Chlorine Isotope Fractionation Research Articles

Dual C–Cl isotope fractionation offers potential to assess biodegradation of 1,2-dichloropropane and 1,2,3-trichloropropane by Dehalogenimonas cultures

1,2-dichloropropane (1,2-DCP) and 1,2,3-trichloropropane (1,2,3-TCP) are hazardous chemicals frequently detected in groundwater near agricultural zones due to their historical use in chlorinated fumigant formulations. In this study, we show that the organohalide-respiring bacterium Dehalogenimonas alkenigignens strain BRE15 M can grow during the dihaloelimination of 1,2-DCP and 1,2,3-TCP to propene and allyl chloride, respectively. Our work also provides the first application of dual isotope approach to investigate the anaerobic reductive dechlorination of 1,2-DCP and 1,2,3-TCP. Stable carbon and chlorine isotope fractionation values for 1,2-DCP (ƐC = −13.6 ± 1.4 ‰ and ƐCl = −27.4 ± 5.2 ‰) and 1,2,3-TCP (ƐC = −3.8 ± 0.6 ‰ and ƐCl = −0.8 ± 0.5 ‰) were obtained resulting in distinct dual isotope slopes (Λ12DCP = 0.5 ± 0.1, Λ123TCP = 4 ± 2). However direct comparison of ΛC-Cl among different substrates is not possible and investigation of the C and Cl apparent kinetic isotope effects lead to the hypothesis that concerted dichloroelimination mechanism is more likely for both compounds. In fact, whole cell activity assays using cells suspensions of the Dehalogenimonas-containing culture grown with 1,2-DCP and methyl viologen as electron donor suggest that the same set of reductive dehalogenases was involved in the transformation of 1,2-DCP and 1,2,3-TCP. This study opens the door to the application of isotope techniques for evaluating biodegradation of 1,2-DCP and 1,2,3-TCP, which often co-occur in groundwaters near agricultural fields.

Comprehensive exploration of the anaerobic biotransformation of polychlorinated biphenyls in Dehalococcoides mccartyi CG1: Kinetics, enantioselectivity, and isotope fractionation

Anaerobic microbial transformation is a key pathway in the natural attenuation of polychlorinated biphenyls (PCBs). Much less is known about the transformation behaviors induced by pure organohalide-respiring bacteria, especially kinetic isotope effects. Therefore, the kinetics, pathways, enantioselectivity, and carbon and chlorine isotope fractionation of PCBs transformation by Dehalococcoides mccartyi CG1 were comprehensively explored. The results indicated that the PCBs were mainly dechlorinated via removing their double-flanked meta-chlorine, with their first-order kinetic constants following the order of PCB132 > PCB174 > PCB85 > PCB183 > PCB138. However, PCBs occurred great loss of stoichiometric mass balance during microbial transformation, suggesting the generation of other non-dehalogenation products and/or stable intermediates. The preferential transformation of (−)-atropisomers and generation of (+)-atropisomers were observed during PCB132 and PCB174 biotransformation with the enantiomeric enrichment factors of −0.8609 ± 0.1077 and −0.4503 ± 0.1334 (first half incubation times)/−0.1888 ± 0.1354 (second half incubation times), respectively, whereas no enantioselectivity occurred during PCB183 biotransformation. More importantly, although there was no carbon and chlorine isotope fractionation occurring for studied substrates, the δ13C values of dechlorination products, including PCB47 (−28.15 ± 0.35‰ ∼ −27.77 ± 0.20‰), PCB91 (−36.36 ± 0.09‰ ∼ −34.71 ± 0.49‰), and PCB149 (−28.08 ± 0.26‰ ∼ −26.83 ± 0.10‰), were all significantly different from those of their corresponding substrates (PCB85: −30.81 ± 0.02‰ ∼ −30.22 ± 0.21‰, PCB132: −33.57 ± 0.15‰ ∼ −33.13 ± 0.14‰, and PCB174: −26.30 ± 0.09‰ ∼ −26.01 ± 0.07‰), which further supported the generation of other non-dehalogenation products and/or stable intermediates with enrichment or depletion of 13C. These findings provide deeper insights into the anaerobic microbial transformation behaviors of PCBs.

