatisfactory performance, since metal ions can potentially inhibit organohalide respiration. To understand the detailed impact of metals on organohalide respiration, we tested the effects of four metal ions (i.e., Cu2+, Cd2+, Cr3+ and Pb2+), as well as their mixtures, on reductive dechlorination of perchloroethene (PCE) and polychlorinated biphenyls (PCBs) in three different cultures, including a pure culture of Dehalococcoides mccartyi CG1, a Dehalococcoides-containing microcosm and a Dehalococcoides-Geobacter coculture. Results showed that the inhibitive impact on organohalide respiration depended on both the type and concentration of metal ions. Interestingly, the metal ions might indirectly inhibit organohalide respiration through affecting non-dechlorinating populations in the Dehalococcoides-containing microcosm. Nonetheless, compared to the CG1 pure culture, the Dehalococcoides-containing microcosm had higher tolerance to the individual metal ions. In addition, no synergistic inhibition was observed for reductive dechlorination of PCE and PCBs in cultures amended with metal ion mixtures. These results provide insights into the impact of metal ions on organohalide respiration, which may be helpful for future in situ bioremediation of organohalide-metal co-contaminated sites.

Integrative isotopic and molecular approach for the diagnosis and implementation of an efficient in-situ enhanced biological reductive dechlorination of chlorinated ethenes

Based on the previously observed intrinsic bioremediation potential of a site originally contaminated with perchloroethene (PCE), field-derived lactate-amended microcosms were performed to test different lactate isomers and concentrations, and find clearer isotopic and molecular parameters proving the feasibility of an in-situ enhanced reductive dechlorination (ERD) from PCE-to-ethene (ETH). According to these laboratory results, which confirmed the presence of Dehalococcoides sp. and the vcrA gene, an in-situ ERD pilot test consisting of a single injection of lactate in a monitoring well was performed and monitored for 190 days. The parameters used to follow the performance of the ERD comprised the analysis of i) hydrochemistry, including redox potential (Eh), and the concentrations of redox sensitive species, chlorinated ethenes (CEs), lactate, and acetate; ii) stable isotope composition of carbon of CEs, and sulphur and oxygen of sulphate; and iii) 16S rRNA gene sequencing from groundwater samples. Thus, it was proved that the injection of lactate promoted sulphate-reducing conditions, with the subsequent decrease in Eh, which allowed for the full reductive dechlorination of PCE to ETH in the injection well. The biodegradation of CEs was also confirmed by the enrichment in 13C and carbon isotopic mass balances. The metagenomic results evidenced the shift in the composition of the microbial population towards the predominance of fermentative bacteria. Given the success of the in-situ pilot test, a full-scale ERD with lactate was then implemented at the site. After one year of treatment, PCE and trichloroethene were mostly depleted, whereas vinyl chloride (VC) and ETH were the predominant metabolites. Most importantly, the shift of the carbon isotopic mass balances towards more positive values confirmed the complete reductive dechlorination, including the VC-to-ETH reaction step. The combination of techniques used here provides complementary lines of evidence for the diagnosis of the intrinsic biodegradation potential of a polluted site, but also to monitor the progress, identify potential difficulties, and evaluate the success of ERD at the field scale.

Reductive dechlorination of perchloroethene (PCE) and bacterial community changes in a continuous-flow, two-stage anaerobic column

This study is aimed at investigating the performance of the continuous-flow, two-stage anaerobic column for the reductive dechlorination of perchloroethene (PCE) in a trichloroethene (TCE)-contaminated aquifer without bioaugmentation. Results indicated that complete dechlorination of PCE to cis-dichloroethene (DCE) was observed within the first 12 days after the addition of lactate and PCE (620:1 mg mg−1 mass ratio). The PCE conversion to ethene was initially observed in the 2nd column after 70 days (during phase II) upon the increase of lactate to PCE mass ratio to 5000:1 (mg mg−1). However, the degradation of PCE to ethene occurred in the 1st column after 30 days operation of phase IV (217–310 d) under the high PCE feed conditions (33.65 ± 2.86 μM). 16S rDNA clone library sequence analysis confirmed that Dehalobacter sp., Desulfuromonas sp., Desulfovibrio sp. Geobacter sp., Methanosaeta sp., Methanobacterium paludism, and M. spelaei were the dominant species in the original aquifer, and their relative abundance were further changed following lactate addition. Dehalococcoides mccartyi sequences were retrieved after lactate stimulation. Overall, the continuous-flow, two-stage anaerobic column process exhibited the maximum dechlorination (6.23 μmol L−1 hr−1) of PCE without bioaugmentation, indicating its potential up-scaling application in PCE dechlorination.

