Reductive Dechlorination Of Trichloroethene Research Articles

Reductive dechlorination of trichloroethene by sulfided microscale zero-valent iron in fresh and saline groundwater: Reactivity, pathways, and selectivity

S/mZVI is a promising material for groundwater remediation due to its excellent properties. However, the reactivity and electron selectivity toward target contaminant are critical. Thus, this study investigated the effect of complex groundwater chemistries (Milli-Q water, fresh groundwater and saline groundwater) on the reactivity of S/mZVI toward trichloroethylene (TCE), dechlorination pathway, hydrogen evolution kinetic, electron efficiency and aging behaviors. Results showed that sulfidation appreciably increased the reactivity and electron selectivity. The major degradation product of TCE dechlorination by S/mZVI was acetylene, which was consistent with TCE dechlorination by β-elimination. Moreover, reductive β-elimination was still the dominant dechlorination pathway for the application of S/mZVI in three groundwater conditions. However, the rates and the quantities of major products from TCE degradation varied significantly. S/mZVI in saline groundwater can maintain the reactivity towardTCE due to the protection of Fe0 by Fe3O4 deposited on the surface. Thus, the higher TCE removal efficiency and less hydrogen accumulation resulted in the greatest electron efficiency (4.3–79.2%). Overall, S/mZVI was more effective for the application in saline groundwater. This study proved insight into the comprehensive evaluation and implications for the application of S/mZVI based technologies in saline contaminated groundwater.

Reductive Dechlorination of Trichloroethene by Sulfided Microscale Zero-Valent Iron in Fresh and Saline Groundwater: Reactivity, Pathways, Mechanism and Selectivity

Reductive Dechlorination of Trichloroethene by Sulfided Microscale Zero-Valent Iron in Fresh and Saline Groundwater: Reactivity, Pathways, Mechanism and Selectivity

Microbiome composition resulting from different substrates influences trichloroethene dechlorination performance

Hydrogen-releasing substrates can stimulate the reductive dechlorination of trichloroethene (TCE) mediated by organohalide-respiring bacteria (OHRB) at contaminated sites. However, how the substrate affects microbiome assembly and the accompanying influences on the growth of OHRB and reductive TCE dechlorination remains unclear. We evaluated the effects of microbial community structures and potential functions on the reductive dechlorination of TCE in three anaerobic reactors with acetate, soybean oil, or molasses as the substrate and no cobalamin or amino acid supplementation. The molasses-fed reactor exhibited superior performance and dechlorination of TCE loadings to ethene, and the oil-fed reactor exhibited a high growth rate of the key OHRB, Dehalococcoides. This finding suggests an effect of the substrate on reductive dechlorination and the growth of Dehalococcoides. The three reactors developed distinct microbial community structures and the predicted metagenomes were distinguished on the basis of vitamin and amino acid metabolisms as well as fermentation pathways. In addition to the diversified hydrogen-producing pathways, the molasses-induced microbiome exhibited high potential to synthesize the cobalamin, which may account for its high Dehalococcoides activity and thus effective dechlorination performance. The substrate dependence of microbiomes may provide insight into strategies of exogenous amino acid supplementation to benefit Dehalococcoides growth. This study adds novel insight into the interplay of hydrogen-releasing substrates and OHRB. The results may contribute to the development of tailored and cost-effective management for the reductive dechlorination of chlorinated solvents in bioremediation.

Reductive dechlorination of trichloroethene under different sulfate-reducing and electron donor conditions

The effect of sulfate presence on reductive dechlorination of chlorinated ethenes has been a matter of conflict among the limited reports found in literature. This paper aims to clarify the misconceptions regarding the performance of trichloroethene biotransformation under sulfate reducing conditions by evaluating the effect of different sulfate concentrations on reductive dechlorination and to assess the influence of electron donor dose on dechlorination rate. To this end, batch experiments containing different sulfate and butyrate concentrations were conducted using trichloroethene-dechlorinating and sulfate-reducing parent cultures. Results demonstrated that if sufficient time and electron donor is provided, complete dechlorination can be achieved, even at up to 400 mg/L initial sulfate concentration. However, the rate of dichloroethene and vinyl chloride degradation is reduced as sulfide concentration increases. Moreover, the excess electron donor dose induced a slightly slower dechlorination rate. The findings of this paper present an explanatory framework for the dechlorination of TCE under sulfate reducing conditions and can contribute to the state-of-art bioremediation of contaminated sites.