Estimates of chlorine isotope fractionation factors using density functional theory: Applications to ore-forming systems

Chloride is the most important anion in ore-forming hydrothermal fluids, and chlorine isotopes are therefore potentially sensitive tracers of the origin and evolution of ore-forming fluids. However, they remain a relatively under-utilized tool in ore deposit geochemistry due to the lack of knowledge of chlorine isotope fractionation during ore-forming processes. Using first-principles density functional theory, this study estimates chlorine isotope fractionation factors for various ore-forming processes. All metal-Cl complexes in hydrothermal fluids are enriched in 37Cl compared to chloride ions, but the change in δ37Cl of the fluid trapped in ore minerals is small after the destabilization of metal-chloride complexes during ore mineral deposition. In the precipitation of evaporite minerals from brine, the sequence of enrichment of the heavy Cl isotope (37Cl) is halite > carnallite > aqueous chloride > kainite > sylvite > bischofite. The results of this study agree with the experimental observations that progressive precipitation of halite from brine lowers the δ37Cl value of the residual fluid until the formation of K-Mg chlorides. In low-temperature deposits, δ37Cl values for fluid inclusions and minerals reflect those of the hydrothermal fluid provenance and the mixing of this fluids with saltwater or basinal brines. In high-temperature magmatic-hydrothermal ore deposits that undergo liquid–vapour phase separation, chlorine isotopes fractionate among phases of silicate melt, vapour and chloride-rich liquid. The considerable range in δ37Cl in fluid inclusions may also reflect fluid mixing and hydrothermal alteration. At ambient temperature, the δ37Cl values may reflect evaporative processes and, in the case of chemical weathering of metallic mineral deposits, may record the supergene enrichment of the metals. This study highlights the use of chlorine isotopes as a new tool for interpreting ore-forming processes.

Oxygen and chlorine isotope fractionation during microbial perchlorate reduction in static and flow through systems

Perchlorate (ClO4−) is an oxyanion with both anthropogenic and natural sources that is widespread on earth and throughout the solar system. Stable isotope ratio measurements are a powerful tool to assess the origin and predict the fate of perchlorate in the environment. The aim of this study was to perform an experimental survey of parameters that influence microbial O and Cl isotope fractionation during microbial perchlorate reduction, covering microbiological (genetic background and community context) and environmental (temperature, salinity, electron donor type and limitation) factors. A new method for the purification of perchlorate for isotope ratio mass spectrometry (IRMS) was developed to allow for more efficient sample preparation and suitable for samples containing small amounts of perchlorate (0.4–2 mg). Although most experiments, regardless of treatment, yielded a consistent isotope enrichment factor ratio (Λ = ε18O/ε37Cl ∼ 2.5), a greater variability was observed of individual isotope enrichment factors (ε18O and ε37Cl). Although most experiments yielded isotope enrichment factors (epsilon) in the larger negative range of previously published values, smaller negative values of ε18O and ε37Cl were observed for a halophilic isolate, Haloarcula marismortui, (ε18O = −15 to −22‰ and ε37Cl = −5 to −7‰), and in sediment column experiments (ε18O = −4 to −15‰ and ε37Cl = −1 to −6‰). Given the complexity of isolating individual parameters from our proposed model, we view our dataset as a survey that points to future opportunities where a particular parameter could be isolated to better understand controls on O and Cl isotope fractionation during microbial perchlorate reduction.

