Hydrogen and carbon isotope fractionation during aerobic biodegradation of benzene

Soil processes modify the composition of volatile organic compounds (VOCs) from CO2- and CH4-dominated geogenic and landfill gases: A comprehensive study

Degradation mechanisms affecting non-methane volatile organic compounds (VOCs) during gas uprising from different hypogenic sources to the surface were investigated through extensive sampling surveys in areas encompassing a high enthalpy hydrothermal system associated with active volcanism, a CH4-rich sedimentary basin and a municipal waste landfill. For a comprehensive framework, published data from medium-to-high enthalpy hydrothermal systems were also included. The investigated systems were characterised by peculiar VOC suites that reflected the conditions of the genetic environments in which temperature, contents of organic matter, and gas fugacity had a major role. Differences in VOC patterns between source (gas vents and landfill gas) and soil gases indicated VOC transformations in soil. Processes acting in soil preferentially degraded high-molecular weight alkanes with respect to the low-molecular weight ones. Alkenes and cyclics roughly behaved like alkanes. Thiophenes were degraded to a larger extent with respect to alkylated benzenes, which were more reactive than benzene. Furan appeared less degraded than its alkylated homologues. Dimethylsulfoxide was generally favoured with respect to dimethylsulfide. Limonene and camphene were relatively unstable under aerobic conditions, while α-pinene was recalcitrant. O-bearing organic compounds (i.e., aldehydes, esters, ketones, alcohols, organic acids and phenol) acted as intermediate products of the ongoing VOC degradations in soil. No evidence for the degradation of halogenated compounds and benzothiazole was observed. This study pointed out how soil degradation processes reduce hypogenic VOC emissions and the important role played by physicochemical and biological parameters on the effective VOC attenuation capacity of the soil.

Comparing simulated shallow subsurface spills of diluted bitumen and conventional crude oil

Increased oil production in Canada has resulted in proposals to extend or develop new oil pipelines. Many of these proposals have been met by concerns from the public over potential environmental impacts related to construction and the potential for oil spills to negatively affect groundwater quality. Crude oil sourced from the Alberta oil sands represents a significant proportion of this increase in production. This crude oil is produced as bitumen, which is subsequently diluted with light hydrocarbons to lower viscosity to allow for pipeline transport producing diluted bitumen. In this study, we pumped water through tanks filled with sand to simulate groundwater flow. Tanks were injected with either conventional crude or diluted bitumen to simulate a crude oil spill from a pipeline rupture occurring below the water table representing a pipeline river crossing scenario. Water samples were collected from the downstream end of the tanks throughout the experiment period (∼two months). Compared to water quality guidelines, effluent waters from both conventional crude and diluted bitumen tanks contained elevated concentrations of dissolved organic compounds, particularly benzene, ethylbenzene, toluene and xylenes (BTEX). The effluent from each tank had similar concentrations of benzene, whereas discharge water from conventional crude tanks contained higher concentrations of ethylbenzene, toluene and xylenes. In both tanks, and as expected, the BTEX concentrations appeared to be proportional to those determined in their injected crude oils. The measured dissolved concentrations of benzene, ethylbenzene and toluene are lower than predicted which is attributed largely due to dilution along the flow path. In addition to organic constituents, effluent sampled from the diluted bitumen tank contained some metals (Co, Cr, Fe and V) which measured constituents of the oil.

Stable Isotope Geochemistry of the Organic Elements within Shales and Crude Oils: A Comprehensive Review

Over time, stable isotopes have proven to be a useful tool in petroleum geochemistry. However, there is currently insufficient literature on stable isotope geochemistry of the organic elements within shales and crude oils in many petroleum systems around the world. As a result, this paper critically reviews the early and recent trends in stable isotope geochemistry of organic elements in shales and crude oils. The bulk and compound-specific stable isotopes of H, C, and S, as well as their uses as source facies, depositional environments, thermal maturity, geological age, and oil–oil and oil–source rock correlation studies, are all taken into account. The applications of the stable isotopes of H and C in gas exploration are also discussed. Then, the experimental and instrumental approaches to the stable isotopes of H, C, and S, are discussed.

Laboratory Experiments to Evaluate the Effectiveness of Persulfate to Oxidize BTEX in Saline Environment and at Elevated Temperature Using Stable Isotopes

In this study, batch experiments were carried out to investigate the effectiveness of persulfate (PS) as an oxidant agent to remediate benzene, toluene, ethylbenzene, and xylenes (BTEX) in saline environments and at high water temperatures (30 °C). This hydrological setting is quite common in contaminated groundwater aquifers in Middle Eastern countries. In general, increasing the system temperature from 10 to 30 °C greatly enhanced the effectiveness of PS, and resulted in a faster oxidation rate for the target contaminants. When PS was added to the reactor at 30 °C, the targeted contaminants were almost completely oxidized over a 98-day reaction period. During the chemical oxidation of the BTEX, carbon and hydrogen isotope fractionations were monitored and utilized as potential proof of contaminant degradation. The calculated carbon-enrichment values were −1.9‰ for benzene, −1.5‰ for ethylbenzene and toluene, −0.4‰ for ρ,m-xylene, and −1.4‰ for o-xylene, while the hydrogen enrichment values were −9.5‰, −6.8‰, −2.1‰, −6.9‰, and −9.1‰, respectively. In comparison with other processes, the hydrogen and carbon isotope fractionations during the chemical oxidation by PS were smaller than the isotope fractionations resulting from sulfate reduction and denitrification. This observation demonstrates the differences in the transformation pathways and isotope fractionations when compounds undergo chemical oxidation or biodegradation. The distinct trend observed on the dual isotope plot (Δδ13C vs. Δδ2H) suggests that compound-specific isotope analysis can be utilized to monitor the chemical oxidation of BTEX by PS, and to distinguish treatment zones where PS and biodegradation technologies are applied simultaneously.

A modified method to calculate dual-isotope slopes for the natural attenuation of organic pollutants in the environment

Two-dimensional compound-specific isotope analysis has become a powerful tool for distinguishing reaction mechanisms. Lambda (Λ), an essential and important parameter for processing two-dimensional isotope fractionation data, exhibits values specific to a reaction mechanism. In the present article, we modified the existing algorithms for calculation of lambdas based on a review of current methods. Specifically, by regressing [(1000+δE_0,2)*(n₁*x₂)*ΔδE_bulk,1] versus [(1000+δE_0,1)*(n₂*x₁)*ΔδE_bulk,2] by the York method, a novel method was developed to calculate Λs. The improved method eliminates both the influence of the nonreacting position and the initial isotope signatures. Furthermore, this method retains the advantages of a two-dimensional isotope plot, which eliminates contributions from commitment to catalysis, does not require determination of the fraction of remaining substrate, and can be constructed even from field data. Additionally, the one-sample t test is applied to generate a 95% confidence interval of the dataset of Λ_ris for various reaction mechanisms. The ranges of 5.67-24.8, 8.54-9.80, 0.51-8.35, 25.2-36.8, and 7.09-21.9 are applicable for the oxidation of C-H bonds (Z_C=1, Z_H=3; Z_C and Z_H are the number of indistinguishable carbon and hydrogen atoms in intramolecular competition, respectively), oxidation of C-H bonds (Z_C=1, Z_H=4), aerobic biodegradation of benzene (Z_C=6, Z_H=6), methanogenic or sulfate-reducing biodegradation of benzene (Z_C=6, Z_H=6), and nitrate-reducing biodegradation of benzene (Z_C=6, Z_H=6). The accumulation and correction of these values will make the data measured in the field easier to interpret.

