Stereoisomer-specific occurrence, distribution, and fate of chiral brominated flame retardants in different wastewater treatment systems in Hong Kong

Polybrominated diphenyl ethers and their alternatives in soil cores from a typical flame-retardant production park: Vertical distribution and potential influencing factors

With the prohibition on the production and use of polybrominated diphenyl ethers (PBDEs), decabromodiphenyl ethane (DBDPE) and organophosphate flame retardants (OPFRs) have emerged as their alternatives. However, the vertical transport and associated influencing factors of these chemicals into soil are not clearly understood. To clarify the vertical distribution of the pollutants and related influencing factors, surface soil and soil core samples were collected at a depth in the range of 0.10-5.00 m in a typical 20-year-old flame-retardant production park and surrounding area. PBDEs and DBDPE show a clear point source distribution around the production park with their central concentrations up to 2.88 × 104 and 8.46 × 104 ng/g, respectively. OPFRs are mainly found in residential areas. The production conversion of PBDEs to DBDPE has obvious environmental characteristics. The vertical distribution revealed that most of the pollutants have penetrated into the soil 5.00 m or even deeper. The median concentrations of deca-BDE and DBDPE reached 50.9 and 9.85 × 103 ng/g, respectively, even at a depth of 5.00 m. Soil organic matter plays a crucial role in determining the vertical distribution, while soil clay particles have a greater impact on the high molecular weight and/or highly brominated compounds.

A review of occurrence, bioaccumulation, and fate of novel brominated flame retardants in aquatic environments: A comparison with legacy brominated flame retardants

Novel brominated flame retardants (NBFRs) have been developed as replacements for legacy brominated flame retardants (BFRs) such as polybrominated diphenyl ethers (PBDEs) and hexabromocyclododecanes (HBCDs). The prevalence of NBFRs in aquatic environments has initiated intense concerns that they resemble to BFRs. To comprehensively elucidate the fate of NBFRs in aquatic environments, this review summarizes the physico-chemical properties, distribution, bioaccumulation, and fates in aquatic environments. 1,2-bis(2,3,4,5,6-pentabromophenyl) ethane (DBDPE) as the major substitute for PBDEs is the primary NBFR. The release from industrial point sources such as e-waste recycling stations is the dominant way for NBFRs to enter the environment, which results in significant differences in the regional distribution of NBFRs. Sediment is the major sink of NBFRs attributed to the high hydrophobicity. Significantly, there is no decreasing trend of NBFRs concentrations, while PBDEs achieved the peak value in 1970-2000 and decreased gradually. The bioaccumulation of NBFRs is reported in both field studies and laboratory studies, which is regulated by the active area, lipid contents, trophic level of aquatic organisms, and the log KOW of NBFRs. The biotransformation of NBFRs showed similar metabolism patterns to that of BFRs, including debromination, hydroxylation, methoxylation, hydrolysis, and glycosylation. In addition, NBFRs show great potential in trophic magnification along the aquatic food chain, which could pose a higher risk to high trophic-level species. The passive uptake by roots dominates the plant uptake of NBFRs, followed by acropetal and basipetal bidirectional transportation between roots and leaves in plants. This review will provide the support to understand the current pollution characteristics of NBFRs and highlight perspectives for future research.

Biotransformation of HBCDs by the microbial communities enriched from mangrove sediments

1,2,5,6,9,10-Hexabromocyclododecanes (HBCDs) are a sort of persistent organic pollutants (POPs). This research investigated 12 microbial communities enriched from sediments of four mangroves in China to transform HBCDs. Six microbial communities gained high transformation rates (27.5-97.7%) after 12 generations of serial transfer. Bacteria were the main contributors to transform HBCDs rather than fungi. Analyses on the bacterial compositions and binning genomes showed that Alcanivorax (55.246-84.942%) harboring haloalkane dehalogenase genes dadAH and dadBH dominated the microbial communities with high transformation rates. Moreover, expressions of dadAH and dadBH in the microbial communities and Alcanivorax isolate could be induced by HBCDs. Further, it was found that purified proteins DadAH and DadBH showed high conversion rates on HBCDs in 36 h (91.9 ± 7.4 and 101.0 ± 1.8%, respectively). The engineered Escherichia coli BL21 strains harbored two genes could convert 5.7 ± 0.4 and 35.1 ± 0.1% HBCDs, respectively, lower than their cell-free crude extracts (61.2 ± 5.2 and 56.5 ± 8.7%, respectively). The diastereoisomer-specific transforming trend by both microbial communities and enzymes were γ- > α- > β-HBCD, differed from α- > β- > γ-HBCD by the Alcanivorax isolate. The identified transformation products indicated that HBCDs were dehalogenated via HBr elimination (dehydrobromination), hydrolytic and reductive debromination pathways in the enriched cultures. Two enzymes converted HBCDs via hydrolytic debromination. The present research provided theoretical bases for the biotransformation of HBCDs by microbial community and the bioremediation of HBCDs contamination in the environment.

