The Light Method cleans Forever Chemicals

Perfluoroalkyl substances (PFAS), nicknamed “forever chemicals”, pose a growing environmental and health threat. Since the invention of Teflon in 1938, PFAS and perfluorinated polymers or PFs have been widely used for their durability and exceptional resistance to water and heat. These properties make them ideal for countless applications, from cookware and clothing to fire retardant foam. However, this very stability has become a major problem. PFASs do not break down easily in the environment, leading to their accumulation in water, soil, and even in human bodies, where they are known to cause carcinogenic effects and hormone disruption. Today, these chemicals can be found in drinking water supplies, food, and even in Antarctic soil. Although there are plans to phase out the production of PFAS, their treatment remains challenging as they only decompose at temperatures above 400 °C. As a result, certain quantities of products containing PFAS and PF end up in landfills, potentially creating future contamination risks.

Now, a room-temperature defluorination method proposed by researchers at Ritsumeikan University could revolutionize PFAS treatment. Their study, published in the journal Angewandte Chemie International Edition on June 19, 2024, details a photocatalytic method that uses visible light to break down PFAS and other fluorinated polymers (FPs) at room temperature into fluoride ions. Using this method, the researchers achieved 100% defluorination of perfluorooctanesulfonate (PFOS) within just 8 hours of light exposure.

“The proposed methodology is promising for the effective decomposition of various perfluoroalkyl substances under mild conditions, contributing significantly towards creating a sustainable fluorine recycling society,” says Professor Yoichi Kobayashi, lead author of the study.

The proposed method involves irradiating visible LED light onto cadmium sulfide (CdS) nanocrystals and copper-doped CdS (Cu-CdS) nanocrystals with mercaptopropionic acid (MPA) surface ligands in a solution containing PFAS, FPs and triethanolamine (TEOA). The researchers found that irradiating these semiconductor nanocrystals generates electrons with a high reduction potential that break the strong carbon-fluorine bonds in the PFAS molecules.

For the photocatalytic reaction, the researchers added 0.8 mg of CdS nanocrystals (NCs), 0.65 mg of PFOS, and 20 mg of TEOA to 1.0 mL of water. They then exposed the solution to 405 nanometer LED light to initiate the photocatalytic reaction. This light excites the nanoparticles, generating electron-hole pairs and prompting the removal of MPA ligands from the surface of the nanocrystals, creating space for PFOS molecules to adsorb onto the NC surface.

To prevent recombination of photoexcited electrons with holes, TEOA is added to trap holes and extend the lifetime of reactive electrons available for PFAS degradation. These electrons undergo an Auger recombination process, where an exciton (an electron-hole pair) recombines non-radiatively, transferring its energy to another electron and creating highly excited electrons. These highly excited electrons possess sufficient energy to participate in chemical reactions with PFOS molecules adsorbed on the NC surface. The reactions lead to the breaking of the carbon-fluorine (CF) bonds in PFOS, resulting in the removal of fluoride ions from the PFAS molecules.

The presence of hydrated electrons, generated by Auger recombination, was confirmed by laser flash photolysis measurements, which identified transient species based on the absorption spectrum after laser pulse excitation. The defluorination efficiency depended on the amount of NCs and TEOA used in the reaction and increased with the light exposure period. For PFOS, defluorination efficiencies were 55%, 70–80%, and 100% for 1-, 2-, and 8-h light irradiation, respectively. Using this method, the researchers also successfully achieved 81% defluorination of Nafion, a fluoropolymer, after 24 hours of light irradiation. Nafion is widely used as an ion exchange membrane in electrolysis and batteries.

Fluorine is a critical component in many industries, from pharmaceuticals to clean energy technologies. By recovering fluoride from PFAS waste, we can reduce dependence on fluoride production and create a more sustainable recycling process. “This technique will contribute to the development of recycling technologies for fluorine elements, which are used in various industries and support our prosperous society”, concludes Prof. Kobayashi.