If you’re disheartened by recent reports that the Earth’s water sources have been infested with dangerous man-made chemicals known as PFAS, which can persist for thousands of years and make even rainwater unsafe to drink, there’s some good news. Chemists at UCLA and Northwestern University have devised a simple method for decomposing nearly a dozen different types of these nearly indestructible “forever chemicals” at low temperatures with no harmful byproducts.
The researchers show in a paper published in the journal Science that in water heated to just 176 to 248 degrees Fahrenheit, common, inexpensive solvents and reagents severed some of the strongest known PFAS molecular bonds and initiated a chemical reaction that “gradually nibbled away at the molecule” until it was gone, according to UCLA distinguished research professor and co-corresponding author Kendall Houk.
Houk added that because the technology is simple, the temperatures are low, and there are no harmful byproducts, there is no limit to how much water can be processed at once. Eventually, the technology could make it easier for water treatment plants to remove PFAS from drinking water.
Per- and polyfluoroalkyl substances (PFAS) are a class of approximately 12,000 synthetic chemicals that have been used in nonstick cookware, waterproof makeup, shampoos, electronics, food packaging, and a variety of other products since the 1940s. Nothing in nature can break the bond between carbon and fluorine atoms.
This proved to be a very complex set of calculations that tested the most modern quantum mechanical methods and fastest computers available to us. Quantum mechanics is the mathematical method that simulates all of chemistry, but we have only been able to take on large mechanistic problems like this in the last decade, evaluating all the possibilities and determining which one can happen at the observed rate.
Kendall Houk
When these chemicals leach into the environment as a result of manufacturing or everyday product use, they enter the Earth’s water cycle. PFAS have contaminated nearly every drop of water on the planet over the last 70 years, and their strong carbon-fluorine bond allows them to pass through most water treatment systems completely unharmed. They can accumulate in the tissues of people and animals over time and cause harm in ways that scientists are just beginning to understand. Certain cancers and thyroid diseases, for example, are associated with PFAS.
For these reasons, finding ways to remove PFAS from water has become particularly urgent. Scientists are experimenting with many remediation technologies, but most of them require extremely high temperatures, special chemicals or ultraviolet light and sometimes produce byproducts that are also harmful and require additional steps to remove.
Leading PFAS to the guillotine
While PFAS molecules have a long “tail” of stubborn carbon-fluorine bonds, their “head” group often contains charged oxygen atoms that react strongly with other molecules, according to Northwestern chemistry professor William Dichtel and doctoral student Brittany Trang. Dichtel’s team created a chemical guillotine by heating the PFAS in water with dimethyl sulfoxide (DMSO) and sodium hydroxide (lye), which lopped off the head and left an exposed, reactive tail.
“That set off all of these reactions, and it began spitting out fluorine atoms from these compounds to form fluoride, which is the safest form of fluorine,” Dichtel explained. “Despite the fact that carbon-fluorine bonds are extremely strong, the charged head group is the Achilles’ heel.”
But the experiments revealed another surprise: The molecules didn’t seem to be falling apart the way conventional wisdom said they should.
To solve this mystery, Dichtel and Trang shared their data with collaborators Houk and Tianjin University student Yuli Li, who was working in Houk’s group remotely from China during the pandemic. The researchers had expected the PFAS molecules would disintegrate one carbon atom at a time, but Li and Houk ran computer simulations that showed two or three carbon molecules peeled off the molecules simultaneously, just as Dichtel and Tang had observed experimentally.
The simulations also revealed that the only byproducts should be fluoride, which is commonly added to drinking water to prevent tooth decay, carbon dioxide, and formic acid, neither of which is harmful. Further experiments by Dichtel and Trang confirmed these predicted byproducts.
“This proved to be a very complex set of calculations that tested the most modern quantum mechanical methods and fastest computers available to us,” said Houk. “Quantum mechanics is the mathematical method that simulates all of chemistry, but we have only been able to take on large mechanistic problems like this in the last decade, evaluating all the possibilities and determining which one can happen at the observed rate.”
Li, Houk said, has mastered these computational methods, and he worked long distance with Trang to solve the fundamental but practically significant problem.
The current work degraded 10 types of perfluoroalkyl carboxylic acids (PFCAs) and perfluoroalkyl ether carboxylic acids (PFECAs), including perfluorooctanoic acid (PFOA). The researchers believe their method will work for most PFAS that contain carboxylic acids and hope it will help identify weak spots in other classes of PFAS. They hope these encouraging results will lead to further research that tests methods for eradicating the thousands of other types of PFAS.
