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

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.

Stable isotope composition of pesticides in commercial formulations: The ISOTOPEST database

By assessing the changes in stable isotope compositions within individual pesticide molecules, Compound Specific Isotope Analysis (CSIA) holds the potential to identify and differentiate sources and quantify pesticide degradation in the environment. However, the environmental application of pesticide CSIA is limited by the general lack of knowledge regarding the initial isotopic composition of active substances in commercially available formulations used by farmers. To address this limitation, we established a database aimed at cataloguing and disseminating isotopic signatures in commercial formulations to expand the use of pesticide CSIA. Our study involved the collection of 25 analytical standards and 120 commercial pesticide formulations from 23 manufacturers. Subsequently, 59 commercial formulations and 25 standards were extracted, and each of their active substance was analyzed for both δ13C (n = 84) and δ15N CSIA (n = 43). The extraction of pesticides did not cause significant isotope fractionation (Δ13C and Δ15N < 1‰). Incorporating existing literature data, stable carbon and nitrogen isotope signatures varied in a relatively narrow range among pesticide formulations for different pesticides (Δ13C and Δ15N < 10‰) and within different formulations for a single substance (Δ13C and Δ15N < 2‰). Overall, this suggests that pesticide CSIA is more suited for identifying pesticide transformation processes rather than differentiating pesticide sources. Moreover, an inter-laboratory comparison showed similar δ13C (Δ13C ≤ 1.2 ‰) for the targeted substances albeit varying GC-IRMS instruments. Insignificant carbon isotopic fractionation (Δ13C < 0.5‰) was observed after 4 years of storing the same pesticide formulations, confirming their viability for long-term storage at 4 °C and future inter-laboratory comparison exercises. Altogether, the ISOTOPEST database, in open access for public use and additional contributions, marks a significant advancement in establishing an environmentally relevant pesticide CSIA approach.

Recent progresses in compound specific isotope analysis of halogenated persistent organic pollutants. Assessing the transformation of halogenated persistent organic pollutants at contaminated sites

Compound specific isotope analysis was extensively used to characterise the environmental processes associated with the abiotic and biotic transformation of persistent halogenated organic pollutants including those of contaminants of emerging concern (CECs). In the last years, the compound specific isotope analysis was applied as tool to evaluate the environmental fate and was expanded to larger molecules like brominated flame retardants and polychlorinated biphenyls. Multi-element (C, H, Cl, Br) CSIA methods have been also employed both in laboratory and field experiments. Nevertheless, despite the instrumental advances of isotope ratio mass spectrometers systems, the instrumental detection limit for gas chromatography-combustion-isotope ratio mass spectrometer (GC-C-IRMS) systems is challenging, especially when it is utilized to δ13C analysis. Liquid chromatography-combustion isotope ratio mass spectrometry methods are challenging, taking into consideration the chromatographic resolution required when analysing complex mixtures. For chiral contaminants, enantioselective stable isotope analysis (ESIA) has turned up as alternative approach but, up to now, it has been used for a limited number of compounds. Taking into consideration the occurrence of new emerging halogenated organic contaminants, new GC and LC methods for non-target screening using high resolution mass spectrometry are needed to be developed prior to the compound specific isotope analysis (CSIA) methods.

Determination of Stable Hydrogen Isotopic Composition and Isotope Enrichment Factor at Low Hydrogen Concentration

Determination of stable hydrogen isotopic compositions (δ2H) is currently challenged to achieve a high detection limit for reaching the linear range where δ2H values are independent of concentration. Therefore, it is difficult to assess precise δ2H values for calculating the hydrogen isotope enrichment factor (εH) and for field application where the concentrations of contaminants are relatively low. In this study, a data treatment approach was developed to obtain accurate δ2H values below the linear range. The core concept was to use a logarithmic function to fit the δ2H values below the linear range and then adjust the δ2H values below the linear range into the linear range by using the fitted logarithmic equation. Moreover, the adjusted δ2H values were calibrated by using laboratory reference materials, e.g., n-alkanes. Tris(2-chloroethyl) phosphate (TCEP) and hexachlorocyclohexane (HCH) isomers were selected as examples of complex heteroatom-bearing compounds to develop the data treatment approach. This data treatment approach was then tested using δ2H values from a TCEP transformation experiment with OH radicals. Comparable δ2H values and εH between the low-concentration experiment and the reference experiment were obtained using the developed approach. Therefore, the developed data treatment approach enables a possibility of determining the hydrogen isotopic compositions of organic components in low concentrations. It is especially valuable for determining organic contaminants in environmental samples, which are usually present in low concentrations.

