Stock image of molecular structure. iStock/BlackJack3D
Researchers have developed a promising new method which uses light to clean up forever chemicals at room temperature.
The method to decompose perfluoroalkyl substances (PFASs) using visible LED light offers a promising solution for sustainable fluorine recycling and PFAS treatment.
The method can be used to recover fluorine from waste PFAS, reducing the need for new fluorine production.
The research was carried out by a team from the Ritsumeikan University, Japan, which was led by Professor Yoichi Kobayashi.
Problems posed by forever chemicals
PFASs, which are also called forever chemicals, are a growing environmental and health threat.
Since the invention of Teflon in 1938 PFASs have been used extensively around the world for their stability and resistance to heat and water.
They are used in countless applications, ranging from cookware and clothing to firefighting foam.
However, the same stability also poses an ecological challenge as the forever chemicals refuse to break down and accumulate in water, soil and even animal bodies. In humans, they have been known to cause carcinogenic effects and hormonal disruptions.
To decompose the forever chemicals, temperatures exceeding 752°F (400°C) are needed, which is rather challenging to achieve.
Therefore, the new method is a path breaking one as it can be used to decompose PFASs at room temperature using visible LED light.
Researchers at achieved a 100% breakdown of perfluorooctanesulfonate (a type of PFAS) in just eight hours and an 81% breakdown of Nafion (a fluoropolymer) in 24 hours, according to a press release by Ritsumeikan University.
The study details a photocatalytic method that uses visible light to break down PFAS and other fluorinated polymers (FPs) at room temperature into fluorine ions.
The method to break PFAS at room temperature
The method involves irradiating visible LED light onto cadmium sulfide (CdS) nanocrystals and copper-doped CdS (Cu-CdS) nanocrystals with surface ligands of mercaptopropionic acid (MPA) 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 down the strong carbon-fluorine bonds in 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.
The light excites the nanoparticles, generating electron-hole pairs and promoting the removal of MPA ligands from the surface of the nanocrystals, creating space for PFOS molecules to adsorb onto the NC surface.
To prevent photoexcited electrons from recombining with holes, TEOA is added to capture the holes and prolong the lifetime of the reactive electrons available for PFAS decomposition.
These electrons undergo an Auger recombination process, where one exciton (an electron-hole pair) recombines non-radiatively, transferring its energy to another electron, and creating highly excited electrons.
These highly excited electrons possess enough energy to participate in chemical reactions with the PFOS molecules adsorbed on the NC surface. The reactions lead to the breaking of carbon-fluorine (C-F) bonds in PFOS, resulting in the removal of fluorine 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 upon laser pulse excitation.
The defluorination efficiency depended on the amount of NCs and TEOA used in the reaction and increased with the period of light exposure.
“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,” Kobayashi said.
“This technique will contribute to the development of recycling technologies for fluorine elements, which are used in various industries and support our prosperous society.”
The study was originally published in the journal Angewandte Chemie International Edition.
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