Bioaccumulation and biotransformation of 1,2-bis (2,4,6-tribromophenoxyethane) (BTBPE) and 1,2-dibromo-4-(1,2-dibromoethyl)-cyclohexane (TBECH) in zebrafish (Danio rerio)

Despite the increasing production, use, and ubiquitous occurrence of novel brominated flame retardants (NBFRs), little information is available regarding their fate in aquatic organisms. In this study, the bioaccumulation and biotransformation of two typical NBFRs, i.e., 1,2-bis (2,4,6-tribromophenoxyethane) (BTBPE) and 1,2-dibromo-4-(1,2-dibromoethyl)-cyclohexane (TBECH), were investigated in tissues of zebrafish (Danio rerio) being administrated a dose of target chemicals through their diet. Linear accumulation was observed for both BTBPE and TBECH in the muscle, liver, gonads, and brain of zebrafish, and the elimination of BTBPE and TBECH in all tissues followed pseudo-first-order kinetics, with the fastest depuration rate occurring in the liver. BTBPE and TBECH showed low bioaccumulation potential in zebrafish, with biomagnification factors (BMFs) < 1 in all tissues. Individual tissues' function and lipid content are vital factors affecting the distribution of BTBPE and TBECH. Stereoselective accumulation of TBECH enantiomers was observed in zebrafish tissues, with first-eluting enantiomers, i.e. E1-α-TBECH and E1-β-TBECH, preferentially accumulated. Additionally, the transformation products (TPs) in the zebrafish liver were comprehensively screened and identified using high-resolution mass spectrometry. Twelve TPs of BTBPE and eight TPs of TBECH were identified: biotransformation pathways involving ether cleavage, debromination, hydroxylation, and methoxylation reactions for BTBPE and hydroxylation, debromination, and oxidation processes for TBECH. Biotransformation is also a vital factor affecting the bioaccumulation potential of these two NBFRs, and the environmental impacts of NBFR TPs should be further investigated in future studies. The findings of this study provide a scientific basis for an accurate assessment of the ecological and environmental risks of BTBPE and TBECH.

Research advances in identification procedures of endocrine disrupting chemicals

Endocrine disrupting chemicals (EDCs) are increasingly concerned substance endangering human health and environment. However, there is no unified standard for identifying chemicals as EDCs, which is also controversial internationally. In this review, the procedures for EDC identification in different organizations/countries were described. Importantly, three aspects to be considered in identifying chemical substances as EDCs were summarized, which were mechanistic data, animal experiments, and epidemiological information. The relationships between them were also discussed. To elaborate more clearly on these three aspects of evidence, scientific data on some chemicals including bisphenol A, 1,2-dibromo-4-(1,2 dibromoethyl) cyclohexane and perchlorate were collected and evaluated. Altogether, the above three chemicals were assessed for interfering with hormones and elaborated their health hazards from macroscopic to microscopic. This review is helpful for standardizing the identification procedure of EDCs.