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 (ε_C=-14.8 and ε_Cl=-5.0) and (2) abiotic reactions with sulfide nucleophiles in water (ε_C=-12.8 and ε_Cl=-5.0). The results suggested a nucleophilic, S_N2-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, Hydrogen and Chlorine Stable Isotope Fingerprinting for Forensic Investigations of Hexachlorocyclohexanes

Multielemental stable isotope analysis of persistent organic pollutants (POPs) has the potential to characterize sources, sinks, and degradation processes in the environment. To verify the applicability of this approach for source identification of hexachlorocyclohexane (HCHs), we provide a data set of carbon, hydrogen, and chlorine stable isotope ratios (δ¹³C, δ²H, δ³⁷Cl) of its main stereoisomers (α-, β-, δ- and γ-HCHs) from a sample collection based on worldwide manufacturing. This sample collection comprises production stocks, agricultural and pharmaceutical products, chemical waste dumps, and analytical-grade material, covering the production time period from the late 1960s until now. Stable isotope ratios of HCHs cover the ranges from -233‰ to +1‰, from -35.9‰ to -22.7‰, and from -6.69‰ to +0.54‰ for δ²H, δ¹³C, and δ³⁷Cl values, respectively. Four groups of samples with distinct multielemental stable isotope fingerprints were differentiated, most probably as a result of purification and isolation processes. No clear temporal trend in the isotope compositions of HCHs was found at the global scale. The multielemental stable isotope fingerprints facilitate the source identification of HCHs at the regional scale and can be used to assess transformation processes. The data set and methodology reported herein provide basic information for the assessment of environmental field sites contaminated with HCHs.

Characterization of toluene and ethylbenzene biodegradation under nitrate-, iron(III)- and manganese(IV)-reducing conditions by compound-specific isotope analysis

Ethylbenzene and toluene degradation under nitrate-, Mn(IV)-, or Fe(III)-reducing conditions was investigated by compound specific stable isotope analysis (CSIA) using three model cultures (Aromatoleum aromaticum EbN1, Georgfuchsia toluolica G5G6, and a Azoarcus-dominated mixed culture). Systematically lower isotope enrichment factors for carbon and hydrogen were observed for particulate Mn(IV). The increasing diffusion distances of toluene or ethylbenzene to the solid Mn(IV) most likely caused limited bioavailability and hence resulted in the observed masking effect. The data suggests further ethylbenzene hydroxylation by ethylbenzene dehydrogenase (EBDH) and toluene activation by benzylsuccinate synthase (BSS) as initial activation steps. Notably, significantly different values in dual isotope analysis were detected for toluene degradation by G. toluolica under the three studied redox conditions, suggesting variations in the enzymatic transition state depending on the available TEA. The results indicate that two-dimensional CSIA has significant potential to assess anaerobic biodegradation of ethylbenzene and toluene at contaminated sites.

New Bio-Indicators for Long Term Natural Attenuation of Monoaromatic Compounds in Deep Terrestrial Aquifers

Deep subsurface aquifers despite difficult access, represent important water resources and, at the same time, are key locations for subsurface engineering activities for the oil and gas industries, geothermal energy, and CO2 or energy storage. Formation water originating from a 760 m-deep geological gas storage aquifer was sampled and microcosms were set up to test the biodegradation potential of BTEX by indigenous microorganisms. The microbial community diversity was studied using molecular approaches based on 16S rRNA genes. After a long incubation period, with several subcultures, a sulfate-reducing consortium composed of only two Desulfotomaculum populations was observed able to degrade benzene, toluene, and ethylbenzene, extending the number of hydrocarbonoclastic-related species among the Desulfotomaculum genus. Furthermore, we were able to couple specific carbon and hydrogen isotopic fractionation during benzene removal and the results obtained by dual compound specific isotope analysis (𝜀C = -2.4‰ ± 0.3‰; 𝜀H = -57‰ ± 0.98‰; AKIEC: 1.0146 ± 0.0009, and AKIEH: 1.5184 ± 0.0283) were close to those obtained previously in sulfate-reducing conditions: this finding could confirm the existence of a common enzymatic reaction involving sulfate-reducers to activate benzene anaerobically. Although we cannot assign the role of each population of Desulfotomaculum in the mono-aromatic hydrocarbon degradation, this study suggests an important role of the genus Desulfotomaculum as potential biodegrader among indigenous populations in subsurface habitats. This community represents the simplest model of benzene-degrading anaerobes originating from the deepest subterranean settings ever described. As Desulfotomaculum species are often encountered in subsurface environments, this study provides some interesting results for assessing the natural response of these specific hydrologic systems in response to BTEX contamination during remediation projects.

Compound specific isotope analysis of hexachlorocyclohexane isomers: a method for source fingerprinting and field investigation of in situ biodegradation

RATIONALE

The manufacturing and uses of hexachlorocyclohexane (HCH) have resulted in a serious environmental challenge and legacy. This study highlights the ability of compound specific isotope analysis (CSIA) to distinguish among various HCH sources and to support the evaluation of the potential for in situ biodegradation in contaminated groundwater.

METHODS

Tests were conducted to verify the absence of significant isotope fractionation during HCH sample pre-concentration including dichloromethane extraction, solvent exchange into iso-octane, and H2SO4 clean-up, and analysis by gas chromatography/combustion-isotope ratio mass spectrometry (GC/C-IRMS). The method was then applied to four Technical Grade (TG) HCH mixtures procured from different sources and to groundwater samples from a contaminated site.

RESULTS

The pre-concentration method enabled determination of carbon isotope ratios (δ(13)C values) of HCH isomers with no significant isotopic fractionation. The TG-HCH mixtures had significantly different δ(13)C values. Moreover, for any given TG-HCH, all isomers had δ(13)C values within 1.1‰ of each other - a distinctly uniform fingerprint. At the HCH-contaminated field site, compared with source wells, downgradient wells showed significant (up to 5.1‰) enrichment in (13)C and the δ(13)C values of the HCH isomers were significantly different from each other.

CONCLUSIONS

A method was successfully developed for the CSIA of HCH isomers that showed potential for HCH source differentiation and identification of HCH in situ biodegradation. At the HCH-contaminated site, the observed preferential isotopic enrichment of certain isomers relative to others for a given source allows differentiation between biodegraded and non-biodegraded HCH.