Transport and release of electron donors and alkalinity during reductive dechlorination by combined emulsified vegetable oil and colloidal Mg(OH)2: Laboratory sand column and microcosm tests

Emulsified vegetable oil combined with colloidal Mg(OH)2 (EVO-CM) can slowly release electron donors and OH– into groundwater and is therefore regarded as a promising amendment for enhanced in situ treatment of chlorinated solvents, such as tetrachloroethene (PCE) and trichloroethene (TCE). However, its migration ability in different porous media and the simultaneous release of electron donor and pH buffer during enhanced reductive dechlorination (ERD) have never been evaluated in detail. In this study, EVO-CM with uniform drop size and desirable stability was prepared. Laboratory-scale column tests were conducted to investigate the transport and spatial distribution of the EVO-CM in different porous media. Batch microcosm experiments were carried out to study the dechlorination efficiency under different EVO:Mg(OH)2 ratios. Experimental results indicate that prepared EVO-CM emulsions can be transported effectively through different porous media with grain size ranging from 0.1–1.0 mm. The emulsified vegetable oil (EVO) and colloidal Mg(OH)2 showed synchronous movement through the porous media column systems, providing both electron donor and alkalinity at the same location. The retention degree of EVO-CM is greater in the finer grained media and decreases with increasing distance from the inlet. The injection of EVO-CM lead to a significant mobilization of TCE in the column. The reductive dechlorination of TCE in the microcosms was remarkably enhanced in the presence of EVO-CM. The coaddition of sufficient colloidal Mg(OH)2 effectively limits the deleterious pH decline caused by acid release. The microcosm achieves an optimum dechlorination efficiency when the EVO:Mg(OH)2 ratio is 1:1.

Effects of 1,1,1-Trichloroethane and Triclocarban on Reductive Dechlorination of Trichloroethene in a TCE-Reducing Culture.

Chlorinated compounds were generally present in the environment due to widespread use in the industry. A short-term study was performed to evaluate the effects of 1,1,1- trichloroethane (TCA) and triclocarban (TCC) on trichloroethene (TCE) removal in a reactor fed with lactate as the sole electron donor. Both TCA and TCC inhibited TCE reduction, but the TCC had a more pronounced effect compared to TCA. The TCE-reducing culture, which had never been exposed to TCA before, reductively dechlorinated TCA to 1,1-dichloroethane (DCA). Below 15 μM, TCA had little effect on the transformation of TCE to cis-dichloroethene (DCE); however, the reduction of cis-DCE and vinyl chloride (VC) were more sensitive to TCA, and ethene production was completely inhibited when the concentration of TCA was above 15 μM. In cultures amended with TCC, the reduction of TCE was severely affected, even at concentrations as low as 0.3 μM; all the cultures stalled at VC, and no ethene was detected. The cultures that fully transformed TCE to ethene contained 5.2–8.1% Dehalococcoides. Geobacter and Desulfovibrio, the bacteria capable of partially reducing TCE to DCE, were detected in all cultures, but both represented a larger proportion of the community in TCC-amended cultures. All cultures were dominated by Clostridium_sensu_stricto_7, a genus that belongs to Firmicutes with proportions ranging from 40.9% (in a high TCC (15 μM) culture) to 88.2%. Methanobacteria was detected at levels of 1.1–12.7%, except in cultures added with 15 and 30 μM TCA, in which they only accounted for ∼0.4%. This study implies further environmental factors needed to be considered in the successful bioremediation of TCE in contaminated sites.

Reductive Dechlorination of Trichloroethene by Zero-valent Iron Nanoparticles: Reactivity Enhancement through Sulfidation Treatment.

