Hexabromocyclododecane: current understanding of chemistry, environmental fate and toxicology and implications for global management

The fate of β-hexabromocyclododecane in female C57BL/6 mice

1,2,5,6,9,10-Hexabromocyclododecane (HBCD) is a high production volume cycloaliphatic used as an additive flame retardant primarily in polystyrene foam building materials. HBCD mixtures contain three major stereoisomers, alpha (α), beta (β), and gamma (γ), at a typical ratio of 1.2:0.6:8.2. The toxicokinetic properties of the α and γ isomers differ. For instance, α-HBCD has greater bioavailability and potential for accumulation in mice than γ-HBCD. The present study reports comparative kinetics data for β-HBCD needed to support toxicological evaluations of HBCD mixtures. Results indicated that a single oral dose of 3mg/kg of [(14)C]-labeled β-HBCD was absorbed rapidly (≥ 85% total dose) in the female C57BL/6 mouse. The C max for β-HBCD-derived radioactivity in tissues, except adipose, was observed 3h following gavage. Approximately 90% of the administered dose was excreted in urine and feces within 24h, primarily as β-HBCD-derived metabolites. A portion of the dose (circa 9%) was excreted in feces as γ-HBCD. Oral administration of 30 or 100mg/kg of β-HBCD resulted initially in slower rates of [(14)C] elimination; however, cumulative excretion data were similar across the dosing range 4 days postdosing. Residual concentrations of [(14)C] in tissues were highest in adipose and liver. β-HBCD-derived radioactivity accumulated in most tissues following four consecutive daily oral doses of 3mg/kg. The extent of metabolism and excretion of β-HBCD in female C57BL/6 mice was similar to that for γ-HBCD. The potential for accumulation of β-HBCD-derived material in most tissues appeared to be less than for α-HBCD.

Preparation of graphene/TiO2 composites by nonionic surfactant strategy and their simulated sunlight and visible light photocatalytic activity towards representative aqueous POPs degradation

A series of graphene/TiO2 composites were fabricated using a single-step nonionic surfactant strategy combined with the solvothermal treatment technique. Their phase structure, morphology, porosity, optical absorption property, as well as composition and structure, were characterized. The as-prepared composites were successfully applied to degrade aqueous persistent organic pollutants (POPs) such as rhodamine B, aldicarb, and norfloxacin in simulated sunlight (λ>320 nm) and visible light (λ>400 nm) irradiation. The degradation mechanism and kinetics of aqueous POPs were studied in detail. The mineralization of aqueous POPs and the recyclability of the composites were also tested in the same condition.

Levels and trends of the emerging contaminants HBCDs (hexabromocyclododecanes) and PFCs (perfluorinated compounds) in marine shellfish along French coasts

The levels and congener patterns of HBCDs (hexabromocyclododecanes) and PFCs (perfluorinated compounds) were determined in filter-feeding molluscs collected in 2008 and 2010 along the coasts of mainland France. α-HBCD and PFOS (perfluorooctane sulfonate) were detected in all samples, revealing widespread contamination of the coastal environment by these emerging contaminants. The spatial distribution of Σ-HBCD concentrations showed higher median levels in samples from the Mediterranean Sea and English Channel respectively, i.e. 0.19 ng g(-1) wet weight (ww) and 0.08 ng g(-1) ww, related to high anthropogenic pressure from urban and industrial activities, while the median concentration was 0.05 ng g(-1) ww in samples from the Atlantic coast. Among PFCs, PFOS was the only compound detected in all samples and PFDA (perfluorodecanoic acid) was the second most frequently-detected compound. PFOS median concentrations were 0.18 ng g(-1) ww, 0.09 ng g(-1) ww and 0.04 ng g(-1) ww in samples from the English Channel, the Atlantic coast and the Mediterranean coast respectively. The highest PFOS concentration was found in the Loire estuary, possibly related to local industrial activities. The Mediterranean samples showed a different pattern, with predominant long-chain PFCAs (perfluorocarboxylic acids), suggesting the presence of alternative sources on the Mediterranean coast. The temporal trends studied in archived samples from the Seine estuary site showed a significant exponential increase in HBCD concentrations between 1981 and 2011, with a doubling time of 7 years, while PFOS levels underwent a significant linear decrease over time. These trends are coherent with current regulations on the use of these compounds. The results presented in this paper provide the first data on the contamination of the French coastal marine environment by the selected emerging compounds, and constitute a reference for the future monitoring of French coastal contamination by emerging contaminants.