Influences of benzene or toluene on dechlorination of perchloroethene and trichloroethene in a simulated zero-valent iron permeable reactive barrier

Abstract Column studies were conducted to investigate the influence of benzene or toluene on the dechlorination of perchloroethene (PCE) and trichloroethene (TCE) in columns packed with zero-valent iron (ZVI) in order to simulate a permeable reactive barrier (PRB). Enhancive and inhibitive influences of benzene and toluene, respectively, on PCE and TCE reduction were observed within 10–80 pore volumes (PV) that flowed through the columns. However, such influences dissipated when the flow-through volume increased above 80 PV. The presence of benzene increased the mean dechlorination kobs of PCE and TCE by 7% and 6%, respectively; in contrast, the presence of toluene decreased the mean dechlorination kobs of PCE and TCE by 21% and 10%, respectively. We presumed that the more competitive adsorption between benzene and toluene in comparison to PCE and TCE on the ZVI particle surface might have caused the disparate influences. With a lower affinity for ZVI, benzene has no substantial influence on PCE and TCE adsorption on the ZVI particle surface. However, toluene has a higher affinity for ZVI and could compete with PCE and TCE by contacting the ZVI particle surface. Moreover, given benzene's higher polarity, it could also benefit electron transfer from ZVI to PCE and TCE.

Isotopic effects of PCE induced by organohalide-respiring bacteria.

Reductive dechlorination performed by organohalide-respiring bacteria (OHRB) enables the complete detoxification of certain emerging groundwater pollutants such as perchloroethene (PCE). Environmental samples from a contaminated site incubated in a lab-scale microcosm (MC) study enable documentation of such reductive dechlorination processes. As compound-specific isotope analysis is used to monitor PCE degradation processes, nucleic acid analysis-like 16S-rDNA analysis-can be used to determine the key OHRB that are present. This study applied both methods to laboratory MCs prepared from environmental samples to investigate OHRB-specific isotope enrichment at PCE dechlorination. This method linkage can enhance the understanding of isotope enrichment patterns of distinct OHRB, which further contribute to more accurate evaluation, characterisation and prospection of natural attenuation processes. Results identified three known OHRB genera (Dehalogenimonas, Desulfuromonas, Geobacter) in diverse abundance within MCs. One species of Dehalogenimonas was potentially involved in complete reductive dechlorination of PCE to ethene. Furthermore, the isotopic effects of PCE degradation were clustered and two isotope enrichment factors (ε) (-11.6‰, -1.7‰) were obtained. Notably, ε values were independent of degradation rates and kinetics, but did reflect the genera of the dechlorinating OHRB.

Effects of tidal operation on pilot-scale horizontal subsurface flow constructed wetland treating sulfate rich wastewater contaminated by chlorinated hydrocarbons.

Three different flow regimes were carried out in a pilot-scale horizontal subsurface flow constructed wetland-treating sulfate rich wastewater contaminated with monochlorobenzene (MCB) and perchloroethene (PCE). The three regimes were continuous flow, 7-day cycle discontinuous flow, and 2.5-day cycle discontinuous flow. The results show that intensifying the tidal regime (2.5-day cycle) significantly enhanced MCB removal before 2m from the inlet and increasing PCE removal efficiency at 0.5m. The PCE dechlorination process was promoted with tidal operation, especially under the 2.5-day cycle regime, with significant increases of cis-1,2- dichloroethenes (DCEs), vinyl chloride (VC), and ethene, but trans-1,2-DCE was significantly decreased after tidal operation. Due to the high sulfate concentration in the influent, sulfide was observed in pore water up to 20 and 23mgL-1 under continuous flow and 7-day cycle regime, respectively. However, sulfide concentrations decreased to less than 4mgL-1 under intensified tidal operation (2.5-day cycle). The increase of oxygen concentration in pore water through intensified tidal operation resulted in better MCB removal performance and the successful inhibition of sulfate reduction. In conclusion, intensifying tidal operation is an effective approach for the treatment of chlorinated hydrocarbons and inhibiting sulfide accumulation in horizontal subsurface flow constructed wetland.