Zero-valent iron nanoparticles (nZVI) synthesized in the presence of reduced sulfur compounds have been shown to degrade trichloroethene (TCE) at significantly higher rates. However, the applicability of sulfidation as a general means to enhance nZVI reactivity under different particle preparation conditions and the underlying cause for this enhancement effect are not well understood. In this study, the effects of sulfidation reagent, time point of sulfidation, and sulfur loading on the resultant particles were assessed through TCE degradation experiments. Up to 60-fold increase in TCE reaction rates was observed upon sulfidation treatment, with products being fully dechlorinated hydrocarbons. While the reactivity of these sulfur-treated nZVI (S-nZVI) was relatively unaffected by the sulfidation reagent (viz., sodium sulfide, dithionite, or thiosulfate) or the sequence of sulfidation relative to iron reduction, TCE reaction rates were found to depend strongly on sulfur to iron ratio. At a low sulfur loading, TCE degradation was accelerated with increasing sulfur dose. The rate constant reached a limiting value, however, as the sulfur to iron mole ratio was greater than 0.025. Different from previous propositions that iron sulfidation leads to more efficient TCE or tetrachloroethene (PCE) degradation by enabling depassivation of iron surface, affording catalytic pathways, or facilitating electron transfer, we show that the role of sulfur in nZVI lies essentially in its ability to poison hydrogen recombination, which drives surface reactions to favor reduction by atomic hydrogen. This implies that the reactivity of S-nZVI is contaminant-specific and is selective against the background reaction of water reduction. As the effect of sulfur manifests through surface processes, sulfidation represents a broadly applicable surface modification approach to modulate or increase the reactivity of nZVI for treating TCE and other related contaminants.

Effects of Aqueous Film-Forming Foams (AFFFs) on Trichloroethene (TCE) Dechlorination by a Dehalococcoides mccartyi-Containing Microbial Community.

The application of aqueous film-forming foams (AFFFs) to extinguish chlorinated solvent-fueled fires has led to the co-contamination of poly- and perfluoroalkyl substances (PFASs) and trichloroethene (TCE) in groundwater and soil. Although reductive dechlorination of TCE by Dehalococcoides mccartyi is a frequently used remediation strategy, the effects of AFFF and PFASs on TCE dechlorination are not well-understood. Various AFFF formulations, PFASs, and ethylene glycols were amended to the growth medium of a D. mccartyi-containing enrichment culture to determine the impact on dechlorination, fermentation, and methanogenesis. The community was capable of fermenting organics (e.g., diethylene glycol butyl ether) in all AFFF formulations to hydrogen and acetate, but the product concentrations varied significantly according to formulation. TCE was dechlorinated in the presence of an AFFF formulation manufactured by 3M but was not dechlorinated in the presence of formulations from two other manufacturers. Experiments amended with AFFF-derived PFASs and perfluoroalkyl acids (PFAAs) indicated that dechlorination could be inhibited by PFASs but that the inhibition depends on surfactant concentration and structure. This study revealed that the fermentable components of AFFF can stimulate TCE dechlorination, while some of the fluorinated compounds in certain AFFF formulations can inhibit dechlorination.

The roles of methanogens and acetogens in dechlorination of trichloroethene using different electron donors.

We evaluated the effects of methanogens and acetogens on the function and structure of microbial communities doing reductive dechlorination of trichloroethene (TCE) by adding four distinct electron donors: lactate, a fermentable organic; acetate, a non-fermentable organic; methanol, a fermentable 1-C (carbon) organic; and hydrogen gas (H2), the direct electron donor for reductive dechlorination by Dehalococcoides. The fermentable electron donors had faster dechlorination rates, more complete dechlorination, and higher bacterial abundances than the non-fermentable electron donors during short-term tests. Phylotypes of Dehalococcoides were relatively abundant (≥9%) for the cultures fed with fermentable electron donors but accounted for only ~1-2% of the reads for the cultures fed by the non-fermentable electron donors. Routing electrons to methanogenesis and a low ratio of Dehalococcoides/methanogenesis (Dhc/mcrA) were associated with slow and incomplete reductive dechlorination with methanol and H2. When fermentable substrates were applied as electron donors, a Dhc/mcrA ratio ≥6.4 was essential to achieve fast and complete dechlorination of TCE to ethene. When methanogenesis was suppressed using 2-bromoethanesulfonate (BES), achieving complete dechlorination of TCE to ethane required a minimum abundance of the mcrA gene. Methanobacterium appeared to be important for maintaining a high dechlorination rate, probably by providing Dehalococcoides with cofactors other than vitamin B12. Furthermore, the presence of homoacetogens also was important to maintain a high dechlorination rate, because they provided acetate as Dehalococcoides's obligatory carbon source and possibly cofactors.

