Carbon Isotope Enrichment Factors Research Articles

Disclosing phototransformation mechanisms of decabromodiphenyl ether (BDE-209) in different media by simulated sunlight: Implication by compound-specific stable isotope analysis.

As one of the typical brominated flame retardants, decabromodiphenyl ether (BDE-209) has been widely detected in environment. However, scarce information was available on BDE-209 phototransformation mechanisms in various media. In this study, compound-specific stable isotope analysis was first applied to investigate BDE-209 phototransformation in n-hexane, MeOH:H2O (v:v, 8:2), and simulated seawater by simulated sunlight. BDE-209 transformation followed pseudo-first-order kinetic, with degradation rate in the following of n-hexane (2.66 × 10-3min-1) > simulated seawater (1.83 × 10-3min-1) > MeOH:H2O (1.41 × 10-3min-1). Pronounced carbon isotope fractionation was first observed for BDE-209 phototransformation, with carbon isotope enrichment factors (εC) of -1.01 ± 0.14‰, -1.77 ± 0.26‰, -2.94 ± 0.38‰ in n-hexane, MeOH:H2O and simulated seawater, respectively. Combination analysis of products and stable carbon isotope, debromination with cleavage of C-Br bonds as rate-limiting step was the main mechanism for BDE-209 phototransformation in n-hexane, debromination and hydroxylation with cleavage of C-Br bonds as rate-limiting steps in MeOH:H2O, and debromination, hydroxylation and chlorination in simulated seawater. This present study confirmed that stable carbon isotope analysis was a robust method to discovery the underlying phototransformation mechanisms of BDE-209 in various solutions.

Carbon isotope fractionation of di-(2-ethylhexyl)-phthalate during photosensitized degradation by •OH and [formula omitted] for characterization of reaction mechanisms

Compound specific isotope analysis (CSIA) has been used to explore the degradation pathways of di-(2-ethylhexyl)-phthalate (DEHP) in photosensitization experiments with H2O2, TiO2, Fe(III) and PMS. The reaction kinetics could be described by first order rate constants [k (h−1)]. The highest degradation rates of DEHP were observed in UV/TiO2 catalyzed reactions.Carbon isotope fractionation factors (εC) and metabolite patterns were examined to analyze the radical reaction. Similar carbon isotope fractionation factors have been found in experiments dominated by OH radical reactions (UV/H2O2, −1.2 ± 0.3‰; UV/TiO2, −1.2 ± 0.5‰). An inverse carbon isotope fractionation has been found for UV/Fe(III) (0.8 ± 0.2‰) and SO4− (0.6 ± 0.2‰ at pH 3) catalyzed reactions indicating that these reactions preferentially taking place at the H atom bond at tert-butyl carbon of the side chain. This hypothesis is further supported by an experiment with isobutyl acetate (εC = 1.8 ± 0.2‰ for UV/Fe(III) at pH 7; εC = -1.1 ± 0.2‰ for UV/PMS at pH 6 and εC = 0.9 ± 0.3‰ for UV/PMS at pH 3).Quenching experiments in the UV/PMS catalyzed reaction indicated a coexistence of SO4− and OH at pH 7. SO4− was the predominant radical at pH 3. OH was found to be the predominant radical in UV/H2O2 and UV/TiO2 reactions. CSIA indicated that the addition of OH to the aromatic ring of DEHP by the RAF pathway is the main reaction mechanism in UV/H2O2 and UV/TiO2 experiments. The SO4− attacks preferentially the C-H bond at the tert-butyl carbon position of the side chain of DEHP which causes an inverse carbon isotope effect. Thus, carbon isotope enrichment factors can be used to distinguish different radical reaction mechanisms with DEHP as a substrate.