Determination of polyparameter linear free energy relationship (pp-LFER) substance descriptors for established and alternative flame retardants

Polyparameter linear free energy relationships (pp-LFERs) can predict partition coefficients for a multitude of environmental and biological phases with high accuracy. In this work, the pp-LFER substance descriptors of 40 established and alternative flame retardants (e.g., polybrominated diphenyl ethers, hexabromocyclododecane, bromobenzenes, trialkyl phosphates) were determined experimentally. In total, 251 data for gas-chromatographic (GC) retention times and liquid/liquid partition coefficients (K) were measured and used to calibrate the pp-LFER substance descriptors. Substance descriptors were validated through a comparison between predicted and experimental log K for the systems octanol/water (K(ow)), water/air (K(wa)), organic carbon/water (K(oc)) and liposome/water (K(lipw)), revealing a high reliability of pp-LFER predictions based on our descriptors. For instance, the difference between predicted and experimental log K(ow) was <0.3 log units for 17 out of 21 compounds for which experimental values were available. Moreover, we found an indication that the H-bond acceptor value (B) depends on the solvent for some compounds. Thus, for predicting environmentally relevant partition coefficients it is important to determine B values using measurements in aqueous systems. The pp-LFER descriptors calibrated in this study can be used to predict partition coefficients for which experimental data are unavailable, and the predicted values can serve as references for further experimental measurements.

Determination of PBDEs, HBB, PBEB, DBDPE, HBCD, TBBPA and related compounds in sewage sludge from Catalonia (Spain)

The objective of this study was to determine different brominated flame retardants (BFRs) in sewage sludge produced in 17 wastewater treatment plants (WWTPs) located in the Northeast of Spain. A total of eight polybrominated diphenyl ether (PBDE) congeners, from tri- to deca-BDEs, were analyzed. The emerging BFR compounds, hexabromobenzene (HBB), pentabromoethylbenzene (PBEB) and decabromodiphenylethane (DBDPE) were also analyzed. The instrumental methodology for the analysis was based on gas chromatography coupled to mass spectrometry with negative chemical ionization (GC-NICI-MS). Moreover, liquid cromathography-quadrupole linear ion trap mass spectrometry (LC-QqLIT-MS/MS) was applied for the determination of hexabromocyclododecanes (HBCDs), tetrabromobisphenol A (TBBPA) and their related compounds, bisphenol A (BPA), monobromobisphenol A (MonoBBPA), dibromobisphenol A (DiBBPA) and tribromobisphenol A (TriBBPA). The most abundant PBDE congener in these sewage sludge samples was BDE-209, with levels ranging from nd to 2303 ng/g dw. In addition, a significant concentrations of DBDPE were also detected, nd-257 ng/g dw, due to their increase production as a Deca-BDE substitute. The emerging compounds HBB and PBEB were detected in 4 WWTPs and their concentrations ranged between nd-5.71 and nd-2.33 ng/g, respectively. TBBPA was detected in quantifiable levels in 15 of the 17 WWTPs analyzed, in concentration range of nd-472 ng/g dw, whereas HBCDs were only found in 8 WWTPs in lower concentrations, between nd and 97.5 ng/g dw. Based on the analysis of sewage sludge samples from different WWTPs and in order to evaluate the impact of these contaminants in the environment, an estimation of BFRs accumulated in the total sewage sludge produced in Catalonia was carried out, with values of 57.8 kg/year, 6.76 kg/year, 13.5 kg/year, 1.56 kg/year, 0.18 kg/year and 0.02 kg/year for PBDEs, DBDPE, TBBPA, HBCDs, HBB and PBEB respectively.

