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Environmental fate & pathways

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In the environment, HBCDD is predicted to partition to soil and sediment (approximately 98%) where it will bind extensively to organic carbon (estimated Koc soil 1.25 x 105) and to be essentially immobile in soil. Aerobic and anaerobic biodegradation in soil and sediment has been demonstrated in microcosms. At low concentrations (ng/g), degradation in anaerobic soil and sediment was rapid with half-lives of 6.9 and 1 day(s) respectively. The aerobic sediment half-life was approximately 1 day. At higher concentrations (mg/kg), no degradation was noted in aerobic soil. The same three degradants as determined in sludge were identified for soil and sediment. Additionally, mineralisation to CO2 was observed. At low concentrations (ng/g), the three HBCDD diastereomers were all found to degrade in aerobic soil with half-lives dependent on the diastereomer and soil type. 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 for HBCDD and half-lives of 162 or 441, 64 or 147 and 126 or 201 days for the alpha, beta and gamma diastereomers.

HBCDD is not expected to volatilise from water based on its river and lake volatilisation half-lives and air-water partition coefficient. HBCDD is expected to partition from water to organic matter (biomass to water partition coefficient 1 x 107). Sewage treatment plants are predicted to remove HBCDD from the influent to a high degree (94% removal) via partitioning to sludge. Aerobic biodegradation in the treatment plant is not expected and HBCDD was not found to be readily biodegradable. Degradation in the anaerobic digester of treatment plants is expected and has been observed in testing.

HBCDD was inherently biodegradable by digester sludge via a sequestration dihalo-elimination with the formation of three degradants (tetrabromocyclododecane, dibromocyclododecadiene and cyclododecatriene). The half-life in sludge was related to concentration: 0.66 days at very low concentration and 15 days at high concentration.

HBCDD's steady-state whole body fish BCF was 8794. Steady-state was reached after a 35 day exposure via water. Applying a kinetic method of calculation, a BCF of 21940 (whole body dry weight) after 35 days exposure was calculated.