Chlorine geochemistry of various geothermal waters in China: Implications for geothermal system geneses

Understanding the geneses of geothermal systems is critical for their efficient and sustainable exploitation. Chlorine is one of the most conservative elements in geothermal waters and capable of providing valuable information related to the formation and evolution of geothermal systems, thereby being a promising indicator for identifying geothermal system geneses. In this study, representative hydrothermal systems located in various tectonic regions in China, including Daggyai, Semi, and Gudui in plate margin as well as Xinzhou and Xiong’an in inland Basins, were selected for investigating the chloride sources of the geothermal waters and possible mechanisms of chlorine isotope fractionation occurring in the relevant geochemical processes. The chloride of geothermal waters in Tibet originated predominantly from magmatic fluids input, whereas water–rock interaction was the primary chloride source for the Xinzhou geothermal waters. As for the chloride in Xiong’an, it was primary derived from the dissolution of halite in carbonate rocks. In addition, the major processes resulting in the chlorine isotope fractionation were supposed to be magma degassing in the Tibetan geothermal systems and ion filtration in the Xiong’an geothermal system. Based on the discovered chloride sources and possible processes inducing non-negligible chlorine isotope fractionation, the geneses of various geothermal systems investigated in this study were proposed. This study suggests that stable chlorine isotopes, coupled with other geochemical parameters, have the potential for discriminating geothermal systems with different geneses.

Effects of Bacterial Growth Conditions on Carbon and Chlorine Isotope Fractionation Associated with TCE Biotransformation

Effects of Bacterial Growth Conditions on Carbon and Chlorine Isotope Fractionation Associated with TCE Biotransformation

Investigation of active site amino acid influence on carbon and chlorine isotope fractionation during reductive dechlorination.

Reductive dehalogenases (RDases) are corrinoid-dependent enzymes that reductively dehalogenate organohalides in respiratory processes. By comparing isotope effects in biotically catalyzed reactions to reference experiments with abiotic corrinoid catalysts, compound-specific isotope analysis (CSIA) has been shown to yield valuable insights into enzyme mechanisms and kinetics, including RDases. Here, we report isotopic fractionation (ε) during biotransformation of chloroform (CF) for carbon (εC = -1.52± 0.34‰) and chlorine (εCl = -1.84± 0.19‰), corresponding to a ΛC/Cl value of 1.13± 0.35. These results are highly suppressed compared to isotope effects observed both during CF biotransformation by another organism with a highly similar RDase (>95% sequence identity) at the amino acid level, and to those observed during abiotic dehalogenation of CF. Amino acid differences occur at four locations within the two different RDases' active sites, and this study examines whether these differences potentially affect the observed εC, εCl, and ΛC/Cl. Structural protein models approximating the locations of the residues elucidate possible controls on reaction mechanisms and/or substrate binding efficiency. These four locations are not conserved among other chloroalkane reducing RDases with high amino acid similarity (>90%), suggesting that these locations may be important in determining isotope fractionation within this homologous group of RDases.

Magmatic chlorine isotope fractionation recorded in apatite from Chang'e-5 basalts

The Moon can have elevated chlorine (Cl) isotope ratios, much higher than any other Solar System objects. Deciphering the Cl isotope compositions of volcanic lunar samples is critical for unraveling the volcanic processes and volatile inventory of the Moon's interior. However, the processes and mechanisms of Cl isotope fractionation are not yet fully understood through previous studies on lunar samples. The China's Chang'e-5 (CE5) basalt samples were collected far from the Apollo and Luna landing sites, and dated at about 2.0 billion years ago (Ga), approximately 1 Ga younger than previously reported lunar basalts. The CE5 samples, therefore, provide an opportunity to investigate Cl isotope characteristics and fractionation mechanisms during a younger lunar volcanism. In this study, we performed systematic petrography, mineral chemistry, volatile abundances and distribution, and Cl isotopic studies on the CE5 apatite via a combination of scanning electron microscopy, electron probe microanalyser, and nanoscale secondary ion mass spectrometry. The CE5 apatite grains from basalt clasts and fragments have subhedral to euhedral shapes with grains sizes mostly less than 10 μm, mainly coexisting with the mesostasis, fayalite olivine, and the margins of pyroxene. These apatites are F-dominated (0.91-3.93 wt%) with a Cl abundance range of 820 to 11989 μg.g−1 and a water abundance range of 134 to 6564 μg.g−1, similar to those in the mare samples previously reported. Chlorine displays notable zoning distributions in some CE5 apatite grains with higher abundance at the rims gradually decrease towards the cores. Chlorine isotopic compositions of CE5 apatite vary from 4.5 to 18.9‰, positively correlated with the Cl abundances. These lines of evidence suggest that magmatic degassing of Cl-bearing species during the crystallisation of apatite at or near the lunar surface could have resulted in a large Cl isotope fractionation. Our new findings highlight a significant role of magmatic fractionation of Cl isotopes during crystallisation of mare lavas and provide clues for determining the primordial Cl isotopic signature of the Moon.