Degradation pathways of atrazine by electrochemical oxidation at different current densities: Identifications from compound-specific isotope analysis and DFT calculation

Current density was the key factor that impacted pollutant degradation by electrochemical oxidation, and reaction contributions at various current densities were non-negligible for the cost-effective treatments of organic pollutants. This research introduced compound specific isotope analysis (CSIA) into atrazine (ATZ) degradation by boron doped diamond (BDD) with current density of 2.5–20 mA/cm2, in order to provide “in-situ” and “fingerprint” analysis of reaction contributions with changed current densities. As results, the increased current density displayed a positive impact on ATZ removal. The ɅC/H values (correlations of Δδ13C and Δδ2H) were 24.58, 9.18 and 8.74 when current densities were 20, 4, and 2.5 mA/cm2, with ·OH contribution of 93.5%, 77.2% and 80.35%, respectively. While DET process favored lower current density with contribution rates up to ∼20%. What's more interesting, though the carbon and hydrogen isotope enrichment factors (εC and εH) were fluctuate, the ɅC/H linearly increased accompanied with applied current densities. Therefore, increasing current density was effective due to the larger ·OH contribution even though side reactions may occur. DFT calculations proved the increase of C–Cl bond length and the delocalization of Cl atom, confirming dechlorination reaction mainly occurred in the direct electron transfer process. While ·OH radical mainly attack the C–N bond on the side chain, which was more benefit to the fast decomposition of ATZ molecule and intermediates. It was forceful to discuss pollutant degradation mechanism by combining CSIA and DFT calculations. Target bond cleavage (i.e., dehalogenation reaction) can be conducted by changing reaction conditions like current density due to the significantly different isotope fractionation and bond cleavage.

Isotopic evidence (δ13C, δ37Cl, δ2H) for distinct transformation mechanisms of chloroform: Catalyzed H2-water system vs. zero-valent iron (ZVI)

Catalytic hydrodechlorination is an efficient technology for degrading organochlorinated compounds, such as chloroform (CF), into harmless products. Compound-specific stable isotope analysis (CSIA) of multiple elements is widely used for the investigation of degradation mechanisms. Yet, its application in the study of catalytic hydrodechlorination is still limited. We have applied CSIA to derive chlorine, carbon, and hydrogen isotope enrichment factors (ε) during the degradation of CF over Pd/Al2O3 and over Rh/Al2O3. In addition, the enrichment factors for the same isotopes were derived for the reaction of CF with zero-valent iron (ZVI) for comparison. For the reactions of CF over Pd/Al2O3 and Rh/Al2O3, εC values (−21.9 ± 0.25 ‰ and −23.4 ± 2.3 ‰) and εCl values (−12.1 ± 1.7 ‰ and −10.3 ± 0.6 ‰) were determined, respectively. The corresponding εC and εCl values, for the dechlorination of CF with ZVI were −22.2 ± 2.8 ‰ and −4.7 ± 0.45 ‰, respectively. The apparent kinetic isotope effects (AKIE) of Cl suggest that the transformation mechanism in the catalyzed hydrogen-water system is a non-concerted reaction, unlike the known reductive dechlorination of CF with ZVI. Moreover, dual-isotope slopes (ΛC/Cl) for both catalyzed reactions (ΛPd = 1.8 ± 0.13 and ΛRh = 2.1 ± 0.14) are markedly different than for the ZVI (ΛZVI = 5.8 ± 0.41), reflecting that the reactions proceed in different mechanisms. For hydrogen isotopes, while there was no clear trend for the catalyzed reactions, an inverse secondary hydrogen isotope effect was observed for the reaction of CF with ZVI.

Mechanistic characterization of anaerobic microbial degradation of BTBPE in coastal wetland soils: Implication by compound-specific stable isotope analysis

As a novel brominate flame retardants, 1,2-bis(2,4,6-tribromophenoxy)ethane (BTBPE) has been extensively used in various consumer products, and frequently detected in various environmental matrices. However, the microbial degradation of BTBPE remains unclear in the environment. This study comprehensively investigated the anaerobic microbial degradation of BTBPE and therein stable carbon isotope effect in the wetland soils. BTBPE degradation followed the pseudo-first-order kinetic, with degradation rate of 0.0085 ± 0.0008 day−1. Based on identification of degradation products, stepwise reductive debromination was the main transformation pathway of BTBPE, and tended to keep the stable of 2,4,6-tribromophenoxy group during the microbial degradation. The pronounced carbon isotope fractionation was observed for BTBPE microbial degradation, and carbon isotope enrichment factor (εC) was determined to be −4.81 ± 0.37‰, indicating cleavage of C–Br bond as the rate-limiting step. Compared to previously reported isotope effects, carbon apparent kinetic isotope effect (AKIEC = 1.072 ± 0.004) suggested that the nucleophilic substitution (SN2 reaction) was the potential reaction mechanism for reductive debromination of BTBPE in the anaerobic microbial degradation. These findings demonstrated that BTBPE could be degraded by the anaerobic microbes in wetland soils, and the compound-specific stable isotope analysis was a robust method to discover the underlying reaction mechanisms.

