Written by Yilin Xie '26
Edited by Anusha Srinivasan '24
Clean water, unpolluted air, safe food—these are all basic and indispensable needs for life on Earth that evolved in a pre-industrial environment. It’s no secret, however, that human activities have left man-made chemicals all over our planet. One class of such chemicals is called PFAS— per- and polyfluoroalkyl substances—but you might know them as ‘forever chemicals’. As the name states, once they are here, they are here to stay. Indeed, according to the U.S. Environmental Protection Agency, PFAS are found in water, air, soil, and the blood of people and animals globally—they accumulate in bodies as well as the environment.
The problem is that, in children and adults alike, exposure to PFAS can lead to negative health consequences, such as increased risks of certain cancers, developmental delay, reduced immunity, and abnormal hormonal levels, among others [4]. Though we don’t know the full extent and degree of the impact of PFAS on health, their persistence in the environment means that they will continue to pose an issue if nothing is done. Thus, it is of interest to prevent more PFAS from being released and to remove existing PFAS from the environment.
But firstly, why don’t PFAS go away? To understand why PFAS stay in the environment, we need to know their chemistry. PFAS are a group of chemicals that contain at least one C atom attached to two or three F atoms [1]. By definition, they have many C-F bonds, which are one of the strongest types of chemical single bonds [2]. These C-F bonds give PFAS often-desired properties such as hydrophobicity and lipophobicity—neither oil nor water would stick to a PFAS-coated surface—leading to the well-known application of PFAS on nonstick cookware and food packaging [3]. However, the strength of C-F bonds also makes PFAS hard to break down. Whereas a chemical with weaker bonds would degrade to smaller components by energy input from the environment—for example, ultraviolet radiation from the Sun—the usual degradation forces in nature are not enough to break the C-F bond. Thus, PFAS stay and accumulate.
One facet of addressing PFAS in the environment is research in PFAS collection. This is done through sorption: we use substances that attract PFAS—sorbents—so PFAS would attach to them and be removed when these substances are removed. One example of a sorbent is activated carbon, which is widely used to attract pollutants due to its high surface area, meaning ample space onto which pollutant molecules could attach [3]. For PFAS, activated carbon is only effective for removing the long-chained types. Another type of established sorbent of PFAS is ion-exchange resins, which are also used to remove inorganic ions (e.g. uranium and arsenic) [5]. In water, PFAS are negatively charged due to the dissociation of some functional groups, so anionic exchange resins, with their positive charge and high surface area, can effectively remove PFAS [6]. Additionally, ion-exchange resins better target the short-chained PFAS, complementing activated carbon in some applications. Finally, filtration techniques like reverse osmosis and membranes can also purify PFAS-contaminated water [7]. In all of these techniques, the end products are cleaned water and a solid or liquid waste stream containing PFAS.
The next question, then, is what to do with the waste stream containing PFAS. If left in landfills, PFAS can leak into the environment over time, so PFAS need to be degraded [7]. Again, due to the high strength of the C-F bonds in PFAS, we need to input a lot of energy to break them. Some current techniques include inputting high energy via heat (high-temperature incineration), sound (ultrasonication), and light (ultraviolet-initiated degradation (with additives)), all of which are harsh and, of course, energy-intensive [3]. Thus, there is a demand for gentler methods of PFAS degradation.
In a recent study by Trang et al., researchers discovered a new method of PFAS degradation. By heating PFOA (perfluorooctanoic acid; a type of PFAS) in polar aprotic solvents, the ‘forever chemicals’ are mineralised, or degraded to forms that are usable to plants. In an analysis of the products (via ion chromatography), the researchers found that 90 ± 6% of the total amount of fluorine atoms exist now as fluoride ions: the vast majority of the F atoms that were in C-F bonds in the reactant are not bonded to a C atom in the products. Without the strong C-F bonds, the products easily degrade.
Compared to traditional methods of PFAS degradation, this novel method is much less harsh. In this method, the solution is heated to 80°C–120°C, around the temperature of hot to boiling water; by comparison, high-temperature incineration, a traditional method of PFAS degradation, has temperatures between 1000°C and 1300°C. The solvents used, like NaOH and DMSO, are common, low-cost, and well-studied. What’s more, this method is selective: it specifically targets PFAS. Overall, the researchers have devised an effective, cheap, and gentler method of PFAS degradation with minimal undesirable side products.
This method is not without shortcomings: a major insufficiency is that this method only works on certain classes of PFAS. Initially, researchers studied this method on PFOA (perfluorooctanoic acid), and they further tested it on PFECAs (perfluoroalkyl ether carboxylic acids) with success, albeit some adjustments were needed. However, this method only works on PFCAs (perfluoroalkyl carboxylic acids), like the two discussed (octanoic acid is also called 1-heptanecarboxylic acid), not PFSAs (perfluorosulfonic acids), another major class of PFAS.
In application, this method has been proposed to be used to treat concentrated PFAS gathered from the discussed collection methods. While this method is new and has not been put to practice yet, it has the potential to become a key step in PFAS degradation processes. What’s more, Trang et al. computationally identified intermediates and gained insight to the mechanism of this method of degradation, paving the road for future research on these forever chemicals. As we come to deeper understandings of PFAS, we can hope to one day undo the damage we unleashed, and to take the story of PFAS as a lesson in our interactions with the world we live in, the only world we have.
References
1. Wang Z, Buser AM, Cousins IT, Demattio S, Drost W, Johansson O, et al. A New OECD Definition for Per- and Polyfluoroalkyl Substances. Environ Sci Technol. 2021 Dec 7;55(23):15575–8.
2. Bond Energies - Chemistry LibreTexts [Internet]. [cited 2022 Nov 13]. Available from: https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Chemical_Bonding/Fundamentals_of_Chemical_Bonding/Bond_Energies
3. Low-temperature mineralization of perfluorocarboxylic acids | Science [Internet]. [cited 2022 Nov 13]. Available from: https://www.science.org/doi/full/10.1126/science.abm8868
4. US EPA O. Our Current Understanding of the Human Health and Environmental Risks of PFAS [Internet]. 2021 [cited 2022 Nov 13]. Available from: https://www.epa.gov/pfas/our-current-understanding-human-health-and-environmental-risks-pfas
5. PFAS removal by ion exchange [Internet]. [cited 2022 Nov 13]. Available from: https://www.lenntech.com/processes/pfas-removal-by-ion-exchange.htm
6. Dixit F, Dutta R, Barbeau B, Berube P, Mohseni M. PFAS removal by ion exchange resins: A review. Chemosphere. 2021 Jun 1;272:129777.
7. TECHNICAL GUIDANCE FOR REMOVAL OF PFAS USING ION EXCHANGE RESINS [Internet]. DuPont; 2020 [cited 2022 Nov 1]. Available from: https://www.dupont.com/content/dam/dupont/amer/us/en/corporate/PFAS/Guide%20for%20PFAS%20Removal%20Using%20Ion%20Exchange%20ResinsFIN.pdf