Tomasz Kuder

3D-CSIA: carbon, chlorine, and hydrogen isotope fractionation in transformation of TCE to ethene by a Dehalococcoides culture.

Carbon (C), chlorine (Cl), and hydrogen (H) isotope effects were determined during dechlorination of TCE to ethene by a mixed Dehalococcoides (Dhc) culture. The C isotope effects for the dechlorination steps were consistent with data published in the past for reductive dechlorination (RD) by Dhc. The Cl effects (combined with an inverse H effect in TCE) suggested that dechlorination proceeded through nucleophilic reactions with cobalamin rather than by an electron transfer mechanism. Depletions of (37)Cl in daughter compounds, resulting from fractionation at positions away from the dechlorination center (secondary isotope effects), further support the nucleophilic dechlorination mechanism. Determination of C and Cl isotope ratios of the reactants and products in the reductive dechlorination chain offers a potential tool for differentiation of Dhc activity from alternative transformation mechanisms (e.g., aerobic degradation and reductive dechlorination proceeding via outer sphere mechanisms), in studies of in situ attenuation of chlorinated ethenes. Hydrogenation of the reaction products (DCE, VC, and ethene) showed a major preference for the (1)H isotope. Detection of depleted dechlorination products could provide a line of evidence in discrimination between alternative sources of TCE (e.g., evolution from DNAPL sources or from conversion of PCE).

The Use of the Isotopic Composition of Individual Compounds for Correlating Spilled Oils and Refined Products in the Environment with Suspected Sources

Correlation of crude oils, or refined products, in the environment with suspected sources is typically undertaken through the use of GC and GCMS and in certain cases bulk carbon isotope compositions. However, with crude condensates, or refined products in particular, the absence, or low concentration, of biomarkers precludes their successful use for making unique correlations. An alternative and, sometimes, complimentary technique for correlation of such products is evolving through the use of combined gas chromatography-isotope ratio mass spectrometry (GCIRMS). This approach permits determination of the carbon and hydrogen isotopic composition of individual compounds in the crude oil or refined product to produce isotopic fingerprints for use in correlation studies. In this paper, it is proposed to review applications of GCIRMS to the correlation of various spilled products with their suspected sources in different environments. Whilst not proposing that this technique will replace GC or GCMS; it is proposed that GCIRMS is a very powerful tool to be used in conjunction with GC and GCMS to make such correlations. Although isotopic fractionation has been observed in some of the lighter components such as benzene and toluene, higher carbon numbered compounds, say above C 10 , do not appear to undergo any significant isotopic fractionation as a result of weathering. Furthermore with refined products, isotopic fractionation of the lighter components has the potential to demonstrate the onset of natural attenuation of refined products in the environment.

Application of CSIA to distinguish between vapor intrusion and indoor sources of VOCs.

At buildings with potential for vapor intrusion of volatile organic chemicals (VOCs) from the subsurface, the ability to accurately distinguish between vapor intrusion and indoor sources of VOCs is needed to support accurate and efficient vapor intrusion investigations. We have developed a method for application of compound-specific stable isotope analysis (CSIA) for this purpose that uses an adsorbent sampler to obtain sufficient sample mass from the air for analysis. Application of this method to five residences near Hill Air Force Base in Utah indicates that subsurface and indoor sources of tricholorethene and tetrachloroethene often exhibit distinct carbon and chlorine isotope ratios. The differences in isotope ratios between indoor and subsurface sources can be used to identify the source of these chemicals when they are present in indoor air.

Demonstration of Compound-Specific Isotope Analysis of Hydrogen Isotope Ratios in Chlorinated Ethenes

High-temperature pyrolysis conversion of organic analytes to H(2) in hydrogen isotope ratio compound-specific isotope analysis (CSIA) is unsuitable for chlorinated compounds such as trichloroethene (TCE) and cis-1,2-dichloroethene (DCE), due to competition from HCl formation. For this reason, the information potential of hydrogen isotope ratios of chlorinated ethenes remains untapped. We present a demonstration of an alternative approach where chlorinated analytes reacted with chromium metal to form H(2) and minor amounts of HCl. The values of δ(2)H were obtained at satisfactory precision (± 10 to 15 per thousand), however the raw data required daily calibration by TCE and/or DCE standards to correct for analytical bias that varies over time. The chromium reactor has been incorporated into a purge and trap-CSIA method that is suitable for CSIA of aqueous environmental samples. A sample data set was obtained for six specimens of commercial product TCE. The resulting values of δ(2)H were between -184 and +682 ‰, which significantly widened the range of manufactured TCE δ(2)H signatures identified by past work. The implications of this finding to the assessment of TCE contamination are discussed.