Using Compound Specific Isotope Analysis to decipher the 1,2,3-trichloropropane-to-Allyl chloride transformation by groundwater microbial communities

1,2,3-trichloropropane (TCP), a refractory contaminant, can be reductive dehalogenated to allyl chloride (AC) by microorganisms, which has been shown a potential in situ bioremediation (ISB) strategy for TCP remediation in groundwater. In practice, however, it is hard to monitor the bioreduction extent because the TCP concentrations may also be decreased by non-biodegradation processes. Compound specific isotope analysis (CSIA) can be promising in determining the extent of degradation by quantifying the isotope enrichment factors (ε) of relevant degradation mechanisms. To date, no CSIA study has been reported on TCP degradation. In this study, a novel TCP-to-AC transformation enrichment culture (dominated by Azotobacter, Parabacteroides, Fusibacter, Hydrogenophaga, Trichococcus Desulfovibrio, etc) in the absence of the already identified TCP anaerobic reductive dechlorinating microorganisms (e.g., Dehalogenimonas) was derived from a chlorinated hydrocarbon-contaminated aquifer. A TCP degradation experiment was carried out by adding yeast extract to produce hydrogen as an electron donor. The TCP-to-AC transformation was found to conform to zero-order conversion kinetics with the rate constant 11 ± 0.34 μmol L−1 d−1 during the main biodegradation stage. The bulk carbon isotope enrichment factor (εbulk) of the TCP-to-AC transformation was firstly evaluated as −5.2 ± 0.1‰. This study for the first time characterized the carbon isotope fractionations during TCP biodegradation using a novel enrichment culture, which would provide a promising tool for the incorporation of ISB for TCP removal in the future.

Two Pathways Compete in the Mn(II)-Catalyzed Oxidation of Aminotrismethylene Phosphonate (ATMP).

Mn(II)-catalyzed oxidation by molecular oxygen is considered a relevant process for the environmental fate of aminopolyphosphonate chelating agents such as aminotrismethylene phosphonate (ATMP). However, the potential roles of Mn(III)ATMP-species in the underlying transformation mechanisms are not fully understood. We combined kinetic studies, compound-specific stable carbon isotope analysis, and equilibrium speciation modeling to shed light on the significance of such Mn-ATMP species for the overall ATMP oxidation by molecular oxygen. The fraction of ATMP complexed with Mn(II) inversely correlated with both (i) the Mn(II)-normalized transformation rate constants of ATMP and (ii) the observed carbon isotope enrichment factors (εc-values). These findings provide evidence for two parallel ATMP transformation pathways exhibiting distinctly different reaction kinetics and carbon isotope fractionation: (i) oxidation of ATMP present in Mn(III)ATMP complexes (εc ≈ -10 ‰) and (ii) oxidation of free ATMP by such Mn(III)ATMP species (εc ≈ -1 ‰) in a catalytic cycle. The higher reaction rate of the latter pathway implies that aminopolyphosphonates can be trapped in catalytic Mn-complexes before being transformed and suggests that Mn(III)ATMP might be a potent oxidant also for other reducible solutes in aqueous environments.

Application of dual carbon-bromine stable isotope analysis to characterize anaerobic micro-degradation mechanisms of PBDEs in wetland bottom-water

