Breakthrough Might Break Down PFAS 'Forever Chemicals'

 

11.17.2022

U.S. News Article 

Scientific Article: Science 2022, 377, 839-845. 

 

I chose an article from U.S. News and World Report titled: “Breakthrough Might Break Down PFAS ‘Forever Chemicals’” that covered a paper published on August 18, 2022 in Science titled “Low-temperature mineralization of perfluorocarboxylic acids.” I chose this article because I was intrigued by the mechanism of how simple basic reaction conditions could induce degradation of highly resilient fluorinated compounds. Perfluoroalkyl carboxylic acids (PFCAs) are one of the most common classes of per- and polyfluoroalkyl substances (PFAS). PFAS are commonplace in our modern society for their favorable properties leading to their use as water and oil-resistant protectants, non-stick cookware, fire-fighting foams, and in electronic devices. The combination of how widely used PFAS are with their high stability has led them to be designated as forever chemicals that are prolific in our environment, especially in drinking water. Therefore, PFAS have been identified to be almost ubiquitous in humans and have been correlated with negative health effects such as cancers, birth complications, and organ diseases (more on PFAS here). Therefore, it is of interest to develop methods for the remediation of drinking water contaminated with PFAS. In an effort to address these concerns, the authors studied a base-induced decarboxylation-defluorination cascade to chemically breakdown PFCAs.

The Science paper focuses on determining the mechanism behind this novel class of reactivity for PFCA degradation (Fig. 3 and 4) and they consider a brief substrate scope of PFCAs of different chain lengths to evaluate the generality of their method and to gain further mechanistic insight (Fig. 2). Several analytical techniques such as ¹H, ¹³C, and ¹⁹F NMR spectroscopy, ion chromatography, electrospray ionization MS, and GC-MS were used to characterize reaction products and transient reaction intermediates. Within the supplementary information, the authors report that 30 equivalents of sodium hydroxide were required for this transformation along with heating to 120 degrees Celsius. Based on this, I find it interesting that the authors claim their transformation to operate under mild reaction conditions as several classes of functional groups could not survive such conditions (e.g. esters, carboxylic acids, Weinreb amides, epoxides, imines, etc…). With that said, considering their degradation method in context of other remediation strategies for PFAS compounds, the temperatures used and high amounts of moderately strong base can be considered quite mild.

The authors’ proposed mechanism, which is supported by computational and experimental work, differs from the most commonly invoked mechanism for PFCA degradation: decarboxylation-hydroxylation-elimination-hydrolysis (DHEH). This pathway involves sequential shortening of PFCAs by a single carbon until the final products are obtained. However, the reduced degradation of three-carbon length compounds and the formation of various carbon-containing byproducts cannot be explained by a DHEH mechanism. Rather, the authors propose a rate-limiting decarboxylation of the carboxylate head group of the PFCA leading to INT1. INT1 undergoes defluorination followed by favorable hydroxylation to form enol INT4. Keto-enol tautomerization leads to INT6, which is the divergent intermediate for the two main observed byproducts of the reaction: fluoroacetate (pathway D) and formate (pathway B). INT6 is electrophilic at the 2 and 4 position and OH^- is on the edge of being a soft or hard nucleophile, so the authors proposal that both pathways are operative is justifiable. Michael-addition leads to pathway D, ultimately forming fluoroacetate and a three-carbon shorter PFCA chain. 1,2-Addition forms INT14 (pathway B) that leads to INT35. INT35 can enter pathway D or undergo a second 1,2-addition to form formate and a one-carbon shorter chain that reenters pathway B. Ultimately, in pathway D one carbon is lost to CO₂ and two carbons are lost to fluoroacetate. In pathway B one carbon is lost to CO₂ and either two carbons are lost to fluoroacetate or one carbon to formate. The other carbon products form through off-cycle processes detailed in the supplementary information. The majority of fluorine atoms becomes fluoride anions.

Image from: Science 2022, 377, 839-845.