Hexabromocyclododecanes in surface soil-maize system around Baiyangdian Lake in North China: Distribution, enantiomer-specific accumulation, transport, temporal trend and dietary risk

This study investigated the occurrence of hexabromocyclododecanes (HBCDs) in soil-maize system around Baiyangdian Lake. The total concentration of ΣHBCDs was in the order of industrial area > residential area > marginal area > Fuhe River estuary in soil. γ-HBCD was predominated in soils, roots and stems, while α-HBCD was the main diastereoisomer in leaves and kernels. Concentration of ΣHBCDs and three diastereoisomer concentrations in soils were significantly reduced and remained low level from 2018 to 2019. Selectivity enrichment of (+)α- and (-)γ-HBCD was found in soils, roots, stems and leaves, whereas only (+)β-HBCD dominated in stems. Most of the total root bioaccumulation factors (ΣRCFs) were less than 1.0, but no significant correlation was showed between translocation factors (TFs) and log K_ow. RCFs and TFs of enantiomers suggested (-)β- and (-)γ-HBCD were easily translocated from soil to roots, while (+)α-, (-)β- and (-)γ-HBCD tended to translocate from stems to leaves. Estimated daily intake (EDI) and of ΣHBCDs, diastereoisomers and enantiomers were all lower than the threshold value, while the Calculated margins of exposure (MOE) were well above the threshold value, which demonstrate the safe consumption of Maize around Baiyangdian Lake.

Occurrence, Spatial Distribution and Health Risk of Hexabromocyclododecane (HBCD) in Source Water in the Lower Yangtze River, China

The occurrence and health risk of hexabromocyclododecane (HBCD), a brominated flame retardant with its three diastereoisomers, in drinking water sources in the lower Yangtze River in China was investigated. Its concentration ranged from 0.58 to 3.71 ng/L and averaged at 1.18 ng/L. Among the three diastereoisomers of α-, β- and γ-HBCD, γ-HBCD was the dominant one accounting for 44% (ranging 27-82%) to the total concentration. Source of HBCD in the contaminated site was discussed according to its spatial distribution and diastereoisomer profile. The margin of exposure (MOE) approach was applied to evaluate the health risk of HBCD through drinking water by estimated exposure and derived reference dose. The MOE was 17 for adults and 12 for children in the worst-case scenario, suggesting a trivial health concern.

Diastereomer- and enantiomer-selective accumulation and depuration of 1,2-dibromo-4-(1,2-dibromoethyl) cyclohexanes (DBE-DBCHs) and 1,2,5,6-tetrabromocyclooctanes (TBCOs) in earthworms (Eisenia fetida)

Due to the regulation of hexabromocyclododecane (HBCD), much attention has been paid to its potential substitutes, 1,2-dibromo-4-(1,2-dibromoethyl) cyclohexane (DBE-DBCH) and 1,2,5,6-tetrabromocyclooctane (TBCO). DBE-DBCH and TBCO contain several diastereomers and enantiomers, which may exhibit different environmental behaviors and biological effects. In this study, the accumulation and depuration of individual DBE-DBCH and TBCO diastereomers by earthworms (Eisenia fetida) from diastereomer-contaminated soils were evaluated. The accumulation and depuration kinetics of DBE-DBCH and TBCO diastereomers followed one-compartment first-order kinetics. The biota soil accumulation factor (BSAF) of β-DBE-DBCH (2.74 g_oc g_lip^-1) was 1.26 times that of α-DBE-DBCH (2.18 g_oc g_lip^-1), while the BSAF of β-TBCO (2.15 g_oc g_lip^-1) was 1.62 times that of α-TBCO (1.3 g_oc g_lip^-1), showing the diastereomer-specific accumulation of DBE-DBCH and TBCO. DBE-DBCH and TBCO diastereomers appeared to be transformed in earthworm-soil systems; however, no evidence of bioisomerization of the four diastereomers in earthworms was found, and no potential metabolites of debromination and hydroxylation were detected. Furthermore, the selective enrichment of E₁-α-DBE-DBCH and E₁-β-DBE-DBCH (E₁ represents the first enantiomer eluted) occurred in earthworms as the enantiomer fractions (EFs) for α-DBE-DBCH (0.562-0.763) and β-DBE-DBCH (0.516-0.647) were significantly greater than those in the technical products (0.501 for α-DBE-DBCH and 0.497 for β-DBE-DBCH, p < 0.05), especially in the depuration stage. The results demonstrated the diastereomer- and enantiomer-selective accumulation of DBE-DBCH and the diastereomer-selective accumulation of TBCO in the earthworm.

