Last Month's Most Accessed Feature: Halogenated carbazoles: Novel compounds in Lake Michigan sediments
- Published: Dec 5, 2014
- Categories: Base Peak
Environmental halo compounds
There are thousands of halogenated compounds in the environment and, contrary to popular belief, many of them are produced naturally. Volcanoes and forest fires are major contributors to the global load but aquatic organisms such as seaweeds, sponges, corals and bacteria, and terrestrial organisms like plants, fungi, lichens, bacteria, insects and animals are also significant sources.
However, industrial processes have produced large amounts of halogenated compounds over the last few decades and many of these are harmful to the environment and to humans. In addition, some of them are slow to degrade so that they accumulate in the environment and have a persistent effect.
Now, the burden of halogenated compounds has been augmented by a group of scientists in North America who have discovered a number of novel halogenated carbazoles in Lake Michigan. Writing in Environmental Science and Technology, senior authors Da Chen and An Li described how they were looking for halogenated flame retardants when they came across these new carbazoles which appear to have been in the lake sediments for some time.
Carbazoles in sediments
The research team were analysing two sediment cores that were drilled from the lake to a depth of 30 cm. Following a comprehensive clean up procedure, the extracts were analysed by low- and high-resolution GC/MS in electron ionisation and electron capture negative ionisation modes which were able to detect the halogenated compounds present and to measure their concentrations in the sediments.
In one of the cores, at a depth thought to be unaffected by the presence of halogenated flame retardants, 15 unknown compounds were detected and they were confirmed as halogenated carbazoles from their mass spectra. Eight of them contained bromine alone and the remaining seven contained more than one type of halogen in the combinations of bromine with chlorine, bromine with iodine, and bromine with chlorine and iodine.
The mixed halocarbazoles were most abundant at depths of 12-16 cm. Below that, they were less concentrated but more-or-less constant and above 10 cm the levels fell considerably. This pattern suggests that these compounds were generated at higher concentrations some time ago but their sources have since become less productive.
At this stage, those sources remain unidentified. Bromo- and chlorocarbazoles are known to be produced by some enzymes but iodine-containing natural products are far less common. With absolute concentrations as high as 95 ng/g and the changing levels throughout the sediment cores, it is likely that the new halocarbazoles are from an anthropogenic source. That’s about as much as can be said at this stage without further investigation into their origins.
The detection of new compounds in the environment in itself may not be a big deal. However, some known carbazoles are mutagenic and carcinogenic, so the news must be treated accordingly. “The occurrence, spatial distribution, temporal trends, sources and emissions, environmental transport and fate, and ecotoxicity and impact on human health of halogenated carbazoles warrant future research,” say the research team.
Environmental Science and Technology 2014, 12807-12815: "Polyhalogenated carbazoles in sediments of Lake Michigan: A new discovery"
Article by Steve Down
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.