Extreme UV light turns water into a weapon against PFAS

PFAS, found in everything from Teflon to firefighting foam, pose a threat to the environment and human health because they can accumulate in organisms and persist in nature for years.

This is why researchers have spent years trying to find ways to break them down.

However, the vast majority of current technologies can do little more than move PFAS from one place to another. And in the few cases in which researchers have succeeded in degrading PFAS, they have had to rely on extremely high-energy UV radiation that, on Earth, can only be produced in laboratories. Even then, the process is slow and often requires harsh chemicals – without researchers truly understanding the underlying chemistry.

However, a new experiment from the Department of Biological and Chemical Engineering at Aarhus University offers reason for hope. Here, Associate Professor Zongsu Wei and his colleagues succeeded in breaking down several types of PFAS without using harmful chemicals. In principle, the experiment requires little more than a container of water and a special lamp that emits highly energetic UV light.

The scientific article in Environmental Science & Technology not only shows that the PFAS were degraded and partly stripped of their fluorine – it also reveals what chemically happens along the way.

The explanation is that the UV light does not attack the PFAS directly but causes the water to form short-lived hydrogen radicals that attack the compounds.

“Our study shows that it is the hydrogen radicals that are capable of breaking down these compounds. This dispels the notion that the process was driven by other reactive substances and gives us a much better understanding of the mechanism behind it,” explains Zongsu Wei, adding:

“This could pave the way for new and more sustainable technologies to destroy PFAS in our environment rather than simply removing them.”

Only the most energetic UV light could crack the PFAS bonds

The explanation for why PFAS compounds are so stubborn lies in the bond between the carbon and fluorine atoms, which is among the strongest in organic chemistry. The atoms hold each other in an iron grip. This is why UV light has long attracted interest, because its energy can be directed very precisely at the bonds.

That is also why it has been the weapon researchers have tried to use to shoot PFAS apart – without fully understanding what it would take to hit the target.

To find an answer, Zongsu Wei and colleagues investigated which wavelengths could break down PFAS. The experiment involved pouring PFAS into a container of water and exposing it to highly energetic UV light.

The light was generated using a special lamp in which an electrical discharge through xenon gas creates a broad light spectrum ranging from 200 to 1000 nanometres. Here, UV light with wavelengths below 300 nanometres proved capable of triggering the process. This wavelength range corresponds to the most energy-rich part of the Sun’s UV spectrum, which is largely absorbed by the atmosphere before reaching the Earth’s surface.

In addition to varying the wavelength within that range, the researchers were also able to increase the light intensity to a level 15 times stronger than the intensity of the corresponding light emitted by the Sun.

The results were hard to mistake: the most energetic UV light, with wavelengths below 300 nanometres, clearly worked best.

Only at wavelengths below this threshold were the researchers able to measure free fluoride – fluorine that had been torn loose from the PFAS molecule. This shows that the strong carbon–fluorine bonds had in fact been broken. If a PFAS compound disappears without a corresponding increase in free fluoride, the light may have rearranged the molecule into another PFAS variant that still retains its fluorine atoms.

PFAS can disappear without truly breaking apart

To fully succeed, the newly formed intermediate products also have to be broken down. The intensity of the process also mattered greatly. When the researchers reduced the lamp’s power, both degradation and fluorine release fell markedly.

For perfluorooctanoic acid (PFOA), degradation fell from 49.0% to 27.8%, and fluoride release fell from 14.7% to 5.8%.

The results therefore suggest that the wavelength determines whether the light can trigger the chemistry at all, and the intensity determines the speed. The researchers also observed clear differences between PFAS, particularly between GenX and the classic compound PFOA.

After five hours of irradiation, GenX had released 21.2% of its fluorine, whereas PFOA, under similar test conditions, had released 14.7%. GenX therefore appears to release fluorine more readily than PFOA, in which the molecule was indeed almost completely degraded after a longer period but far less fluorine was actually torn free: here, it took 12 hours to achieve 25.5% defluorination and 86% degradation.

