This discovery focuses on a specific type of chemical pollution called perfluoroalkyl substances (PFAS), which are infamously hard to break down due to the strong bonds between its atoms.
Used in cosmetics for water resistance, smoothing, and product longevity, PFAS are increasingly scrutinised and being phased out due to health and environmental concerns.
By coating ZnO NCs with a special layer and shining a simple LED light on them, the research team at Ritsumeikan University in Japan was able to break down 92% of a common PFAS known as perfluorooctanesulfonic acid (PFOS).
This method is a major step forward as it works at room temperature and does not require the extreme heat or dangerous materials used in the past. As these crystals are inexpensive to make and safe to handle, this technology has potential to be used on a large scale.
Solving the deadlock
PFAS are a group of man-made chemicals valued across industries — from semiconductor fabrication and cooking gear to water- and oil-repellent materials — for their heat resistance and exceptional chemical stability.
These molecules consist of a chain of carbon and fluorine atoms linked together. The energy required to break the carbon-fluorine (C-F) bond is extremely high, making these compounds incredibly durable and highly resistant to biological degradation.
Consequently, they accumulate in the environment and human bodies, raising global concerns about long-term exposure and contamination cycles for ecosystems.
Traditional techniques for PFAS degradation are notoriously challenging. They typically require harsh chemicals, high pressure, or extreme temperatures often exceeding 800°C. Other light-based methods have relied on short-wavelength UVC light, which requires specialised quartz equipment and often uses toxic mercury lamps.
These conventional approaches are not only energy-intensive but are also becoming less feasible due to strict regulatory constraints, such as the Minamata Convention on Mercury.
The industry has long required a novel, sustainable, and energy-efficient method to enable PFAS recycling and mitigate environmental risks.
Surface engineering drives efficiency
The research team chose to work with ZnO NCs because of their natural ability to use light to trigger chemical reactions. ZnO is an ideal choice for industrial use because it is safe, affordable, and easy to produce in large quantities compared to other catalysts.
The real breakthrough, however, was not just using zinc oxide but also how the scientists engineered its surface. They did this by “capping” the tiny crystals with special organic layers called ligands, which drastically improved their ability to break down PFOS.
The study compared two specific types of coatings: one using acetic acid and another using 3-mercaptopropionic acid. While both could start the cleaning process, the results were markedly different.
After 24 hours of exposure to a near-UV LED light, the crystals coated with mercaptopropionic acid only broke down 8.4% of the chemicals. In contrast, the acetic acid version reached a massive 92% breakdown rate under the best conditions.
The difference comes down to how well the pollution “sticks” to the catalyst — a process called adsorption. The researchers discovered that the acetic acid coating made the surface of the crystals much more attractive to the PFOS molecules.
In fact, over 80% of the PFOS stuck to the acetic acid crystals before the light was even switched on, while only about 14% stuck to the other version. Getting these molecules physically close to the crystals was essential for the reaction to work quickly and effectively.
A repeatable, light-driven process
This system works like a solar-powered engine. When near-UV light hit the NCs, it created “excited electrons” that attacked the chemical bonds. This process systematically stripped away the fluorine atoms that made PFAS indestructible, causing the molecule to fall apart.
Another key feature of these crystals is that they are durable and can be used repeatedly. The team tested the crystals through several cycles and found that they remained highly effective.
They estimated that a single tiny crystal could break about 8,250 chemical bonds before wearing out. This high level of repeatability suggested the technology would be cost-effective for industrial use.
Additionally, the entire process happens at ambient temperature. It does not require the massive, expensive, and sometimes dangerous furnaces used in traditional high-heat incineration. This provides a safe way to solve the global PFAS crisis without needing large, costly new infrastructure.
This research highlighted a shift towards “green catalysis” in the fight against industrial pollution. By moving away from toxic, cadmium-based catalysts, the Ritsumeikan University team has provided a blueprint for energy-efficient environmental protection.
They wrote in conclusion: “Experiments suggest that the enhanced performance is attributed to efficient PFOS adsorption and Auger-induced multiphoton processes. The reaction proceeds via a reductive mechanism driven by excited electrons, with minimal contribution from ligand desorption.
“These results highlight the potential of AA-ZnO NCs as a low-toxicity, energy-efficient photocatalyst for PFAS degradation and fluorine recycling.”
Source: Chemical Science
“Photocatalytic defluorination of perfluoroalkyl substances by surface-engineered ZnO nanocrystals”
https://doi.org/10.1039/D5SC05781g
Authors: Shuhei Kanao, et al.
