New techniques use visible light to destroy PFAS

Toxic per- and polyfluoroalkyl substances (PFAS) persist forever in the environment; even when they’re removed, it’s difficult to keep them from eventually winding up back in the environment. Annihilating PFAS is the only sure way to keep them out of drinking water and our bodies. In recent years, researchers and start-ups have devised several techniques to destroy these forever chemicals.

Two independent research groups now report for the first time that visible light can break apart PFAS into benign by-products (Nature 2024, DOI: 10.1038/s41586-024-08179-1 and 10.1038/s41586-024-08327-7). Both teams have developed photocatalysts that, when excited by purple light, energetically lob electrons at PFAS to sever the stubborn carbon-fluorine bonds that make the substances resistant to heat and water.

It’s still early, but the advances hint at a low-cost, large-scale route to destroy PFAS directly in water. “We use light as the energy source, and the sun could give this light,” says Yan-Biao Kang, a chemist at the University of Science and Technology of China. “Our dream is to put our catalyst, immobilized on some material, in water, where it would slowly destroy PFAS.”

Four years ago, Kang, Jian-Ping Qu of Nanjing Tech University, and colleagues made a carbazole-based catalyst that, under purple light, efficiently ruptured the carbon-fluorine bond in fluorobenzene. The researchers then set their sights on PFAS, but that catalyst wasn’t great at cleaving all the C–F bonds in the compounds.

The team has now designed a version of the catalyst with a twisted carbazole ring and more electrons to donate. The researchers tested the new photocatalyst on various PFAS, including polytetrafluoroethylene (PTFE), known as Teflon, as well as multiple long-chain PFAS with eight or more carbons, including perfluorooctane sulfonic acid and polyfluorooctanoic acid (shown).

They mix each PFAS separately in a solvent containing the photocatalyst and potassium formate as an electron donor to replenish the electrons that the photocatalyst sends off to cleave the carbon-fluorine bond. They then shine a purple light-emitting diode (LED) on the mixtures. The PTFE breaks down to give amorphous carbon and reusable potassium fluoride; more than 95% of the fluorine is converted into the fluoride salt. The long-chain PFAS are degraded and transformed into carbonate, formate, oxalate, and trifluoroacetate as the end products.

Chemists Garret M. Miyake and Robert S. Paton at Colorado State University, Niels H. Damrauer at the University of Colorado Boulder, and colleagues have followed a trajectory similar to that of the group in China. About 8 years ago, the Colorado team developed a benzoperylene-based photocatalyst; they have now tweaked and tamed its reactivity so it can shear carbon-fluorine bonds in PFAS when excited by purple light.

The researchers use tetrabutylammonium fluoride as the electron donor. Their reaction gives benign hydrocarbons and fluoride ions as the major by-products.

Neither catalyst system is close to practical right now. The reactions are sluggish in water, and both Kang and Miyake say the first step will be getting their respective catalyst to work efficiently on PFAS dissolved in water.

“From a practical perspective there are a lot of grand challenges,” Miyake says. But an efficient system that uses LEDs, and maybe sunlight, offers a tantalizing economical route to destroy PFAS, he says.

Jinyong Liu, a chemical engineer at the University of California, Riverside, who wrote a commentary about the research for Nature, calls these advances “exciting.” Although the photocatalysts have complicated structures and might not be immediately ready for real-world application, he says, these two studies pave the way for safe, low-energy technologies to demolish PFAS compared with incineration.

Several low-temperature technologies—ultraviolet light combined with photocatalysts, plasma destruction, and electrochemical oxidation—are also being scaled up. “Each room-temperature process has limitations in terms of what kind of PFAS structure they can destroy effectively,” Liu says. “Currently, we expect to have an integrated system that combines the advantages of different technologies to achieve thorough destruction of PFAS.”