Trichloromethane dechlorination by a novel Dehalobacter sp. strain 8M reveals a third contrasting C and Cl isotope fractionation pattern within this genus

Trichloromethane (TCM) is a pollutant frequently detected in contaminated aquifers, and only four bacterial strains are known to respire it. Here, we obtained a novel Dehalobacter strain capable of transforming TCM to dichloromethane, which was denominated Dehalobacter sp. strain 8M. Besides TCM, strain 8M also completely transformed 1,1,2-trichloroethane to vinyl chloride and 1,2-dichloroethane. Quantitative PCR analysis for the 16S rRNA genes confirmed growth of Dehalobacter with TCM and 1,1,2-trichloroethane as electron acceptors. Carbon and chlorine isotope fractionation during TCM transformation was studied in cultured cells and in enzymatic assays with cell suspensions and crude protein extracts. TCM transformation in the three studied systems resulted in small but significant carbon (εC = −2.7 ± 0.1‰ for respiring cells, −3.1 ± 0.1‰ for cell suspensions, and − 4.1 ± 0.5‰ for crude protein extracts) and chlorine (εCl = −0.9 ± 0.1‰, −1.1 ± 0.1‰, and − 1.2 ± 0.2‰, respectively) isotope fractionation. A characteristic and consistent dual CCl isotope fractionation pattern was observed for the three systems (combined ΛC/Cl = 2.8 ± 0.3). This ΛC/Cl differed significantly from previously reported values for anaerobic dechlorination of TCM by the corrinoid cofactor vitamin B12 and other Dehalobacter strains. These findings widen our knowledge on the existence of different enzyme binding mechanisms underlying TCM-dechlorination within the genus Dehalobacter and demonstrates that dual isotope analysis could be a feasible tool to differentiate TCM degraders at field studies.

Isotopic Tracing of Perchlorate Sources in the Environment

Perchlorate (ClO4−) is an emerging persistent pollutant that is ubiquitous in the environment at trace concentrations. Perchlorate ingestion poses a risk to human health because it interferes with thyroidal hormone production. The identification of perchlorate sources in groundwater is a primary concern. Chlorine and multi-oxygen isotopic tracing of perchlorate (δ37Cl, 36Cl/Cl, δ18O, and Δ17O) can provide a unique tool for identifying the origin and transport of perchlorate in groundwater. Along with the kinetic fractionation of chlorine and oxygen isotopes, the Δ17O value, 36Cl/Cl ratio, and ε18O/ε37Cl (the fractionation coefficient of oxygen and chlorine isotopes) are constant, potentially indicating the biodegradation of perchlorate, without disguising its source information. Therefore, comprehensive characterization of stable chlorine and poly-oxygen isotopes is expected to provide direct evidence for identifying the source of perchlorate in groundwater. However, further studies are needed to increase the amount of isotopic data of different perchlorate sources, to make the end-member model available to broader regions. It is critically important to understand the range of values and differences of isotopes among natural perchlorate sources and the perchlorate formation mechanisms.