Bioaccumulation and biotransformation of tetrabromoethylcyclohexane (TBECH) in maize (Zea mays L.): Stereoselective driving roles of plant biomacromolecules

The bioaccumulation and biotransformation of tetrabromoethylcyclohexane (TBECH) in maize were investigated. Furthermore, the roles of plant biomacromolecules such as lipid transfer proteins (LTPs), CYP and GST enzymes in driving the biological processes of TBECH stereoisomers were explored. The uptake and translocation of TBECH in maize were diastereo- and enantio-selective. Isomerization from α- to δ-TBECH and β- to γ-TBECH, and metabolites of debromination, hydroxylation and TBECH-GSH adducts were identified in maize roots. The gene expressions of LTPs, CYPs and GSTs were extensively changed in maize after exposure to technical TBECH. CYP and GST enzyme activities as well as GST31 and CYP71C3v2 gene expressions were selectively induced or inhibited by TBECH diastereomers over time. TBECH was able to dock into the active sites and bind with specific residues of the typical biomacromolecules ZmLTP1.6, GST31 and CYP71C3v2, indicating their roles in the bioaccumulation and metabolization of TBECH. Binding modes and affinities to biomacromolecules were significantly different between α- and β-TBECH, which contributed to their stereo-selectivity. This study provided a deep understanding of the biological fate of TBECH, and revealed the driving molecular mechanisms of the selectivity of TBECH stereoisomers in plants.

Potential environmental fate and risk based on the hydroxyl radical-initiated transformation of atmospheric 1,2-dibromo-4-(1,2dibromoethyl)cyclohexane stereoisomers

1,2-dibromo-4-(1,2-dibromoethyl)cyclohexane (TBECH), as an emerging brominated flame retardant (EBFR) pollutant, has been often observed in the air, and to comprehend its fate in the environment is still challenging due to the diversity of its stereoisomers. In this work, the environmental transformation behavior and potential toxicological implications of TBECH stereoisomers under the oxidation of OH· in the gas phase were investigated by computational chemistry. Our results indicate the complexity of the TBECH transformation reactions and the diversity of its transformation products in the atmosphere. Although the reactions of TBECH enantiomers with OH· exhibit highly consistency, it is obvious that the reactions of the four diastereoisomers of TBECH with OH· and their subsequent reactions have both specificity and similarity. The dehydrogenation intermediates produced by H-abstraction of OH· in the initial reactions may undergo oxidative debromination, hydroxylation and decomposition reactions, leading to the transformation into low bromine and monohydroxy substituted compounds, as well as debrominated or unbrominated unsaturated fatty ketones. The toxicity assessments show that all transformation products are less toxic to aquatic organisms than TBECH, but some of them are still classified at toxic or harmful levels. More importantly, some transformation products still exhibit carcinogenic and teratogenic activity. To our knowledge, this study provides, for the first time, a deep insight into the transformation mechanism, kinetics, and environmental impacts of atmospheric TBECH by theoretical calculations.

Occurrence, bioaccumulation, fate, and risk assessment of novel brominated flame retardants (NBFRs) in aquatic environments - A critical review

Novel brominated flame retardants (NBFRs), which have been developed as replacements for legacy flame retardants such as polybrominated diphenyl ethers (PBDEs), are a class of alternative flame retardants with emerging and widespread applications. The ubiquitous occurrence of NBFRs in the aquatic environments and the potential adverse effects on aquatic organisms have initiated intense global concerns. The present article, therefore, identifies and analyzes the current state of knowledge on the occurrence, bioaccumulation, fates, and environmental and health risks of NBFRs in aquatic environments. The key findings from this review are that (1) the distribution of NBFRs are source-dependent in the global aquatic environments, and several NBFRs have been reported at higher concentrations than that of the legacy flame retardants; (2) high bioaccumulative properties have been found for all of the discussed NBFRs due to their strong hydrophobic characteristics and weak metabolic rates; (3) the limited information available suggests that NBFRs are resistant to biotic and abiotic degradation processes and that sorption to sludge and sediments are the main fate of NBFRs in the aquatic environments; (4) the results of ecological risk assessments have indicated the potential risks of NBFRs and have suggested that source areas are the most vulnerable environmental compartments. Knowledge gaps and perspectives for future research regarding the monitoring, toxicokinetics, transformation processes, and development of ecological risk assessments of NBFRs in aquatic environments are proposed.

