Thread by @brittanytrang

Today our paper about #PFAS destruction (at much milder conditions than previously thought possible) is out in @ScienceMagazine.

We also present a detailed, evidence-based degradation mechanism.

This paper is my entire PhD and I'm very proud of it. A 🧵:
science.org/doi/10.1126/sc…

PFAS are environmental pollutants linked to negative health effects at exposure levels so low that the US EPA recently set a drinking water health advisory limit that is lower than we can measure

(a nice way of saying there's no safe level)

https://twitter.com/brittanytrang/status/1539745916227391488?t=2PbAjHaKKnxuve2w6vsPAA&s=19

In the last few years, the field has gotten pretty good at removing PFAS from water.

Our group in particular has made several regenerable PFAS adsorbents. But every time we present this research, someone asks 🙋‍♂️ "What do you do with the PFAS after you've desorbed it?"

Ideally, we want to degrade the PFAS so they can't go back into the environment.

However, as you can imagine, destroying something nicknamed "forever chemicals" is, uh, difficult.

As a broad generalization, until now, PFAS destruction methods have focused on injecting a ton of energy to break the very unreactive carbon–fluorine bonds in PFAS.

Incineration? yes.
Electricity? yes.
Plasma? yes.
UV? yes.
Strong chemical oxidants/reductants? yes.
Sonication? yes.
Subcritical/supercritical water? yes.

Combinations of all of the above? heck yea

Unfortunately, these processes aren't very efficient, produce a lot of smaller/harder-to-degrade PFAS byproducts, and don't always mineralize PFAS.

(again, a generalization; pls read the paper/my thesis/this review): tandfonline.com/doi/abs/10.108…

Instead of focusing on the C–F bond, we decided to first target perfluorocarboxylic acid decarboxylation.

Literature shows that most carboxylic acid salts will decarboxylate in polar aprotic solvents:
science.org/doi/10.1126/sc…

As it turns out, perfluorocarboxylic acids are SUPER good at decarboxylating in polar aprotic solvent; some (GenX) will even do it *at room temperature* pubs.acs.org/doi/abs/10.102…

However, no one has ever tried to use this reactivity quirk for PFAS degradation.

Turns out, when we added NaOH (as suggested in ball-milling, HALT, & incineration studies) to this decarboxylation, we were able to completely defluorinate most PFCAs, some up to 100% ✨ with no remaining fluorocarbon byproducts ✨

Control experiments indicate this defluorination activity isn't activated solely by exposure to NaOH; in order to flip this PFCA kill switch, you do have to decarboxylate first.

When we degraded PFCAs with 2–9 carbons, we noticed there were some byproducts—trifluoroacetate and formate—and (most importantly) that this byproduct formation had *patterns* 👀

Stumped because this didn't fit previously proposed (DHEH) mechanisms, we recruited our friends Ken Houk from UCLA and his undergrad student Yuli Li from Tianjin University, who helped us computationally piece together the mechanism based on our experimental results.

The mechanism is complex (to say the least) and I'll leave it for those who read the paper (and SI!)

(This is what I imagine I look like every time I try to explain the mechanism):

If you'll induge one result:

The most stunning finding was that the decarboxylation, not the defluorination, was the rate-limiting step.

In fact, some of the steps have no enthalpic barrier 🤯

As a result, we were able to degrade perfluoro-1H-alkanes (decarboxylated PFCAs) & a perfluoroalkene at *40°C* with up to 70% fluoride recovery.

This mineralization shows that defluorination can take place at low temps without irradiation or other energy input.

We hope that by investigating this new method and mechanism, we've shown that low-energy-input PFAS degradation is possible & that others will be inspired to leverage this chemistry in new ways.

science.org/doi/10.1126/sc…

This paper would not have been possible without my co-first-author Yuli Li, who again did this excellent computational work as an undergrad(!) and my advisor @Dichtel, as well as @houk1000.

Check out my other co-authors too: @XiaosongX @moha_ateia

If you want to hear more about this paper at #ACSFall2022 next week, @Dichtel will be presenting some of these results at his talks.

I'll also be speaking at the Developed & Emerging PFAS Treatment Technologies symposium at 4:40 pm on Wed 8/24 (No. 3735705)

See you there!