Triple-Element Compound-Specific Stable Isotope Analysis (3D-CSIA): Added Value of Cl Isotope Ratios to Assess Herbicide Degradation.

Multielement isotope fractionation studies to assess pollutant transformation are well-established for point-source pollution but are only emerging for diffuse pollution by micropollutants like pesticides. Specifically, chlorine isotope fractionation is hardly explored but promising, because many pesticides contain only few chlorine atoms so that "undiluted" position-specific Cl isotope effects can be expected in compound-average data. This study explored combined Cl, N, and C isotope fractionation to sensitively detect biotic and abiotic transformation of the widespread herbicides and groundwater contaminants acetochlor, metolachlor, and atrazine. For chloroacetanilides, abiotic hydrolysis pathways studied under acidic, neutral, and alkaline conditions as well as biodegradation in two soils resulted in pronounced Cl isotope fractionation (εCl from -5.0 ± 2.3 to -6.5 ± 0.7‰). The characteristic dual C-Cl isotope fractionation patterns (ΛC-Cl from 0.39 ± 0.15 to 0.67 ± 0.08) reveal that Cl isotope analysis provides a robust indicator of chloroacetanilide degradation. For atrazine, distinct ΛC-Cl values were observed for abiotic hydrolysis (7.4 ± 1.9) compared to previous reports for biotic hydrolysis and oxidative dealkylation (1.7 ± 0.9 and 0.6 ± 0.1, respectively). The 3D isotope approach allowed differentiating transformations that would not be distinguishable based on C and N isotope data alone. This first data set on Cl isotope fractionation in chloroacetanilides, together with new data in atrazine degradation, highlights the potential of using compound-specific chlorine isotope analysis for studying in situ pesticide degradation.

Chlorine isotope fractionation during serpentinization and hydrothermal mineralization: A density functional theory study

Because of the large-scale recycling of volatile chlorine from the interior to the surface of the Earth, it is possible to use the isotopic composition of this element (δ37Cl) in serpentinite to study mantle-crust interactions in subduction zones. It is also possible to use chlorine isotopes to track the evolution of fluids (through fluid inclusions/minerals) in hydrothermal ore-forming systems. Here, we report the results of a study of equilibrium chlorine isotope fractionation during serpentinization and hydrothermal mineralization based on density functional theory (DFT) and ab initio molecular dynamics simulations (AIMD). The chlorine isotope fractionation between lizardite and brine under variable thermodynamic conditions is described by the relationship 1000lnαlizardite-fluid = 0.4170 × (1000/T)2–0.0281 × (1000/T) + 0.0582, which yielded Δ37Cllizardite-fluid values of +0.49 to +4.46‰, +0.41 to +0.59‰ and + 0.49 to +3.57‰ for conditions at the seafloor, in the mantle wedge and in subduction zones. As a proxy for the diversity of metal-chloride complexes in hydrothermal fluids, the stable configurations of ferrous chloride complexes were acquired from long trajectories of AIMD simulation. Using this information, the chlorine isotope fractionation between minerals (i.e., apatite-group minerals, muscovite, phlogopite, tremolite, lizardite, marialite and metal halides) and ore-forming fluid was estimated. The relatively large chlorine isotope fractionation (Δ37Cl minerals-hydrothermal fluid values from −1.99 to +2.18‰) might partly explain the large variation of δ37Cl in individual fluid inclusions observed in hydrothermal ore deposits. Because of the limited chlorine isotope fractionation between apatite and hydrothermal fluid (i.e., Δ37Cl apatite-ore-forming fluid of 0.06‰–0.69‰), apatite-group minerals might be an alternative to fluid inclusions for constraining the origin and evolution of hydrothermal fluids using δ37Cl values. The theoretical constraints provided in this paper will facilitate the use of chlorine isotopes in tracking the sources of chloride in subduction zones and the origin of mineralizing fluids in ore deposits.

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.

