The prevalence of perfluoroalkyl substances (PFASs), also known as ‘forever chemicals,’ presents a pressing environmental and health concern. Ever since the creation of Teflon in 1938, PFASs and perfluorinated polymers or PFs have been extensively used due to their exceptional stability and resistance to water and heat. These attributes have rendered them indispensable in numerous applications, ranging from cookware and clothing to firefighting foam.
However, their enduring stability has now become a significant issue. PFASs persist in the environment without breaking down easily, leading to their accumulation in water, soil, and even in the human body, where they are known to induce carcinogenic effects and hormonal disruptions.
These harmful chemicals are present in our drinking water, food, and even the soil of Antarctica. While there are plans to stop producing PFAS, getting rid of them is difficult because they only break down at very high temperatures. As a result, products containing PFASs and PFs end up in landfills, posing contamination risks.
Now, researchers at Ritsumeikan University have developed a room-temperature defluorination method to break down PFAS using visible light, achieving 100% defluorination of fluorine ions. Using this method, the researchers achieved 100% defluorination of perfluorooctanesulfonate (PFOS) within just 8 hours.
“The proposed methodology is promising for the effective decomposition of diverse perfluoroalkyl substances under gentle conditions, thereby significantly contributing towards the establishment of a sustainable fluorine-recycling society,” says Professor Yoichi Kobayashi, the lead author of the study.
The innovative approach involves exposing cadmium sulfide (CdS) and copper-doped CdS (Cu-CdS) nanocrystals to visible LED light in a solution containing PFAS, FPs, and triethanolamine (TEOA). The results show that this process generates high-reduction potential electrons, which effectively degrade the strong carbon-fluorine bonds in PFAS molecules.
In the photocatalytic reaction, 0.8 mg of CdS nanocrystals (NCs), 0.65 mg of PFOS, and 20 mg of TEOA were added to 1.0 ml of water. Upon exposure to 405-nanometer LED light, the nanoparticles are excited, creating electron-hole pairs and facilitating the removal of MPA ligands from the nanocrystals’ surface. This creates space for PFOS molecules to adsorb onto the nanocrystal surface.
In order to prevent the recombination of photoexcited electrons with holes, TEOA is included to capture the holes and extend the lifespan of the reactive electrons available for PFAS decomposition. These electrons undergo an Auger recombination process, in which one exciton (an electron-hole pair) recombines non-radiatively, transferring its energy to another electron and generating highly excited electrons.
These highly excited electrons have the ability to engage in chemical reactions with the PFOS molecules adsorbed on the NC surface. These reactions result in the cleavage of carbon-fluorine (C-F) bonds in PFOS, leading to the elimination of fluorine ions from the PFAS molecules.
The presence of hydrated electrons, resulting from Auger recombination, has been verified through laser flash photolysis measurements. This method identified transient species based on the absorption spectrum upon laser pulse excitation.
The efficiency of defluorination depended on the amount of nanocrystals (NCs) and triethanolamine (TEOA) used, and it increased with the duration of light exposure. In the case of PFOS, the defluorination efficiency was 55%, 70–80%, and 100% for 1-, 2-, and 8-hour light exposure, respectively. This process also led to 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 plays a crucial role in various industries, including pharmaceuticals and clean energy technologies. Recovering fluorine from waste PFAS can reduce our reliance on fluorine production and establish 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.
Journal reference:
- Yuzo Arima, Yoshinori Okayasu, Daisuke Yoshioka, Yuki Nagai, Yoichi Kobayashi. Multiphoton-driven Photocatalytic Defluorination of Persistent Perfluoroalkyl Substances and Polymers by Visible Light. Angewandte Chemie International Edition, 2024; DOI: 10.1002/anie.202408687