Polybrominated diphenyl ethers (PBDEs), one kind of persistent organic pollutants, were widely detected in coastal wetlands. Microbial reductive debromination is one of the most important attenuation processes for PBDEs in anaerobic environment, whereas the underlying reaction mechanisms remain elusive. Dual-element stable isotope analysis was recently recognized to distinguish different reaction mechanism for degradation of organic pollutants. In this study, the dual carbon-bromine isotope effects associated with the anaerobic microbial degradation were first investigated to characterize the reaction mechanisms for BDE-47 and BDE-153. Presence of lower brominated congeners indicated stepwise debromination as the main degradation pathway, with the preferential removal of bromine in para position > meta/ortho position. The pronounced isotope fractionation was observed for both carbon and bromine, with similar carbon (εC) and bromine isotope enrichment factor (εBr) between BDE-47 (εC = -5.98‰, εBr = -2.44‰) and BDE-153 (εC = -5.57‰, εBr = -2.06‰) during the microbial degradation. Compared to εC and εBr, the correlation of carbon and isotope effects (ΛC/Br = Δδ81Br/Δδ13C) was almost the same between BDE-47 (0.436) and BDE-153 (0.435), indicating the similar reaction mechanism. The calculated carbon and bromine apparent kinetic isotope effects (AKIEC and AKIEBr) were 1.0773 and 1.0098 for BDE-47 and 1.0716 and 1.0125 for BDE-153, within range reported for degradation of halogenated compounds following nucleophilic substitution. Combination analysis of degradation products, ΛC/Br and AKIE, all the results pointed to that the anaerobic reductive debromination of BDE-47 and BDE-153 followed the nucleophilic aromatic substitution, with the addition of cofactor to the benzene ring concomitant with dissociation of carbon-bromine bond via the inner-sphere electron transfer, and the cleavage of C-Br bond was the rate-determining step. This study contributed to the development of dual carbon-bromine isotope analysis as a robust approach to probe the fate of PBDEs in contaminated sites.

Carbon, hydrogen and nitrogen stable isotope fractionation allow characterizing the reaction mechanisms of 1H-benzotriazole aqueous phototransformation

1H-benzotriazole is part of a larger family of benzotriazoles, which are widely used as lubricants, polymer stabilizers, corrosion inhibitors, and anti-icing fluid components. It is frequently detected in urban runoff, wastewater, and receiving aquatic environments. 1H-benzotriazole is typically resistant to biodegradation and hydrolysis, but can be transformed via direct photolysis and photoinduced mechanisms. In this study, the phototransformation mechanisms of 1H-benzotriazole were characterized using multi-element compound-specific isotope analysis (CSIA). The kinetics, transformation products, and isotope fractionation results altogether revealed that 1H-benzotriazole direct photolysis and indirect photolysis induced by OH radicals involved two alternative pathways. In indirect photolysis, aromatic hydroxylation dominated and was associated with small carbon (εC = -0.65 ± 0.03‰), moderate hydrogen (εH = -21.6‰), and negligible nitrogen isotope enrichment factors and led to hydroxylated forms of benzotriazole. In direct photolysis of 1H-benzotriazole, significant nitrogen (εN = -8.4 ± 0.4 to -4.2 ± 0.3‰) and carbon (εC = -4.3 ± 0.2 to -1.64 ± 0.04‰) isotope enrichment factors indicated an initial N-N bond cleavage followed by nitrogen elimination with a C-N bond cleavage. The results of this study highlight the potential for multi-element CSIA application to track 1H-benzotriazole degradation in aquatic environments.

Mechanistic Aspects Regarding the Ultraviolet Degradation of Polychlorinated Biphenyls in Different Media: Insights from Carbon and Chlorine Isotope Fractionation.

In this study, the carbon and chlorine isotope fractionation during ultraviolet-photolysis of polychlorinated biphenyls (PCBs, including PCB18, PCB77, PCB110, and PCB138) in n-hexane (Hex), methanol/water (MeOH/H2O), and silica gel was first investigated to explore their mechanistic processes. We observed a significant variation in ΛCl-C (εCl/εC) for the same PCBs in different photochemical systems, implying that PCB degradation processes in various photoreaction systems could differ. Although all substrates showed normal apparent carbon/chlorine kinetic isotope effects (C-/Cl-AKIE >1), the putative inverse C-AKIE of nondechlorinated pathways was suggested by 13C depletion of the average carbon isotope composition of PCB138 and corresponding dechlorinated products in MeOH/H2O, which might originate from the magnetic isotope effect. Significant negative correlations were found between C-AKIE and relative disappearance quantum yields ("Φ") of ortho-dechlorinated substrates (PCB18, PCB110, and PCB138) in Hex and MeOH/H2O. However, the C-AKIE and "Φ" of PCB77 (meta/para-dechlorinated congener) obviously deviated from the above correlations. Furthermore, significantly different product-related carbon isotope enrichment factors of PCB77 in Hex were found. These results demonstrated the existence of dechlorination position-specific and masking effects in carbon isotope fractionations.