Novel Fe-Pd/SiO2 catalytic materials for degradation of chlorinated organic compounds in water

AbstractNovel reactive materials for catalytic degradation of chlorinated organic compounds in water at ambient conditions have been prepared on the basis of silica-supported Pd-Fe nanoparticles. Nanoscale Fe-Pd particles were synthesized inside porous silica supports using (NH4)3[Fe(C2O4)3] and [Pd(NH3)4]Cl2 or Pd acetate as reaction precursors. According to temperature programmed reduction (TPR) studies, Pd introduction decreased the reduction temperature of the supported Fen+ species and nearly complete reduction with H2 was observed at 400 °C. The successful surface loading with Pd was confirmed by X-ray photoelectron spectroscopy (XPS) analysis. Characterization of the samples by X-ray diffraction (XRD) and X-ray absorption near-edge structure + extended X-ray absorption fine structure (XANES + EXAFS) verified the presence of highly dispersed Pd0, Pdx Fe1–x and Fe0 phases. Reduction of the supported precursors in hydrogen resulted in materials that were highly active in perchloroethene (PCE) degradation and 2-chlorobiphenyl (2-ClBP) dechlorination. It was found that highly dispersed amorphous Fe-Pd bimetallic nanoparticles on silica support showed superior catalytic activity against PCE dechlorination in comparison to the free-standing Fe-Pd nanoparticles. For the samples with the same Fe content, the conversion of chlorinated organics as well as the stability increased with the Pd loading, e.g., the most effective degradation of PCEs and 2-ClBP was achieved at a Pd loading of 2.3–3.2 wt. %.

Successful microcosm demonstration of a strategy for biodegradation of a mixture of carbon tetrachloride and perchloroethene harnessing sulfate reducing and dehalorespiring bacteria

Carbon tetrachloride (CT) is known to inhibit the transformation of perchloroethene (PCE) to ethene by dehalorespiring bacteria, creating a challenge for the bioremediation of environments contaminated with both compounds. We report on the sequential use of sulfate reduction and dehalorespiration as a microbial strategy for the transformation of a mixture of CT (10μM) and PCE (14μM). Sulfide production in Desulfovibrio vulgaris cultures led to complete CT disappearance in as little as 12 days. The addition of amorphous ferric oxide decreased the proportion of chloroform (CF) produced from 65% to 30%. CT conversion rates were enhanced more than 13-fold where vitamin B12 (5μM) was added. In vitamin B12-containing D. vulgaris cultures, no chlorinated products were detected and carbon disulfide was the major product of CT transformation. PCE concentrations were not impacted upon by D. vulgaris activity. The subsequent inoculation of a PCE-respiring enrichment culture resulted in microbial PCE dechlorination to ethene.

Sequential Reductive and Oxidative Biodegradation of Chloroethenes Stimulated in a Coupled Bioelectro-Process

This article for the first time demonstrates successful application of electrochemical processes to stimulate sequential reductive/oxidative microbial degradation of perchloroethene (PCE) in mineral medium and in contaminated groundwater. In a flow-through column system, hydrogen generation at the cathode supported reductive dechlorination of PCE to cis-dichloroethene (cDCE), vinyl chloride (VC), and ethene (ETH). Electrolytically generated oxygen at the anode allowed subsequent oxidative degradation of the lower chlorinated metabolites. Aerobic cometabolic degradation of cDCE proved to be the bottleneck for complete metabolite elimination. Total removal of chloroethenes was demonstrated for a PCE load of approximately 1.5 μmol/d. In mineral medium, long-term operation with stainless steel electrodes was demonstrated for more than 300 days. In contaminated groundwater, corrosion of the stainless steel anode occurred, whereas DSA (dimensionally stable anodes) proved to be stable. Precipitation of calcareous deposits was observed at the cathode, resulting in a higher voltage demand and reduced dechlorination activity. With DSA and groundwater from a contaminated site, complete degradation of chloroethenes in groundwater was obtained for two months thus demonstrating the feasibility of the sequential bioelectro-approach for field application.