Acute effects of hexabromocyclododecane on Leydig cell cyclic nucleotide signaling and steroidogenesis in vitro

Hexabromocyclododecane (HBCDD), an additive brominated flame retardant routinely added to various consumer products, was reported to have toxic effects upon biota, including endocrine disruption. In this study, the potential toxicity of HBCDD was tested in peripubertal rat Leydig cells in vitro during 6h exposure. HBCDD inhibited human chorionic gonadotropin- and forskolin-supported cAMP accumulation and steroidogenesis. It also inhibited basal cAMP production, but elevated basal steroidogenesis. The expression of several cAMP-dependent genes, including steroidogenic acute regulatory protein, cholesterol side chain cleavage enzyme, and 3β-hydroxysteroid dehydrogenase, was also inhibited by HBCDD treatment. Nevertheless, this was not accompanied by a decrease in steroidogenic acute regulatory protein expression, as documented by western blot analysis, and activity of steroidogenic enzymes, as documented by unaffected steroidogenesis in the presence of permeable 22(R)-hydroxycholesterol. However, HBCDD caused significant decrease in mitochondrial membrane potential in untreated and human chorionic gonadotropin-treated cells. This indicates that HBCDD acute toxicity in Leydig cells reflects changes in mitochondrial membrane potential-dependent cAMP production and basal and cAMP-regulated cholesterol transport. This in turn facilitates basal but inhibits cAMP-dependent steroidogenesis. Acute effects of HBCDD treatment on transcription are also indicative of its sustained effects on Leydig cell function.

Environmental fate of hexabromocyclododecane from a new Canadian electronic recycling facility

An electronics recycling facility began operation at the municipal landfill site for the City of Edmonton, Canada in March 2008 with the goal of processing 30,000 tonnes of electronic wastes per year. Of the many by-products from the process, brominated fire retardants such as hexabromocyclododecane (HBCD) can evolve off of e-wastes and be released into the environmental media. HBCD has been identified by many countries and international bodies as a chemical of concern because of its ability to bioaccumulate in the ecosystem. An evaluation of the potential emission of HBCD indicates that up to 500 kg per year may be released from a landfill and recycling facility such as that operating in Edmonton. A multimedia fugacity model was used to evaluate the dispersion and fate of atmospherically emitted HBCD traveling into surrounding agricultural land and forested parkland. The model indicates that the three isomers of HBCD partitioned into environmental media similarly. Much of the HBCD is lost through atmospheric advection, but it is also found in soil and sediment. Modeled air concentrations are similar to those measured at locations with a history of e-waste recycling. Since HBCD has been shown to bioaccumulate, the HBCD released from this source has the long-term potential to affect agricultural food crops and the park ecosystem.

The cytological effects of HBCDs on human hepatocyte L02 and the potential molecular mechanism

The concentration of hexabromocyclododecanes (HBCDs) in the environment media and organism samples are gradually rising with the increase of HBCDs usage. This study is designed to investigate the cytological effects of HBCDs on human hepatocyte L02 and explore the potential molecular mechanism. The results of CCK-8 assay showed that high concentration of HBCDs (>20 μM) significantly suppressed cell survival, while comparatively lower dose of HBCDs (10(-13)-10(-7)M) slightly stimulated cell proliferation (P < 0.05). In the mean time, high concentration HBCDs markedly induced cell apoptosis and DNA damage, accompanying with increase of intracellular Ca(2+)level and decrease of mitochondrial membrane potential (P < 0.05). ROS level induced by low concentration of HBCDs was comparatively lower than that by high concentration of HBCDs. In addition, low concentration HBCDs exposure (10(-13)-10(-7)M) resulted in up-regulation of PCNA protein expression level in a time-dependent manner. However, high concentration HBCDs exposure led to increase of Apaf-1 expression level. In conclusion, loss of mitochondrial membrane potential and activation of Apaf-1 mediated pathway involve the L02 cell apoptosis induced by high concentration HBCDs exposure. However, low concentration HBCDs exposure could stimulate cell proliferation of L02 cells, which might be associated with enhancement of PCNA expression.