Integrating compound-specific stable isotope and enantiomer-specific analysis to characterize the isomeric and enantiomeric signatures of hexachlorocyclohexanes (HCHs) in paddy soils

Organic pollutants in paddy fields may undergo different processes from those in dryland due to the anaerobic environment. The integrated use of compound-specific stable isotope analysis (CSIA) and enantiomer-specific analysis is a promising technique for understanding the behavior and fate of organic pollutants in soils. In this study, soil samples were collected from paddy fields in three major rice cultivation regions of China, spanning a transect of 4000 km. The mean concentrations of ƩHCHs in paddy soils from the Taihu Plain were the highest (1.44 ng/g). The ratios of α-HCH/β-HCH (all below 11.8) and α-HCH/γ-HCH (92% below 4.64), as well as the enantiomeric fractions (EFs) of chiral α-HCH (mean of 0.81), reflected that the distribution of HCHs was affected by the use of both technical HCHs and lindane. The preferential depletion of (-)-α-HCH and pronounced carbon isotope fractionation of α-HCH (δ13C of -28.22 ± 0.92‰ -23.63 ± 1.89‰) demonstrated its effective transformation. Factors such as altitude, soil temperature, soil pH, soil conductivity and soil organic matter significantly influenced the fate and transformation of HCHs. The current study highlights the integrated application of CSIA and enantiomer-specific analysis to provide multiple lines of evidence for the transformation of HCHs in soils.

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.

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 (ε_C) 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 (AKIE_C = 1.072 ± 0.004) suggested that the nucleophilic substitution (S_N2 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.

Compound-specific stable isotope analysis for characterization of the transformation of γ-HCH induced by biochar

The role of biochar as the redox catalyst in the removal of reductive pollutants from soil and water system has been extensively studied recently, but there is still a lack of qualitative description of its specific mechanisms in redox processes. In this study, the mechanism of biochar in the transformation process of γ-HCH under anoxic condition was revealed by the compound-specific isotope analysis. The concentration and carbon isotopic composition (δ¹³C) of γ-HCH were detected in the treatments with different initial concentrations of γ-HCH and biochar materials with different redox properties and varied doses. The surface functional groups and electrochemical properties of biochar before and after the reaction were also characterized. The addition amount of biochar could affect the reduction of γ-HCH concentration, which were 59.1%, 34.6% and 22.4% in treatments with the addition of 5%, 1% and 0.2% biochar, respectively. Meanwhile, the δ¹³C value of γ-HCH also increased from -26.6 ± 0.2‰ to -23.8 ± 0.2‰ with the addition amount of biochar, especially in the treatment with 5% biochar. As evidenced by X-ray diffraction analysis and electrochemical analysis, biochar promoted the adsorption and transformation of γ-HCH simultaneously, and the oxygen-containing functional groups on the surface of biochar played an important role in the redox process. The isotopic fractionation value (ε_C) of γ-HCH transformation by biochar was first reported as -3.4 ± 0.4‰. The results will enable the quantitative description of the transformation degree of organic pollutants induced by biochar, and provide a new approach for evaluating the in-situ remediation effects of biochar in a complex environment.

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).

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 Carbon Isotope Analysis of Hexachlorocyclohexanes by Liquid–Liquid Extraction Gas Chromatography Isotope Ratio Mass Spectrometry: Method Evaluation and Applications

Compound specific isotope analysis (CSIA) and enantiomer specific isotope analysis (ESIA) are powerful tools for assessing the fate of hexachlorocyclohexanes (HCHs) in the environment. However, there is no systematic study on the CSIA and ESIA analysis test methods of the carbon isotopes of HCHs in water and soil environments, in particular the isotope fractionation in the pre-concentration process. We endeavored to test the compatibility of CSIA and ESIA with the liquid–liquid extraction method of HCHs in water. The results showed that there were negligible changes in the δ13C of HCHs after extraction, indicating that liquid–liquid extraction can be used as a pre-concentration method for the determination of δ13C of HCHs in water. The optimized method was validated and then applied to differentiate three HCHs from different manufacturers, to identify in situ degradation of HCHs of groundwater from a contaminated site and to resolve the carbon isotope fractionation occurring in the α-HCH oxidation by CaO2/Fe(II) Fenton system. The results showed that the same reagents from different manufacturers have different carbon isotope compositions, and different isomers from the same manufacturer also have different isotope compositions, showing useful evidence in identifying the source of HCHs. The more enriched δ13C in the down-gradient wells indicated that HCHs have undergone biodegradation or/and chemical reactions in the groundwater system of the site. Carbon isotopic enrichment factors (εC) of −1.90 ± 0.10‰ were obtained in the oxidation process. Hence, the method validated in this study has great potential as a method for identifying the degradation of HCHs in a water environment.