Kinetics of dechlorination by Dehalococcoides mccartyi using different carbon sources

Stimulated anaerobic dechlorination is generally considered a valuable step for the remediation of aquifers polluted with chlorinated ethenes (CEs). Correct simulation and prediction of this process in situ, however, require good knowledge of the associated biological reactions. The aim of this study was to evaluate the dechlorination reaction in an aquifer contaminated with trichloroethene (TCE) and its daughter products, discharging into the Zenne River. Different carbon sources were used in batch cultures and these were related to the dechlorination reaction, together with the monitored biomarkers. Appropriate kinetic formulations were assessed. Reductive dechlorination of TCE took place only when external carbon sources were added to microcosms, and occurred concomitant with a pronounced increase in the Dehalococcoides mccartyi cell count as determined by 16S rRNA gene-targeted qPCR. This indicates that native dechlorinating bacteria are present in the aquifer of the Zenne site and that the oligotrophic nature of the aquifer prevents a complete degradation to ethene. The type of carbon source, the cell number of D. mccartyi or the reductive dehalogenase genes, however, did not unequivocally explain the observed differences in degradation rates or the extent of dechlorination. Neither first-order, Michaelis-Menten nor Monod kinetics could perfectly simulate the dechlorination reactions in TCE spiked microcosms. A sensitivity analysis indicated that the inclusion of donor limitation would not significantly enhance the simulations without a clear process understanding. Results point to the role of the supporting microbial community but it remains to be verified how the complexity of the microbial (inter)actions should be represented in a model framework.

Effects of Varying Growth Conditions on Stable Carbon Isotope Fractionation of Trichloroethene (TCE) by tceA-containing Dehalococcoides mccartyi strains

To quantify in situ bioremediation using compound specific isotope analysis (CSIA), isotope fractionation data obtained from the field is interpreted according to laboratory-derived enrichment factors. Although previous studies that have quantified dynamic isotopic shifts during the reductive dechlorination of trichloroethene (TCE) indicate that fractionation factors can be highly variable from culture-to-culture and site-to-site, the effects of growth condition on the isotope fractionation during reductive dechlorination have not been previously examined. Here, carbon isotope fractionation by Dehalococcoides mccartyi 195 (Dhc195) maintained under a variety of growth conditions was examined. Enrichment factors quantified when Dhc195 was subjected to four suboptimal growth conditions, including decreased temperature (-13.3 ± 0.9‰), trace vitamin B12 availability (-12.7 ± 1.0‰), limited fixed nitrogen (-14.4 ± 0.8‰), and elevated vinyl chloride exposure (-12.5 ± 0.4‰), indicate that the fractionation is similar across a range of tested conditions. The TCE enrichment factors for two syntrophic cocultures, Dhc195 with Desulfovibrio vulgaris Hildenborough (-13.0 ± 2.0‰) and Dhc195 with Syntrophomonas wolfei (-10.4 ± 1.2‰ and -13.3 ± 1.0‰), were also similar to a control experiment. In order to test the stability of enrichment factors in microbial communities, the isotope fractionation was quantified for Dhc-containing groundwater communities before and after two-year enrichment periods under different growth conditions. Although these enrichment factors (-8.9 ± 0.4‰, -6.8 ± 0.8‰, -8.7 ± 1.3‰, -9.4 ± 0.7‰, and -7.2 ± 0.3‰) were predominantly outside the range of values quantified for the isolate and cocultures, all tested enrichment conditions within the communities produced nearly similar fractionations. Enrichment factors were not significantly affected by changes in any of the tested growth conditions for the pure cultures, cocultures or the mixed communities, indicating that despite a variety of temperature, nutrient, and cofactor-limiting conditions, stable carbon isotope fractionations remain consistent for given Dehalococcoides cultures.

