A concept for studying the transformation reaction of hexachlorocyclohexanes in food webs using multi-element compound-specific isotope analysis

Tracking the transformation of persistent organic pollutants in food webs using multi element isotope and enantiomer fractionation

In order to track the transformation of persistent organic pollutants (POPs) in food webs, field experiments were conducted at two sites using stable isotope and enantiomer fractionation concepts. The enantiomers of α-hexachlorocyclohexane (α-HCH) were selected as representative compounds for POPs. Isotope and enantiomer fractionation allowed the characterization of α-HCH enantiomer biotransformation processes along trophic levels of the food web - from soil and plants to animal livers, fat tissues and milk. The enrichment of heavy isotopes in soils, plants and sediments as well as the changes of enantiomer fractionation indicate that the biotransformation of α-HCH occurred in these compartments. Moreover, the increase of carbon and chlorine isotopic compositions as well as the changes of enantiomer fractionation of liver, fat tissues and milk demonstrated that the overall HCH exposure was much higher than estimates based on concentration levels, while the isotope and enantiomer fractionation revealed the enantiomer specific enantiomer uptake across the blood-brain barriers. Dual element isotope analysis suggested that complex transformation processes have occurred along the potential food web from the HCH sources over different environmental compartments to animal livers, fat tissues and milk. The results imply that the analyses of stable isotope compositions and concentrations has potential to reconstruct the exposure of higher organisms to POPs.

Anaerobic biotransformation of hexachlorocyclohexane isomers in aqueous condition: Dual CCl isotope fractionation and impact on microbial community compositions

Hexachlorocyclohexane (HCH) isomers are persistent organic pollutants (POPs) with high toxicity, lipid solubility, chemical stability. Despite the current ban on usage of Lindane, residual contamination cannot be ignored, and HCH are frequently detected in groundwater and threaten human health. Cultures capable of degrading α-HCH, β-HCH, γ-HCH, and δ-HCH individually have been enriched in anoxic aqueous conditions. Compound-Specific Isotope Analysis (CSIA) was applied to examine the transformation mechanisms of different HCH isomers by the four enrichment cultures. 16S rRNA sequencing techniques were employed to examine the community composition of the enrichment cultures and detect changes in these communities resulting from adding individual HCH isomers. The results indicated that the ability of the enrichment cultures for dichloroelimination of HCH isomers was inconsistent. During dichloroelimination, different bond cleavage mode of β- and δ-HCH led to distinct isotopic effects. HCH isomers had significant impact on the microbial community, while different microbial communities showed comparable isotopic effects during the transformation of a specific HCH isomer. In addition, bacteria in the phyla Proteobacteria and Firmicutes were proposed as the dominant dechlorinators. This study provides a novel perspective on the mode of bond cleavage during HCH dichloroelimination and the effect of HCH on microbial communities, which could potentially support the evaluation of HCH transformation by CSIA and their effects on the microecosystems of groundwater.

Adaptive evolution of Sphingopyxis sp. MC4 conferred degradation potential for persistent β- and δ-Hexachlorocyclohexane (HCH) isomers

Hexachlorocyclohexane (HCH), an organochlorine pesticide imposes several harmful impacts on the ecosystem. β- and δ-isomers of HCH are highly toxic, persistent, and recalcitrant to biodegradation, slow and incomplete degradation of β- and δ- isomers have been reported in a few strains. We have isolated a strain designated as Sphingopyxis strain MC4 that can tolerate and degrade high concentrations of α-, β-, γ- and δ-HCH isomers. To date, no other Sphingopyxis strain has been reported to degrade β- and δ-isomers. To understand the underlying genetic makeup contributing to adaptations, the whole genome of strain MC4 was sequenced. Comparative genome analysis showed that strain MC4 harbors the complete pathway (lin genes) required for HCH degradation. Genetic footprints such as presence of lin genes on genomic islands, IS6100 elements in close proximity of lin genes, and synteny in lin flanking regions with other strains reflects the horizontal gene transfer in strain MC4. Positive selection and HGT drive the adaptive evolution of strain MC4 under the pressure of HCH contamination that it experienced in its surrounding niche. In silico analyses showed efficient binding of β- and δ-isomers with enzymes leading to rapid degradation that need further validation by cloning and biochemical experiments.