Dual C–Cl isotope analysis for characterizing the anaerobic transformation of α, β, γ, and δ-hexachlorocyclohexane in contaminated aquifers

Hexachlorocyclohexanes (HCHs) are widespread and persistent environmental pollutants, which cause heavy contamination in soil, sediment and groundwater. An anaerobic consortium, which was enriched on β-HCH using a soil sample from a contaminated area of a former pesticide factory, was capable to transform α, β, γ, and δ-HCH via tetrachlorocyclohexene isomers stoichiometrically to benzene and chlorobenzene. The carbon and chlorine isotope enrichment factors (εC and εCl) of the dehalogenation of the four isomers ranged from −1.9 ± 0.3 to −6.4 ± 0.7‰ and from −1.6 ± 0.2 to −3.2 ± 0.6‰, respectively, and the correlation of δ37Cl and δ13C (Λ values) of the four isomers ranged from 1.1 ± 0.1 to 2.4 ± 0.2. The evaluation of Λ and the apparent kinetic isotope effects (AKIE) for carbon and chlorine may lead to the hypothesis that the two eliminated chlorine atoms of α- and γ-HCH were in axial positions, the same as for the β-HCH conformer which has six chlorine atoms in axial positions after ring flip. The dichloroelimination of δ-HCH resulted in distinct AKIE and Λ values as one chlorine atom is in axial whereas the other chlorine atoms are in the equatorial positions. Significant chlorine and carbon isotope fractionations of HCH isomers were observed in the samples from a contaminated aquifer (Bitterfeld, Germany). The 37Cl/35Cl and 13C/12C isotope fractionation patterns of HCH isomers from laboratory experiments were used diagnostically in a model to characterize microbial dichloroelimination in the field study. The comparison of isotope fractionation patterns indicates that the transformation of HCH isomers at the field was mainly governed by microbial dichloroelimination transformation.

Compound-specific carbon isotope analysis for mechanistic characterization of debromination of decabrominated diphenyl ether.

Decabrominated diphenyl ether (BDE-209) is a notorious persistent organic pollutant widely found in the environment. Developing a compound-specific isotope analysis (CSIA) method is much needed in order to trace its transport and degradation processes and to evaluate the effectiveness of the remediation of BDE-209 in the environment. However, the conventional CSIA method, i.e. gas chromatography (GC) combustion isotope ratio mass spectrometry, is not appropriate for BDE-209 because of its high thermal instability and incomplete combustion. We developed a high-performance liquid chromatography (HPLC) method for the separation and purification of BDE-209 that prevents its thermal reactivity as occurred in prior GC-based methods. The δ13 C value of the purified BDE-209 was determined using offline elemental analyzer isotope ratio mass spectrometry (EA/IRMS). This two-step method was applied to determine the δ13 C values of BDE-209 in two commercial samples and to characterize carbon isotope fractionation associated with the debromination of BDE-209 via nanoscale zero-valent iron. The mean values of daily δ13 C analyses of six replicates of a BDE-209 standard varied from -27.66‰ to -27.92‰, with a standard deviation ranging from 0.07‰ to 0.16‰, indicating a good reproducibility of EA/IRMS. The EA/IRMS analysis of the purified BDE-209 standard indicated no obvious isotope fractionation during the sample purification. The impurity content in commercial BDE-209 samples may contribute additional variation of the δ13 C values of BDE-209. The δ13 C values of BDE-209 gradually changed from -27.47 ± 0.37‰ to -24.59 ± 0.19‰ when 74% of the BDE-209 standard was degraded within 36 h. The estimated carbon isotope enrichment factor was -1.72 ± 0.18‰. The two-step method based on HPLC and EA/IRMS avoids the thermal instability of BDE-209 in the traditional CSIA method. It offers a novel approach for elucidating the degradation mechanisms of BDE-209 in the environment and for source identification in contaminated sites.