Insights into the Occurrence, Fate, and Impacts of Halogenated Flame Retardants in Municipal Wastewater Treatment Plants

Halogenated flame retardants (HFRs) have been extensively used in various consumer products and many are classified as persistent organic pollutants due to their resistance to degradation, bioaccumulation potential and toxicity. HFRs have been widely detected in the municipal wastewater and wastewater treatment solids in wastewater treatment plants (WWTPs), the discharge and agricultural application of which represent a primary source of environmental HFRs contamination. This review seeks to provide a current overview on the occurrence, fate, and impacts of HFRs in WWTPs around the globe. We first summarize studies recording the occurrence of representative HFRs in wastewater and wastewater treatment solids, revealing temporal and geographical trends in HFRs distribution. Then, the efficiency and mechanism of HFRs removal by biosorption, which is known to be the primary process for HFRs removal from wastewater, during biological wastewater treatment processes, are discussed. Transformation of HFRs via abiotic and biotic processes in laboratory tests and full-scale WWTPs is reviewed with particular emphasis on the transformation pathways and functional microorganisms responsible for HFRs biotransformation. Finally, the potential impacts of HFRs on reactor performance (i.e., nitrogen removal and methanogenesis) and microbiome in bioreactors are discussed. This review aims to advance our understanding of the fate and impacts of HFRs in WWTPs and shed light on important questions warranting further investigation.

A review of 1,2-dibromo-4-(1,2-dibromoethyl)cyclohexane in the environment and assessment of its persistence, bioaccumulation and toxicity

Following the ban of many historically-used flame retardants (FRs), numerous replacement chemicals have been produced and used in products, with some being identified as environmental contaminants. One of these replacement flame retardants is 1,2-dibromo-4-(1,2-dibromoethyl)-cyclohexane (DBE-DBCH; formerly abbreviated as TBECH), which to date has not been identified for risk assessment and potential regulation. DBE-DBCH technical mixtures consist largely of α- and β-diastereomers with trace amounts of γ- and δ-DBE-DBCH. The α- and β-isomers are known contaminants in various environmental media. While current global use and production volumes of DBE-DBCH are unknown, recent studies identified that DBE-DBCH concentrations were among the highest of the measured bromine-based FRs in indoor and urban air in Europe. Yet our mass balance fugacity model and modeling of the physical-chemical properties of DBE-DBCH estimated only 1% partitioning to air with a half-life of 2.2 d atmospherically. In contrast, our modeling characterized DBE-DBCH adsorbing strongly to suspended particulates in the water column (~12%), settling onto sediment (2.5%) with minimal volatilization, but with most partitioning and adsorbing strongly to soil (~85%) with negligible volatilization and slow biodegradation. Our modeling further predicted that organisms would be exposed to DBE-DBCH through partitioning from the dissolved aquatic phase, soil, and by diet, and given its estimated logKow (5.24) and a half-life of 1.7 d in fish, DBE-DBCH is expected to bioaccumulate into lipophilic tissues. Low concentrations of DBE-DBCH are commonly measured in biota and humans, possibly because evidence suggests rapid metabolism. Yet toxicological effects are evident at low exposure concentrations: DBE-DBCH is a proven endocrine disruptor of sex and thyroid hormone pathways, with in vivo toxic effects on reproductive, metabolic, and other endpoints. The objectives of this review are to identify the current state of knowledge concerning DBE-DBCH through an evaluation of its persistence, potential for bioaccumulation, and characterization of its toxicity, while identifying areas for future research.

Occurrence and spatial distribution of legacy and novel brominated flame retardants in seawater and sediment of the South China sea

The occurrence and spatial distribution of polybrominated diphenyl ethers (PBDEs) and novel brominated flame retardants (NBFRs) in seawater and surficial sediment samples (N = 19 and 45, respectively) from the South China Sea (SCS) in 2018 were investigated, and the correlation between BFRs and site parameters (total organic carbon, depth, etc.) were assessed by principal component analysis. The concentration ranges of ΣPBDEs in seawater and sediments were 0.90-4.40 ng/L and 0.52-22.67 ng/g dry weight (dw), respectively, while those of ΣNBFRs were 0.49-37.42 ng/L and 0.78-82.29 ng/g dw, respectively. BDE-209 and decabromodiphenyl ethane were the predominant BFRs, accounting for 38.65% and 36.94% in seawater and 26.71% and 68.42% in sediments, respectively. Notably, tris(2,3-dibromopropyl)isocyanurate and 2,4,6-tris(2,4,6-tribromophenoxy)-1,3,5-triazine, seldomly detected in aquatic matrices worldwide, were detected for the first time in the study area, and their relatively high levels and detection frequencies indicate the ubiquitous application of these NBFRs in the Pearl River Delta. Zhuhai and Jiangmen are the main sources of NBFRs in the SCS. Preliminary risk assessment on NBFRs using hazard quotient indicates low to medium risks to marine organisms at some sites. The occurrence of NBFRs in the SCS highlights the prioritization of more toxicological information on these compounds.