The major appeal of this degradation method compared to other remediation strategies is that methods such as using filters and adsorbents creates PFAS and PFCA contaminated waste that needs to be disposed of. Likewise, other methods require significantly harsher reaction conditions and have the potential to form fluorinated organics as byproducts. In contrast, fluoride anions and small organics produced (e.g. carbonate, glycolate, oxalate, etc…) are not harmful and do not require subsequent disposal. Their degradation method works for historically problematic perfluorooctanoic acid (PFOA) and GenX, a PFCA of modern concern. Furthermore, the reaction is operationally simple and is not air- or moisture-sensitive, which is appealing for scale in applying this to real-world water samples. However, the authors only conducted the reaction on 0.5-1 mmol scale (see SI) that limits insight into scalability. Ultimately, further work will need to be conducted to expand the practicality of this method for removal of PFCAs and potentially expand to other classes of PFAS that lack a carboxylate head group through alternate pathways of forming perfluoroalkyl carbanions.  

The U.S. News article overall does a good job of introducing the major topic under study and some of the takeaways from the scientific paper, but misses the main point. The popular press article starts with a hook about why we should care about PFAS, namely that PFAS sticks around in our environment and is harmful. The news article then does a nice job presenting the main appeal of the author’s method compared to previous methods being that their degradation strategy proceeds under mild conditions using readily available reagents. The news article also highlights that the degradation products are not harmful to people. The U.S. News article then transitions into discussing why one should care about PFAS in more detail and some of the specifics/limitations for the method presented in the Science article (e.g. that it only works for carboxylate capped PFAS). The news report shifts to a brief mention of the computational work done to probe the reaction mechanism and considerations of applying this chemistry to remediation of drinking water such as looking at the limitations. The article concludes with a single statement quoting the corresponding author about what they think the main finding of the paper is: fundamental understanding of the reaction mechanism of degradation.

U.S. News should be complimented for their strong presentation of why readers should care about PFAS. The main concerns with PFAS are how prevalent they are in drinking water, health concerns, and difficulty in removal, which were all presented. Furthermore, U.S. News linked further reading on PFAS from the National Institute of Environmental Health Sciences and the USEPA that provides readers with further information if they like without distracting from the science. The news article decently presents the benefits of this method compared to others (considers thermal inputs), but a deeper discussion is lacking. For example, I think it would be beneficial for readers to understand the benefits of chemical degradation compared to filtration/adsorption that is also commonly employed for PFAS remediation. Likewise, the news article balanced discussion with quotations from the authors of the paper quite well giving a sense of expertise in their writing. However, they do not include outside experts and only gives the authors perspective, which limits the overall confidence in this work. Overall, the main strengths of the article are the succinct presentation of contextual information about PFAS and a partial inclusion of the benefits of this method compared to others. The article did a mediocre job at presenting the actual science done. The main point of the research is that the authors discovered a novel decomposition pathway for PFCAs in the presence of base and they elucidated its mechanism. It is a fundamental discovery that will inform future research in PFAS remediation. The news report gives a brief commentary on the work done computationally to probe the mechanism, but leaves out the experimental work done in parallel. Likewise, the news article misrepresents the authors consideration of different plausible mechanistic pathways when they wrote, “The researchers settled on one process and then used it to successfully degrade 10 different perfluoroalkyl carboxylic acids (PFCAs) and perfluoroalkyl ether carboxylic acids (PFECAs).” The Science authors did not “settle” on a process, rather they optimized reaction conditions and then elucidated the operative mechanism from a series of possible pathways and then applied it to different types of PFCAs. This has the potential to lead readers to believe that there are other similar strategies to degrade PFCAs, rather than other possible mechanisms. The news article also only briefly mentions that this method is limited to carboxylate capped PFCAs, which is a major limitation of this work when U.S. News makes this method seem as “the” solution to PFAS remediation through the title and beginning of the article. I understand the news report is trying to sell this work and make the discovery seem instantly applicable, but ultimately using this method for actual PFAS remediation is a long way off.

With the aforementioned in mind, I give the news report a score of a 5/10. The U.S. News article earns points for an effective discussion on what PFAS is, the concerns of PFAS, and linking to further readings on the topic. Likewise, points are earned for containing some discussion on the benefits of the method. However, I cannot justify a higher score because I feel the news report focused too much on problems with PFAS and the potential applications of this method rather than focusing on the main discovery of this paper: a novel reaction mechanism that will provide insight into future research. Similarly, points are deducted for mispresenting processes vs mechanism and for exaggerating the current applicability of this method, albeit with some minor discussion of future directions. A larger discussion of what needs to be studied in the future is needed along with a greater focus on the main finding of this seminal work.