Spatial distribution and inter-year variation of hexabromocyclododecane (HBCD) and tris-(2,3-dibromopropyl) isocyanurate (TBC) in farm soils at a peri-urban region

Hexabromocyclododecane (HBCD) is a high production volume brominated flame retardant (BFR) which has been of increasing environmental and public health concern due to its potential environmental persistency, bioaccumulation and toxicity. Tris-(2,3-dibromopropyl) isocyanurate (TBC) is another BFR which has recently been found in environmental matrices near a manufacturing plant, but its production volume and environmental distribution is currently not well known. This study was conducted to investigate the presence and distribution of these two BFRs in farm soils at a region in southeast Beijing. Total HBCD levels ranged from 0.17 to 34.5 ng g(-1) on a dry weight basis (dw) with a median level of 2.97 ng g(-1)dw. The composition profile of HBCD diastereoisomers was, on average, 28%, 13% and 59% for α-, β- and γ-HBCD, respectively. Detection frequency of TBC was only 25% in 2010 but was detected in all soil samples in 2011, and the median level was 0.19 ng g(-1)dw with the range between below detection limit to 1.62 ng g(-1) dw. There were no significant differences of HBCD and TBC levels among different irrigation sources in the region. The soil HBCD and TBC levels in samples collected in 2011 were significantly higher than in 2010. The increasing short-term temporal levels in farm soil might be due to the rapid urbanization in this region or could also reflect the increasing usage of HBCD and TBC after the phase out of other BFRs.

After the PBDE phase-out: a broad suite of flame retardants in repeat house dust samples from California

Higher house dust levels of PBDE flame retardants (FRs) have been reported in California than other parts of the world, due to the state's furniture flammability standard. However, changing levels of these and other FRs have not been evaluated following the 2004 U.S. phase-out of PentaBDE and OctaBDE. We analyzed dust collected in 16 California homes in 2006 and again in 2011 for 62 FRs and organohalogens, which represents the broadest investigation of FRs in homes. Fifty-five compounds were detected in at least one sample; 41 in at least 50% of samples. Concentrations of chlorinated OPFRs, including two (TCEP and TDCIPP) listed as carcinogens under California's Proposition 65, were found up to 0.01% in dust, higher than previously reported in the U.S. In 75% of the homes, we detected TDBPP, or brominated "Tris," which was banned in children's sleepwear because of carcinogenicity. To our knowledge, this is the first report on TDBPP in house dust. Concentrations of Firemaster 550 components (EH-TBB, BEH-TEBP, and TPHP) were higher in 2011 than 2006, consistent with its use as a PentaBDE replacement. Results highlight the evolving nature of FR exposures and suggest that manufacturers continue to use hazardous chemicals and replace chemicals of concern with chemicals with uncharacterized toxicity.

Hexabromocyclododecane in terrestrial passerine birds from e-waste, urban and rural locations in the Pearl River Delta, South China: levels, biomagnification, diastereoisomer- and enantiomer-specific accumulation

Diastereoisomers and enantiomers of hexabromocyclododecane (HBCD) were determined in muscle and stomach contents of three terrestrial passerine birds from e-waste, urban and rural locations in South China. The highest HBCD concentration was found at the urban site, followed by the e-waste site, suggesting that HBCD was linked to urbanization and industrialization, as well as e-waste recycling activities. Trophic magnification for α-HBCD was observed in the terrestrial food chain. Diastereisomeric pattern in birds showed the predominance of α-isomer, with a minor contribution of γ-isomer. The enantiomeric analysis revealed a preferential enrichment of (-)-α-HBCD and (+)-γ-HBCD. The similarity in enantiomeric fractions of HBCD isomers between muscle and stomach contents of birds suggested that dietary uptake may be responsible for the observed nonracemic distribution of HBCD isomers in the studied birds although in vivo enantioselective biotic processes cannot be ruled out.