Assessing microbial degradation degree and bioavailability of BDE-153 in natural wetland soils: Implication by compound-specific stable isotope analysis

Microbial degradation is an important pathway for the attenuation of polybrominated diphenyl ethers (PBDEs) in natural soils. In this study, the compound-specific stable isotope analysis (CSIA) was applied to characterize microbial degradation of BDE-153, one of the prevailing and toxic PBDE congeners, in natural wetland soils. During the 45-day incubation, the residual percentages of BDE-153 decreased to 67.9% and 73.6% in non-sterilized soils spiked with 1.0 and 5.0 μg/g, respectively, which were both much lower than those in sterilized soils (96.0% and 97.2%). This result indicated that microbial degradation could accelerate BDE-153 elimination in wetland soils. Meanwhile, the significant carbon isotope fractionation was observed in non-sterilized soils, with δ13C of BDE-153 shifting from −29.4‰ to −26.7‰ for 1.0 μg/g and to −27.2‰ for 5.0 μg/g, respectively, whilst not in sterilized soils. This phenomenon indicated microbial degradation could induce stable carbon isotope fractionation of BDE-153. The carbon isotope enrichment factor (εc) for BDE-153 microbial degradation was first determined as −7.58‰, which could be used to assess the microbial degradation and bioavailability of BDE-153 in wetland soils. Based on δ13C and εc, the new methods were developed to dynamically and quantitatively estimate degradation degree and bioavailability of BDE-153 during degradation process, respectively, which could exclude interference of physical processes. This work revealed that CSIA was a promising method to investigate in situ microbial degradation of PBDEs in field studies.

Transformation of Chlorofluorocarbons Investigated via Stable Carbon Compound-Specific Isotope Analysis.

Compound-specific isotope analysis (CSIA) is a valuable tool in contaminant remediation studies. Chlorofluorocarbons (CFCs) are ozone-depleting substances previously thought to be persistent in groundwater under most geochemical conditions but more recently have been found to (bio)transform in some laboratory experiments. To date, limited applications of CSIA to CFCs have been undertaken. Here, biotransformation-associated carbon isotope enrichment factors, εC,bulk for CFC-113 (εC,bulk = -8.5 ± 0.4‰) and CFC-11 (εC,bulk = -14.5 ± 1.9‰), were determined. δ13C signatures of pure-phase CFCs and hydrochlorofluorocarbons were measured to establish source signatures. These findings were applied to investigate potential in situ CFC transformation in groundwater at a field site, where carbon isotope fractionation of CFC-11 suggests naturally occurring biotransformation by indigenous microorganisms. The maximum extent of CFC-11 transformation is estimated to be up to 86% by an approximate calculation using the Rayleigh concept. CFC-113 δ13C values in contrast were not resolvably different from pure-phase sources measured to date, demonstrating that CSIA can aid in identifying which compounds may, or may not, be undergoing reactive processes at field sites. Science and public attention remains focused on CFCs, as unexplained source inputs to the atmosphere have been recently reported, and the potential for CFC biotransformation in surface and groundwaters remains unclear. This study proposes δ13C CSIA as a novel application to study the fate of CFCs in groundwater.

Enantiomer and Carbon Isotope Fractionation of α-Hexachlorocyclohexane by Sphingobium indicum Strain B90A and the Corresponding Enzymes.

Chiral organic contaminants, like α-hexachlorocyclohexane (α-HCH), showed isotope fractionation and enantiomer fractionation during biodegradation. This study aims to understand the correlation between these two processes. Initial tests of α-HCH degradation by six Sphingobium strains (with different LinA variants) were conducted. Results showed variable enantiomer selectivity over the time course. In contrast, constant enantiomer selectivity was observed in experiments employing (i) cell suspensions, (ii) crude extracts, or (iii) LinA1 and LinA2 enzymes of strain B90A for α-HCH degradation in enzyme activity assay buffer. The average value of enantioselectivity (ES) were -0.45 ± 0.03 (cell suspensions), -0.60 ± 0.05 (crude extracts), and 1 (LinA1) or -1 (LinA2). The average carbon isotope enrichment factors (εc) of (+)α- and (-)α-HCH were increased from cells suspensions (-6.3 ± 0.1‰ and -2.3 ± 0.03‰) over crude extracts (-7.7 ± 0.4‰ and -3.4 ± 0.02‰) to purified enzymes (-11.1 ± 0.3‰ and -3.8 ± 0.2‰). The variability of ES and the εc were discussed based on the effect of mass transport and degradation rates. Our study demonstrates that enantiomer and isotope fractionation of α-HCH are two independent processes and both are affected by underlying reactions of individual enzymes and mass transport to a different extent.