Removal of monochlorobenzene and perchloroethene in wetland rhizosphere model systems

Constructed wetlands are a promising technology to protect river flood plains against the impact of contaminated groundwater. They are suitable for the treatment of waters contaminated with monochlorobenzene and perchloroethene. However, the removal performance differs with the operation conditions, and generally, transferable performance data are not yet available. In this study, removal efficiencies were determined and the dominant removal processes for monochlorobenzene and perchloroethene were evaluated under various operation conditions in helophyte rhizosphere reactors. Monochlorobenzene removal was very efficient (>99%) under low carbon load (overall oxic) and moderate carbon load (overall reduced) conditions. Higher loads of easily degradable carbon (acetate, 300 mg/L) impaired the elimination of monochlorobenzene (removal of 72−96%). Microbial reductive dechlorination of perchloroethene was not detected in the rhizosphere under low carbon load, sulphate reduction, and high-carbon load conditions. Nonetheless, considerable amounts of perchloroethene were eliminated (79−87%), presumably by plant uptake and phytovolatilisation. Under fluctuating moderate carbon load conditions, perchloroethene dechlorination was initiated, and trichloroethene and cis-dichloroethene production showed that a minimum of 10% of the perchloroethene inflow load was dechlorinated. Sulphate reduction and the associated sulphide toxicity showed to constitute a hazard for constructed wetland treatment of sulphate containing groundwater contaminated with chlorinated volatile organic compounds, causing a decrease in removal efficiencies by 50 and 20% for monochlorobenzene and perchloroethene, respectively.

Transformation and Carbon Isotope Fractionation of Tetra- and Trichloroethene to Trans-Dichloroethene by Dehalococcoides sp. Strain CBDB1

Dehalococcoides sp. strain CBDB1 reductively dechlorinated perchloroethene (PCE) and trichloroethene (TCE) to predominantly trans-1,2-dichloroethene (trans-DCE). Cell counting by direct microscopy showed that strain CBDB1 used PCE and TCE as electron acceptors for respiratory growth obtaining a growth yield of 3.9 × 10(12) cells per mol of chloride released in both cases. PCE and TCE were dechlorinated to trans- and cis-DCE at an average constant ratio of 3.4 (±0.2):1, which is consistent with the ratios found in several trans-DCE-producing sediments and soils containing uncultured Dehalococcoides-like species. Significant carbon isotope fractionation was observed during PCE and TCE reductive dehalogenation. The enrichment factor of TCE (εC = -11.2) was within the range of previously reported values for TCE dechlorination by other Dehalococcoides species although the tceA gene responsible for ethene generation in the latter cultures was absent in strain CBDB1. On the contrary, the enrichment factor of PCE (εC = -1.6) was 3.8-times lower than that obtained for Dehalococcoides sp. strain 195 although both strains shared a high similarity in the pceA gene responsible for PCE dechlorination in strain 195. In addition, the product-related enrichment factors for TCE dehalogenation were calculated based on product isotope signature of the two accumulated products cis-DCE (εC TCE→cis-DCE = -11.0) and trans-DCE (εC TCE→trans-DCE = -15.9). These results are of particular interest since strain CBDB1 constitutes, together with the recent isolated strain MB, the unique Dehalococcoides species unable to dechlorinate PCE and TCE beyond DCE.

Density Functional Theory Studies on the Relative Reactivity of Chloroethenes on Zerovalent Iron

The gas-phase dissociation of perchloroethene (PCE), trichloroethene (TCE), and cis-dichloroethene (cis-DCE) on zerovalent iron Fe(110) was investigated using periodic density functional theory (DFT) with the generalized gradient approximation (GGA) and climbing image nudged elastic band method (CI-NEB). Activation energies and dechlorination rate constants for reductive beta-elimination of the chloroethene compounds were calculated using an Arrhenius equation with theoretically calculated vibrational frequencies for the compounds. Activation energies were found to decrease as the chlorination number increases. The reaction rate-limiting step for PCE dissociation occurs at the second chlorine cleavage, while the rate-limiting steps for TCE and cis-DCE occur at the first chlorine cleavage. The activation energies of PCE, TCE, and cis-DCE at their rate-limiting steps are 9.9, 16.6, and 23.8 kJ/mol, respectively. Energy profiles along the reaction coordinate for the dechlorination paths are presented. The relative gas-phase reactivity order among chlorothenes on Fe(110) was found to be PCE > TCE > cis-DCE. At room temperature (300 K), the PCE dechlorination rate is 14 and 338 times faster, respectively, than that of TCE and cis-DCE. Details regarding the electronic properties of the transition states of the dechlorinated compounds are reported.