Uptake and Transformation of Hexachlorocyclohexane Isomers (HCHs) in Tree Growth Rings at a Contaminated Field Site

The potential transformation of hexachlorocyclohexane isomers (HCHs) within tree trunks could have a significant impact on the use of phytoscreening. However, the transformation mechanisms of HCH in trunks particularly in growth rings are not yet well understood. Therefore, a field study on an HCH-contaminated field site was conducted to investigate the fate of HCH, particularly α-HCH in tree trunks using multielement compound-specific isotope analysis (ME-CSIA) and enantiomer fractionation. The results indicate that α-HCH was transformed, as evidenced by higher δ¹³C and δ³⁷Cl values detected across different growth ring sections and in the bark compared to those in muck and soil. Remarkably, in the middle growth ring section, δ¹³C values of HCH were only marginally higher or comparable to those in muck, whereas δ³⁷Cl values were higher than those of the muck, indicating a different transformation mechanism. Moreover, the δ³⁷Cl values of β-HCH also increased in the tree trunks compared to those in soil and muck, implying a transformation of β-HCH. Additionally, dual-element isotope analysis revealed that there are different transformation mechanisms between the middle growth rings and other sections. Our findings suggest that the transformation of HCHs in trunks could bias quantitative phytoscreening approaches; however, ME-CISA offers an option to estimate the degradation extent.

Characterization of Hexachlorocyclohexane Isomer Dehydrochlorination by LinA1 and LinA2 Using Multi-element Compound-Specific Stable Isotope Analysis

Dehydrochlorination is one of the main (thus far discovered) processes for aerobic microbial transformation of hexachlorocyclohexane (HCH) which is mainly catalyzed by LinA enzymes. In order to gain a better understanding of the reaction mechanisms, multi-element compound-specific stable isotope analysis was applied for evaluating α- and γ-HCH transformations catalyzed by LinA1 and LinA2 enzymes. The isotopic fractionation (ε_E) values for particular elements of (+)α-HCH (ε_C = -10.8 ± 1.0‰, ε_Cl = -4.2 ± 0.5‰, ε_H = -154 ± 16‰) were distinct from the values for (-)α-HCH (ε_C = -4.1 ± 0.7‰, ε_Cl = -1.6 ± 0.2‰, ε_H = -68 ± 10‰), whereas the dual-isotope fractionation patterns were almost identical for both enantiomers (Λ_C-Cl = 2.4 ± 0.4 and 2.5 ± 0.2, Λ_H-C = 12.9 ± 2.4 and 14.9 ± 1.1). The ε_E of γ-HCH transformation by LinA1 and LinA2 were -7.8 ± 1.0‰ and -7.5 ± 0.8‰ (ε_C), -2.7 ± 0.3‰ and -2.5 ± 0.4‰ (ε_Cl), -170 ± 25‰ and -150 ± 13‰ (ε_H), respectively. Similar Λ_C-Cl values (2.7 ± 0.2 and 2.9 ± 0.2) were observed as well as similar Λ_H-C values (20.1 ± 2.0 and 18.4 ± 1.9), indicating a similar reaction mechanism by both enzymes during γ-HCH transformation. This is the first data set on 3D isotope fractionation of α- and γ-HCH enzymatic dehydrochlorination, which gave a more precise characterization of the bond cleavages, highlighting the potential of multi-element compound-specific stable isotope analysis to characterize different transformation processes (e.g., dehydrochlorination and reductive dehalogenation).

Simple extraction methods for pesticide compound-specific isotope analysis from environmental samples

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Extractions from soil, plants, and water were tested for pesticide C and N CSIA.

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Pesticide recoveries strongly varied among compounds and matrices properties.

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Tested extraction methods caused no effect on δ¹³C and δ¹⁵N of pesticides.

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C and N pesticide CSIA can be applied in situ to agricultural water samples.

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Pesticide CSIA for soil and sediment samples are limited to source areas.

