Using low temperatures and inexpensive, common reagents, a team of chemists from Northwestern University (Evanston, Illinois, USA) have developed a process that causes two major classes of per- and polyfluoroalkyl substances (PFAS) compounds to fall apart, leaving behind only benign end products. Their research was recently published in the journalScience.
Commonly used since the 1940s, PFAS are a group of manufactured chemicals that are commonly used as nonstick and waterproofing agents in a variety of consumer products. They are also known as “forever chemicals” due to their ability to withstand bacteria, fire, and other elements. However, these harmful chemicals have been linked to many dangerous health effects in humans, livestock, and the environment.
“PFAS has become a major societal problem,” says William Dichtel, the Robert L. Letsinger Professor of Chemistry in Northwestern’s Weinberg College of Arts and Sciences. “Even just a tiny, tiny amount of PFAS causes negative health effects, and it does not break down. We can’t just wait out this problem. We wanted to use chemistry to address this problem and create a solution that the world can use. It’s exciting because of how simple—yet unrecognized—our solution is.”
Over the years, PFAS has made its way out of consumer goods and into drinking water and even the blood of 97 percent of the U.S. population. Although the health effects are not fully understood, PFAS exposure is strongly associated with decreased fertility, developmental effects in children, increased risks of various types of cancer, and other adverse health effects. With these effects in mind, the U.S. Environmental Protection Agency (EPA) recently declared several PFAS as unsafe, even at trace levels.
“Recently, the EPA revised its recommendations for PFOA [perfluorooctanoic acid] essentially down to zero,” said Dichtel, who led the Northwestern study. “This puts several PFAS in the same category as lead.”
Although efforts to filter PFAS from water have been successful, few options have emerged for disposing of it once it is removed. These options include PFAS destruction at high temperatures and pressures, or other methods that require large energy outputs. Rather than solving the problem, however, many of these solutions result in having “just kicked the can down the road,” says Dichtel.
The reason that PFAS are “forever chemicals” is that they contain many carbon-fluorine bonds, which are the strongest bonds in organic chemistry. But while studying the compounds, Dichtel’s team found a weakness. PFAS contains a long tail of unyielding carbon-fluorine bonds, with a charged group of oxygen atoms at one end of the molecule. The team targeted this head group by heating the PFAS in dimethyl sulfoxide—an unusual solvent for PFAS destruction—with sodium hydroxide, a common reagent. The process decapitated the head group, leaving behind a reactive trail.
“That triggered all of these reactions, and it all started with spitting out fluorine atoms from these compounds to form fluoride, which is the safest form of fluorine,” Dichtel says. ‘Although carbon-fluorine bonds are super strong, that charged head group is the Achilles’ heel.”
In previous attempts to destroy PFAS, other researchers used high temperatures up to 400 °C (752 °F). Dichtel is excited that the new technique relies on milder conditions and a simple, inexpensive reagent, making the solution more practical for widespread use.
Along with Northwestern doctoral student Brittany Trang, Dichtel discovered that fluorinated pollutants fall apart by different processes than commonly assumed. Using computer models to simulate PFAS degradation, they found that PFAS actually falls part two or three carbons at a time, a discovery that matched Dichtel and Trang’s experiments. Through this discovery, researchers can confirm that only benign products remained, which can help guide further improvements to the method.
Going forward, Dichtel’s team will test the effectiveness of its new strategy on other types of PFAS, including perfluoroalkyl carboxylic acids (PFCAs) and perfluoroalkyl ether carboxylic acids (PFECAs). With the U.S. EPA having identified more than 12,000 PFAS compounds, the research team will have a daunting workload on its hands—and yet Dichtel remains hopeful.
“Our work addressed one of the largest classes of PFAS, including many we are most concerned about,” he says. “There are other classes that don’t have the same Achilles’ heel, but each one will have its own weakness. If we can identify it, then we know how to activate it to destroy it.”
Source: Northwestern Now, https://news.northwestern.edu.