Using Tree Core Samples to Monitor Natural Attenuation and Plume Distribution After a PCE Spill

The potential of using tree core samples to detect and monitor natural attenuation of perchloroethene (PCE) in groundwater was investigated at a PCE-contaminated site. In the area of the known plume with PCE concentrations between 0.004 and > 40 mg/L, cores were collected from tree trunks at a height of about 1 m above ground surface. Tree sampling of the site was completed in under six hours. Chlorinated ethenes were analyzed by headspace GC/MS. PCE (0.001 to 7 mg/ kg) and natural attenuation products, TCE (< 0.001 to 0.4 mg/ kg) and c-DCE (< 0.001 to 0.46 mg/kg), were detected in tree cores. 1,1-dichloroethene and vinyl chloride were not detected, corresponding to very low concentrations in the groundwater. The contaminant plume was mapped from the concentrations measured in trees, which delineated a probable hot spot area that had been undetected in decades of traditional groundwater monitoring. Natural attenuation products in tree cores increased with distance from the known source area. Concentrations of PCE and reductive dechlorination products in tree cores were correlated with the corresponding groundwater concentrations. Within a range of limitations, tree-core sampling provides a rapid, reliable and inexpensive method to investigate the extent of shallow contamination by chlorinated ethenes in soil and groundwater.

Comparison of pulsed and continuous addition of H 2 gas via membranes for stimulating PCE biodegradation in soil columns

Column experiments were performed to investigate a technology for remediating aquifers contaminated with chlorinated solvents. The technology involves installation of hollow-fiber membranes in the subsurface to supply hydrogen gas (H 2) to groundwater to support biological reductive dechlorination in situ. Three laboratory-scale columns [control (N 2 only), continuous H 2, and pulsed H 2] were packed with aquifer material from a trichloroethene (TCE)-contaminated wetland in Minnesota and supplied with perchloroethene (PCE)-contaminated synthetic groundwater. The main goals of the research were: (1) evaluate the long-term performance of the H 2 supply system and (2) compare the effects of pulsed (4 h on, 20 h off) versus continuous H 2 supply (lumen partial pressure ∼1.2 atm) on PCE dechlorination and production of by-products (i.e. methane and acetate). The silicone-coated fiberglass membranes employed in these experiments were robust, delivering H 2 steadily over the entire 349-day experiment. Methane production decreased when H 2 was added in a pulsed manner. Nevertheless, the percentage of added H 2 used to support methanogenesis was similar in both H 2-fed columns (92–93%). For much of the experiment, PCE dechlorination (observed end product=dichloroethene) in the continuous and pulsed H 2 columns was comparable, and enhanced in comparison to the natural attenuation observed in the control column. Dechlorination began to decline in the pulsed H 2 column after 210 days, however, while dechlorination in the continuous H 2 column was sustained. Acetate was detected only in the continuous H 2 column, at concentrations of up to 36 μM. The results of this research suggest that in situ stimulation of PCE dechlorination by direct H 2 addition requires the continuous application of H 2 at high partial pressures, favoring the production of bioavailable organic matter such as acetate to provide a carbon source, electron donor, or both for dechlorinators. Unfortunately, this strategy has proven to be inefficient, with the bulk of the added H 2 used to support methanogenesis.

Effects of surfactants on the dechlorination of chlorinated ethenes.

