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Coal tar paving sealant and PAHs in the environment – an example of limited science breeding poor science March 3, 2011

Posted by fetzthechemist in Uncategorized.
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There has been quite a firestorm over some recent work by the US Geological Survey that claimed that the sealant used for asphalt paving was the major source of polycyclic aromatic hydrocarbons in the runoff water in urban areas. I will not go over that work much, except as background for a very recent posting that claims otherwise:

www.starseal.com/…/PCTC_NPE%20Symposium_OReilly.pdf

The USGS work, like most analytical work done for environmental PAHs is the usual vanilla list of the 16 US EPA PAHs. This list was developed about 40 years ago and is a GC-MS method because that EPA group was a GC-MS one (in Cincinnati, if I remember_. These 16 PAHs have been beaten to death, only because the US EPA still sticks solely to this antiquated list. It bears little support from decades of later PAH work on many counts. These PAHs are few – the European Union has a listing of more than three-times the PAHs and also has some sulfur and nitrogen analogues. These 16 contain little variety, even for the 2 to 6 ring PAHs it supposedly cpvers. There are few isomer sets and only a few with highly condensed structures and few with five-member rings (nonalterant PAHs). Its use is like having someone analyze the writing styles of different authors by looking at the use of only fifty or a hundred words.

Forensic chemists have tried to overcome this shortfall by looking at these 6 in more detail, using a wide variety of pattern recognition statistics. This is instead of looking at the much wider variety of PAHs that would easily show specific compounds being produced or absent from various materials. Coal tar pitch is the most obvious and glaring material that is misanalyzed – GC-MS for the 16 and statistics. Coal tar pitch is the one material that has every coceivable PAH in it. Max Zander, who worked for decades on coal tar pitch composition (he was employed by a German coal company), has repeated stated that this is the case and his work showed hundreds of PAhs in coal tar pitch. My own work showed that exotic PAHs like benzo[a]perylene are in coal tar pitch, but only in ultratrace amounts in most combustion material.

So this link above is a report, by a laboratory funded by a paving industry group, that says the USGS work really is not definitive. The results do not unambiguously point to coal tar pitch as the source. Well, of course, because GC-MS for the 16 is a very poor way to try to do that. HPLC with UV or fluorescence detection looking for all the PAHs or even GC-MS that looks at a much wider variety of PAHs will give a better answer.

But my opinion is that coal tar pitch, being used as this sealant and being composed of PAHs is large amounts (typically 0.1 to 1 percent for each specific PAH) ought to give off lots of soluble PAHs….and yes, I’ve done myriads of PAH analyses and PAH aqueous solubility studies. That it does not would be the big surprise.

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