Diastereoisomer- and enantiomer-specific accumulation, depuration, and bioisomerization of hexabromocyclododecanes in zebrafish (Danio rerio)

In this study, zebrafish (Danio rerio) were exposed to two dietary concentrations of individual HBCD diastereoisomers (α-, β-, and γ-HBCD) for 42 days, followed by clean food for 21 days, to examine bioaccumulation, depuration, and enantiomer fractions (EFs) of HBCD diastereoisomers and to test the bioisomerization of HBCDs in fish. The depuration of α-, β-, and γ-HBCD in zebrafish followed the first-order process. Bioaccumulation parameters of the three diastereoisomers differed between low and high dose, suggesting that the bioaccumulation of them is concentration dependent. Calculated assimilation efficiencies (AEs), biomagnification factors (BMFs), and half-lives (t(1/2)) of α-HBCD were the highest among the three diastereoisomers. Furthermore, the study showed that zebrafish could biotransform γ-HBCD to α-HBCD. The highest AE, BMF, and t(1/2) of α-HBCD and bioisomerization of γ-HBCD to α-HBCD could explain why α-HBCD appears to be dominant in biota samples. The EFs for α- and γ-HBCD in zebrafish estimated at different times of bioaccumulation and depuration were all significantly greater than those in corresponding food (P < 0.05), indicating selective enrichment of (+) α-enantiomer and (+) γ-enantiomer relative to (-) α-enantiomer and (-) γ-enantiomer, respectively.

Co-release of hexabromocyclododecane (HBCD) and Nano- and microparticles from thermal cutting of polystyrene foams

Polystyrene foam is a very important insulation material, and hexabromocyclododecane (HBCD) is frequently used as its flame retardant. HBCD is persistent, bioaccumulative, and toxic, and therefore workplace exposure and environmental emission should be avoided. In this study, we investigated the co-release of HBCD and aerosol particles during the thermal cutting of expanded polystyrene foam (EPS) and extruded polystyrene foam (XPS). The generated particles were simultaneously measured by a fast mobility particle sizer (FMPS) and collected by a cascade impactor (NanoMoudi). In the breathing zone of a cutting worker, the number concentration of aerosol particles was above 1 × 10(12) particles m(-3), and the air concentration of HBCD was more than 50 μg m(-3). Most of the released HBCD was partitioned into particles with an aerodynamic diameter at the nanometer scale. The average concentrations of HBCD in these submicrometer particles generated from the thermal cutting of EPS and XPS were 13 times and 15 times higher than the concentrations in raw foams, respectively. An occupational exposure assessment indicated that more than 60% of HBCD and 70% of particles deposited in the lung of cutting worker would be allocated to the alveolar region. The potential subchronic (or chronic) toxicity jointly caused by the particles and HBCD calls for future studies.

Species-specific time trends and enantiomer fractions of hexabromocyclododecane (HBCD) in biota from East Greenland

Time trends of hexabromocyclododecane (HBCD) isomers were studied for glaucous gull and ringed seal from East Greenland. The ringed seal results extended a previous time trend (1986-2008) to 2010. α-HBCD was the only isomer consistently above quantification limits. For glaucous gull liver, annual median values of α-HBCD (1994-2010) ranged from 22 to 120 ng/g lipid weight (lw) with no significant trend, while HBCD in ringed seal blubber from the same area showed a significant increase from 3.9 to 11 ng/g lw (1986-2010). Reasons for this difference are unknown, but might include different feeding habits and species-specific metabolisation processes. Concentrations of several organochlorine (OC) compounds were determined for glaucous gull and ringed seal samples collected from the same area in 2004. HBCD concentrations in glaucous gull liver appeared relatively low when compared to OC concentrations in the same tissue and to both HBCD and OCs in ringed seal blubber from the same area. Enantiomer fractions (EF) deviated significantly from racemic for all annual mean EFs in glaucous gull suggesting metabolisation processes toward an enrichment of (-)α-HBCD. For ringed seal, this enrichment was less pronounced and only significant for two of the ten years. For neither species, significant changes in EF were found over time.

