Per- and polyfluoroalkyl compounds (PFAS) have been used as non-stick and waterproofing agents since the 1950s. However, since they could hardly be degraded using previous methods, they accumulate in drinking water and in the food chain. Even in small amounts, they can be harmful to health. A research team has now found a way to chemically destroy certain PFAS in a simple and environmentally friendly way.
Whether cookware with a non-stick coating, waterproof cosmetics or fire-fighting foam: the areas of application for PFAS are diverse. But there is growing evidence that chemicals in this group have adverse effects on human health. In addition, they cannot be degraded with the biological, chemical and physical processes that have been customary up to now. Plants that were designed to incinerate PFAS at high temperatures and with a great deal of energy have been found to instead release some of the compounds into the air unmodified. In the absence of suitable degradation processes, PFAS accumulate in the environment, get into drinking water and our food.
Simple solution to a longstanding problem
“PFAS have become a major societal problem,” says William Dichtel of Northwestern University in Evanston, Illinois. “Even a tiny amount of PFAS has negative health effects and they are not broken down. We can’t just sit out this problem. We wanted to use chemistry to tackle this problem and find a solution that the world can use.” The researchers succeeded: A team led by Dichtel’s colleague Brittany Trang actually found a way to produce different types of PFAS using uncomplicated chemical processes into harmless substances.
“Destruction of PFAS is a challenging task because the strong bonds between the carbon and fluorine atoms they contain not only give the PFAS their desirable properties, but also make these compounds resistant to degradation,” the researchers explain. But Trang and her team have found a weak point in the actually almost indestructible molecules: While the tail of the molecule consists of the extremely stable carbon-fluorine bonds, there is a charged group at the head of the molecule, which often contains charged oxygen atoms.
Achilles heel on the head
This is where the researchers came in: in the solvent dimethyl sulfoxide, they heated the PFAS together with sodium hydroxide, a common reagent that reacts as a strong base with the charged head of the PFAS molecule. This removed the head of the molecule, leaving a reactive tail. “That triggered all these reactions. The fluorine atoms were practically spat out of these compounds and formed fluoride, the safest form of fluorine,” explains Dichtel. “Although carbon-fluorine bonds are very strong, the charged head group is the Achilles’ heel.” Contrary to what was previously assumed, the tail of the molecule was not split one carbon after the other, but several carbon atoms at once.
In complex calculations and simulations, the researchers confirmed this experimental result and uncovered the associated reaction pathways. In this way, they could be sure that the resulting end products are actually harmless to people and the environment. In future studies, the team hopes to find degradation pathways for other types of PFAS. “Our current work is already addressing one of the largest classes of PFAS, including many that are among the most problematic,” says Dichtel. “There are other classes that don’t share the same Achilles heel, but each has its own weakness. If we can identify them, then we will know how to activate them in order to destroy them.”
“Trang et al. provide insight into how these seemingly robust compounds can be almost completely degraded under unexpectedly mild conditions,” write Shira Joudan of York University in Toronto and Rylan Lundgren of the University of Alberta in Edmonton in a commentary accompanying the study , also published in the journal Science. “It is hoped that the fundamental findings can be combined with an efficient collection of PFAS from contaminated environmental sites to find a possible solution to the problem of persistent chemicals.”
Source: Brittany Trang (Northwestern University, Evanston, Illinois) et al., Science, doi: 10.1126/science.abm8868