The influence of surfactants on a perchloroethene (PCE) dechlorinating mixed culture was investigated in laboratory experiments. Surfactants (Steol CS-330, Aerosol MA 80-I, alpha olefin sulfonate 14 to 16, Neodol 25-7, Tween 80, alkyl polyglycoside, C16TAB [trimethylammonium bromide], and sodium dodecyl sulfate) were evaluated for their effects on the rate and extent of PCE reductive dechlorination and their potential biodegradation by the mixed culture. Limited, if any, surfactant biodegradation was observed for the surfactants tested, and all surfactants impaired dechlorination in either the rate of PCE dechlorination or the terminal dechlorination products observed. Based on initial testing, a nonionic surfactant, Tween 80, and an anionic surfactant, Steol CS-330, were selected for additional investigation. Dechlorination of PCE to dichloroethene (DCE), vinyl chloride (VC), and ethene (ETH) occurred in all Tween 80-amended microcosms, with a depressed rate of ETH production as the only adverse effect. Steol CS-330, however, inhibited dechlorination beyond DCE at all surfactant concentrations exceeding 25 mg/L. Attempts to acclimate a culture to Steol CS-330 were unsuccessful. Inhibition of VC and ETH production was reversible on dilution of the surfactant to a concentration of 10 mg/L or less, indicating that surfactant interactions with the enzyme system responsible for reductive dechlorination of DCE may be the cause of inhibition.

Evaluation of polyethylene hollow-fiber membranes for hydrogen delivery to support reductive dechlorination in a soil column

Engineered systems are often needed to supply an electron donor, such as hydrogen (H 2), to the subsurface to stimulate the biological dehalogenation of perchloroethene (PCE) to ethene. A column study was performed to evaluate the ability of gas permeable hollow-fiber membranes to supply H 2 directly to PCE-contaminated groundwater to facilitate bioremediation. Two glass columns were packed with soil obtained from a trichloroethene-contaminated site at Cape Canaveral, Florida, and were fed a minimal medium spiked with PCE (7 μM) for 391 days. The columns were operated in parallel, with one column receiving H 2 via polyethylene hollow-fiber membranes (lumen H 2 pressure of approximately 1 atm) and a control column receiving no H 2. PCE was initially dechlorinated at a similar rate and to a similar extent in both columns, likely due to the presence of soil organic matter that was able to support dechlorination. After 265 days of operation, dechlorination performance declined in the control column and the benefits of membrane-supplied H 2 became evident. Although the membrane-supplied H 2 effectively stimulated PCE dechlorination at the end of the experiment (days 359–391), the system was inefficient in that only 5% of the supplied H 2 was used for dechlorination. Most of the remainder was used to support methanogenesis (94%). Despite the dominance of methanogens, nearly complete dechlorination of PCE to ethene was observed in the H 2-fed column. In addition to the inefficient use of H 2, operational problems included excessive foulant accumulation on the outside of the membrane fibers and water condensation inside the fibers. Use of alternative membrane materials and changes to the operating approach (e.g. pulsing or supplying H 2 at low partial pressures) may help to overcome these problems so that this technology can provide effective and stable remediation of aquifers contaminated with chlorinated ethenes.

Effect of Nitrate and Sulfate on Dechlorination by a Mixed Hydrogen-Fed Culture

A novel hollow-fiber membrane remediation technology developed in our laboratory for hydrogen delivery to the subsurface was shown to support the dechlorination of perchloroethene (PCE) to cis-dichloroethene. In previous research, the presence of nitrate or sulfate has been observed to inhibit biological reductive dechlorination. In this study hollow-fiber membranes were used to supply hydrogen to a mixed culture to investigate whether adequate hydrogen could be added to support dechlorination in the presence of alternative electron acceptors. By continuously supplying hydrogen through the membrane, the hydrogen concentrations within the reactor were maintained well above the hydrogen thresholds reported to sustain reductive dechlorination. It was hypothesized that by preventing nitrate and sulfate reducers from decreasing hydrogen concentrations to below the dehalorespirer threshold, the inhibition of PCE dechlorination by nitrate and sulfate might be avoided and dechlorination could be stimulated more effectively. Enough membrane-fed hydrogen was supplied to completely degrade the alternative electron acceptors present and initiate dechlorination. Nevertheless, nitrate and sulfate inhibited dechlorinating activity even when hydrogen was not limiting. This suggests that competition for hydrogen was not responsible for the observed inhibition. Subsequent microcosm experiments demonstrated that the denitrification intermediate nitrous oxide was inhibitory at 13 µM.