Occurrence of alternative flame retardants in indoor dust from New Zealand: indoor sources and human exposure assessment

Due to worldwide restrictions on polybrominated diphenyl ethers (PBDEs), the demand for alternative flame retardants (AFRs), such as organophosphate flame retardants (OPFRs), novel brominated FRs (NBFRs) and hexabromocyclododecanes (HBCDs), has recently increased. Little is known about human exposure to NBFRs and OPFRs and that their levels in dust have been scarcely evaluated worldwide. To increase the knowledge regarding these chemicals, we measured concentrations of five major NBFRs, ten OPFRs and three HBCD isomers in indoor dust from New Zealand homes. Dust samples were taken from living room floors (n=34) and from mattresses of the same houses (n=16). Concentrations (ngg(-1)) of NBFRs were: 1,2-bis(2,4,6-tribromophenoxy)ethane (BTBPE) (<2-175), decabromodiphenyl ethane (DBDPE) (<5-1430), 2-ethylhexyl-2,3,4,5-tetrabromobenzoate (TBB) (<2-2285) and bis(2-ethylhexyl)-3,4,5,6-tetrabromophthalate (TBPH) (<2-640). For OPFRs, concentrations (ngg(-1)) ranged between: tri-ethyl-phosphate (TEP) (<10-235), tri-n-butyl-phosphate (TnBP) (<20-7545), tris-(2-chloroethyl)-phosphate (TCEP) (<20-7605), tris-(1-chloro-2-propyl) phosphate (TCPP) (20-7615), tri-(2-butoxyethyl)-phosphate (TBEP) (50-27325), tris-(2,3-dichloropropyl)-phosphate (TDCPP) (20-16560), tri-phenyl-phosphate (TPhP) (20-35190), and tri-cresyl-phosphate (TCP) (<50-3760). HBCD concentrations fell in the range <2-4100ngg(-1). BTBPE, DBDPE, TBPH, TBEP, and TnBP showed significant positive correlation (p<0.05) between their concentrations in mattresses and the corresponding floor dust (n=16). These data were used to derive a range of plausible exposure scenarios. Although the estimated exposure is well below the corresponding reference doses (RfDs), caution is needed given the likely future increase in use of these FRs and the currently unknown contribution to human exposure by other pathways such as inhalation and diet.

Diastereomer-specific uptake, translocation, and toxicity of hexabromocyclododecane diastereoisomers to maize

Hexabromocyclododecane (HBCD), a brominated flame retardant, has become a ubiquitous contaminant due to its wide application, persistence, and toxicity. HBCD diastereoisomers have different physical and chemical properties and may differ in their bioaccumulation and toxicity in plants. Accumulation and toxicity of α-, β-, and γ-HBCDs in maize were investigated in the present study. The accumulation was in the order β-HBCD > α-HBCD > γ-HBCD in roots and β-HBCD > γ-HBCD > α-HBCD in shoots. Both the inhibitory effect of the diastereoisomers on the early development of maize and the intensities of hydroxyl radical and histone H2AX phosphorylation in maize exposed to 2 μg L(-1) HBCD followed the order α-HBCD > β-HBCD > γ-HBCD, indicating the diastereomer-specific oxidative stress and DNA damage in maize. It was further confirmed that the generation of reactive oxygen species was one, but not the only, mechanism for DNA damage in maize exposed to HBCDs.