Compound-specific isotope analysis (CSIA) is a powerful approach to evaluate the transformation of organic pollutants in the environment. However, the application of CSIA to micropollutants, such as pesticides, remains limited because appropriate extraction methods are currently lacking. Such methods should address a wide range of pesticides and environmental matrices, while recovering sufficient mass for reliable CSIA without inducing stable isotope fractionation. Here, we present simple extraction methods for carbon and nitrogen CSIA for different environmental matrices and six commonly used herbicides, i.e., atrazine, terbutryn, acetochlor, alachlor, butachlor, and S-metolachlor, and three fungicides, i.e., dimethomorph, tebuconazole, and metalaxyl. We examined the potential of several extraction methods for four types of soils or sediments, three types of environmental waters and aerial and root plant samples for multielement (ME)-CSIA.•

Pesticide extraction recoveries varied depending on the physical characteristics of the pesticides and matrix properties for environmental water (77 to 87%), soil and sediment (35 to 82%), and plant (40 to 59%) extraction.

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The tested extraction methods did not significantly affect the carbon and nitrogen stable isotope signatures of pesticides (Δ(¹³C) <0.9‰ for Δ(¹⁵N) <1.0‰).

Uptake and Metabolization of HCH Isomers in Trees Examined over an Annual Growth Period by Compound-Specific Isotope Analysis and Enantiomer Fractionation

To understand the role of plants for natural attenuation, a field study was conducted to characterize the fate of HCH in trees over an annual growth period using compound-specific isotope analysis and enantiomer fractionation. Stable and slightly higher δ¹³C and δ³⁷Cl values of HCH of host soil samples compared to the muck (consisting nearly exclusively of HCH) revealed that masking isotope effects caused by the limited bioavailability may underestimate the real extent of HCH transformation in soil. In contrast, an increase of δ¹³C and δ³⁷Cl values in trees indicated the transformation of HCH. A large variability of δ¹³C and δ³⁷Cl values in trees over the growth period was observed, representing different transformation extents among different growth times, which is further supported by the shift of the enantiomer fraction (EF), indicating the preferential transformation of enantiomers also varied over the different growth periods. Based on dual-element isotope analysis, different predominant transformation mechanisms were observed during the growing seasons. Our observation implies that plants are acting as biological pumps driving a cycle of uptake and metabolization of HCH and refeed during littering to soil catalyzing their transformation. The changes of the transformation mechanism in different seasons have implications for phytoscreening and shed new light on phytoremediation of HCH at field sites.

Perfluoroalkyl and polyfluoroalkyl substances (PFASs) in groundwater: current understandings and challenges to overcome

Perfluoroalkyl and polyfluoroalkyl substances (PFASs) have been frequently detected in groundwater globally. With the phase-out of perfluorooctane sulfonate (PFOS) and perfluorooctanate (PFOA) due to their risk to the ecosystem and human population, various novel PFASs have been used as replacements and detected in groundwater. In order to summarize the current understanding and knowledge gaps on PFASs in groundwater, we reviewed the studies about environmental occurrence, transport, and risk of legacy and novel PFASs in groundwater published from 1999 to 2021. Our review suggests that PFOS and PFOA could still be detected in groundwater due to the long residence time and the retention in the soil-groundwater system. Firefighting training sites, industrial parks, and landfills were commonly hotspots of PFASs in groundwater. More novel PFASs have been detected via nontarget analysis using high-resolution mass spectrometry. Some novel PFASs had concentrations comparable to that of PFOS and PFOA. Both legacy and novel PFASs can pose a risk to human population who rely on contaminated groundwater as drinking water. Transport of PFASs to groundwater is influenced by various factors, i.e., the compound structure, the hydrochemical condition, and terrain. The exchange of PFASs between groundwater and surface water needs to be better characterized. Field monitoring, isotope tracing, nontarget screening, and modeling are useful approaches and should be integrated to get a comprehensive understanding of PFASs sources and behaviors in groundwater.