“The difference highlights an important point in PFAS chemistry: a substance can disappear as the original molecule without all the fluorine being released. The challenge is therefore not only to make the PFAS molecule disappear, but to break the carbon–fluorine bonds,” says Wei, continuing:

“Although highly energetic UV light has the potential to degrade PFAS, the process is still relatively slow. Intermediate products may also form along the way, and these themselves need to be broken down. The results therefore do not solve the PFAS problem overnight.”

The researchers had to follow the fluorine atom by atom

After showing that PFAS could break down under the lamp’s short-wavelength UV light, the researchers were left with the next question: what actually drives the process?

The dominant explanation for years has been that highly energetic UV light knocks electrons loose from water or added chemicals. In water, the free electrons are quickly enveloped by water molecules, causing them to behave as hydrated electrons – small, highly reactive electrons surrounded by water molecules. They have therefore long been considered the central driving force behind PFAS degradation.

However, the breakdown of PFAS and the release of fluoride still did not tell the researchers what was actually driving the attack. They therefore shifted their attention one step further back in the process. They also investigated which short-lived compounds formed in the water immediately after exposure to light.

They did this by introducing chemical blockers into the experiment. Certain substances can capture specific reactive compounds before they reach the PFAS. If the degradation stops when a particular blocker is added, it reveals which chemical attacker was playing the leading role.

In addition, the researchers measured traces of short-lived radicals, analysed the PFAS fragments along the way and compared the results with chemical simulations.

The water itself turned out to be the attacker

To show that the radicals really were being formed, the team had to combine advanced computer simulations of the chemical reactions with a special form of mass spectrometry carried out by Lu Bai, the study’s lead author and Wei’s PhD student.

The equipment functions like an ultrasensitive chemical balance capable of identifying individual molecules in fractions of a second. By sending electrical charges through the molecules while they were being exposed to light, she was able to measure the mass of the short-lived intermediate steps and track the route the PFAS molecule followed as the fluorine gradually lost its grip.

This led the researchers to question the conventional explanation. Instead, the explanation appeared to be that the short-wavelength UV radiation caused the water itself to form hydrogen radicals – extremely short-lived hydrogen compounds with an unpaired electron – powerful enough to tear fluorine atoms from the grip of the PFAS.

In this system, the degradation therefore appears to be driven by hydrogen radicals originating from the water itself. Under the right light conditions, the water is therefore not merely a solvent – it delivers the attack itself – without the researchers having to add auxiliary chemicals or create a strongly basic environment.

“By identifying the true driving force behind the reaction, this research provides a crucial piece of the puzzle. The study shows that even the most persistent pollutants can be vulnerable if we understand the chemistry well enough to target them precisely,” says Zongsu Wei.

Now begins the battle to move the technology beyond the laboratory

Although the results offer hope for a new way to destroy PFAS, the researchers emphasise that this is still basic research carried out under tightly controlled laboratory conditions. According to the researchers, the next step will be to determine whether the method also works in natural groundwater and real wastewater, in which the water contains many other substances.

If the discovery is to be translated into practice and make a real difference in aquatic environments, the researchers say that this will require entirely new types of treatment facilities.

“The challenge now is to scale up the process. To bring the technology into use outside the laboratory, we need to design closed reactors for wastewater treatment plants, in which specialised UV lamps can artificially recreate the necessary radiation,” explains Zongsu Wei.

The very composition of the water pumped up from underground also presents the researchers with new puzzles to solve. Zongsu Wei explains that natural raw water is never as clean as the laboratory test water. It contains natural components that can interfere with the process.

The team’s initial tests show that ordinary nitrate in the water can act as a kind of light shield and radical scavenger: it absorbs part of the light and scavenges the hydrogen radicals that would otherwise have attacked the PFAS before they get the chance to attack the forever chemicals. In addition, the researchers observed major differences in how resistant the PFAS compounds were to the treatment.

“The central role that hydrogen radicals play in our experiments gives us a completely new and promising direction, but we need to understand in detail how other substances in the raw water interfere with the process before we can design the final treatment systems,” concludes Zongsu Wei.