Carbon isotope fractionation in reactions of 1,2-dibromoethane with FeS and hydrogen sulfide.

EDB (1,2-dibromoethane) is frequently detected at sites impacted by leaded gasoline. In reducing environments, EDB is highly susceptible to abiotic degradation. A study was conducted to evaluate the potential of compound-specific isotope analysis (CSIA) in assessing abiotic degradation of EDB in sulfate-reducing environments. Water containing EDB was incubated in sealed vials with various combinations of Na(2)S (<0.7 mM) and mackinawite (FeS) (180 mM). Degradation rates in vials containing FeS exceeded those in Na(2)S-only controls. In the presence of FeS, first-order constants ranged from 0.034 ± 0.002 d(-1) at pH 6 to 0.081 ± 0.005 d(-1) at pH 8.5. In the presence of FeS, products from reductive debromination (ethylene) and from S(N)2 substitution with S(II) nucleophiles were detected (1,2-dithioethane, DTA). Relatively high yields of DTA suggested that the S(N)2 reactions were not mediated by HS(-) only but likely also included reactions mediated by FeS surface. Significant carbon isotope effects were observed for nucleophilic substitution by HS(-) (ε = -31.6 ± 3.7‰) and for a combination of reductive and substitution pathways in the presence of FeS (-30.9 ± 0.7‰), indicating good site assessment potential of CSIA. The isotope effects (KIEs) observed in the presence of FeS corroborated the predominance of S(N)2 substitution by nucleophiles combined with two-electron transfer reductive debromination.

Validation of adsorbents for sample preconcentration in compound-specific isotope analysis of common vapor intrusion pollutants

Isotope ratios of volatile organic compounds (VOCs) in the environment are often of interest in contaminant fate studies. Adsorbent preconcentration-thermal desorption of VOCs can be used to collect environmental vapor samples for compound-specific isotope analysis (CSIA). While active adsorbent samplers offer logistic benefits in handling large volumes of air, their performance in preserving VOCs isotope ratios was not previously tested under sampling conditions corresponding to typical indoor air sampling conditions. In this study, the performance of selected adsorbents was tested for preconcentration of TCE (for determination of C and Cl isotope ratios), PCE (C and Cl) and benzene (C and H). The key objective of the study was to identify the adsorbent(s) permitting preconcentration of the target VOCs present in air at low μg/m(3) concentrations, without significant alteration of their isotope ratios. Carboxen 1016 was found to perform well for the full range of tested parameters. Carboxen 1016 can be recommended for sampling of TCE, PCE and benzene, for CSIA, from air volumes up to 100 L. Variable extent of isotope ratio alteration was observed in the preconcentration of the target VOCs on Carbopack B and Carbopack X, resulting from partial analyte loss via adsorbent bed breakthrough and (possibly) via incomplete desorption. The results from testing the Carbopack B and Carbopack X highlight the need of adsorbent performance validation at conditions fully representative of actual sample collection conditions, and caution against extrapolation of performance data toward more challenging sampling conditions.

Assessment of anaerobic biodegradation of bis(2-chloroethyl) ether in groundwater using carbon and chlorine compound-specific isotope analysis

Carbon and chlorine compound specific isotope analysis (CSIA) of bis(2-chloroethyl) ether (BCEE) was performed to distinguish the primary processes contributing to observed concentration reductions in an anaerobic groundwater plume. Laboratory microcosms were constructed to demonstrate and obtain isotopic enrichment factors and dual-element CSIA trends from two potential transformation processes (1) anaerobic biodegradation using saturated sediment samples from the field site (εC=-14.8 and εCl=-5.0) and (2) abiotic reactions with sulfide nucleophiles in water (εC=-12.8 and εCl=-5.0). The results suggested a nucleophilic, SN2-type dechlorination as the mechanism of biodegradation of BCEE. Identical dual-element CSIA trends observed in the field and in the microcosm samples suggested that the same degradation mechanism was responsible for BCEE degradation in the field. While biodegradation was the likely dominant mechanism of BCEE mass destruction in the aquifer, potential contribution of abiotic hydrolysis to the net budget of degradation could not be confidently excluded. To our knowledge, this is the first unequivocal demonstration of BCEE biodegradation at a field site.

