NNNS Chemistry blog
Prevous: Novel alkene aminocarboxylation
PFAS not so forever
20 August 2022 - Cleanup on aisle five
In 2019 this blog reported on the PFAS lockdown, that has brought the country of the The Netherlands to a halt because all building locations for housing, roads and whatnot are seemingly contaminated by a group of perfluoroalkyls. These chemical compounds are collectively known as the forever chemicals because they are very stable environmentally and there are no known avenues for biological degradation. In the meanwhile they do degrade human health as they tend to bio-accumulate. A health crisis then back in 2019.
Building locations are still unavailable to developers today but PFAS compounds have been taking a back seat in favor of the new national menace: the reactive nitrogen compounds. This is a health crisis and an environmental crisis that has resulted in a nitrogen lockdown. That does not mean the PFAS problem went away and it still needs to be addressed. To say that chemists will eventually clean up their own mess would be a controversial statement but I will it anyway: chemists will eventually clean up their own mess and even PFAS will be dealt with. That brings me to the topic of this blog: a group of researchers from Northwestern University and the University of California have reported a new way to degrade PFAS compounds (Trang et al. DOI).
More specifically the report concerns the perfluoroether carboxylic acids subgroup and a degradation medium of water, dimethylsulfoxide (DMSO) and sodium hydroxide at mild conditions (80-120 degrees Celsius). The base is key: in absence of the strong base the only reaction product for per example perfluorooctanoic acid (PFOA) is the decarboxylated perfluoro-1H-heptane. In presence of the base PFOA degrades to nothing but sodium trifluoroacetate and hydrocarbon formate, carbonate, oxlate and glycolate sodium salts. The trifluoroacetate will eventually degrade to sodium fluoride. For PFOA fluoride recovery is 90% and with fluorine badboy GenX (no, not generation X) the number is 41%. Another notorious perfluoro chemical PFOS is a sulfonic acid and not a carboxylic acid and remains as forever.
As for reaction mechanism the researchers propose that in step 1 carbon dioxide is removed to form a perfluoro carbanion. It is emphasized that this terminal anion does not gobble up the chain carbon atom by carbon atom. Instead one fluoride ion is removed in an elimination reaction to a terminal alkene and then another fluor atom is substituted by an hydroxide ion setting the stage for an fluorinated acetate break-away. The hydroxylation step seems outlandish but a consultation of the DFT oracle revealed this step is actually highly exothermic. Take that, conventional wisdom! The mechanism was corroborated by mass spectroscopy which showed a dominant CF3CF2 fragment. Small carbon frames also did not work and a gamma carbon atom is a prerequisite. NMR analysis of intermediate products only showed the presence of the trifluoroacetate compound.
A practical method for global PFAS cleanup? This is of course just one avenue of research and for example co-author of this study William Dichtel is also a co-founder of Cyclopure inc., a company that is marketing a PFAS adsorbent based on corn. The research funding section lists several donors but conspicuously absent are two big PFAS producing polluting companies, 3M (PFOS) and DuPont / Chemours (GenX). Apparently not all chemists do clean up their own mess but rather let others clean it up for them. These two companies must think they have a better way to spent their money: lawyers.