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 (ε_C) and bromine isotope enrichment factor (ε_Br) between BDE-47 (ε_C = -5.98‰, ε_Br = -2.44‰) and BDE-153 (ε_C = -5.57‰, ε_Br = -2.06‰) during the microbial degradation. Compared to ε_C and ε_Br, the correlation of carbon and isotope effects (Λ_C/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 (AKIE_C and AKIE_Br) 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, Λ_C/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.

Dehalogenation of α-hexachlorocyclohexane by iron sulfide nanoparticles: Study of reaction mechanism with stable carbon isotopes and pH variations

The biodegradation of hexachlorocyclohexanes (HCHs) is known to be accompanied by isotope fractionation of carbon (¹³C/¹²C), but no systematic studies were performed on abiotic degradation of HCH isomers by iron (II) minerals. In this study, we explored the carbon isotope fractionation of α-HCH during dechlorination by FeS nanoparticles at different pH values. The results of three different experiments showed that the apparent rate constants during dehalogenation of α-HCH by FeS increased with pH. The lowest apparent rate constant value α-HCH during dehalogenation by FeS was 0.009 d^-1 at pH value of 2.4, while the highest was 1.098 d^-1 at pH 11.8. α-HCH was completely dechlorinated by FeS only at pH values 9.9 and 11.8, while the corresponding apparent rate constants were 0.253 d^-1 and 1.098 d^-1, respectively. Regardless of the pH used, the 1,2,4-trichlorobenzene (1,2,4-TCB), 1,2-dichlorobenzene (1,2-DCB), and benzene were the dominant degradation products of α-HCH. An enrichment factor (ε_C) of -4.7 ± 1.3‰ was obtained for α-HCH using Rayleigh model, which is equivalent to an apparent kinetic isotope effect (AKIE_C) value of 1.029 ± 0.008 for dehydrohalogenation, and of 1.014 ± 0.004 for dihaloelimination, respectively. The magnitude of isotope fractionation from this study suggests that abiotic isotope fractionation by FeS must be taken into account in anoxic sediments and aquifers contaminated with HCH isomers, when high concentrations of FeS are present in the above-mentioned anoxic environments.

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.

Competitive microbial degradation among PBDE congeners in anaerobic wetland sediments: Implication by multiple-line evidences including compound-specific stable isotope analysis

Polybrominated diphenyl ethers (PBDEs) are widespread contaminants in the environment. Microbial reductive debromination is one of the important attenuation processes for PBDEs in the anaerobic sediments. This study first investigated the interaction between BDE-47 and BDE-153 during the microbial degradation in wetland sediments by the multiple-line approaches including biodegradation kinetics, microbial community structures and stable isotope composition. BDE-47 and BDE-153 biodegradation fitted pseudo-zero-order kinetics, with the higher degradation rates in single than combined exposure, indicating the mutual inhibition in co-exposure condition. BDE-47 and BDE-153 shared the common dehalogenators (genus Dehalococcoides and Acinetobacter) with enrichment in combined exposure, indicating the potential competition in dehalogenating bacteria during biodegradation. Microbial degradation could lead to the isotopic fractionation of BDE-47 and BDE-153, with the smaller changes in δ¹³C in combined than single exposure. The apparent kinetic isotope effect of carbon (AKIE_C) was different between BDE-47 and BDE-153 in single exposure, whilst identical in combined exposure, indicating the similar degradation mechanism for BDE-47 and BDE-153 in co-exposure condition. These results revealed that the competition on microbial degradation occurred among PBDEs in co-exposure condition, which was important for the comprehensive risk assessment of simultaneous exposure to multiple PBDE congeners in the environment.

A critical review on the formation, fate and degradation of the persistent organic pollutant hexachlorocyclohexane in water systems and waste streams

The environmental impacts of persistent organic pollutants (POPs) is an increasingly prominent topic in the scientific community. POPs are stable chemicals that are accumulated in living beings and can act as endocrine disruptors or carcinogens on prolonged exposure. Although efforts have been taken to minimize or ban the use of certain POPs, their use is still widespread due to their importance in several industries. As a result, it is imperative that POPs in the ecosystem are degraded efficiently and safely in order to avoid long-lasting environmental damage. This review focuses on the degradation techniques of hexachlorocyclohexane (HCH), a pollutant that has strong adverse effects on a variety of organisms. Different technologies such as adsorption, bioremediation and advanced oxidation process have been critically analyzed in this study. All 3 techniques have exhibited near complete removal of HCH under ideal conditions, and the median removal efficiency values for adsorption, bioremediation and advanced oxidation process were found to be 80%, 93% and 82% respectively. However, it must be noted that there is no ideal HCH removal technique and the selection of removal method depends on several factors. Furthermore, the fates of HCH in the environment and challenges faced by HCH degradation have also been explained in this study. The future scope for research in this field has also received attention.