Model based evaluation of a contaminant plume development under aerobic and anaerobic conditions in 2D bench-scale tank experiments

The influence of transverse mixing on competitive aerobic and anaerobic biodegradation of a hydrocarbon plume was investigated using a two-dimensional, bench-scale flow-through laboratory tank experiment. In the first part of the experiment aerobic degradation of increasing toluene concentrations was carried out by the aerobic strain Pseudomonas putida F1. Successively, ethylbenzene (injected as a mixture of unlabeled and fully deuterium-labeled isotopologues) substituted toluene; nitrate was added as additional electron acceptor and the anaerobic denitrifying strain Aromatoleum aromaticum EbN1 was inoculated to study competitive degradation under aerobic /anaerobic conditions. The spatial distribution of anaerobic degradation was resolved by measurements of compound-specific stable isotope fractionation induced by the anaerobic strain as well as compound concentrations. A fully transient numerical reactive transport model was employed and calibrated using measurements of electron donors, acceptors and isotope fractionation. The aerobic phases of the experiment were successfully reproduced using a double Monod kinetic growth model and assuming an initial homogeneous distribution of P. putida F1. Investigation of the competitive degradation phase shows that the observed isotopic pattern cannot be explained by transverse mixing driven biodegradation only, but also depends on the inoculation process of the anaerobic strain. Transient concentrations of electron acceptors and donors are well reproduced by the model, showing its ability to simulate transient competitive biodegradation.

Identification of multiple sources of groundwater contamination by dual isotopes

Chlorinated solvents are one of the most commonly detected groundwater contaminants in industrial areas. Identification of polluters and allocation of contaminant sources are important concerns in the evaluation of complex subsurface contamination with multiple sources. In recent years, compound-specific isotope analyses (CSIA) have been employed to discriminate among different contaminant sources and to better understand the fate of contaminants in field-site studies. In this study, the usefulness of dual isotopes (carbon and chlorine) was shown in assessments of groundwater contamination at an industrial complex in Wonju, Korea, where groundwater contamination with chlorinated solvents such as trichloroethene (TCE) and carbon tetrachloride (CT) was observed. In November 2009, the detected TCE concentrations at the study site ranged between nondetected and 10,066 µg/L, and the CT concentrations ranged between nondetected and 985 µg/L. In the upgradient area, TCE and CT metabolites were detected, whereas only TCE metabolites were detected in the downgradient area. The study revealed the presence of separate small but concentrated TCE pockets in the downgradient area, suggesting the possibility of multiple contaminant sources that created multiple comingling plumes. Furthermore, the variation of the isotopic (δ(13) C and δ(37) Cl) TCE values between the upgradient and downgradient areas lends support to the idea of multiple contamination sources even in the presence of detectable biodegradation. This case study found it useful to apply a spatial distribution of contaminants coupled with their dual isotopic values for evaluation of the contaminated sites and identification of the presence of multiple sources in the study area.

Compound-specific isotope analysis as a tool to characterize biodegradation of ethylbenzene

This study applied one- and two-dimensional compound-specific isotope analysis (CSIA) for the elements carbon and hydrogen to assess different means of microbial ethylbenzene activation. Cultures incubated under nitrate-reducing conditions showed significant carbon and highly pronounced hydrogen isotope fractionation of comparable magnitudes, leading to nearly identical slopes in dual-isotope plots. The results imply that Georgfuchsia toluolica G5G6 and an enrichment culture dominated by an Azoarcus species activate ethylbenzene by anaerobic hydroxylation catalyzed by ethylbenzene dehydrogenase, similar to Aromatoleum aromaticum EbN1. The isotope enrichment pattern in dual plots from two strictly anaerobic enrichment cultures differed considerably from those for benzylic hydroxylation, indicating an alternative anaerobic activation step, most likely fumarate addition. Large hydrogen fractionation was quantified using a recently developed Rayleigh-based approach considering hydrogen atoms at reactive sites. Data from nine investigated microbial cultures clearly suggest that two-dimensional CSIA in combination with the magnitude of hydrogen isotope fractionation is a valuable tool to distinguish ethylbenzene degradation and may be of practical use for monitoring natural or technological remediation processes at field sites.

Diffusion sampler for compound specific carbon isotope analysis of dissolved hydrocarbon contaminants

Compound Specific Isotope Analysis (CSIA) is widely utilized to study the fate of organic contaminants in groundwater. To date, however, no method is available to obtain CSIA samples at a fine (cm) spatial scale across the sediment-surface water interface (SWI), a key boundary for discharge of contaminated groundwater to surface water. Dissolved contaminants in such discharged zones undergo rapid temporal and spatial changes due to heterogeneity in redox conditions and microbial populations. The compatibility of a passive sediment pore water sampler ("peeper") to collect 40 mL samples for CSIA of benzene, toluene, monochlorobenzene, and 1,2-dichlorobenzene at field-relevant concentrations (0.1-5 mg L(-1)) was evaluated in laboratory experiments. Results demonstrate that physical diffusion across the polysulfone membrane does not alter the carbon isotope values (±0.5‰). Measured δ(13)C values also remain invariant despite significant adsorption of the compounds on the peeper material, an effect which increased with higher numbers of chlorine atoms and sorption coefficient (Koc) values. In addition, isotope equilibrium between the peeper chamber and the sediment pore water occurred in less than a day, indicating the peeper method can be used to provide samples for CSIA analysis at fine spatial and temporal sampling resolutions in contaminated sediments.

Quantifying the contribution of grape hexoses to wine volatiles by high-precision [U¹³C]-glucose tracer studies

Many fermentation volatiles important to wine aroma potentially arise from yeast metabolism of hexose sugars, but assessing the relative importance of these pathways is challenging due to high endogenous hexose substrate concentrations. To overcome this problem, gas chromatography combustion isotope ratio mass spectrometry (GC-C-IRMS) was used to measure high-precision (13)C/(12)C isotope ratios of volatiles in wines produced from juices spiked with tracer levels (0.01-1 APE) of uniformly labeled [U-(13)C]-glucose. The contribution of hexose to individual volatiles was determined from the degree of (13)C enrichment. As expected, straight-chain fatty acids and their corresponding ethyl esters were derived almost exclusively from hexoses. Most fusel alcohols and their acetate esters were also majority hexose-derived, indicating the importance of anabolic pathways for their formation. Only two compounds were not derived primarily from hexoses (hexanol and isobutyric acid). This approach can be extended to other food systems or substrates for studying precursor-product relationships.

Isotopic analysis of oxidative pollutant degradation pathways exhibiting large H isotope fractionation

Oxidation of aromatic rings and its alkyl substituents are often competing initial steps of organic pollutant transformation. The use of compound-specific isotope analysis (CSIA) to distinguish between these two pathways quantitatively, however, can be hampered by large H isotope fractionation that precludes calculation of apparent (2)H-kinetic isotope effects (KIE) as well as the process identification in multi-element isotope fractionation analysis. Here, we investigated the C and H isotope fractionation associated with the transformation of toluene, nitrobenzene, and four substituted nitrotoluenes by permanganate, MnO4(-), to propose a refined evaluation procedure for the quantitative distinction of CH3-group oxidation and dioxygenation. On the basis of batch experiments, an isotopomer-specific kinetic model, and density functional theory (DFT) calculations, we successfully derived the large apparent (2)H-KIE of 4.033 ± 0.20 for the CH3-group oxidation of toluene from H isotope fractionation exceeding >1300‰ as well as the corresponding (13)C-KIE (1.0324 ± 0.0011). Experiment and theory also agreed well for the dioxygenation of nitrobenzene, which was associated with (2)H- and (13)C-KIEs of 0.9410 ± 0.0030 (0.9228 obtained by DFT) and 1.0289 ± 0.0003 (1.025). Consistent branching ratios for the competing CH3-group oxidation and dioxygenation of nitrotoluenes by MnO4(-) were obtained from the combined modeling of concentration as well as C and H isotope signature trends. Our approach offers improved estimates for the identification of contaminant microbial and abiotic oxidation pathways by CSIA.

