PFAS, a group of manufactured chemicals commonly used since the 1940s, are called “eternal chemicals” for a reason. Bacteria cannot eat them; fire cannot incinerate them; and water cannot dilute them. And, if these toxic chemicals are buried, they seep into the surrounding soil, becoming a lingering problem for generations to come.
Now, chemists at Northwestern University have done what seemed impossible. Using low temperatures and inexpensive common reagents, the research team has developed a process that causes two major classes of PFAS compounds to collapse, leaving only benign end products behind.
This simple technique could potentially be a powerful solution to finally eliminating these harmful chemicals, which are linked to numerous dangerous health effects in humans, livestock and the environment.
The research will be published Aug. 19 in the journal Science.
“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.
Dichtel is the Robert L. Letsinger Professor of Chemistry at Northwestern’s Weinberg College of Arts and Sciences. 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.
“The same category as lead”
Short for per- and polyfluoroalkyl substances, PFAS have been used for 70 years as release and waterproofing agents. They are commonly found in nonstick cookware, waterproof cosmetics, fire-fighting foams, water-repellent fabrics, and grease- and oil-resistant products.
“PFAS has become a major societal problem. We wanted to use chemistry to solve this problem and create a solution that the world could use. —William Dichtel, chemist
Over the years, however, PFAS has made its way out of consumer goods and into our drinking water and even into the blood of 97% of the American population. Although the health effects are not yet fully understood, exposure to PFAS is strongly 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.
“Recently, the EPA revised its guidelines for PFOA to essentially zero,” Dichtel said. “That puts several PFAS in the same category as lead.”
Although community efforts to filter PFAS from water have been successful, there are few solutions to remove PFAS once it is removed. The few options currently emerging have typically involved destruction of PFAS at high temperatures and pressures or other methods requiring large energy inputs.
“In New York State, a plant claiming to incinerate PFAS was found to be releasing some of these compounds into the air,” Dichtel said. “The compounds were emitted from chimneys and into the local community. Another failed strategy was to bury the compounds in landfills. When you do that, you’re just guaranteeing you’ll have a problem 30 years from now because it’s going to slowly dissipate. You have not solved the problem. You just kicked the box on the road.
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.
“When you have that kind of difference between two atoms — and they’re roughly the same size, which is carbon and fluorine — that’s the recipe for a really strong bond,” Dichtel explained.
Spotting the Achilles heel of PFAS
But, 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 spewing 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 conditions for PFAS degradation, 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.
“It turned out to be a very complex set of calculations that challenged the most modern quantum mechanical methods and the fastest computers available to us,” said Houk, a distinguished research professor in organic chemistry. “Quantum mechanics is the mathematical method that simulates all of chemistry, but it’s only in the last decade that we’ve been able to tackle big mechanistic problems like this, evaluating all the possibilities and determining which can occur at the observed rate. Yuli mastered these computational methods and worked with Brittany Long Distance to solve this fundamental but practically significant problem.
Ten down, 11,990 left
Next, the Dichtel team will test the effectiveness of their new strategy on other types of PFAS. In the current study, they successfully degraded 10 perfluoroalkyl carboxylic acids (PFCAs) and perfluoroalkyl ether carboxylic acids (PFECAs), including perfluorooctanoic acid (PFOA) and one of its common substitutes, known as GenX – two of the most important PFAS compounds. The US EPA, however, has identified over 12,000 PFAS compounds.
Although it may seem daunting, Dichtel remains hopeful.
“Our work has focused on one of the largest classes of PFAS, many of which concern us most,” he said. “There are other classes that don’t have the same Achilles heel, but each will have its own weakness. If we can identify it, then we know how to activate it to destroy it.
Dichtel is a member of the Institute for Sustainability and Energy of Northwestern’s Plastics, Ecosystems and Public Health Program; the Water Research Center and the International Institute of Nanotechnology
The study, “Low Temperature Mineralization of Perfluorocarboxylic Acids,” was supported by the National Science Foundation.