Occurrence and mass balance of emerging brominated flame retardants in a municipal wastewater treatment plant

As a class of alternative flame retardants, "novel" brominated flame retardants (NBFRs) have been widely used in clothing, textiles, electronics, building materials, and plastics and are frequently found in environmental samples. Although the use and discharge of NBFRs are increasing all over the world, little information is available about their fates and removal in wastewater treatment plants (WWTPs). This study investigated the occurrence and behavior of 2,4,6-tribromophenyl allyl ether (ATE), 2,3-dibromopropyl tribromophenyl ether (DPTE), tetrabromo-o-chlorotoluene (TBCT), pentabromobenzyl acrylate (PBBA), 1,2-bis(2,4,6-tribromophenoxy) ethane (BTBPE), bis(2-ethylhexyl)-3,4,5,6-tetrabromo-phthalate (TBPH), and decabromodiphenyl ethane (DBDPE) in a municipal WWTP in Beijing, China. Four of the NBFRs (ATE, BTBPE, TBPH, and DBDPE) were detected in all wastewater and biosolid samples. The dominant compound in both wastewater and biosolid samples was DBDPE, with concentrations in the ranges of 6.4-18 ng/L and 83-288 ng/g dry weight (dw), respectively. A mass flow analysis indicated that the overall removal efficiencies were 31±18% for ATE, 97±1.5% for DPTE, 79±11% for BTBPE, 87±5.3% for TBPH, and 93±3.3% for DBDPE, but the removal efficiency in aqueous phase only ranged from -43 (ATE) to 57% (DBDPE). The low and negative removal efficiencies of ATE were possibly due to the transformation of DPTE during the anaerobic treatment processes. Under the aerobic conditions, BTBPE, TBPH and DBDPE underwent a very slow biodegradation. A total of 68-91% of the initial mass loadings of ATE, BTBPE, TBPH and DBDPE were found in the biosolid samples, suggesting that the four NBFRs were removed mainly via sorption in the WWTP.The biosolid was a huge reservoir of target NBFRs, and effective removal of NBFRs during treatment in the WWTP is critical.

Application of enantiomeric fractions in environmental forensics: Uncertainties and inconsistencies

The assumption that only biological processes are enantioselective introduces challenges in the reliability of enantioselective analysis as a tool for discriminating biotic and abiotic processes in the environmental fate of chiral pollutants. Enantioselectivity does not depend on the nature of the fate process a chiral contaminant undergoes but on the interaction of the chiral contaminant with homochirality inducing external agents (e.g. chiral molecules, macromolecules or surfaces such as enzymes, blood plasma, proteins, chiral co-pollutants, humic acid and soil organominerals). The environmental behavior of a chiral contaminant is difficult to anticipate because the interactions between the chiral contaminants and the homochirality inducing external agents is often complex and strongly influenced by local environment conditions such as pH, redox conditions, organic carbon, organic nitrogen, humic acid, and redox conditions. Furthermore, the use of enantioselective analysis in environmental forensics depend on the adequate separation and accurate identification and quantification of the enantiomers of the chiral contaminant. Matrix effects, instrument effects, inadequate enantioselective separation, and poor quantification techniques introduce uncertainties in the determination of enantiomeric composition. Here we present the weaknesses of this assumption and recommend using enantiomeric fractions as chemical markers of biotransformation with caution. We recommend using stable isotopes, including abiotic controls to determine if enantioselective sorption occurs, and determining stability of enantiomers in solvent or at elevated temperatures to account for confounding factors arising from matrix effects, enantioselective abiotic processes, and enantiomerization due solvent and thermal lability of the chiral analyte, respectively to maintain the integrity of the utility of enantiomeric composition changes as an environmental forensics tool.