Carbon Isotope Fractionation of 1,2-Dibromoethane by Biological and Abiotic Processes

1,2-Dibromethane (EDB) is a toxic fuel additive that likely occurs at many sites where leaded fuels have impacted groundwater. This study quantified carbon (C) isotope fractionation of EDB associated with anaerobic and aerobic biodegradation, abiotic degradation by iron sulfides, and abiotic hydrolysis. These processes likely contribute to EDB degradation in source zones (biodegradation) and in more dilute plumes (hydrolysis). Mixed anaerobic cultures containing dehalogenating organisms (e.g., Dehaloccoides spp.) were examined, as were aerobic cultures that degrade EDB cometabolically. Bulk C isotope enrichment factors (εbulk) associated with biological degradation covered a large range, with mixed anaerobic cultures fractionating more (εbulk from -8 to -20‰) than aerobic cultures (εbulk from -3 to -6‰). εbulk magnitudes associated with the abiotic processes (dihaloelimination by FeS/FeS2 and hydrolysis) were large but fairly well constrained (εbulk from -19 to -29‰). As expected, oxidative mechanisms fractionated EDB less than dihaloelimination and substitution mechanisms, and biological systems exhibited a larger range of fractionation, potentially due to isotope masking effects. In addition to quantifying and discussing εbulk values, which are highly relevant for quantifying in situ EDB degradation, an innovative approach for constraining the age of EDB in the aqueous phase, based on fractionation during hydrolysis, is described.

Degradation of 4-bromophenol by Ochrobactrum sp. HI1 isolated from desert soil: pathway and isotope effects

Anthropogenic activities have introduced elevated levels of brominated phenols to the environment. These compounds are associated with toxic and endocrine effects, and their environmental fate is of interest. An aerobic strain Ochrobactrum sp. HI1 was isolated from soils in the vicinity of a bromophenol production plant and tested for its ability to degrade 4-bromophenol (4-BP). A ring hydroxylation pathway of degradation was proposed, using the evidence from degradation intermediates analysis and multi-element (C, Br, H) compound-specific isotope analysis. Benzenetriol and 4-bromocatechol were detected during degradation of 4-bromophenol. Degradation resulted in a normal carbon isotope effect (εC = −1.11 ± 0.09‰), and in insignificant bromine and hydrogen isotope fractionation. The dual C–Br isotope trend for ring hydroxylation obtained in the present study differs from the trends expected for reductive debromination or photolysis. Thus, the isotope data reported herein can be applied in future field studies to delineate aerobic biodegradation processes and differentiate them from other natural attenuation processes.

Modeling 3D-CSIA data: Carbon, chlorine, and hydrogen isotope fractionation during reductive dechlorination of TCE to ethene

Reactive transport modeling of multi-element, compound-specific isotope analysis (CSIA) data has great potential to quantify sequential microbial reductive dechlorination (SRD) and alternative pathways such as oxidation, in support of remediation of chlorinated solvents in groundwater. As a key step towards this goal, a model was developed that simulates simultaneous carbon, chlorine, and hydrogen isotope fractionation during SRD of trichloroethene, via cis-1,2-dichloroethene (and trans-DCE as minor pathway), and vinyl chloride to ethene, following Monod kinetics. A simple correction term for individual isotope/isotopologue rates avoided multi-element isotopologue modeling. The model was successfully validated with data from a mixed culture Dehalococcoides microcosm. Simulation of Cl-CSIA required incorporation of secondary kinetic isotope effects (SKIEs). Assuming a limited degree of intramolecular heterogeneity of δ37Cl in TCE decreased the magnitudes of SKIEs required at the non-reacting Cl positions, without compromising the goodness of model fit, whereas a good fit of a model involving intramolecular CCl bond competition required an unlikely degree of intramolecular heterogeneity. Simulation of H-CSIA required SKIEs in H atoms originally present in the reacting compounds, especially for TCE, together with imprints of strongly depleted δ2H during protonation in the products. Scenario modeling illustrates the potential of H-CSIA for source apportionment.