Hydrogen and carbon isotope fractionation during degradation of chloromethane by methylotrophic bacteria

Chloromethane (CH3 Cl) is a widely studied volatile halocarbon involved in the destruction of ozone in the stratosphere. Nevertheless, its global budget still remains debated. Stable isotope analysis is a powerful tool to constrain fluxes of chloromethane between various environmental compartments which involve a multiplicity of sources and sinks, and both biotic and abiotic processes. In this study, we measured hydrogen and carbon isotope fractionation of the remaining untransformed chloromethane following its degradation by methylotrophic bacterial strains Methylobacterium extorquens CM4 and Hyphomicrobium sp. MC1, which belong to different genera but both use the cmu pathway, the only pathway for bacterial degradation of chloromethane characterized so far. Hydrogen isotope fractionation for degradation of chloromethane was determined for the first time, and yielded enrichment factors (ε) of -29‰ and -27‰ for strains CM4 and MC1, respectively. In agreement with previous studies, enrichment in (13) C of untransformed CH3 Cl was also observed, and similar isotope enrichment factors (ε) of -41‰ and -38‰ were obtained for degradation of chloromethane by strains CM4 and MC1, respectively. These combined hydrogen and carbon isotopic data for bacterial degradation of chloromethane will contribute to refine models of the global atmospheric budget of chloromethane.

Estimating pathway-specific contributions to biodegradation in aquifers based on dual isotope analysis: theoretical analysis and reactive transport simulations

At field sites with varying redox conditions, different redox-specific microbial degradation pathways contribute to total contaminant degradation. The identification of pathway-specific contributions to total contaminant removal is of high practical relevance, yet difficult to achieve with current methods. Current stable-isotope-fractionation-based techniques focus on the identification of dominant biodegradation pathways under constant environmental conditions. We present an approach based on dual stable isotope data to estimate the individual contributions of two redox-specific pathways. We apply this approach to carbon and hydrogen isotope data obtained from reactive transport simulations of an organic contaminant plume in a two-dimensional aquifer cross section to test the applicability of the method. To take aspects typically encountered at field sites into account, additional simulations addressed the effects of transverse mixing, diffusion-induced stable-isotope fractionation, heterogeneities in the flow field, and mixing in sampling wells on isotope-based estimates for aerobic and anaerobic pathway contributions to total contaminant biodegradation. Results confirm the general applicability of the presented estimation method which is most accurate along the plume core and less accurate towards the fringe where flow paths receive contaminant mass and associated isotope signatures from the core by transverse dispersion. The presented method complements the stable-isotope-fractionation-based analysis toolbox. At field sites with varying redox conditions, it provides a means to identify the relative importance of individual, redox-specific degradation pathways.

Online methodology for determining compound-specific hydrogen stable isotope ratios of trichloroethene and 1,2-cis-dichloroethene by continuous-flow isotope ratio mass spectrometry

RATIONALE

Carbon and chlorine compound-specific isotope analysis (CSIA) is utilized in chlorinated solvent contamination studies of soil and groundwater contaminated sites. However, in field studies, hydrogen CSIA has been used only in non-chlorinated volatile organic compound (VOC) investigations, due to low conversion yields into hydrogen gas and poor reproducibility. Therefore, it is important to develop hydrogen CSIA methodology for soil and subsurface contamination studies.

METHODS

A new analytical method for determining compound-specific hydrogen stable isotope ratios is presented. The isotopic ratios were measured by gas chromatography/isotope ratio mass spectrometry (GC/IRMS) coupled with a chromium reduction system. The method was used to determine the δ(2) H values of trichloroethene (TCE) and 1,2-cis-dichloroethene (cis-DCE).

RESULTS

The accuracy of the method was verified by conducting comparison measurements of standards by the conventional offline technique and the new method. The precision of the new analytical method (better than ±7 ‰) is better than that obtained from the offline method. The quantification limits of the headspace-solid-phase microextraction (SPME) are 400 µg/L and 200 µg/L for TCE and cis-DCE, respectively. The quantification limits can be improved by adopting a more efficient pre-concentration system such as purge-and-trap or thermal adsorption.

CONCLUSIONS

This analytical method will facilitate the use of hydrogen CSIA on chlorinated solvents, which can be beneficial in multi-isotope approaches (coupling δ(2)H values with δ(13)C and/or δ(37)Cl values) in field site investigations where source identifications and contaminant behaviours are questioned.

Carbon and nitrogen isotope effects associated with the dioxygenation of aniline and diphenylamine

Dioxygenation of aromatic rings is frequently the initial step of biodegradation of organic subsurface pollutants. This process can be tracked by compound-specific isotope analysis to assess the extent of contaminant transformation, but the corresponding isotope effects, especially for dioxygenation of N-substituted, aromatic contaminants, are not well understood. We investigated the C and N isotope fractionation associated with the biodegradation of aniline and diphenylamine using pure cultures of Burkholderia sp. strain JS667, which can biodegrade both compounds, each by a distinct dioxygenase enzyme. For diphenylamine, the C and N isotope enrichment was normal with ε(C)- and ε(N)-values of -0.6 ± 0.1‰ and -1.0 ± 0.1‰, respectively. In contrast, N isotopes of aniline were subject to substantial inverse fractionation (ε(N) of +13 ± 0.5‰), whereas the ε(C)-value was identical to that of diphenylamine. A comparison of the apparent kinetic isotope effects for aniline and diphenylamine dioxygenation with those from abiotic oxidation by manganese oxide (MnO(2)) suggest that the oxidation of a diarylamine system leads to distinct C-N bonding changes compared to aniline regardless of reaction mechanism and oxidant involved. Combined evaluation of the C and N isotope signatures of the contaminants reveals characteristic Δδ(15)N/Δδ(13)C-trends for the identification of diphenylamine and aniline oxidation in contaminated subsurfaces and for the distinction of aniline oxidation from its formation by microbial and/or abiotic reduction of nitrobenzene.

Degradation of γ-HCH spiked soil using stabilized Pd/Fe0 bimetallic nanoparticles: pathways, kinetics and effect of reaction conditions

This study investigates the degradation pathway of gamma-hexachlorocyclohexane (γ-HCH) in spiked soil using carboxymethyl cellulose stabilized Pd/Fe(0) bimetallic nanoparticles (CMC-Pd/nFe(0)). GC-MS analysis of γ-HCH degradation products showed the formation of pentachlorocyclohexene, tri- and di-chlorobenzene as intermediate products while benzene was formed as the most stable end product. On the basis of identified intermediates and final products, degradation pathway of γ-HCH has been proposed. Batch studies showed complete γ-HCH degradation at a loading of 0.20 g/L CMC-Pd/nFe(0) within 6h of incubation. The surface area normalized rate constant (k(SA)) was found to be 7.6 × 10(-2) L min(-1)m(-2). CMC-Pd/nFe(0) displayed ≈ 7-fold greater efficiency for γ-HCH degradation in comparison to Fe(0) nanoparticles (nFe(0)), synthesized without CMC and Pd. Further studies showed that increase in CMC-Pd/nFe(0) loading and reaction temperature facilitates γ-HCH degradation, whereas a declining trend in degradation was noticed with the increase in pH, initial γ-HCH concentration and in the presence of cations. The data on activation energy (33.7 kJ/mol) suggests that γ-HCH degradation is a surface mediated reaction. The significance of the study with respect to remediation of γ-HCH contaminated soil using CMC-Pd/nFe(0) has been discussed.

