Environmental Distribution And Bioaccumulation Of Hexabromocyclododecane Diastereomers And Enantiomers

Hexabromocyclododecanes (HBCDs), a kind of hydrophobic brominated flame retardants, are widely used in plastic materials, textiles and electronics. Environmental investigations reveal that HBCDs can be extensively detected in sediments, water, soils and biotic samples. HBCDs were listed as candidate persistent organic pollutants (POPs) in Stockholm Convention on POPs in May2009, which increased their concern in the world. Further studies found that HBCDs are of characteristics for POPs, i.e. high persistence, high bioaccumulation potential, semi-volatility and high toxicity. Besides, HBCDs have different diastereomers and enantiomers and show selectivity for these species during environmental processes.Based on a lot of field studies, it has been found that the profiles of three HBCD diastereomers (α-, β-and γ-HBCD) are very different between sediments and organisms. In the abiotic environmental matrices (sediments and soils), y-HBCD is the dominant isomer with a ratio similar to that (-80%) in the technical products. However, a-HBCD is found as the dominant species in organisms, and the percentage of a-HBCD increases with ascending trophic level, and the ratio up to100%was found in birds at the highest trophic level. Each of the three diastereomers have two enantiomers, the distinct enantioselectivity has been found in different species and even in different tissues.Although there are a lot of field investigation data on the environmental distribution of HBCDs in the world, only few studies can be found in China. Moreover, as stated above, the bioaccumulation of different HBCD diastereomers and enantiomers are different based on field investigation. However, the selectivity tendency for different diastereomers and enantiomers should be checked for different organism species, and the process and mechanism controlling the selectivity are still needed to clarify. Hence, in the present thesis, a systematic study on the distribution in different environmental media in China and bioaccumulation characteristics of HBCD diastereomers and enantiomers was conducted, and main results acquired are as follows(1) To elucidate the pollution characteristics of HBCDs in soils, HBCD diastereomers and enantiomers were measured and discussed in soil samples from a sewage irrigation area and an electronic recycling district in Tianjin. The concentration of HBCDs in soils from the electronic recycling district ranged from1.57to3.76ng/g dw; and9.33to44.6ng/g dw in farmland from agricultural district. The unexpected higher HBCD concentration in farmland as compared with those from electronic recycling district may result from the sewage irrigation from Dagu Drainage Canal. In addition, this suggested that HBCDs were not widely used in electronic products in China. Diastereomer-specific selectivity occurred in the soils from electronic recycling district with higher a-HBCD ratio, which was contrary to that in technical products with higher y-HBCD ratio, indicating the transformation between HBCD diastereomers. Combined with the low concentration in these soils, the contamination should be a historic issue. However, an opposite trend was found in the soils from agricultural district, indicating a fresh contamination. There was no consistent tendency of enantioselectivity in soils, indicating that the enantioselectivity is determined by multiple processes controlled by variable factors. As for β-HBCD, agricultural soils exhibited enantioselectivity of (+)-HBCD and vice versa in the soils from electronic product recycling district.(2) To grasp the pollution characteristics in aquatic sediment, HBCDs were investigated in the sediment from Haihe River (HR), Dagu Drainage Canal (DDC) and Tianjin Harbor (TH) at BohaiBay using a total of51samples of surface sediments and a sediment core collected from May to September in2010. The concentrations of total HBCDs were generally high, with mean value of31.0and ranges of1.35to634ng/g dw, respectively. The contamination followed the order of TH>DDC>HR. Higher levels (up to634ng/g dw) occurred in the lower reach of HR and DDC located in an industrial area of Tianjin. This is the first time to report such a concentration of HBCDs in sediment in Southeast Asia. Only7out of51samples exhibited γ-diastereomer ratio similar to that (75-89%) in technical products, and other samples showed significant difference with technical products. The sample sites with high difference of diastereomer ratio is nearby the discharge outlets of waste water treatment plants. These indicated that the pollution history and biodegradation extent both determined the diastereomer profiles at different sites, and the high ratio of y-diastereomer could be used as an indicator for fresh contamination input. Enantiomeric factors (EFs) of HBCD isomers in most of the samples were statistically different from technical products (p<0.05), showing a trend of more easily enrichment of the (-)-HBCD-enantiorner compared to the (+)-HBCD-enantiomer. The δ-and ε-diastereomers were frequently detected but at low levels. The HBCD jagged in the sediment core, and the highest value was in accordance with the time of the establishment of a related factory, and other small peaks were related with the economic developing speeed of China and Tianjin.