A new method decapitates PFAS, causing it to break down into benign end products.
The term “eternal chemicals” refers to a group of manufactured chemicals that have been widely used since the 1940s. They cannot be destroyed by fire, eaten by bacteria, or diluted by water. Moreover, if these harmful chemicals are buried, they seep into the earth around them and persist for future generations.
Chemists at Northwestern University have now accomplished what seemed impossible. The study team created a technique that causes two key classes of PFAS compounds to break down, leaving only benign end products. It requires low temperatures and inexpensive common reagents.
The simple method can prove to be an effective way to finally get rid of these harmful chemicals, which have been linked to several harmful impacts on human health, livestock and the environment.
The results were published 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.
But over time, PFAS found its way out of consumer goods and into our water supply and even into the blood of 97% of Americans. Exposure to PFAS is strongly linked to decreased fertility, impacts on child development, higher risks for many forms of cancer, reduced immunity to infections, and high cholesterol levels , although the health implications are not yet fully understood. The United States Environmental Protection Agency (EPA) has deemed many PFAS to be unsafe – even at low levels – in light of these adverse health effects.
“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 this, you are simply guaranteeing that you will have a problem 30 years from now because it will 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 about 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 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.
“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 present study, they successfully degraded 10 perfluoroalkyl carboxylic acids (PFCAs) and perfluoroalkyl ether carboxylic acids (PFECAs), including perfluorooctanoics 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.
Reference: “Low-temperature mineralization of perfluorocarboxylic acids” by Brittany Trang, Yuli Li, Xiao-Song Xue, Mohamed Ateia, KN Houk and William R. Dichtel, August 18, 2022, Science.
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 was funded by the National Science Foundation.