Compound Specific Isotope Analysis (CSIA) for chlorine and bromine: a review of techniques and applications to elucidate environmental sources and processes

Chlorinated and brominated compounds belong to the class of organohalogen compounds that have received attention because of their widespread occurrence, use and applications. Understanding the sources and transformation processes of these contaminants in the environment enables assessment of their possible impact on humans and ecosystems. Recently new and innovative methods of Compound Specific Isotope Analysis have started to be applied to characterize the origin and fate of compounds, their breakdown products and degradation rates in different environmental compartments. Almost all studies have focussed on determination of isotopes of C and H, only recently new methodologies have been developed to measure isotopes of Cl and Br. This review firstly gives a brief description of chemistry properties and geochemical cycle of chlorine and bromine followed by a summary of their uses and applications. In the second section, an overview of CSIA techniques and new challenges and successful applications are also presented.

Characterization of the relationship between microbial degradation processes at a hydrocarbon contaminated site using isotopic methods

Decisions to employ monitored natural attenuation (MNA) as a remediation strategy at contaminated field sites require a comprehensive characterization of the site-specific biodegradation processes. In the present study, compound-specific carbon and hydrogen isotope analysis (CSIA) was used to investigate intrinsic biodegradation of benzene and ethylbenzene in an aquifer with high levels of aromatic and aliphatic hydrocarbon contamination. Hydrochemical data and isotope fractionation analysis of sulfate and methane was used complementarily to elucidate microbial degradation processes over the course of a three year period, consisting of six sampling campaigns, in the industrial area of Weißandt-Gölzau (Saxony-Anhalt, Germany). Enrichment of (13)C and (2)H isotopes in the residual benzene and ethylbenzene pool downgradient from the pollution sources provided evidence of biodegradation of BTEX compounds at this site, targeting both compounds as the key contaminants of concern. The enrichment of heavy sulfur isotopes accompanied by decreasing sulfate concentrations and the accumulation of isotopically light methane suggested that sulfate-reducing and methanogenic processes are the major contributors to overall biodegradation in this aquifer. Along the contaminant plume, the oxidation of methane with δ(13)C(CH4) values of up to +17.5‰ was detected. This demonstrates that methane formed in the contaminant source can be transported along groundwater flow paths and be oxidized in areas with higher redox potentials, thereby competing directly with the pollutants for electron acceptors. Hydrochemical and isotope data was summarized in a conceptual model to assess whether MNA can be used as viable remediation strategy in Weißandt-Gölzau. The presented results demonstrate the benefits of combining different isotopic methods and hydrochemical approaches to evaluate the fate of organic pollutants in contaminated aquifers.

Field applicability of Compound-Specific Isotope Analysis (CSIA) for characterization and quantification of in situ contaminant degradation in aquifers

Microbial processes govern the fate of organic contaminants in aquifers to a major extent. Therefore, the evaluation of in situ biodegradation is essential for the implementation of Natural Attenuation (NA) concepts in groundwater management. Laboratory degradation experiments and biogeochemical approaches are often biased and provide only indirect evidence of in situ degradation potential. Compound-Specific Isotope Analysis (CSIA) is at present among the most promising tools for assessment of the in situ contaminant degradation within aquifers. One- and two-dimensional (2D) CSIA provides qualitative and quantitative information on in situ contaminant transformation; it is applicable for proving in situ degradation and characterizing degradation conditions and reaction mechanisms. However, field application of CSIA is challenging due to a number of influencing factors, namely those affecting the observed isotope fractionation during biodegradation (e.g., non-isotope-fractionating rate-limiting steps, limited bioavailability), potential isotope effects caused by processes other than biodegradation (e.g., sorption, volatilization, diffusion), as well as non-isotope-fractionating physical processes such as dispersion and dilution. This mini-review aims at guiding practical users towards the sound interpretation of CSIA field data for the characterization of in situ contaminant degradation. It focuses on the relevance of various constraints and influencing factors in CSIA field applications and provides advice on when and how to account for these constraints. We first evaluate factors that can influence isotope fractionation during biodegradation, as well as potential isotope-fractionating and non-isotope-fractionating physical processes governing observed isotope fractionation in the field. Finally, the potentials of the CSIA approach for site characterization and the proper ways to account for various constraints are illustrated by means of a comprehensive CSIA field study at the benzene, toluene, ethylbenzene, and xylene (BTEX)-contaminated site Zeitz.

Origin and fate of organic compounds in water: characterization by compound-specific stable isotope analysis

Within the past 15 years, compound-specific stable isotope analysis has continued to increase in popularity in the area of contaminant hydrology of organic molecules. In particular, in cases where concentration data alone are insufficient to elucidate environmental processes unequivocally, the isotope signature can provide additional unique information. Specifically, it can help answer questions about contaminant source apportionment, quantification of biotic and abiotic processes, and identification of transformation reactions on a mechanistic level. We review advances in laboratory and field investigations and exemplary applications in contaminant hydrology via stable isotope analysis. We also highlight future directions in the field.

Bioremediation of benzene-, MTBE- and ammonia-contaminated groundwater with pilot-scale constructed wetlands

In this pilot-scale constructed wetland (CW) study for treating groundwater contaminated with benzene, MTBE, and ammonia-N, the performance of two types of CWs (a wetland with gravel matrix and a plant root mat) was investigated. Hypothesized stimulative effects of filter material additives (charcoal, iron(III)) on pollutant removal were also tested. Increased contaminant loss was found during summer; the best treatment performance was achieved by the plant root mat. Concentration decrease in the planted gravel filter/plant root mat, respectively, amounted to 81/99% for benzene, 17/82% for MTBE, and 54/41% for ammonia-N at calculated inflow loads of 525/603 mg/m(2)/d, 97/112 mg/m(2)/d, and 1167/1342 mg/m(2)/d for benzene, MTBE, and ammonia-N. Filter additives did not improve contaminant depletion, although sorption processes were observed and elevated iron(II) formation indicated iron reduction. Bacterial and stable isotope analysis provided evidence for microbial benzene degradation in the CW, emphasizing the promising potential of this treatment technique.

Pathway-dependent isotope fractionation during aerobic and anaerobic degradation of monochlorobenzene and 1,2,4-trichlorobenzene

Stable carbon isotope fractionation is a valuable tool for monitoring natural attenuation and to establish the fate of groundwater contaminants. In this study, we measured carbon isotope fractionation during aerobic and anaerobic degradation of two chlorinated benzenes: monochlorobenzene (MCB) and 1,2,4-trichlorobenzene (1,2,4-TCB). MCB isotope fractionation was measured in anaerobic methanogenic microcosms, while 1,2,4-TCB isotope experiments were carried out in both aerobic and anaerobic microcosms. Large isotope fractionation was observed in both the anaerobic microcosm experiments. Enrichment factors (ε) for anaerobic reductive dechlorination of MCB and 1,2,4-TCB were -5.0‰ ± 0.2‰ and -3.0‰ ± 0.4‰, respectively. In contrast, no significant isotope fractionation was found during aerobic microbial degradation of 1,2,4-TCB. The cleavage of a C-Cl σ bond occurs during anaerobic reductive dechlorination of MCB and 1,2,4-TCB, while no σ bond cleavage is involved during aerobic degradation via dioxygenase. The difference in isotope fractionation for aerobic versus anaerobic biodegradation of MCB and 1,2,4-TCB can be explained by the difference in the initial step of aerobic versus anaerobic biodegradation pathways.

