A recent study shows zinc oxide nanocrystals can efficiently break down PFAS chemicals like PFOS under ambient light; a new, low-energy solution to tackle "forever chemicals".
Study: Photocatalytic defluorination of perfluoroalkyl substances by surface-engineered ZnO nanocrystals. Image Credit: veeart/Shutterstock.com
A group of researchers from Ritsumeikan University has demonstrated that zinc oxide nanocrystals capped with specific ligands can effectively defluorinate perfluorooctanesulfonic acid, a prominent perfluoroalkyl substance.
This method has the potential to address the challenges associated with precycling erfluoroalkyl substances (PFAS). The study was published in the journal Chemical Science.
Perfluoroalkyl substances (PFASs) are a category of synthetic chemicals that have been widely used for their water- and stain-resistant properties, as well as their exceptional chemical stability.
The PFASs were previously appreciated for their capacity to resist heat, water, and oil. These substances have been used in the manufacturing of both common and industrial products. PFAS molecules consist of a chain of carbon and fluorine atoms interconnected.
However, these substances often accumulate in the environment, posing significant environmental and health risks. Over the last decade or so, countries have begun introducing legislation to phase out their use.
The energy necessary to sever the carbon-fluorine (C-F) bond is exceedingly high, rendering these compounds robust and exceptionally resistant to biological degradation.
PFASs are frequently referred to as "forever chemicals" due to their difficulty in degradation. This persistence contributes to continuous pollution and bioaccumulation, heightening global apprehensions regarding prolonged exposure and contamination cycles affecting ecosystems and human health.
PFAS-defluorination refers to the process of extracting fluorine atoms from the molecule, which results in decreased stability and increased vulnerability to further decomposition.
Conventional PFAS degradation methods pose challenges as they necessitate harsh chemicals or substantial energy. The advancement of innovative, sustainable, and energy-efficient techniques is essential to facilitate the recycling of PFAS and alleviate the environmental risks associated with these substances.
The recent study investigated the potential application of zinc oxide (ZnO) nanocrystals (NCs) in the process of PFAS defluorination. These NCs, recognized for their photocatalytic characteristics, can harness light to produce reactive species that break down organic contaminants. To improve their efficiency, NCs were capped with various ligands.
Perfluorooctanesulfonic acid or PFOS is a PFAS compound that was once widely used but is now strictly regulated, and we wanted to see if ligand-capped ZnO NCs can defluorinate it.
Yoichi Kobayashi, Professor, Ritsumeikan University
The research primarily concentrated on the defluorination efficiency of ZnO nanocrystals (NCs), which were capped with either acetic acid (AA–ZnO NCs) or 3-mercaptopropionic acid (MPA–ZnO NCs). Additionally, various other organic ligands were employed to cap the NCs for the purpose of comparative analysis.
The defluorination experiments were carried out using a 365 nm LED light source, as it simulates ambient lighting conditions. The defluorination efficacy of these ligand-capped NCs was also evaluated on several other per- and polyfluoroalkyl substances (PFASs), including trifluoroacetic acid and Nafion.
AA–ZnO NCs demonstrated a high efficiency in defluorinating PFOS when subjected to near-UV light irradiation under ambient conditions.
The acetic acid ligand was found to be significantly more effective than the 3-mercaptopropionic acid, as MPA–ZnO NCs achieved merely 8.4 % defluorination after a 24-hour period, whereas AA-ZnO NCs reached an impressive defluorination rate of up to 92 % after 24 hours under optimized conditions.
To assess the sustainability of these NCs, their durability and the reduction in catalytic efficiency over time were also investigated. The results indicated that the decomposition reaction occurred over multiple cycles, with a single ZnO NC capable of breaking up to 8,250 C-F bonds, highlighting its potential for reusability.
Zinc oxide nanocrystals (ZnO NCs) are highly effective in the defluorination process owing to their distinctive characteristics. They possess low toxicity, are cost-effective, and can be manufactured on a large scale, unlike many earlier catalysts.
“The reaction occurs at room temperature and does not require high-energy light sources, which can be costly, fragile, or hazardous,” mentioned Mr. Shuhei Kanao.
This gentle photodegradation system is designed to address the globally significant PFAS recycling challenge. It is applicable for mitigating industrial PFAS contamination and can be used in fluorochemical production facilities, semiconductor manufacturing plants, the recycling sector, wastewater treatment plants, and beyond.
PFAS pollution is a worldwide concern, and this simple NC-based technology could contribute significantly to tackling this issue.
Yoichi Kobayashi, Professor, Ritsumeikan University
Journal Reference
Kanao, S., et al. (2025) Photocatalytic defluorination of perfluoroalkyl substances by surface-engineered ZnO nanocrystals. Chemical Science. DOI: 10.1039/D5SC05781G.