Hexabromocyclododecane in consumer fish from South China: implications for human exposure via dietary intake

Levels of hexabromocyclododecane (HBCD) were determined in 12 consumer fish species from South China. The concentrations of the sum of HBCD diastereoisomers (ΣHBCD) ranged from nondetectable to 194 pg/g wet weight, with a detectable frequency of 70%. This was at the low end of the concentration range globally observed. Carnivorous fish species contained higher HBCD concentrations than herbivorous and detritivorous ones, suggesting the potential for biomagnification of HBCDs via the trophic chain. Moreover, ΣHBCD concentrations were higher in both freshwater and seawater farmed fish than in wild marine fish, indicating that human activities were probably an important input source of HBCD in aquaculture. Among the three diastereoisomers, α-HBCD dominated in most samples, with a mean relative abundance of 70%. In agreement with previous studies, a difference in the diastereomer patterns was observed from commercial products and several environmental media to biota samples. For farm-raised snubnose pompano, the concentrations of ΣHBCD were moderately linearly correlated with lipid contents but were not significantly correlated with fish body lengths and weights. In the worst-case exposure, the mean estimated daily intake of ΣHBCD via fish consumption for residents of South China ranged from 13 to 16 pg/kg body weight/d for various age groups, much lower than its lowest observed adverse effect level derived from a two-generation reproductive toxicity study on rats.

Biotransformation of Hexabromocyclododecanes (HBCDs) with LinB--an HCH-converting bacterial enzyme

Hexabromocyclododecanes (HBCDs) and hexachlorocyclohexanes (HCHs) are polyhalogenated hydrocarbons with similar stereochemistry. Both classes of compounds are considered biologically persistent and bioaccumulating pollutants. In 2009, the major HCH stereoisomers came under regulation of the Stockholm convention. Despite their persistence, HCHs are susceptible to bacterial biotransformations. Here we show that LinB, an HCH-converting haloalkane dehalogenase from Sphingobium indicum B90A, is also able to transform HBCDs. Racemic mixtures of α-, β-, and γ-HBCDs were exposed to LinB under various conditions. All stereoisomers were converted, but (-)α-, (+)β-, and (+)γ-HBCDs were transformed faster by LinB than their enantiomers. The enantiomeric excess increased to 8 ± 4%, 27 ± 1%, and 20 ± 2% in 32 h comparable to values of 7.1%, 27.0%, and 22.9% as obtained from respective kinetic models. Initially formed pentabromocyclododecanols (PBCDOHs) were further transformed to tetrabromocyclododecadiols (TBCDDOHs). At least, seven mono- and five dihydroxylated products were distinguished by LC-MS so far. The widespread occurrence of HCHs has led to the evolution of bacterial degradation pathways for such compounds. It remains to be shown if LinB-catalyzed HBCD transformations in vitro can also be observed in vivo, for example, in contaminated soils or in other words if such HBCD biotransformations are important environmental processes.

Hexabromocyclododecane (HBCD) and tetrabromobisphenol A (TBBPA) in riverine and estuarine sediments of the Pearl River Delta in southern China, with emphasis on spatial variability in diastereoisomer- and enantiomer-specific distribution of HBCD

Hexabromocyclododecane (HBCD) and tetrabromobisphenol A (TBBPA) were measured in surface sediments in the Pearl River Delta, southern China. The concentrations ranged from 0.06 to 304 ng/g dry weight for TBBPA and from 0.03 to 31.6 ng/g for total HBCD. The correlations between the TBBPA and HBCD concentrations were significant in rivers (Dongjiang, Zhujiang, Beijiang, and Dayanhe Rivers) with local input sources but not significant in the Xijiang River and Pearl River Estuary without local sources. HBCD was dominated by γ-HBCD (averaged 52.5-75.0%), whereas α-HBCD contributions were relatively high in sediments from the urban areas. The enantiomeric fractions of γ-HBCD in the sediments (averaged from 0.431 ± 0.035 to 0.479 ± 0.010) differed significantly from that in HBCD technical product except for samples from the e-waste area (Dayanhe River, 0.488 ± 0.091). This suggests that enantioselective biotransformation of HBCD occurs in the aquatic environment in this region.