PFAS destruction techniques – can UV be part of a solution?

PFAS stands for a group of perfluoroalkyl and polyfluoroalkyl substances. Known as forever chemicals, PFAS are incredibly durable and do not break down over time which means they are now attracting international attention over the environmental impact PFAS are having on our water supplies.
Posted: Tuesday, 1 November 2022

What are PFAS?

PFAS are used in commercial outputs such as oil, water, temperature, chemical and fire resistance products, paintings, clothing, Teflon® coatings, paper packaging products, as well as electrical insulating properties.

PFAS were invented in the 1930’s as part of a foam substance that was very good at putting out fires and were in commercial production by the 1950’s. By the 1970’s measurement of PFAS levels was in progress and in the 1990’s research advancements made it clear that these compounds were beginning to be found in more and more unexpected places causing unforeseen health and environmental damage.

From the 2000’s onwards there has been international recognition of the emerging environmental crisis, and work introduced to help reduce usage and increase actions to help the environment recover. This is ongoing to this day.

In the monitoring and research, it became clear that PFAS has reached the water supply and are now widely found in both drinking and commercial supplies.

While whole industries have sprung up to monitor and measure PFAS, they remain very difficult to remove from our environment and various organisations continue to look into how to do this including the EPA in the United States. The EPA’s statement on PFAS on their website gives us a good understanding of the scale of the task facing us all.

EPA’s researchers and partners across the country are working hard to answer critical questions about PFAS:

  • How to better and more efficiently detect and measure PFAS in our air, water, soil, and fish and wildlifeHow much people are exposed to PFAS
  • How harmful PFAS are to people and the environment
  • How to remove PFAS from drinking water
  • How to manage and dispose of PFAS

Governments are also involved in regulating the uses of PFAS such as in the UK where various pieces of legislation have been introduced over the past 13 years, to help restrict usage, and control standards. For example, UK drinking water must contain PFAS chemicals at no more than 100 nanograms per litre (ng/l).

What’s the challenge to the water industry?

Like other forever chemicals, PFAS are found in low concentrations but build up over the course of many years. So, it is only now that we are beginning to see the impact these chemicals may have on human conditions.One of the ways PFAS are finding their way into humans is via water. PFAS have been found in both influent and effluent of wastewater treatment plants which are considered as one of the major sources for their occurrence in surface and groundwater. Like other forever chemicals, PFAS compositions make for a very challenging degradation and/or removal from the water.

Can PFAS be broken down?

Part of the research around how to reduce contamination levels has included testing how ultraviolet-C (UVC) and vacuum ultraviolet (VUV) light could affect PFAS.

Various research papers have been published looking at this subject and all conclude that when testing PFAS oxidative and reductive degradation using ultraviolet light, low-pressure lamps providing only 254nm UVC light (and any UV light over 220nm) produced little to no degradation of PFAS (ref 1.)

However, testing at below 200nm (using VUV light) produced a much stronger absorption rate due to the higher photon energy generated during VUV photolysis (ref 2.). Although it is worth noting that the process produces ozone which may influence the results.

Research has also looked at the use of medium-pressure UV lamps in the degradation of PFAS. These lamps have a wider range of ultraviolet light (220-460nm) and results showed a similar effect to the above. It was found that absorption of UV light was much higher for the deep-UV region to 220 nm but was much lower for 220–270 nm. However, with the addition of oxygen through ferric ion (Fe3+) to the process, the rate of degradation increased between 2.7 and 3.8 fold (Ref 3.).

Even better results were seen when medium-pressure UV lamps were used with the addition of titanium dioxide. When composite materials were used to improve the photocatalytic degradation of PFASs, such as TiO2-MWCNT and TiO2-rGO it was found that the degradation efficiency of PFOA could reach 100% when using TiO2-MWCNT after 8 h under irradiation of UV at 265 nm [Ref 4.]. This is potentially excellent news for the direction of future treatment developments.

So where does this leave us?

The research carried out so far shows us that ultraviolet light using the wider range of medium-pressure UV lamps, combined with composite materials such as titanium dioxide can be part of the solution to removing PFAS from our water supplies. The challenge is that the research undertaken so far to understand this potential has not translated into real-world applications. Much more work is needed to make this effective and viable on a scale that can be commercially useful. 

Research references:
Scholarly Community Encyclopedia – https://encyclopedia.pub/entry/164381. 
1. Giri, R.R.; Ozaki, H.; Okada, T.; Taniguchi, S.; Takanami, R. Factors influencing UV photodecomposition of perfluorooctanoic acid in water. Chem. Eng. J. 2012, 180, 197–203.2.
2. Chen, J.; Zhang, P.-Y.; Liu, J. Photodegradation of perfluorooctanoic acid by 185 nm vacuum ultraviolet light. J. Environ. Sci. 2007, 19, 387–390.3.
3. Hori, H.; Yamamoto, A.; Koike, K.; Kutsuna, S.; Osaka, I.; Arakawa, R. Photochemical decomposition of environmentally persistent short-chain perfluorocarboxylic acids in water mediated by iron(II)/(III) redox reactions. Chemosphere 2007, 68, 572–578.4.
4. Song, C.; Chen, P.; Wang, C.; Zhu, L. Photodegradation of perfluorooctanoic acid by synthesized TiO2–MWCNT composites under 365nm UV irradiation. Chemosphere 2011, 86, 853–859. [CrossRef] [PubMed] 

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