Stable isotope fractionation to investigate natural transformation mechanisms of organic contaminants: principles, prospects and limitations

Gas chromatography-isotope ratio mass spectrometry (GC-IRMS) has made it possible to analyze natural stable isotope ratios (e.g., (13)C/(12)C, (15)N/(14)N, (2)H/(1)H) of individual organic contaminants in environmental samples. They may be used as fingerprints to infer contamination sources, and may demonstrate, and even quantify, the occurrence of natural contaminant transformation by the enrichment of heavy isotopes that arises from degradation-induced isotope fractionation. This review highlights an additional powerful feature of stable isotope fractionation: the study of environmental transformation mechanisms. Isotope effects reflect the energy difference of isotopologues (i.e., molecules carrying a light versus a heavy isotope in a particular molecular position) when moving from reactant to transition state. Measuring isotope fractionation, therefore, essentially allows a glimpse at transition states! It is shown how such position-specific isotope effects are "diluted out" in the compound average measured by GC-IRMS, and how a careful evaluation in mechanistic scenarios and by dual isotope plots can recover the underlying mechanistic information. The mathematical framework for multistep isotope fractionation in environmental transformations is reviewed. Case studies demonstrate how isotope fractionation changes in the presence of mass transfer, enzymatic commitment to catalysis, multiple chemical reaction steps or limited bioavailability, and how this gives information about the individual process steps. Finally, it is discussed how isotope ratios of individual products evolve in sequential or parallel transformations, and what mechanistic insight they contain. A concluding session gives an outlook on current developments, future research directions and the potential for bridging the gap between laboratory and real world systems.

Carbon isotope fractionation during dechlorination of 1,2,3,4-tetrachlorodibenzo-p-dioxin by a Dehalococcoides-containing culture

Carbon isotope fractionation was observed during dechlorination of 1,2,3,4-tetrachlorodibenzo-p-dioxin (1,2,3,4-TeCDD) by a mixed culture containing Dehalococcoides ethenogenes strain 195. Fractionation was examined when 1,2,3,4-TeCDD was added as the only chlorinated compound and when 1,2,3,4-TeCDD was added with a known growth substrate, tetrachloroethene (PCE). The 1,2,3,4-TeCDD was dechlorinated to 1,2,4-trichlorodibenzo-p-dioxin (1,2,4-TrCDD) which was enriched in (13)C relative to 1,2,3,4-TeCDD with isotope separation factors, epsilon(C), of 1.3+/-0.2 per thousand and 1.7+/-0.4 per thousand (average+/-95% confidence interval (CI)) in cultures with and without PCE, respectively. The 1,2,4-TrCDD was further dechlorinated to 1,3-dichlorodibenzo-p-dioxin (1,3-DCDD) which was depleted in (13)C relative to 1,2,4-TrCDD with epsilon(C) of -2.4+/-0.4 per thousand and -2.9+/-0.8 per thousand (average+/-95% CI) in cultures with and without PCE, respectively. This demonstrates carbon isotope fractionation during sequential reductive dechlorination of PCDDs, where isotope fractionation during dechlorination of the intermediate was substantial and a (13)C depleted lightly chlorinated PCDD congener was ultimately formed during dechlorination of more highly chlorinated PCDD congeners. Despite reproducible, statistically significant differences between isotope compositions of the parent, 1,2,3,4-TeCDD and daughter, 1,2,4-TrCDD and 1,3-DCDD congeners in triplicate bottles of both treatments, fractionation factors for 1,2,3,4-TeCDD could not be determined for all replicates by regression analysis of the plot of the Rayleigh equation. It is possible that dissolution of 1,2,3,4-TeCDD imposed a kinetic limitation on dechlorination, thus masking isotope fractionation during its dechlorination.

Development of an advanced on-line position-specific stable carbon isotope system and application to methyl tert-butyl ether

We present an advanced system for on-line position-specific carbon isotope analysis. The main limitation of on-line intramolecular isotope ratio measurements has been that optimal pyrolytic fragments are obtained mostly at temperatures where the analyte has not completely reacted. As a result of undetermined isotopic fractionation, the isotopic signatures of the pyrolysis products are not strictly equal to these of the equivalent moieties in the parent molecule. We designed a pyrolytic unit in which both temperature and reaction time are variable parameters, enabling determination of the enrichment factor of the pyrolysis at optimal temperature by construction of a Rayleigh plot. In the case of methyl tert-butyl ether (MTBE) presented here, a 'pre-pyrolysis' fractionation of MTBE leading to a depletion of 0.9 per thousand was discovered and the enrichment factor of the optimal pyrolysis reaction was determined at -1.7 per thousand. Absolute delta(13)C values of two functional groups of MTBE - the methoxy group and the 2-methylpropane group - could be determined with 95% confidence intervals of 0.4 per thousand and 0.5 per thousand, respectively.

Carbon and hydrogen isotope fractionation of benzene during biodegradation under sulfate-reducing conditions: a laboratory to field site approach

The microbial carbon and hydrogen isotope fractionation of benzene under sulfate-reducing conditions was investigated within systems of increasing complexity: (i) batch laboratory microcosms, (ii) a groundwater-percolated column system, and (iii) an aquifer transect. Recent molecular biological studies indicate that, at least in the laboratory microcosms and the column system, benzene is degraded by similar bacterial communities. Carbon and hydrogen enrichment factors (epsilon(C), epsilon(H)) obtained from laboratory microcosms and from the column study varied significantly although experiments were performed under similar redox and temperature conditions. Thus, enrichment factors for only a single element could not be used to distinguish benzene degradation under sulfate-reducing conditions from other redox conditions. In contrast, using correlation of changes of hydrogen vs. carbon isotope ratios (Lambda = Delta delta(2)H/Delta delta(13)C), similar Lambda-values were derived for the benzene biodegradation under sulfate-reducing conditions in all three experimental systems (Lambda(laboratory microcosms) = 23 +/- 5, Lambda(column) = 28 +/- 3, Lambda(aquifer) = 24 +/- 2), showing the robustness of the two-dimensional compound-specific stable isotope analysis (2D-CSIA) for elucidating distinct biodegradation pathways. Comparing carbon and hydrogen isotope fractionation data from recent studies, an overlap in Lambda-values was observed for benzene biodegradation under sulfate-reducing (Lambda = 23 +/- 5 to Lambda = 29 +/- 3) and methanogenic (Lambda = 28 +/- 1 to Lambda = 39 +/- 5) conditions, indicating a similar initial benzene reaction mechanism for both electron-acceptor conditions.