Stable chlorine isotope analysis of triclosan using GC-qMS: Method development and applications

Occurrence of triclosan (TCS) in water environment caused a serious concern due to TCS toxic properties as well as yield of toxic byproducts during natural attenuation. Here we developed an online approach for chlorine compound specific isotope analysis (CSIA) of TCS based on gas chromatography quadrupole mass spectrometry (GC-qMS). Specifically, the instrumental parameters including dwell time, ionization energy and split ratio values were tested and optimized. Moreover, the calculation scheme considering all the TCS molecular ions presented better precision compared with the method using the most abundant neighboring ion pairs. The optimized method was validated by applying it to differentiate various TCS manufacturers, and was used to resolve chlorine isotope fractionation occurring during TCS photodegradation induced by simulated solar radiation. Chlorine isotopic enrichment factor (εCl) of −2.61 ± 0.37‰ and −0.72 ± 1.95‰ were obtained at pH values of 8.3 and 4.5, respectively. This indicates that preferential cleavage of C-Cl bonds occurred at a higher pH value. Thus, this study provides a robust and sensitive approach for chlorine CSIA of TCS, and could be readily applied to monitor chlorine isotope effects during TCS environmental transformations.

Chlorine isotope mantle heterogeneity: Constraints from theoretical first-principles calculations

The processes that caused the heterogeneity of δ37Cl in the Earth's mantle are unclear. Here we report theoretical estimates of equilibrium chlorine isotope fractionation among common chlorine-bearing minerals, namely apatite-group minerals (Cl-Ap, F-Cl-Ap, OH-Cl-Ap), muscovite, phlogopite, tremolite, lizardite, marialite and metal halides, based on first-principles calculations, and use them to provide an explanation for this heterogeneity. Our results show that at ambient P-T conditions, the reduced isotopic partition function ratio (β-factor) is strongly correlated with the metal-Cl bond length/bond strength of the above minerals and that phlogopite and muscovite are more enriched in 37Cl than other minerals. As a result of a number of factors, including the adjacent atomic environment, the sites occupied by chlorine atoms in crystals, the Cl-metal coordination and the crystal density, the sensitivity of 103lnβ to pressure follows the sequence: halite > fluoro-chlorapatite > chlorapatite > marialite > hydroxyl-chlorapatite > lizardite > tremolite > muscovite > phlogopite. Estimates of the chlorine isotope fractionation between chlorine-bearing minerals and aqueous fluid at the pressure and temperature prevailing during subduction indicate that the δ37Cl values of mantle minerals could vary between −6‰ and + 3‰, assuming equilibrium. In contrast, the average δ37Cl value of the bulk mantle is −0.53 ± 0.16‰. Thus, large-scale recycling of volatile Cl from the deep mantle to the continental/oceanic crust and ocean, and the isotopic fractionation of chlorine with increasing metamorphic grade during subduction, could explain the heterogeneity of δ37Cl values observed in mantle materials.