Detecting and quantifying reductive dechlorination under monitored natural attenuation conditions

AbstractField sampling and testing were used to investigate the relationship between baseline geochemical and microbial community data and in situ reductive dechlorination rates at a site contaminated with trichloroethene (TCE) and carbon tetrachloride (CTET). Ten monitoring wells were selected to represent conditions along groundwater flow paths from the contaminant source zone to a wetlands groundwater discharge zone. Groundwater samples were analyzed for a suite of geochemical and microbial parameters; then push‐pull tests with fluorinated reactive tracers were conducted in each well to measure in situ reductive dechlorination rates. No exogenous electron donors were added in these tests, as the goal was to assess in situ reductive dechlorination rates under natural attenuation conditions. Geochemical data provided preliminary evidence that reductive dechlorination of TCE and CTET was occurring at the site, and microbial data confirmed the presence of known dechlorinating organisms in groundwater. Push‐pull tests were conducted using trichlorofluoroethene (TCFE) as a reactive tracer for TCE and, in one well, trichlorofluoromethane (TCFM) as a reactive tracer for CTET. Injected TCFE was transformed to cis‐ and trans‐dichlorofluoroethene and chlorofluoroethene, and, in one test, injected TCFE was completely dechlorinated to fluoroethene (FE). In situ TCFE transformation rates ranged from less than 0.005 to 0.004/day. In the single well tested, injected TCFM was transformed in situ to dichlorofluoromethane and chlorofluoromethane; the TCFM transformation rate was estimated as 0.001/day. The results indicate that it is possible to use push‐pull tests with reactive tracers to directly detect and quantify reductive dechlorination of chlorinated ethenes and ethanes under monitored natural attenuation conditions, which has not previously been demonstrated. Transformation rate estimates obtained with these techniques should improve the accuracy of contaminant transport modeling. © 2012 Wiley Periodicals, Inc.

Investigating the biodegradability of perfluorooctanoic acid

Perfluorooctanoic acid (PFOA) is an industrial chemical that has become disseminated globally in aquatic and terrestrial habitats, humans, and wildlife. Understanding PFOA’s biodegradability (susceptibility to microbial metabolic attack) is a crucial element in developing an informed strategy for predicting and managing this compound’s environmental fate. Reasoning that PFOA might be susceptible to reductive defluorination by anaerobic microbial communities, we embarked on a 2-phase experimental approach examining the potential of five different microbial communities (from a municipal waste-water treatment plant, industrial site sediment, an agricultural soil, and soils from two fire training areas) to alter PFOA’s molecular structure. A series of primarily anaerobic incubations (up to 259 d in duration) were established with acetate, lactate, ethanol, and/or hydrogen gas as electron donors and PFOA (at concentrations of 100 ppm and 100 ppb) as the electron acceptor. Cometabolism of PFOA during reductive dechlorination of trichloroethene (TCE) and during reduction of nitrate, iron, sulfate, and methanogenesis were also examined. Endpoints of potential PFOA transformation included release of fluoride and detection of potential transformation products by LC/Orbitrap MS and LC/accurate radioisotope counting in a 14C radiotracer study. The strongest indication of PFOA transformation occurred during its potential cometabolism at the 100 ppb concentration during reductive dechlorination of TCE. Despite an extensive search for transformation products to corroborate potential cometabolism of PFOA, we were unable to document any alteration of PFOA’s chemical structure. We conclude that, under conditions examined, PFOA is microbiologically inert, hence environmentally persistent.

Dechlorination kinetics of TCE at toxic TCE concentrations: Assessment of different models

The reductive dechlorination of trichloroethene (TCE) in a TCE source zone can be self-inhibited by TCE toxicity. A study was set up to examine the toxicity of TCE in terms of species specific degradation kinetics and microbial growth and to evaluate models that describe this self-inhibition. A batch experiment was performed using the TCE dechlorinating KB-1 culture at initial TCE concentrations ranging from 0.04mM to saturation (8.4mM). Biodegradation activity was highest at 0.3mM TCE and no activity was found at concentrations from 4 to 8mM. Species specific TCE and cis-DCE (cis-dichloroethene) degradation rates and Dehalococcoides numbers were modeled with Monod kinetics combined with either Haldane inhibition or a log-logistic dose-response inhibition on these rates. The log-logistic toxicity model appeared the most appropriate model and predicts that the species specific degradation activities are reduced by a factor 2 at about 1mM TCE, respectively cis-DCE. However, the model showed that the inhibitive effects on the time for TCE to ethene degradation are a complex function of degradation kinetics and the initial cell densities of the dechlorinating species. Our analysis suggests that the self-inhibition on biodegradation cannot be predicted by a single concentration threshold without information on the cell densities.

