Scientists at Northwestern University claim to have developed a process that destroys PFAS compounds – leaving only benign end products behind.
Using low temperatures and inexpensive common reagents, the research team has developed a process that causes two major classes of PFAS compounds to collapse.
“PFAS has become a major societal problem,” said William Dichtel of Northwestern, who led the study. “Even a very small amount of PFAS has negative health effects and does not break down. We can’t wait for this issue to be resolved. We wanted to use chemistry to solve this problem and create a solution that the world could use. This is exciting because of the simplicity – but little known – of our solution.
Brittany Trang, who led the project as part of her recently completed doctoral dissertation in Dichtel’s lab, is the paper’s co-first author.
Although the health effects are not yet fully understood, exposure to PFAS has been associated with decreased fertility, developmental effects in children, increased risks of various types of cancer, reduced immunity to fight infections and increased cholesterol levels. With these adverse health effects in mind, the US Environmental Protection Agency (EPA) recently declared several PFAS as hazardous, even at trace levels.
Although some of the most dangerous PFAS, PFOS and PFOA compounds are already restricted in the EU, Germany, the Netherlands, Denmark, Sweden and Norway are currently working to broadly restrict the use of PFAS where they are not considered essential. This initiative is in line with the EU Chemicals Strategy for Sustainability, which outlines the Commission’s ambition to limit non-essential uses of PFAS chemicals.
The secret to PFAS’s indestructibility lies in its chemical bonds. PFAS contains many carbon-fluorine bonds, which are the strongest bonds in organic chemistry. As the most electronegative element on the periodic table, fluorine wants electrons – and badly. Carbon, on the other hand, is more willing to give up its electrons.
While studying the compounds, Dichtel’s team found a weakness. PFAS contains a long tail of inflexible carbon-fluorine bonds. But at one end of the molecule there is a charged group which often contains charged oxygen atoms. Dichtel’s team targeted this head group by heating PFAS in dimethyl sulfoxide – an unusual solvent for PFAS destruction – with sodium hydroxide, a common reagent. The process decapitated the leading group, leaving behind a reactive tail.
“It started all these reactions and started spitting out fluorine atoms from these compounds to form fluoride, which is the safest form of fluorine,” Dichtel said. “Although the carbon-fluorine bonds are super strong, this charged head group is the Achilles’ heel.”
In previous attempts to destroy PFAS, other researchers have used high temperatures – up to 400 degrees Celsius. Dichtel is delighted that the new technique relies on milder conditions and a simple, inexpensive reagent, which makes the solution potentially more practical for widespread use.
After discovering the degradation conditions for PFAS, Dichtel and Trang also discovered that fluorinated pollutants break down by different processes than those generally assumed. Using powerful computational methods, collaborators Ken Houk from UCLA and Yuli Li, a student from Tianjin University who virtually visited Houk’s group, simulated the degradation of PFAS.
Their calculations suggest that PFAS collapses through more complex processes than expected. Although it had previously been assumed that PFAS should break down one carbon at a time, the simulation showed that PFAS actually breaks down two or three carbons at a time – a finding that matched the experiments of Dichtel and Trang. . By understanding these pathways, researchers can confirm that only benign products remain. This new knowledge could also help guide further improvements to the method.