Japanese researchers have found a new way to break down persistent PFAS compounds. Using light and zinc oxide nanocrystals coated with organic ligands, they were able to efficiently remove fluorine atoms from PFOS. The method works at room temperature and with simple UV light.
PFAS (Per- and Polyfluoroalkyl substances) are water, grease and dirt repellent, which is why they are found in many products such as pans, clothing and fire-fighting foam. PFAS molecules consist of a CF chain of carbon and fluorine atoms linked together. It takes a lot of energy to break that connection. As a result, PFAS hardly breaks down and accumulates in the environment and body, where the substance can cause damage.
One way to break down PFAS is through defluorination. Defluorination is the process of removing fluorine atoms from molecules to make them unstable. But that requires persistent chemicals or a lot of energy.
Nanocrystals break down PFAS compounds
Researchers from Japan’s Ritsumeikan University investigated whether nanocrystals of zinc oxide (ZnO) – which are thousands of times smaller than a human hair – could help remove fluorine atoms from PFOS (perfluorooctane sulfonates) – a subgroup of PFAS compounds. Nanocrystals generate reactive particles by shining light on them. These particles can in turn break down pollutants such as PFAS.
This is done more effectively – the Japanese assumed – by covering ZnO with so-called ligands. These are molecules or ions with a lone pair of electrons that fulfill a certain function. In this case, these were organic molecules (such as acetic acid or 3-mercaptopropionate ligands) that bind to the surface of the ZnO nanocrystals and enhance their photocatalytic (light-activated) degradation of PFAS.
“Perfluorooctane sulfonic acid or PFOS is a PFAS compound that was once widely used but is now heavily regulated. We wanted to see whether zinc oxide nanocrystals with ligand caps can defluorine this compound,” said Prof. Kobayashi, who led the study.
Illuminating crystals with “black light”.
Kobayashi and colleagues illuminated the ZnO nanocrystals with LED light of different wavelengths. ZnO with acetic acid showed a defluorination percentage of up to 92 percent after 24 hours of exposure to ultraviolet light (“black light”). ZnO with 3-mercaptopropionate performed somewhat less: after a day under the same light, the ZnO crystal plus ligand reached less than 9 percent.
The ZnO nanocrystals are also reusable, the researchers say. A single nanocrystal could break more than eight thousand CF bonds before it was played out. In addition, the crystals are non-toxic, cheap and can be produced on a large scale. Or as co-author Shuhei Kanao says: “The reaction takes place at room temperature and does not require high-energy light sources, which can be expensive, fragile or dangerous.”
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