Rock Fragments in Trichloroethene Microcosms for Bedrock Aquifers

ABSTRACT The purpose of this study was to evaluate microcosm methods that can be used to predict the in situ anaerobic reductive dechlorination of trichloroethene (TCE) in bedrock aquifers with and without amendments. Microcosm conditions (e.g., surface area:volume ratio, initial TCE concentration [C 0], incubation temperature) were selected to simulate in situ conditions at the site. The effect of rock media in the microcosms as a source of surface area and nutrients was also assessed. Comparison was made between microcosms constructed with sterile rock media and rock media submerged for 45 days in a groundwater well within a TCE plume for field colonization. Statistically significant biotic degradation of TCE was observed in microcosms with both groundwater only and non-field-colonized rock media, when amended with lactate. Measured half-lives were similar to half-lives observed in field conditions. Normalizing biotic results to abiotic results provides a method that deducts the abiotic losses and allows complete comparison between sample rounds, assuming samples were randomly selected for analysis within the sample round to avoid systematic errors or variation.

Evaluation of Three Electron-Donor Permeable Reactive Barrier Materials for Enhanced Reductive Dechlorination of Trichloroethene

ABSTRACT Understanding the fate of complex electron-donor materials is important for developing efficient biostimulation strategies to treat ground water contamination by chlorinated ethenes (CEs). The fermentation product distributions and H2 production of common permeable reactive barrier (PRB) carbon substrates (dairy whey, sodium lactate syrup, and Hydrogen Release Compound [HRC]) were monitored as measures of substrate efficiency in aquifer microcosms spiked with trichloroethene (TCE). In long-term experiments, the fermentation of PRB substrates to slow-degrading organic acids maintained low H2 partial pressures (≤ 10−3.5) that, as previous studies suggest, may give competitive advantage to dechlorinators over hydrogenotrophic methanogens. Whey-amended and lactate-amended microcosms exhibited faster complete dechlorination and, according to organic acid carbon flow, higher rates of fermentation to acetate. In HRC-amended microcosms, propionate appeared to serve as a carbon sink that prolonged dechlorination. Upon complete dechlorination, whey microcosms contained the highest percentage of organic acid carbon. Native Dehalococcoides populations increased by 3 orders of magnitude (per g sediment) in whey-amended microcosms. Whey's efficiency improved in microcosms prepared with aquifer sediment and water from within a downgradient whey PRB. Results suggested whey loading values of 0.2 kg/m3 may be appropriate under sufficiently reducing conditions to efficiently stimulate hydrogenotrophic and potentially actetotrophic dechlorinating populations. Renewal of whey PRBs may, however, be required. Implications for further long-term study of cost-efficiencies are discussed.

Bioaugmentation for chlorinated ethenes using Dehalococcoides sp.: Comparison between batch and column experiments

Batch and column experiments were performed to evaluate the transport, growth and dechlorination activity of Dehalococcoides sp. (DHC) during bioaugmentation for chlorinated ethenes. Batch experiments showed that the reductive dechlorination of trichloroethene (TCE), cis-1,2-dichloroethene (DCE), and vinyl chloride (VC), as well as growth of the DHC, were well described by the Monod kinetic model. The measured maximum utilization rate coefficients for TCE, DCE, and VC were 1.3 × 10 −12, 5.2 × 10 −13, and 1.4 × 10 −12 mmol Cl − (cell h) −1, respectively. Results of the column experiments showed that dechlorination occurred throughout the length of the column, and that extractable DHC concentrations associated with the soil phase throughout the column were negligible relative to the aqueous phase concentrations. Dechlorination rates relative to aqueous DHC concentrations in the column were approximately 200-times greater than in the batch experiments. Additional batch experiments performed using column effluent water confirmed this result. Incorporation of these enhanced dechlorination kinetics in the transport model provided a reasonable prediction of the column data. Overall results of this study suggest that aqueous phase (as opposed to soil phase) DHC concentrations can be used to estimate dechlorination activity in saturated soils, and DHC dechlorination activity in porous media may be substantially greater than DHC dechlorination activity measured in batch experiments.

