Subset of “Forever Chemicals” Destroyed by Efficient New Method
A team of experimental and computational chemists has developed a simple, new method to degrade a subset of perfluoroalkyl and polyfluoroalkyl substances (PFAS) – toxic and durable pollutants colloquially referred to as “forever chemicals” – into benign end products. The results are published in the journal Science.
Potential hazards of PFAS
PFAS are a large family of over 9,000 artificial chemicals. First commercially produced in the 1940s, PFAS have an incredibly stable chemical structure and don’t react easily, making them suitable for a wide array of applications including producing non-stick coatings for cookware, waterproofing clothing and as key components in fire-fighting foams. Each PFAS molecule has a hydrophilic “head” group and hydrophobic “tail” group, with these compounds owing their stability to the incredibly strong molecular “tail” of carbon–fluorine bonds, which renders them mostly inert.
Nevertheless, their durable properties also present problems for their disposal. PFAS barely degrade in the natural environment, leading to their accumulation in the environment and living things including livestock. Indeed, one US study detected PFAS in 97% of human blood samples and another detected PFAS in 60% of US public-supply drinking water wells. This may have important implications for health, as PFAS has been associated with various ill effects such as an increased risk of developing certain cancers, reduced fertility, low birth weight and elevated cholesterol levels. The US Environmental Protection Agency (EPA) has even updated its health advisories to declare some PFAS as unsafe even in trace amounts, placing them at a similar level to lead.
Presently, PFAS can be extracted during wastewater treatment using techniques such as adsorption using activated carbon or ion-exchange resins. However, there is a lack of safe and effective methods to destroy PFAS once removed. Previous efforts to destroy or dispose of PFAS include incineration, which releases harmful pollutants into the air, and burial in landfill, which only delays the problem and leaches PFAS into the soil. More recent methods that endeavor to degrade PFAS are exceptionally energy intensive and require operation at high temperatures and pressures to be effective. Now, researchers have developed a new low-temperature method using inexpensive, common reagents to safely degrade a subtype of carboxylated PFAS – known as perfluorocarboxylic acids (PFCAs) – which they hope to one day extend to other PFAS compounds.
Identifying the chemical Achilles’ heel
When studying PFAS for weaknesses that could be exploited for degradation, the researchers identified a potential vulnerability in PFCAs – namely their polar (i.e., unevenly electrically charged), carboxylated “head” groups. These contain perfluoroalkyl ions, making them more reactive – and therefore more easily degraded – than the inert fluorocarbon “tail” group. Having potentially found the Achilles’ heel for these compounds, the researchers found that once they removed these carboxyl groups (also known as decarboxylation), this led to the head of the molecule being “chopped off”, and allowing it to be degraded.
Dipolar aprotic solvents – such as dimethyl sulfoxide (DMSO), commonly used in these kinds of reactions – were used to decarboxylate PFCAs at relatively low temperatures (80 to 120 °C) in the presence of sodium hydroxide (NaOH) under normal pressure. This effectively “decapitates” the head group, leaving behind the tail group, which then rapidly degrades in the presence of NaOH. For example, one such PFCA, perfluorooctanoic acid (PFOA), was efficiently degraded with >90% removal of fluorine at 120 °C. This produced a mixture of products: fluoride, trifluoroacetate ions and carbon-containing by-products.
19F NMR spectroscopy – a technique used to identify fluorine-containing compounds – was carried out on samples collected from the reaction over a 24-hour period to investigate how the fluorine-containing compounds degraded. This showed that they were no longer able to detect PFOA after 14 hours of the reaction. PFOA was therefore rapidly destroyed, and any identifiable by-products continued to degrade over time into fluoride, the safest form of fluorine. Further analysis showed that approximately 90% of the fluorine atoms that originated from PFOA were recovered as fluoride ions after 24 hours of reaction time. This high level of fluorine recovery at the end of the reaction shows that PFAS was successfully degraded, without producing smaller chain PFAS.
Additionally, computer modeling to simulate these chemical reactions using density functional theory (DFT) calculations suggested that these compounds do not degrade as previously thought and that the mechanism is in fact much more complex. Different PFAS compounds vary in the length of their tails, and it was previously thought that PFCAs degraded through iterative removal of single carbons along these tails. However, the computational chemistry suggested these chains degrade by two to three carbons at a time, highlighting a more complex mechanism than initially thought. These models also confirmed experimental data that showed only benign by-products remain at the end of the reaction.
Together, this study demonstrated in total the degradation of 10 PFCAs and perfluoroalkyl ether carboxylic acids (PFECAs) – another major class of PFAS contaminants. The findings suggest that this method may be generalizable and could be applied to other types of PFAS – such as perfluorooctane sulfonate (PFOS) – once approaches to activate their “head” groups are discovered. Despite there being approximately 11,990 other PFAS compounds identified by the US EPA that were not investigated in this analysis, Dr. William Dichtel, professor of chemistry at Northwestern University and senior author of the study, is optimistic about the application of similar techniques to other PFAS compounds. “Our work addressed one of the largest classes of PFAS, including many we are most concerned about,” he explained in a press release, “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.”
Clearing up PFAS contamination
Discussing the potential applications of this method in an interview with Technology Networks, the lead author of the study, Dr. Brittany Trang, said, “Our method slots into the water purification process after PFAS has already been removed from the water. It is much more energy-efficient to treat PFAS after a filter or other method removed it from water. For example, a reverse-osmosis system could collect PFAS in its concentrate, or an ion-exchange resin or adsorbent could be regenerated to create a concentrated PFAS waste solution. Our method could then treat that concentrated PFAS waste; however, integrating this into any industrial system would take much more optimization than we currently have done.”
She continues, “What I think is much more immediately promising – and what I personally hope this method will be used for – is getting people in the PFAS degradation field to think about designing destruction methods differently. Now that we know more about potential PFAS degradation mechanisms and know we can access degradation pathways that occur at much lower temperatures than previously thought possible, I hope that other researchers will either be able to iterate upon our research with new ideas for similar systems or look at their own research with the idea that some of the things we found in our study might be appliable to their methods.”
Dr. Brittany Trang was speaking to Sarah Whelan, Science Writer for Technology Networks.
Reference: Trang B, Li Y, Xue XS, Ateia M, Houk KN, Dichtel WR. Low-temperature mineralization of perfluorocarboxylic acids. Science. 2022;377(6608):839-845. doi: 10.1126/science.abm8868