Hexabromocyclododecanes (HBCDs) are brominated
aliphatic cyclic hydrocarbons used as flame retardants in
thermal insulation building materials, upholstery textiles,
and electronics. As a result of their widespread use and
their physical and chemical properties, HBCDs are
now ubiquitous contaminants in the environment and
humans. This review summarizes HBCD concentrations in
several environmental compartments and analyzes
these data in terms of point sources versus diffuse sources,
biomagnification potential, stereoisomer profiles, time
trends, and global distribution. Generally, higher concentra
tions were measured in samples (air, sediment, and fish)
collected near point sources (plants producing or processing
HBCDs), while lower concentrations were recorded in
samples from locations with no obvious sources of HBCDs.
High concentrations were measured in top predators,
such as marine mammals and birds of prey (up to 9600
and 19 200 ng/g lipid weight, respectively), suggesting a
biomagnification potential for HBCDs. Relatively low HBCD
concentrations were reported in the few human studies
conducted to date (median values varied between 0.35 and
1.1 ng/g lipid weight). HBCD levels in biota are increasing
slowly and seem to reflect the local market demand.
One important observation is the shift from the high percentage
of the γ-HBCD stereoisomer in the technical products to
a dominance of the α-HBCD stereoisomer in biological
samples. A combination of factors such as variations in
solubility, partitioning behavior, uptake, and, possibly, selective
metabolism of individual isomers may explain the observed
changes in stereoisomer patterns. Recommendations
for further work include research on how HBCDs are
transferred from products into the environment upon
production, use, and disposal. Time trends need to be
analyzed more in detail, including HBCD stereoisomers,
and more data on terrestrial organisms are needed, especially
for humans. Whenever possible, HBCDs should be
analyzed as individual stereoisomers in order to address
their fate and effects.
The environmental safety of decabromodiphenyl ether (BDE-209), a widely used flame retardant, has been the topic of controversial discussions during the past several years. Degradation of BDE-209 into lower brominated diphenyl ether congeners, exhibiting a higher bioaccumulation potential, has been a critical issue. Here, we report on the degradation of BDE-209 and the formation of octa- and nonabromodiphenyl ether congeners under anaerobic conditions. Sewage sludge collected from a mesophilic digester was used as the inoculum and incubated up to 238 days with and without a set of five primers. Following Soxhlet extraction and a liquid chromatography cleanup procedure, parent compounds and debromination products were analyzed by GC/HRMS. In experiments with primers, concentrations of BDE-209 decreased by 30% within 238 days. This corresponds to a pseudo-first-order degradation rate constant of 1 x 10(-3) d(-1). Without primers, the degradation rate constant was 50% lower. Formation of two nonabromodiphenyl ether and six octabromodiphenyl ether congeners proved that BDE-209 underwent reductive debromination in these experiments. Debromination occurred at the para and the meta positions, whereas debromination at the ortho position was not statistically significant. All three nonabromodiphenyl ether congeners (BDE-206, BDE-207, and BDE-208) were found to undergo reductive debromination as well. No significant change of the BDE-209 concentration and no formation of lower brominated congeners was observed in sterile control experiments. To our knowledge, this is the first report demonstrating microbially mediated reductive debromination of BDE-209 under anaerobic conditions.
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