Membrane permeation continuous-flow isotope ratio mass spectrometry for on-line carbon isotope ratio determination

Gaseous membrane permeation (MP) technologies have been combined with continuous-flow isotope ratio mass spectrometry for on-line delta13C measurements. The experimental setup of membrane permeation-gas chromatography/combustion/isotope ratio mass spectrometry (MP-GC/C/IRMS) quantitatively traps gas streams in membrane permeation experiments under steady-state conditions and performs on-line gas transfer into a GC/C/IRMS system. A commercial polydimethylsiloxane (PDMS) membrane sheet was used for the experiments. Laboratory tests using CO2 demonstrate that the whole process does not fractionate the C isotopes of CO2. Moreover, the delta13C values of CO2 permeated on-line give the same isotopic results as off-line static dual-inlet IRMS delta13C measurements. Formaldehyde generated from aqueous formaldehyde solutions has also been used as the feed gas for permeation experiments and on-line delta13C determination. The feed-formaldehyde delta13C value was pre-determined by sampling the headspace of the thermostated aqueous formaldehyde solution. Comparison of the results obtained by headspace with those from direct aqueous formaldehyde injection confirms that the headspace sampling does not generate isotopic fractionation, but the permeated formaldehyde analyzed on-line yields a 13C enrichment relative to the feed delta13C value, the isotopic fractionation being 1.0026 +/- 0.0003. The delta13C values have been normalized using an adapted two-point isotopic calibration for delta13C values ranging from -42 to -10 per thousand. The MP-GC/C/IRMS system allows the delta13C determination of formaldehyde without chemical derivatization or additional analytical imprecision.

Effect of molecule size on carbon isotope fractionation during biodegradation of chlorinated alkanes by Xanthobacter autotrophicus GJ10

The effect of the number of carbon and chlorine atoms on carbon isotope fractionation during dechlorination of chlorinated alkanes by Xanthobacter autotrophicus GJ10 was studied using pure culture and cell-free extract experiments. The magnitude of carbon isotope fractionation decreased with increasing carbon number. The decrease can be explained by an increasing probability that the heavy isotope is located at a non-reacting position for increasing molecule size. The isotope data were corrected for the number of carbons as well as the number of reactive sites to obtain reacting-site-specific values denoted as apparent kinetic isotope effect (AKIE). Even after the correction, the obtained AKIE values varied (on average 1.0608, 1.0477, 1.0616, and 1.0555 for 1,2-dichloroethane, chloropentane, 1,3-dichloropentane and chlorobutane, respectively). Cell-free extract experiments were carried out to evaluate the effect of transport across the cell membrane on the observed variability in the AKIE values, which revealed that variability still persisted. The study demonstrates that even after differences related to the carbon number and structure of the molecule are taken into account, there still remain differences in AKIE values even for compounds that are degraded by the same pure culture and an identical reaction mechanism.

Carbon and chlorine isotope fractionation during aerobic oxidation and reductive dechlorination of vinyl chloride and cis-1,2-dichloroethene

The study investigated carbon and chlorine isotope fractionation during aerobic oxidation and reductive dechlorination of vinyl chloride (VC) and cis-1,2-dichloroethene (cDCE). The experimental data followed a Rayleigh trend. For aerobic oxidation, the average carbon isotope enrichment factors were -7.2 per thousand and -8.5% for VC and cDCE, respectively, while average chlorine isotope enrichment factors were only -0.3 per thousand for both compounds. These values are consistent with an initial transformation by epoxidation for which a significant primary carbon isotope effect and only a small secondary chlorine isotope effect is expected. For reductive dechlorination, larger carbon isotope enrichment factors of -25.2 per thousand for VC and -18.5 per thousand for cDCE were observed consistent with previous studies. Although the average chlorine isotope enrichmentfactors were larger than those of aerobic oxidation (-1.8 per thousand for VC, -1.5 per thousand for cDCE), they were not as large as typically expected for a primary chlorine isotope effect suggesting that no cleavage of C-Cl bonds takes place during the initial rate-limiting step. The ratio of isotope enrichment factors for chlorine and carbon were substantially different for the two reaction mechanisms suggesting that the reaction mechanisms can be differentiated at the field scale using a dual isotope approach.

Isotopic evidence suggests different initial reaction mechanisms for anaerobic benzene biodegradation

The initial metabolic reactions for anaerobic benzene biodegradation remain uncharacterized. Isotopic data for carbon and hydrogen fractionation from nitrate-reducing, sulfate-reducing, and methanogenic benzene-degrading enrichment cultures and phylogenic information were used to investigate the initial reaction step in anaerobic benzene biodegradation. Dual parameter plots of carbon and hydrogen isotopic data (deltadelta2H/ deltadelta13C) from each culture were linear, suggesting a consistent reaction mechanism as degradation proceeded. Methanogenic and sulfate-reducing cultures showed consistently higher slopes (m = 29 +/- 2) compared to nitrate-reducing cultures (m = 13 +/- 2) providing evidence for different initial reaction mechanisms. Phylogenetic analyses confirmed that culture conditions were strictly anaerobic, precluding any involvement of molecular oxygen in the observed differences. Using published kinetic data, we explored the possibility of attributing such slopes to reaction mechanisms. The higher slopes found under methanogenic and sulfate-reducing conditions suggest against an alkylation mechanism for these cultures. Observed differences between the methanogenic and nitrate-reducing cultures may not represent distinct reactions of different bonds, but rather subtle differences in relative reaction kinetics. Additional mechanistic conclusions could not be made because kinetic isotope effect data for carboxylation and other putative mechanisms are not available.

Precise and accurate compound specific carbon and nitrogen isotope analysis of atrazine: critical role of combustion oven conditions

Compound-specific stable isotope analysis by gas chromatography-isotope ratio mass spectrometry (GC-IRMS) is increasingly used to assess origin and fate of organic substances in the environment. Although analysis without isotopic discrimination is essential, it cannot be taken for granted for new target compounds. We developed and validated carbon isotope analysis of atrazine, a herbicide widely used in agriculture. Combustion was tested with reactors containing (i) CuO/NiO/Pt operating at 940 degrees C; (ii) CuO operating at 800 degrees C; (iii) Ni/NiO operating at 1150 degrees C and being reoxidized for 2 min during each gas chromatographic run. Accurate and precise carbon isotope measurements were only obtained with Ni/NiO reactors giving a mean deviation delta delta(13)C from dual inlet measurements of -0.1-0.2% per hundred and a standard deviation (SD) of +/- 0.4% per hundred. CuO at 800 degrees C gave precise, but inaccurate values (delta delta(13)C = -1.3% per hundred, SD +/- 0.4% per hundred), whereas CuO/NiO/Pt reactors at 940 degrees C gave inaccurate and imprecise data. Accurate (delta delta(15)N = 0.2% per hundred) and precise (SD +/- 0.3% per hundred) nitrogen isotope analysis was accomplished with a Ni/NiO-reactor previously used for carbon isotope analysis. The applicability of the method was demonstrated for alkaline hydrolysis of atrazine at 20 degrees C and pH 12 (nucleophilic aromatic substitution) giving epsilon(carbon) = -5.6% per hundred +/- 0.1% per hundred (SD) and epsilon(nitrogen) = -1.2% per hundred +/- 0.1% per hundred (SD).