Stable isotope fractionation associated with the synthesis of hexachlorocyclohexane isomers for characterizing sources

The stable isotope fingerprints of hexachlorocyclohexane (HCH) isomers have potential for identifying sources as they are related to the synthesis processes and isotopic compositions of raw materials. However, the isotopic fractionation associated with the synthesis processes has not been investigated. Therefore, photochemical synthesis experiments using benzene and chlorine gas were conducted to characterize the associated isotopic fractionation under different conditions. Different patterns of isotopic fractionation factors (α_C, α_Cl, and α_H) were observed in each experiment. The large variability of α_H is related to the accumulating secondary hydrogen isotope effects or the rearrangement of C-H bonds at the cyclohexane ring. An increase of δ¹³C and δ³⁷Cl values of HCH isomers was observed during synthesis, which is related to the C-Cl bond formation in the radical dichlorination forming HCH and the subsequent chlorine substitution forming heptachlorocyclohexanes. The large variability of δ²H values is related to the secondary and primary hydrogen isotope effects. Different δ¹³C, δ³⁷Cl and δ²H values among HCH isomers were observed, indicating that conformational complexity of HCH caused by arrangement of C-Cl bonds in planar and axial positions also influence the isotope values. The understanding of isotopic fractionation during HCH synthesis can be indicative for source identification in the field.

Soil from a Hexachlorocyclohexane Contaminated Field Site Inoculates Wheat in a Pot Experiment to Facilitate the Microbial Transformation of β-Hexachlorocyclohexane Examined by Compound-Specific Isotope Analysis

β-Hexachlorocyclohexane (β-HCH) is a remnant from former HCH pesticide production. Its removal from the environment gained attention in the last few years since it is the most stable HCH isomer. However, knowledge about the transformation of β-HCH in soil-plant systems is still limited. Therefore, experiments with a contaminated field soil were conducted to investigate the transformation of β-HCH in soil-plant systems by compound specific isotope analysis (CSIA). The results showed that the δ¹³C and δ³⁷Cl values of β-HCH in the soil of the planted control remained stable, revealing no transformation due to a low bioavailability. Remarkably, an increase of the δ¹³C and δ³⁷Cl values in soil and plant tissues of the spiked treatments were observed, indicating the transformation of β-HCH in both the soil and the plant. This was surprising as previously it was shown that wheat is unable to transform β-HCH when growing in hydroponic culture or garden soil. Thus, results of this work indicate for the first time that a microbial community of the soil inoculated the wheat and then facilitated the transformation of β-HCH in the wheat, which may have implications for the development of phytoremediation concepts. A high abundance of HCH degraders belonging to Sphingomonas sp., Mycobacterium sp., and others was detected in the β-HCH-treated bulk and rhizosphere soil, potentially supporting the biotransformation.

Photodegradation of pesticides using compound-specific isotope analysis (CSIA): a review

Pesticides are commonly applied in agriculture to protect crops from pests, weeds, and harmful pathogens. However, chronic, low-level exposure to pesticides can be toxic to humans. Photochemical degradation of pesticides in water, soil, and other environmental media can alter their environmental fate and toxicity. Compound-specific isotope analysis (CSIA) is an advanced diagnostic tool to quantify the degradation of organic pollutants and provide insight into reaction mechanisms without the need to identify transformation products. CSIA allows for the direct quantification of organic degradation, including pesticides. This review summarizes the recent developments observed in photodegradation studies on different categories of pesticides using CSIA technology. Only seven pesticides have been studied using photodegradation, and these studies have mostly occurred in the last five years. Knowledge gaps in the current literature, as well as potential approaches for CSIA technology for pesticide monitoring, are discussed in this review. Furthermore, the CSIA analytical method is challenged by chemical element types, the accuracy of instrument analysis, reaction conditions, and the stability of degradation products. Finally, future research applications and the operability of this method are also discussed.

Characterizing the biotransformation of hexachlorocyclohexanes in wheat using compound-specific stable isotope analysis and enantiomer fraction analysis

Hexachlorocyclohexane isomers (HCHs) are persistent organic pollutants being responsible for environmental contamination worldwide. In order to characterize transformation of HCHs in different plant compartments during uptake, a hydroponic experimental setup was designed using wheat as the test plant. The extent of transformation was determined by using compound-specific isotope analysis (CSIA) and enantiomer fraction (EF) analysis. In nutrient solutions, no change of carbon (δ¹³C) and chlorine isotope ratios (δ³⁷Cl) of α-HCH and β-HCH was detected throughout the experiment indicating no transformation there. In wheat leaves, stems and roots, however, transformation of α-HCH due to a C‒Cl bond cleavage was indicated by increasing δ¹³C and δ³⁷Cl compared to the nutrient solution. In addition, 1,3,4,5,6-pentachlorocyclohexene (PCCH) was identified as the major metabolite of α-HCH transformation. For β-HCH, in contrast, no transformation was detected. The evaluation of enantiomer fraction analysis revealed no change of the EF(-) in the nutrient solution or on root surface but a decrease in the wheat compartments, providing an evidence for the preferential biological transformation of (-)α-HCH in wheat. The current study provides the first experimental evidence for biotransformation of α-HCH in wheat using CSIA and EF and provides a concept to evaluate processes during phytoremediation.