Chlorine isotope data of chlorides challenge the pore fluid paradigm

In order to examine the seawater-seafloor sediment interactions that influence the chemical composition of seawater through time, we examined hundreds of pore fluid geochemical analyses from 13 clay-rich sedimentary successions drilled by the ODP-IODP. Chemical trends such as monotonous increases in Ca2+, and decreases in Mg2+ and δ18O with depth are traditionally interpreted to result from water-rock interaction. In this view, the release of Ca2+ into fluids and the uptake of Mg2+ and 18O mainly results from the formation of low-temperature clays in the sediment and within underlying basalts. Chloride concentration profiles and isotopic compositions, however, suggest that different processes may influence pore water geochemistry. The data examined here show relatively constant chloride contents but with a systematic decrease in δ37Cl of chlorides with depth from 0 permil (the seawater value) down to −8.5 permil. The δ37Cl data are highly correlated with δ18O (with δ18O down to −5.7 permil).The δ37Cl-depletions of pore fluid chlorides are found in all studied sedimentary piles regardless of tectonic or sedimentary history. These trends cannot be explained by water–rock exchange reactions because minerals formed at low temperature have Cl contents that are too low to compensate for δ37Cl depletions observed in pore fluids. Accordingly, we hypothesize that fluid-specific processes are responsible for the δ37Cl-depletions of the fluids and that δ37Cl-enriched chlorides were expelled out of the sediments into the ocean. After reviewing the fluid-specific processes that are known to change the chlorine isotope ratios in chlorides, we rule out diffusion and gravitational isotope fractionations of chlorides could generate this isotope pattern. The flow of a δ37Cl -depleted fluid from the underlying basaltic basement into the sediments could explain the δ37Cl data. But the mechanism that produces depletion in δ37Cl of the fluid remains unknown. It cannot be chloride exchanges between fluids and rocks.Here we show that compaction-induced ion filtration of chlorides through clay-rich membranes can produce the observed pore fluid δ37Cl-depletions, with isotope fractionation factors ranging from 1.000 to 1.008 between the chlorides of the expelled fluid (the permeate) and those of the residual fluid (the retentate). We find that smectite-rich sediments are associated with higher isotopic fractionation factors, while illite/chlorite-rich sediments are associated with intermediate values and with clay-poor sediments associated with lower values. This suggests that chlorine isotope fractionation might be controlled by surface charge associated with specific clay minerals. Our calculations show that compaction-induced filtration has the capacity to produce 18O-depletion for oxygen isotope fractionation factors between the expelled fluid and the retentate ranging from 1.000 to 1.005. 18O-enrichment in the expelled fluid is in agreement with the experimental data of Haydon and Graf (1986). Overall, although further experimental work on both chlorine and oxygen isotopes is certainly needed, the results of this study indicate that ion-filtration should be considered as a potential mechanism for fractionating isotopic species in sediment pore waters, particularly for oxygen isotope ratios whose variations are often commonly attributed to water–rock exchange.

Extraction and analysis of hydrosoluble chlorine and its stable isotopic composition in soil using thermal ionization mass spectrometry

Chlorine is an essential micronutrient for high plant growth. The uptake of chlorine by plants is controlled by the hydrosoluble chlorine in soil, which can lead to the cycle change of chlorine isotopes in surficial processes. In this study, a novel method using water extraction coupled with concatenated ion exchange chromatography for the extraction of hydrosoluble chlorine in soil samples is established for the determination of chorine isotopes by thermal ionization mass spectrometry. The results show that no loss of Cl− ion and the fractionation of chlorine isotopes occur in the extraction, with recoveries ranging from 96.4% to 106.4%. The variations of δ37Cl values in soil samples range from −2.40‰ to +1.31‰ with an average of −0.44‰ at a standard deviation of 1.08‰. Stepwise linear regression analysis reveals a positive linear relationship between δ37Cl values and Cl− amount in HT soil profile, which indicates that the diffusion of Cl− ion along with soil water flow controls the chlorine isotope in the range of less than 2‰ in soils. The correlation in WF soils shows that except the seawater invasion, the 37Cl value variation is attributable to the mixing of different sources including anthropogenic activities.

Dual C-Cl Isotope Analysis for Characterizing the Reductive Dechlorination of α- and γ-Hexachlorocyclohexane by Two Dehalococcoides mccartyi Strains and an Enrichment Culture.

