Uncovering a Byproduct of the Blues: Synthetic Indigo and Halogenated Carbazoles

Better analytical capabilities, or novel or advanced detection techniques have often been an important factor for the discovery of environmental contaminants. For example PCBs were initially called “DDT ghost peaks” in analytical samples, but were eventually identified by the Swedish chemist, Soren Jensen, who utilized advances in gas chromatography and mass spectrometry to unmask the mystery compounds in the late 1960’s (Spears, 2014). Widespread groundwater contamination from the use of the industrial solvent, TCE, made headlines as analytical methods in the 1970’s allowed for low-level detection. Likewise, a similar history can be found for the rocket propellant, and firework ingredient, perchlorate. Prior to 1997 perchlorate could only be reliably detected to approximately 100 parts per billion (ppb) (Motzer, 2001) but “breakthrough developments in analytical instrumentation and methods” dropped that detection limit to 4 ppb (Halden, 2014). Such contaminants often remain invisible to the public until analytical techniques sufficiently advance to shed light on their presence.

Recently, certain contaminants of emerging concern called halogenated carbazoles have been discovered in both domestic and international soils and sediments. Although sediment dating indicates their presence as a contaminant as early as the late 19th century, with one exception (Kuehl, 1984) halogenated carbazoles have not been identified in the scientific literature or popular press as a contaminant prior to 2004. It has been suggested that in general the laboratory procedures used in the analysis may have inhibited their detection, and in part may have prevented more widespread investigation into their presence, origins, and behavior.

With the increase in the identification of halogenated carbazoles in environmental samples appearing in the scientific literature, researchers have started to investigate potential sources, particularly because the contaminants exhibit both persistence and dioxin-like toxicity potential. Thus far, both natural and anthropogenic sources have been proposed, but these hypotheses are still being explored.

With expertise in environmental forensics – particularly in reconstructing historical industrial chemical processes – Dr. Robert Parette of Matson & Associates, hypothesized that halogenated indigo dyes, produced beginning just prior to World War I and peaking prior to mid-century, were the likely sources of certain halogenated carbazoles in sediments in North American samples. With the assistance of the following coauthors – Dr. Robert McCrindle, Katherine S. McMahon (yes, that’s me), Dr. Miren Pena-Abaurrea, Dr. Eric Reiner, Brock Chittim, Nicole Riddell, Dr. Gundula Voss, Dr. Frank Dorman, and M&A’s President, Wendy Pearson – the hypothesis was further explored and developed, and published online this January in the journal Chemosphere (see link for abstract and article), entitled “Halogenated indigo dyes: A likely source of 1,3,6,8-tetrabromocarbazole and some other halogenated carbazoles in the environment.”

Utilizing a review of patents and historical scientific and industrial literature, the article identifies numerous pathways and mechanisms in the manufacture of synthetic indigos that may result in the inadvertent production of certain halogenated carbazoles. The article also reviews the history of U.S. production in order to compare patterns in dated sediments, suggests pathways to the environment, and explores potential sources of other halogenated carbazoles not explained by synthetic indigo. Future research directions include greater historical sample analysis and deeper investigation into potential pathways to the environment. Hopefully M&A will be able to share more of this exciting work soon!

Thanks to advances in analysis and detection, today, researchers and scientists are better equipped to identify contaminants of emerging concern, and trace their origins in order to understand fate and transport, and ultimately better protect human health and the environment. PCBs, TCE, Perchlorate, and many other well-known pollutants were once “contaminants of emerging concern,” and today there is a vast amount of literature dedicated specifically to understanding their historical use, behavior in the environment, and toxicity, as well as methods of remediating environmental media. As scientific inquiry furthers our knowledge of halogenated carbazoles, the better informed we will be to evaluate potential concerns as well as the need for further action.

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[1] Spears, E. 2014. Baptized in PCBs: Race, Pollution, and Justice in an All-American Town. University of North Carolina Press, Chapel Hill. 158-9.
[2] Motzer, W.E. 2001. Perchlorate: Problems, Detection, and Solutions. Environ Forensics. 2, 301–311.
[3] Halden RU. 2015. Epistemology of contaminants of emerging concern and literature meta-analysis. J Hazard Mater. 282, 2-9.
[4] Kuehl, D.W., Durham, E., Butterworth, B.C., Linn, D., 1984. Tetrachloro-9Hcarbazole, a previously unrecognized contaminant in sediments of the Buffalo River. J. Great Lakes Res. 10, 210–214.