Microbial degradation of four dispersed crude oils by Rhodococcus sp. evaluated using carbon stable isotope analysis

AbstractBACKGROUNDThe high frequency and wide distribution of oil spills pose serious threats to the marine environmental ecosystem and human health. Microbial degradation is considered to be the most applicable means to process oil spills. Carbon isotope fractionation was employed as a crucial tool for evaluating the biodegradation of four crude oils by Rhodococcus sp. with Corexit 9500A as oil dispersant.RESULTSFollowing biodegradation, carbon isotope fractionation of n‐alkanes in four dispersed crude oil samples was measured by gas chromatography–isotope ratio mass spectrometry. A relationship between the carbon isotope ratios and residual concentrations of n‐alkanes was established based upon the Rayleigh equation, with a correlation coefficient, R2, ranging from 0.67 to 0.83. The carbon isotope enrichment factors for n‐alkanes of the four crude oil samples tended to be 0.04–0.25‰ in the residuals. The biodegradation level of n‐alkanes calculated from the Rayleigh equation agreed with the biodegradation results analyzed by gas chromatography–mass spectrometry.CONCLUSIONSThe degree of microbial degradation was positively correlated to the carbon isotope fractionation in n‐alkanes. Compound‐specific stable isotope analysis is capable of quantifying the biodegradation level of crude oil or oil spills. © 2019 Society of Chemical Industry

Carbon and hydrogen stable isotope analysis for characterizing the chemical degradation of tributyl phosphate

Tributyl phosphate (TBP) belongs to the group of trialkyl substituted organophosphate esters. Its chemical reactivity depends on the stability of various chemical bonds. TBP was used as a model compound for the development of a concept using stable isotope fractionation associated with bond cleavage reactions for better understanding the fate of TBP in the environment. Carbon isotope enrichment factors (εC) of TBP hydrolysis were found to be pH dependent (−3.8 ± 0.3‰ at pH 2, -4.6 ± 0.5‰ at pH 7, -2.8 ± 0.1‰ at pH 9, no isotope fractionation at pH 12), which is in accordance with the mode of a SN2 hydrolytic bond cleavage. Hydrogen isotope fractionation was negligible as no H bond cleavage is involved during hydrolysis. The apparent carbon kinetic isotope effect (AKIEC) ranged from 1.045 to 1.058. In contrast to hydrolysis, both carbon and hydrogen isotope fractionation were observed during radical oxidation of TBP by OH and SO4–, yielding εC from −0.9 ± 0.1‰ to −0.5 ± 0.1‰ and εH from −20 ± 2‰ to −11 ± 1‰. AKIEC and AKIEH varied from 1.007 to 1.011 and from 1.594 to 2.174, respectively. The correlation of 2H and 13C isotope fractionation revealed Λ values ranging from 17 ± 1 to 25 ± 6. Results demonstrated that the correlation of 2H and 13C isotope fractionation of TBP allowed to identify radical reactions and to distinguish them from hydrolysis. The presented dual isotope analysis approach has diagnostic value for characterizing the chemical transformation of TBP in the environment.

Dual Carbon-Chlorine Isotope Analysis Indicates Distinct Anaerobic Dichloromethane Degradation Pathways in Two Members of Peptococcaceae.