(3) To interpret the distribution of HBCDs in various organisms, we measured the concentrations and diastereomer and enantiomer profiles of HBCDs in21different species of limnic and marine cohorts from Tianjin, China.The concentration ranges of HBCDs in limnic and marine organisms were64.3-1111ng/g1w and85.5-989ng/g1w, respectively. Living habitat and feeding habits had important impacts on HBCD diastereomerdistribution. Most of the species appeared to preferentially select (+)-a-,(-)-β-and (-)-γ-HBCD. There is a tendency that the total anda-HBCDs weremagnified astrophic level increased with trophic magnification factors (TMFs) around2. The concentrations of HBCDs in the limnic and marine fishes were highest in the liver, followed by the gill, skin, and muscle. Terrestrial plants cound concentrate HBCDs, with the total HBCD concentration being1.47-27.8ng/g dw. The highest concentrations were observed in leaves, followed by roots and rhizosphere soils. Plants showed selectivity for HBCD enantiomers, which varied with plant species and organs (leaf vs. root) of the same plant. Higher estimated daily intakes (EDIs) of HBCDs were observed from fish than from wheat.(4) To understand the bioconcentration of HBCD diastereomers and enantiomers in aquatic producers (at low trophic levels), we acquired the single diastereomers by semi-preparative HPLC, and studied algae bioaccumulation by exposing two microalgae, Spirulina subsalsa and Scenedesmus obliguus, inthe plateau phase to single HBCD diastereomers (a paired enantiomers) for7days. The accumulation ability of S. obliquus was higher than that of S. subsalsa, with steady state BCF values in the range of174to350and390to469, respectively. For three diastereomers, the accumulation trend of a-> β-> γ-HBCD was observed in S. subsalsa, which is the same with other aquatic organisms. Whereas the accumulation trend of β-> a-> y-HBCD was observed in S. obliquus, which is in accordance with those of terrestrial species. Contrary to the process typically occurring in fish and other mammals, the bioisomerization from β-or γ-HBCD to a-HBCD was not observed in algae. a-HBCD exhibited selective enrichment of (+)enantiomer in S. subsalsand (-)enantiomer in S. obliquus. No significant enantioselectivity was observed for β-and γ-HBCD. The selectivity on diastereomers and enantiomers in the algae may be transferred to high trophic level through food chain. New metabolites, tetrabromocyclododecadiene (TBCDi) and tribromocyclododecatriene (TriBCDie), were observed in S. subsalsa for the first time, but these were not observed in S. obliquus.(5) In order to study the bioaccumulation characteristics of HBCD diastereomers and enantiomersin fish exposed to water phase, we conducted a bioaccumulation test by exposing mirror carp to three HBCD diastereomers in water during30d of accumulation and30d of depuration according to Organization for Economic Co-operation and Development (OECD) Test Guidelines305(Bioaccumulation in Fish:Aqueous and Dietary Exposure). It was found that the BCFKL values (bioconcentration factor calculated from kinetic data and adjusted to lipid content) of a-HBCD in different tissues of the carp were in the range of3.07-4.52×104, much higher than those of β-HBCDs (1.03-1.90×103) and γ-HBCD (0.95-1.73×103), as was true for t1/2. The order of BCFK for α-,β-and γ-HBCD in different tissues was viscera> gill> skin> muscle. β-and γ-HBCD were transformed to a-HBCD, with50.0-92.9%and96.2-98.6%bioisomerization efficiencies at the end of the experiment, respectively. Isomerization products from a-HBCD were not found. Selective enrichment of the (+)α-and γ-HBCD was found, whereas β-HBCD shows weak enantioselectivity of (-)β-HBCD. New metabolites such as tetrabromocyclododecadiene (TBCDi), tribromocyclododecadiene (TriBCDi) and tribromocyclododecatriene (TriBCDie) were found in mirror carp under multiple reaction monitoring (MRM) mode.(6) To demonstrate the bioaccumulation of HBCD diastereomer and enantiomer in fish by food exposing, a simple food chain with two levels(Talapia/Spirulina subsalsa) was set up, bioaccumulation test was conducted for35d and depuration for30d in the laboratory. The trend of the three diastereomers in viscera, muscle and skin was α-> β-> γ-HBCD, while the trend in gill was γ->α-> β-HBCD. In the depuration phase, the half-time (ti/2) of a-HBCD was significantly higher than β-HBCD and y-HBCD. Depuration ability of HBCDs by Talapia was higher than mirror carp, zebrafish and rainbow trout. Because the content of lipid and exposure route were different between Talapia and mirror carp, the BMF trend in different tissues of Talapia (gill> viscera> muscle> skin) was various with mirror carp (viscera> gill> skin> muscle). No bioisomerized products were found in algae but the a-HBCD was isomerized from β-and y-HBCD in Talapia. In the algae, significant enantioselectivity for (+)α-HBCD was found, and β-and γ-HBCD showed no enantioselectivity. While in Talapia, which preyed on the algae, α-and β-HBCD showed significant enantioselectivity of (-)-HBCD, y-HBCD showed (+)-HBCD enantioselectivity. The decline of EFs for α-and β-HBCD and increase for γ-HBCD was caused by diet behavior.(7) To assess the fate of HBCD diastereomers and enantiomers in multimedia of aquatic system, we built up microcosms to simulate a pond system, which contained water phase, sediment, mudfish and mirror carp. The bioaccumulation in carp was compared in the presence and without the presence of mudfish. In the group with the mudfish, HBCD concentrations in the carp were much higher than those in the group without mudfish, so as for the uptake rate constant of k1. The suspended particular matter (SPM) and dissolved organic matter (DOM) promoted uptake of HBCDs, and the promotion of SPM was higher than DOM. The order of HBCDs in the uptake tissues in absorption phase was intestine> gill>skin and the order of the distribution tissues in steady state was adipose>brain> muscle. The bioisomerization from P- and y-HBCD to a-HBCD was found in mirror carps but not in mudfish. In mirror carps, BCFSS of a-HBCD was higher than β-and γ-HBCD, the BCFk showed the same order with BCFSS. a-and y-HBCD showed significant enantioselectivity for (+)-HBCD, β-HBCD showed obvious preference for (-)-HBCD, the enantioselectivity of HBCD diastereomers in mirror carps was not significantly correlated with uptake way. In mudfish, the accumulation order was a-> β-> γ-HBCD and only γ-HBCD exhibited significant enantioselectivity for (+)-HBCD.