Monitoring and evaluation of dechlorination processes using compound‐specific chlorine isotope analysis

A simple, quick and sensitive method for the compound-specific stable chlorine isotope analysis of chlorinated solvents by conventional quadrupole gas chromatography/mass spectrometry (GC/MS) is presented. With this method, compound-specific stable chlorine isotope ratios of typical chlorinated solvents like tetrachloroethene (PCE) and trichloroethene (TCE) can be determined quantitatively within 30 min by direct injection. The chlorine isotope ratios of target substances are calculated from the peak areas of several selected molecular ions and fragment ions of the substances, using a set of unique mathematical equations. The precision of the method was demonstrated through reproducibility tests. An internal precision of +/-0.4 per thousand to +/-1.1 per thousand was obtained when analyzing PCE and TCE in the 10-1000 pmol range. The validity of the method was further demonstrated by determining the chlorine isotopic fractionation factor during the reductive dechlorination of TCE in a batch experiment using zero-valent iron. The chlorine isotopic fractionation factor was calculated as 0.9976 +/- 0.0011 with a correlation coefficient of 0.9469 (n = 38). The high correlation coefficient indicates that compound-specific stable chlorine isotope analysis can be performed with sufficient accuracy using conventional quadrupole GC/MS when significant fractionation takes place during a reaction. For the first time, the chlorine isotope fractionation factor of TCE during an abiotic anaerobic dechlorination process was determined using quadrupole GC/MS, without offline sample preparation.

Anaerobic Biodegradation and Hydrogeochemical Controls on Natural Attenuation of Trichloroethene in an Inland Forested Wetland

ABSTRACT A field and laboratory investigation of natural attenuation, focusing on anaerobic biodegradation, was conducted in a forested wetland where a plume of trichloroethene discharges from a sand aquifer through organic-rich wetland and stream-bottom sediments. The rapid response of the wetland hydrology to precipitation events altered groundwater flow and geochemistry during wet conditions in the spring compared to the drier conditions in the summer and fall. During dry conditions, partial reductive dechlorination of trichloroethene to cis-1,2-dichloroethene occurred in methanogenic wetland porewater. Influx of oxygenated recharge during wet conditions resulted in a change from methanogenic to iron-reducing conditions and a lack of 1,2-dichloroethene production in the wet spring conditions. During these wet conditions, dilution was the primary attenuation mechanism evident for trichloroethene in the wetland porewater. Trichloroethene degradation was insignificant in anaerobic microcosms constructed with the shallow wetland sediment, and microbiological analyses showed a low microbial biomass and absence of known dehalorespiring microorganisms. Despite the typically organic-rich characteristic of wetland sediments, natural attenuation by anaerobic degradation may not be an effective groundwater remediation for chlorinated solvents at all sites.

Carbon isotope fractionation during reductive dechlorination of TCE in batch experiments with iron samples from reactive barriers

Reductive dechlorination of trichloroethene (TCE) by zero-valent iron produces a systematic enrichment of 13C in the remaining substrate that can be described using a Rayleigh model. In this study, fractionation factors for TCE dechlorination with iron samples from two permeable reactive barriers (PRBs) were established in batch experiments. Samples included original unused iron as well as material from a barrier in Belfast after almost 4 years of operation. Despite the variety of samples, carbon isotope fractionations of TCE were remarkably similar and seemed to be independent of iron origin, reaction rate, and formation of precipitates on the iron surfaces. The average enrichment factor for all experiments was −10.1‰ (±0.4‰). These results indicate that the enrichment factor provides a powerful tool to monitor the reaction progress, and thus the performance, of an iron-reactive barrier over time. The strong fractionation observed may also serve as a tool to distinguish between insufficient residence time in the wall and a possible bypassing of the wall by the plume, which should result in an unchanged isotopic signature of the TCE. Although further work is necessary to apply this stable isotope method in the field, it has potential to serve as a unique monitoring tool for PRBs based on zero-valent iron.