Dichloromethane (DCM) is a probable human carcinogen and frequent groundwater contaminant and contributes to stratospheric ozone layer depletion. DCM is degraded by aerobes harboring glutathione-dependent DCM dehalogenases; however, DCM contamination occurs in oxygen-deprived environments, and much less is known about anaerobic DCM metabolism. Some members of the Peptococcaceae family convert DCM to environmentally benign products including acetate, formate, hydrogen (H2), and inorganic chloride under strictly anoxic conditions. The current study applied stable carbon and chlorine isotope fractionation measurements to the axenic culture Dehalobacterium formicoaceticum and to the consortium RM comprising DCM degrader Candidatus Dichloromethanomonas elyunquensis. Degradation-associated carbon and chlorine isotope enrichment factors (εC and εCl) of -42.4 ± 0.7‰ and -5.3 ± 0.1‰, respectively, were measured in D. formicoaceticum cultures. A similar εCl of -5.2 ± 0.1‰, but a substantially lower εC of -18.3 ± 0.2‰, were determined for Ca. Dichloromethanomonas elyunquensis. The εC and εCl values resulted in distinctly different dual element C-Cl isotope correlations (ΛC/Cl = Δδ13C/Δδ37Cl) of 7.89 ± 0.12 and 3.40 ± 0.03 for D. formicoaceticum and Ca. Dichloromethanomonas elyunquensis, respectively. The distinct ΛC/Cl values obtained for the two cultures imply mechanistically distinct C-Cl bond cleavage reactions, suggesting that members of Peptococcaceae employ different pathways to metabolize DCM. These findings emphasize the utility of dual carbon-chlorine isotope analysis to pinpoint DCM degradation mechanisms and to provide an additional line of evidence that detoxification is occurring at DCM-contaminated sites.

Chloromethane formation and degradation in the fern phyllosphere

Chloromethane (CH3Cl) is the most abundant halogenated trace gas in the atmosphere. It plays an important role in natural stratospheric ozone destruction. Current estimates of the global CH3Cl budget are approximate. The strength of the CH3Cl global sink by microbial degradation in soils and plants is under discussion. Some plants, particularly ferns, have been identified as substantial emitters of CH3Cl. Their ability to degrade CH3Cl remains uncertain. In this study, we investigated the potential of leaves from 3 abundant ferns (Osmunda regalis, Cyathea cooperi, Dryopteris filix-mas) to produce and degrade CH3Cl by measuring their production and consumption rates and their stable carbon and hydrogen isotope signatures. Investigated ferns are able to degrade CH3Cl at rates from 2.1 to 17 and 0.3 to 0.9μggdw−1day−1 for C. cooperi and D. filix-mas respectively, depending on CH3Cl supplementation and temperature. The stable carbon isotope enrichment factor of remaining CH3Cl was −39±13‰, whereas negligible isotope fractionation was observed for hydrogen (−8±19‰). In contrast, O. regalis did not consume CH3Cl, but produced it at rates ranging from 0.6 to 128μggdw−1day−1, with stable isotope values of −97±8‰ for carbon and −202±10‰ for hydrogen, respectively. Even though the 3 ferns showed clearly different formation and consumption patterns, their leaf-associated bacterial diversity was not notably different. Moreover, we did not detect genes associated with the only known chloromethane utilization pathway “cmu” in the microbial phyllosphere of the investigated ferns. Our study suggests that still unknown CH3Cl biodegradation processes on plants play an important role in global cycling of atmospheric CH3Cl.

δ13C and δ37Cl Isotope Fractionation To Characterize Aerobic vs Anaerobic Degradation of Trichloroethylene

Trichloroethylene (TCE) is a carcinogenic organic chemical impacting water resources worldwide. Its breakdown by reductive vs oxidative degradation involves different types of chemical bonds. Hence, if distinct isotope effects are reflected in dual element (carbon and chlorine) isotope values, such trends could help distinguishing both processes in the environment. This work explored dual element isotope trends associated with TCE oxidation by two pure bacterial cultures: Pseudomonas putida F1 and Methylosinus trichosporium OB3b, where the latter expresses either soluble methane-monooxygenase (sMMO) or particulate methane-monooxygenase (pMMO). Carbon and chlorine isotope enrichment factors of TCE (e13C = −11.5, −2.4, and −4.2‰; e37Cl = 0.3, −1.3, and −2.4‰, respectively) differed strongly between the strains. The dual element isotope trend for strain F1 (e13C/e37Cl = −38) reflected, as expected, primary carbon and negligible chlorine isotope effects, whereas unexpectedly large chlorine isotope effects bec...