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Environmental fate & pathways

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Biodegradation in water: screening

HBCDD is not readily biodegradable in a 28 day aerobic sewage sludge study (Schafer and Haberlein, 1996). However, inherent biodegradation was observed in aerobic and anaerobic digester sludge (Davis et al., 2004; 2006; 2006 Environ. Sci. Technol.). Within 28 days, HBCDD had decreased to approximately 10% of the starting value while a 50% loss occurred by day 15. Abiotic degradation processes appeared to play an important role. All three diastereomers were degraded. Gerecke et al. (2006) reported a much shorter half-life (0.66 days) in aerobic sewage sludge. The difference in half-lives, 15 days versus 0.66 days, was attributed to the 90-fold higher HBCDD concentration used by Davis et al. At higher concentrations for poorly soluble substances such as HBCDD, biodegradation rates are more dependent upon mass transfer limitations than on true biodegradation kinetics. Environmentally relevant concentrations should be used to generate meaningful kinetic data.

The Davis et al. (2006) work reported three degradants: tetrabromocyclododecane, dibromocyclododecadine and cyclododecatriene. The authors proposed that HBCDD was sequentially debrominated via dihaloelimination where at each step there is the loss of two bromines from vicinal carbons with the subsequent formation of a double bond between the adjacent carbon atoms. The conclusion was that microorganisms naturally occurring in anaerobic digester sludge mediate complete debromination of HBCDD.

Biodegradation in water and sediment: simulation tests

In studies according to OECD Guidelines 307 and 308, HBCDD was tested at concentrations ranging from approximately 10 to 80 ng/g dry weight (Davis et al., 2005 Water Research; 2003). Using LC-MS, HBCDD loss was observed in all with faster rates under anaerobic conditions. Biologically mediated transformation accelerated the rate loss of HBCDD compared to biologically inhibited (i.e. autoclaved) soils and sediments. Biotransformation half-lives were 63 and 6.9 days in the aerobic and anaerobic soils, respectively, and 11 to 32 and 1.1 to 1.5 days in aerobic and anaerobic sediments. Brominated degradation products were not detected in any of the soils or sediments during the study.

In a further investigation with 14C-HBCDD, the formation and identification of degradants were assessed in activated digester sludge, river sediment and surface soil under aerobic and anaerobic conditions at 3-5 mg/kg to generate sufficient products for identification (Davis et al., 2004; 2006 Environ. Sci. Technol.). HPLC with radiochemical detection, HPLC-APPI-MS and GC-EI-MS were utilised. Substantial biological transformation was observed in the anaerobic digester sludge and in aerobic and anaerobic freshwater sediment. No degradation was noted in aerobic soil. In sludge and sediment, degradation of each of the three diastereomers occurred with little difference in rates. Formation of three products was observed in the sludge and sediments: tetrabormocyclododecane, dibromocyclododecandiene and cyclodecatriene.

Biodegradation in soil

In studies conducted according to OECD Guidelines 307 and 308, HBCDD was tested at concentrations ranging from approximately 10 to 80 ng/g dry weight (Davis et al., 2005 Water Research; 2003). Using LC-MS, HBCDD loss was observed in all with faster rates under anaerobic conditions. Biologically mediated transformation accelerated the rate loss of HBCDD compared to biologically inhibited (i.e. autoclaved) soils and sediments. Biotransformation half-lives were 63 and 6.9 days in the aerobic and anaerobic soils, respectively, and 11 to 32 and 1.1 to 1.5 days in aerobic and anaerobic sediments. Brominated degradation products were not detected in any of the soils or sediments during the study.

In a further investigation with 14C-HBCDD, the formation and identification of degradants were assessed in activated digester sludge, river sediment and surface soil under aerobic and anaerobic conditions at 3-5 mg/kg to generate sufficient products for identification (Davis et al., 2004; 2006 Environ. Sci. Technol.). HPLC with radiochemical detection, HPLC-APPI-MS and GC-EI-MS were utilised. Substantial biological transformation was observed in the anaerobic digester sludge and in aerobic and anaerobic freshwater sediment. No degradation was noted in aerobic soil. In sludge and sediment, degradation of each of the three diastereomers occurred with little difference in rates. Formation of three products was observed in the sludge and sediments: tetrabormocyclododecane, dibromocyclododecandiene and cyclodecatriene.

The primary degradation and mineralisation of the three major HBCDD diastereomers (alpha, beta and gamma) were further characterised in two aerobic soils (Davis et al., 2010). Soil microcosms (25-85 ng/g) were prepared and incubated for up to 221 days, extracted and analysed at intervals using HPLC-ESI with MS operating in the MRM mode. The concentrations of the three diastereomers decreased in both soil types in both viable and biologically inhibited microcosms. Each diastereomer behaved differently in the two soils and degradation was a function of the specific diastereomer and soil type. The half-lives for alpha, beta and gamma in the sandy clay loam viable soil microcosms were 441, 64 and 126 days respectively while in the viable loamy soil were 162, 147 and 201 days respectively. Loss of all three was also observed in the biologically inhibited microcosms, however, greater biological loss was noted for the beta and gamma diastereomers. Loss in the inhibited microcosms could be attributed to biodegradation resulting from slow recovery and growth of the microbial populations. Mineralisation to CO2 was observed in the aerobic soils (27%-16% after 218 days). 5-10% of the 14C-activity could not be extracted from soils was and was approximately 6-fold higher in the viable soils demonstrating the importance of the biological processes in transformation of HBCDD in aerobic soils.

Thus, biodegradation of HBCDD in anaerobic soils is rapid with a half-life of approximately 7 days. Slower biodegradation was observed in aerobic soils with an overall half-life of 63 days of HBCDD and half-lives of 162 or 441, 64 or 147 and 126 or 210 days for the alpha, beta and gamma diastereomers, respectively, depending on soil type.

In the EU Risk Assessment Report and the SVHC support document (ECHA, 2008) temperature corrected half-lives to 12

ºC (according to the guidance documents TGD and ECHA guidance). This yielded half-lives for viable aerobic sediments of 21 and 61 days, for anaerobic sediments of 2.8 and 2.1 days and for viable aerobic soils of 119 days and anaerobic soils of 13 days. The temperature correction is, however debated in the scientific community and is commonly not regarded as scientifically justified.