Uptake of α-HCH by wheat from the gas phase and translocation to soil analyzed by a stable carbon isotope labeling experiment

Hexachlorocyclohexane isomers (HCH) are persistent organic pollutants which cause serious environmental pollution. Phytoextraction is one of the strategies of phytoremediation, which was considered as a promising method for the clean-up of HCH contaminated field sites. To understand the uptake and translocation mechanisms of HCH in soil-plant system, the uptake of HCH from the gas phase was investigated in a tracer experiment with ¹³C-labeled α-HCH. The results provide new insights on the uptake mechanism of HCH and allow the elucidation of transport pathways of POPs from the leaves to the rhizosphere. A higher dissipation of α-HCH in planted set-ups versus unplanted controls indicated next to intensive biodegradation in the rhizosphere the removal of HCH by root uptake, accumulation and possible transformation within plants. Analyzing the carbon isotopic composition (δ¹³C) of α-HCH in the soil of unplanted controls revealed a change of 15.8-28.6‰ compared to the initial δ¹³C value, indicating that a soil gas phase transportation of α-HCH occurred. Additionally, higher δ¹³C values of α-HCH were observed in bulk and rhizosphere soil in non-labeled treatments compared to unplanted controls, revealing the uptake of α-HCH from the gas phase by the leaves and the further translocation to the roots and finally release to the rhizosphere. This uptake by the leaves and the subsequent translocation of α-HCH within the plant is further indicated by the observed variations of the δ¹³C value of α-HCH in different plant tissues at different growth stages. The uptake and translocation pathways of α-HCH from the gas phase need to be considered in phytoremediation.

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 (ε_C and ε_Cl) 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 Isotope Analysis and Enantiomer Fractionation to Characterize the Transformation of Hexachlorocyclohexane Isomers in a Soil-Wheat Pot System

The uptake by plants from soil is one of the first steps for hexachlorocyclohexane (HCH) isomers to enter the food web. However, the HCH transformation associated with the uptake process is still not well understood. Therefore, a soil-wheat pot experiment was conducted to characterize the HCH transformation during wheat growth using compound-specific isotope analysis (CSIA) and enantiomer fractionation. The results showed that the δ¹³C and δ³⁷Cl values of β-HCH remained stable in soil and wheat, revealing no transformation. In contrast, an increase of δ¹³C and δ³⁷Cl values of α-HCH indicated its transformation in soil and wheat. A shift of the enantiomer fraction (EF) (-) from 0.50 to 0.35 in soil at the jointing stage and 0.35 to 0.57 at the harvest stage suggested that the preferential transformation of enantiomers varied at different growth stages. Based on the dual element isotope analysis, the transformation mechanism in the soil-wheat system was different from that in wheat in hydroponic systems. The high abundance of HCH degraders, Sphingomonas sp. and Novosphingobium sp., was detected in the α-HCH-treated rhizosphere soil, supporting the potential for biotransformation. The application of CSIA and EF allows characterizing the transformation of organic pollutants such as HCHs in the complex soil-plant systems.

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 (ε_C and ε_Cl) were derived from the transformation of γ-HCH (ε_C, from -4.0 ± 0.5 to -4.4 ± 0.6 ‰; ε_Cl, from -2.9 ± 0.4 to -3.3 ± 0.4 ‰) and α-HCH (ε_C, from -2.4 ± 0.2 to -3.0 ± 0.4 ‰; ε_Cl, from -1.4 ± 0.3 to -1.8 ± 0.2 ‰). During α-HCH transformation, no enantioselectivity was observed, and similar ε_c values were obtained for both enantiomers. The correlation of ¹³C and ³⁷Cl fractionation (Λ = Δδ¹³C/Δδ³⁷Cl ≈ ε_C/ε_Cl) 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.