Hexachlorocyclohexanes (HCHs) are persistent organic contaminants that threaten human health. Microbial reductive dehalogenation is one of the most important attenuation processes in contaminated environments. This study investigated carbon and chlorine isotope fractionation of α- and γ-HCH during the reductive dehalogenation by three anaerobic cultures. The presence of tetrachlorocyclohexene (TeCCH) indicated that reductive dichloroelimination was the first step of bond cleavage. Isotope enrichment factors (εC and εCl) were derived from the transformation of γ-HCH (εC, from -4.0 ± 0.5 to -4.4 ± 0.6 ‰; εCl, from -2.9 ± 0.4 to -3.3 ± 0.4 ‰) and α-HCH (εC, from -2.4 ± 0.2 to -3.0 ± 0.4 ‰; εCl, from -1.4 ± 0.3 to -1.8 ± 0.2 ‰). During α-HCH transformation, no enantioselectivity was observed, and similar εc values were obtained for both enantiomers. The correlation of 13C and 37Cl fractionation (Λ = Δδ13C/Δδ37Cl ≈ εC/εCl) of γ-HCH (from 1.1 ± 0.3 to 1.2 ± 0.1) indicates similar bond cleavage during the reductive dichloroelimination by the three cultures, similar to α-HCH (1.7 ± 0.2 to 2.0 ± 0.3). The different isotope fractionation patterns during reductive dichloroelimination and dehydrochlorination indicates that dual-element stable isotope analysis can potentially be used to evaluate HCH transformation pathways at contaminated field sites.

Compound-specific chlorine isotope fractionation in biodegradation of atrazine.

Atrazine is a frequently detected groundwater contaminant. It can be microbially degraded by oxidative dealkylation or by hydrolytic dechlorination. Compound-specific isotope analysis is a powerful tool to assess its transformation. In previous work, carbon and nitrogen isotope effects were found to reflect these different transformation pathways. However, chlorine isotope fractionation could be a particularly sensitive indicator of natural transformation since chlorine isotope effects are fully represented in the molecular average while carbon and nitrogen isotope effects are diluted by non-reacting atoms. Therefore, this study explored chlorine isotope effects during atrazine hydrolysis with Arthrobacter aurescens TC1 and oxidative dealkylation with Rhodococcus sp. NI86/21. Dual element isotope slopes of chlorine vs. carbon isotope fractionation (Λ = 1.7 ± 0.9 vs. Λ = 0.6 ± 0.1) and chlorine vs. nitrogen isotope fractionation (Λ = -1.2 ± 0.7 vs. Λ = 0.4 ± 0.2) provided reliable indicators of different pathways. Observed chlorine isotope effects in oxidative dealkylation (εCl = -4.3 ± 1.8‰) were surprisingly large, whereas in hydrolysis (εCl = -1.4 ± 0.6‰) they were small, indicating that C-Cl bond cleavage was not the rate-determining step. This demonstrates the importance of constraining expected isotope effects of new elements before using the approach in the field. Overall, the triple element isotope information brought forward here enables a more reliable identification of atrazine sources and degradation pathways.

Chlorine Isotope Fractionation of the Major Chloromethane Degradation Processes in the Environment.

Chloromethane (CH3Cl) is an important source of chlorine in the stratosphere, but detailed knowledge of the magnitude of its sources and sinks is missing. Here, we measured the stable chlorine isotope fractionation (εCl) associated with the major abiotic and biotic CH3Cl sinks in the environment, namely, CH3Cl degradation by hydroxyl (·OH) and chlorine (·Cl) radicals in the troposphere and by reference bacteria Methylorubrum extorquens CM4 and Leisingera methylohalidivorans MB2 from terrestrial and marine environments, respectively. No chlorine isotope fractionation was detected for reaction of CH3Cl with ·OH and ·Cl radicals, whereas a large chlorine isotope fractionation (εCl) of -10.9 ± 0.7‰ (n = 3) and -9.4 ± 0.9 (n = 3) was found for CH3Cl degradation by M. extorquens CM4 and L. methylohalidivorans MB2, respectively. The large difference in chlorine isotope fractionation observed between tropospheric and bacterial degradation of CH3Cl provides an effective isotopic tool to characterize and distinguish between major abiotic and biotic processes contributing to the CH3Cl sink in the environment. Our findings demonstrate the potential of emerging triple-element isotopic approaches including chlorine to carbon and hydrogen analysis for the assessment of global cycling of organochlorines.