Impact of bioavailability restrictions on microbially induced stable isotope fractionation. 2. Experimental evidence

Stable isotope fractionation analysis (SIFA) of contaminants is an emerging technique to characterize in situ microbial activity. The kinetic isotope effect in microbial degradation reactions, or enzyme catalysis, is caused by the preferential cleavage of bonds containing light rather than heavy isotopes. This leads to a relative enrichment of the heavier isotopes in the residual substrate pool. However, a number of nonisotopically sensitive steps preceding the isotopically sensitive bond cleavage may affect the reaction kinetics of a degradation process, thus reducing the observed (i.e., the macroscopically detectable) isotope fractionation. Low bioavailability of contaminants poses kinetic limitations on the biodegradation process and can significantly reduce the observed kinetic isotope fractionation. Here we present experimental evidence for the influence of bioavailability-limited pollutant biodegradation on observed stable isotope fractionation. Batch laboratory experiments were performed to quantify the toluene hydrogen isotope fractionation of Pseudomonas putida mt-2 (pWWO) subjected to different small concentrations of toluene with and without deuterium label, which corresponded to realistic environmental mass transfer scenarios. Detected isotope fractionations depended significantly on the toluene concentration, hence confirming the influence of substrate mass transfer limitation on observed isotope fractionation, hypothesized by Thullner et al. (Environ. Sci. Technol. 2008, 42,6544-6551). Our results indicate that the bioavailability of a substrate should be considered during quantitative analysis of microbial degradation based on SIFA.

Combined carbon and hydrogen isotope fractionation investigations for elucidating benzene biodegradation pathways

Recently, combined carbon and hydrogen isotope fractionation investigations have emerged as a powerful tool for the characterization of reaction mechanisms relevant for the removal of organic pollutants. Here, we applied this approach in order to differentiate benzene biodegradation pathways under oxic and anoxic conditions in laboratory experiments. Carbon and hydrogen isotope fractionation of benzene was studied with four different aerobic strains using a monooxygenase or a dioxygenase for the initial benzene attack, a facultative anaerobic chlorate-reducing strain as well as a sulfate-reducing mixed culture. Carbon and hydrogen enrichment factors (epsilon(C), epsilon(H)) varied for the specific pathways and degradation conditions, respectively, so that from the individual enrichment factors only limited information could be obtained for the identification of benzene biodegradation pathways. However, using the slope derived from hydrogen vs carbon isotope discriminations or the ratio of hydrogen to carbon enrichment factors (lambda = deltaH/ deltaC approximately epsilon(H)/epsilon(C)), benzene degradation mechanisms could be distinguished. Although experimentally determined lambda values partially overlapped, ranges could be determined for different benzene biodegradation pathways. Specific lambda values were < 2 for dihydroxylation, between 7 and 9 for monohydroxylation, and > 17 for anaerobic degradation. Moreover, variations in lambda values suggest that more than one reaction mechanism exists for monohydroxylation as well as for anaerobic benzene degradation under nitrate-reducing, sulfate-reducing, or methanogenic conditions. Our results show that the combined carbon and hydrogen isotope fractionation approach has potential to elucidate biodegradation pathways of pollutants in field and laboratory microcosm studies.

Pitfalls in compound-specific isotope analysis of environmental samples

In the last decade compound-specific stable isotope analysis (CSIA) has evolved as a valuable technique in the field of environmental science, especially in contaminated site assessment. Instrumentation and methods exist for highly precise measurements of the isotopic composition of organic contaminants even in a very low concentration range. Nevertheless, the determination of precise and accurate isotope data of environmental samples can be a challenge. Since CSIA is gaining more and more popularity in the assessment of in situ biodegradation of organic contaminants, an increasing number of authorities and environmental consulting offices are interested in the application of the method for contaminated site remediation. Because of this, it is important to demonstrate the problems and limitations associated with compound-specific isotope measurements of environmental samples. In this review, potential pitfalls of the analytical procedure are critically discussed and strategies to avoid possible sources of error are provided. In order to maintain the analytical quality and to ensure the basis for reliable stable isotope data, recommendations on groundwater sampling, and sample preservation and storage are given. Important aspects of sample preparation and preconcentration techniques to improve sensitivity are highlighted. Problems related to chromatographic resolution and matrix interference are discussed that have to be considered in order to achieve accurate gas chromatography/isotope ratio mass spectrometry measurements. As a result, the need for a thorough investigation of compound-specific isotope fractionation effects introduced by any step of the overall analytical method by standards with known isotopic composition is emphasized. Finally, we address some important points that have to be considered when interpreting data from field investigations.

Estimation of nitrogen removal rate in aqueous phase based on delta15N in microorganisms in solid phase

Microbial nitrification and denitrification are important processes for removing nitrogenous compounds in aqueous systems. Nitrogen removal rate estimation is essential for controlling nitrogen removal processes and modeling the nitrogen cycle in ecosystems. The model described the relationship between ammonium removal rate (aqueous phase) and the nitrogen stable isotope ratio (delta15N) of microorganisms (solid phase) when a coupled nitrification-denitrification process occurs and assimilation and advections are maintained in a steady state. An oxidation ditch in a municipal wastewater treatment plant was evaluated for 3 years using the model. The ammonium removal rate was calculated from the data of delta15N of the activated sludge, it correlated significantly with the observed removal rate. The isotope fractionation factor (epsilon) was determined to be -5.5 per thousand by using a nonlinear method. The model and obtained factor value were applicable for standard activated-sludge processes performed in parallel in the oxidation ditch and a river watershed. The model may help illustrate nitrogen behavior in ecosystems.

Potential Use of Environmental Isotopes in Pollutant Migration Studies

This article presents the use of natural abundance stable isotope (hydrogen, carbon, nitrogen, oxygen, chlorine) analysis data as a tool for providing important information about the origin of contaminants, the contribution of different sources to a multi-source plume, characterisation of their complex transport (rate and mechanisms) and for evaluating the success of contaminated site remediation. Isotopic signatures of contaminants are useful tracers of their sources, while isotopic fractionation can be used to quantitatively assess the progress of an environmental process such as biodegradation. This new isotopic approach is reliable and can offer more information than traditional techniques in pollutant migration studies, particularly after waste disposal. During biological degradation of any organic compound, molecules containing lighter isotopes are degraded, and the portion of heavier isotopes in the substrate is increased, identifying specific microbial roles in biogeochemical cycling. Since isotopic fractionation is proportional to degradation, depending on the type of contamination, a microbial degradation of 50% to 99% of the initial concentration can be quantified using isotope ratio measurements.

Assessment of the natural attenuation of chlorinated ethenes in an anaerobic contaminated aquifer in the Bitterfeld/Wolfen area using stable isotope techniques, microcosm studies and molecular biomarkers

The in situ degradation of chlorinated ethenes was assessed in an anaerobic aquifer using stable isotope fractionation approaches, microcosm studies and taxon specific detection of specific dehalogenating groups of bacteria. The aquifer in the Bitterfeld/Wolfen region in Germany contained all chlorinated ethenes, benzene and toluene as contaminants. The concentrations and isotope composition of the chlorinated ethenes indicated biodegradation of the contaminants. Microcosm studies confirmed the presence of in situ microbial communities capable of the complete dechlorination of tetrachloroethene. Taxon specific investigation of the microbial communities indicated the presence of various potential dechlorinating organisms including Dehalococcoides, Desulfuromonas, Desulfitobacterium and Dehalobacter. The integrated approach, using metabolite spectra, molecular marker analysis and isotope studies, provided several lines of evidence for natural attenuation of the chlorinated ethenes.