Biotransformation of hexachlorocyclohexanes contaminated biomass for energetic utilization demonstrated in continuous anaerobic digestion system

Lindane, the γ-hexachlorocyclohexane (HCH) isomer, was among the most used pesticides worldwide. Although it was banned in 2009, residues of Lindane and other HCH-isomers are still found with high concentrations in contaminated fields. For clean-up, phytoremediation combined with anaerobic digestion (AD) of contaminated biomass to produce biogas and fertilizer could be a promising strategy and was tested in two 15 L laboratory-scale continuous stirred tank reactors. During operation over one year by adding HCH isomers (γ, α and β) consecutively, no negative influence on conventional reactor parameters was observed. The γ- and α-HCH isomers were transformed to chlorobenzene and benzene, and transformation became faster along with time, while β-HCH was not removed. Genus Methanosaeta and order Clostridiales, showing significant enhancement on abundance with HCH addition, may be used as bioindicators for HCH dehalogenation in AD process. The potential for HCH degradation in AD system was restricted to axial Cl atoms of HCH and it showed slight enantioselective preference towards transformation of (+) α-HCH. Moreover, metabolite benzene was mineralized to CO₂ and methane, deducing from tracer experiments with benzene-¹³C₆. Overall, AD appears to be a feasible option for treatment of γ and α-HCHs contaminated biomass.

Can Alkaline Hydrolysis of γ-HCH Serve as a Model Reaction to Study Its Aerobic Enzymatic Dehydrochlorination by LinA?

Hexachlorocyclohexane (HCH) isomers constitute a group of persistent organic pollutants. Their mass production and treatment have led to a global environmental problem that continues to this day. The characterization of modes of degradation of HCH by isotope fractionation is a current challenge. Multi isotope fractionation analysis provides a concept to characterize the nature of enzymatic and chemical transformation reactions. The understanding of the kinetic isotope effects (KIE) on bond cleavage reaction contributes to analyses of the mechanism of chemical and enzymatic reactions. Herein, carbon, chlorine, and hydrogen kinetic isotope effects are measured and predicted for the dehydrochlorination reaction of γ-HCH promoted by the hydroxyl ion in aqueous solution. Quantum mechanical (QM) microsolvation with an implicit solvation model and path integral formalism in combination with free-energy perturbation and umbrella sampling (PI-FEP/UM) and quantum mechanical/molecular mechanical QM/MM potentials for including solvent effects as well as calculating isotope effects are used and analyzed with respect to their performance in reproducing measured values. Reaction characterization is discussed based on the magnitudes of obtained isotope effects. The comparative analysis between the chemical dehydrochlorination of γ-HCH in aqueous media and catalyzed reaction by dehydrochlorinase, LinA is presented and discussed. Based on the values of isotope effects, these two processes seem to occur via the same net mechanism.

Isotope fractionation approach to characterize the reactive transport processes governing the fate of hexachlorocyclohexanes at a contaminated site in India

The transformation processes of hexachlorocyclohexane isomers (HCHs) from production sites of Lindane across the landscape and along the food web were studied as an example to understand the fate of POPs in the environment. Therefore, we studied the concentration and isotope composition of HCHs in different matrices in the vicinity of a dumpsite and a chemical plant producing HCHs in India. Carbon isotope compositions (δ¹³C) of HCHs and the enantiomer fraction (EF) of α-HCH were used as indicators to characterize in situ degradation in soil, groundwater, and sediment as well as along the food web. The HCHs were detected in plants growing on contaminated soil. Elevated concentrations of HCHs were found in a number of crops, which indicates an important transfer pathway of HCHs entering food webs. The EF value of α-HCH and the δ¹³C signature of HCHs indicated that degradation processes occurred in the rhizosphere or within the plants potentially attenuating the contamination of HCHs. The isotope enrichment of HCHs in dung and milk samples showed that degradation of HCHs may take place in the digestive track of cow and buffalo as well as during their metabolism. The δ¹³C of HCHs was used to analyze the potential dispersion routes on the landscape scale in order to understand the reactive transport pathways starting at the source of HCHs. In this study, the potential of carbon isotope fractionation and EF for characterizing uptake of HCHs into plants and accumulation in the food web were examined. To the best of our knowledge, this is the first study using the combination of stable isotope fractionation and EF to track the reactive transport processes in a complex environment including the food web.