Ask an expert: What is PFAS?
Per- and polyfluoroalkyl substance
Per- and polyfluoroalkyl substance
Per- and polyfluoroalkyl substances (PFAS) have gained notoriety in recent years as an environmental contaminant. Its use in firefighting foams is of particular concern, with a number of contaminated sites across Australia currently under investigation.
We asked UNSW Canberra Associate Professor Terry Frankcombe to explain the chemistry behind PFAS and his team’s latest research into reducing its pollution.
What is PFAS?
PFAS stands for “per- and polyfluoroalkyl substance”. These are molecules that contain at least one chain of carbon atoms that are fully saturated with bonds to fluorine atoms. There are thousands of different types of molecules that are described by the generic term PFAS. PFAS molecules that have recently received attention include perfluorooctane sulfonate (PFOS), perfluorooctanoic acid (PFOA) and perfluorohexane sulfonate (PFHxS).
Where does it come from?
Lots of PFASs have very useful properties. They were developed by the US Navy and the 3M company in the 1940s. Since then, they have found widespread use to provide fire resistance and stain resistance to products, to make non-wetting and non-stick surfaces, to improve emulsions, etc. For example, the Teflon coating on cookware and Scotchgard treatments on furniture are made from a PFAS. PFASs are manufactured by industrial-scale chemical processes. Recently, PFAS use in firefighting foams has gained a lot of attention, particularly with regard to environmental contamination. How can PFAS be harmful to our health and the environment?
The carbon-fluorine bonds in a PFAS are very stable. That means that when PFAS enters our environment it stays there for a long time. PFASs are therefore what are called Persistent Organic Pollutants. Some PFASs (notably PFOS) are explicitly mentioned in the Stockholm Convention, which is a United Nations treaty aimed at reducing Persistent Organic Pollutants.
There is no conclusive evidence that PFASs are harmful to humans. However, PFAS elimination from the body is slow, meaning that environmental exposure can lead to accumulation. Research has shown that high levels of PFAS causes harm to animals, including causing cancer. That makes PFAS a suspected human carcinogen.
Where is it a problem in Australia?
Like in most parts of the world, low-level PFAS pollution is widespread in Australia. Higher concentrations of PFAS can be found around many sites within Australia. Often these are around sites like airports and Defence bases, where firefighting training and exercises has led to uncontained release of PFAS-based fire retardant into the local area.
What previous attempts have been used to remove these substances from the environment?
The state-of-the-art practical method for removing PFAS from the environment is physical removal. PFAS molecules in water can be adsorbed on to solid materials (activated carbon) or exchanged with ions in porous materials (ion exchange resin). The PFAS can be removed from the environment using these methods, by PFAS-contaminated material is produced, which still needs to be dealt with. This is called secondary contamination.
The stability of PFAS makes it hard to destroy. Incineration at temperatures above 1000 degrees Celsius does do the trick, at considerable expense.
Describe the method you have outlined in your latest research.
We have created a photocatalyst that degrades the carbon-fluorine bond in PFAS. By adding nitrogen atoms to the common material titanium dioxide in a particular way, we have created a material that absorbs light at a range of wavelengths, including visible light. The energy from this light breaks the carbon-fluorine bonds of PFAS molecules that approach the surface of the photocatalyst, destroying the PFAS.
What happens to the fluorine atoms once the carbon-fluorine bonds are broken?
The immediate product is hydrogen fluoride, a highly toxic and very strong acid! That's actually easy to deal with, by adding sodium or calcium salts to the products, which rapidly form inert solid precipitates and water.
What are the advantages and limitations of the photocatalyst?
As our photocatalyst works with visible wavelength light, it can be powered by sunlight. A big advantage of using sunlight is energy efficiency. Most photocatalysts work best or only work with ultraviolet light, which generally means artificial light sources have to be used. Our catalyst can be deployed in the field to decompose PFAS without using any power to produce the light that powers the actual chemical reaction that is being catalysed. The downside is that you are limited by the day-night cycle and subject to weather conditions, as well as limitations on the light intensity that is available.
What is the next phase of research for this topic?
We know the photocatalyst works. Next we need to work out the best way to deploy it in contaminated areas. We also need to demonstrate that the sunlight-driven catalysis design can reduce PFAS contamination to acceptable levels and ensure that there are no harmful environmental side effects.
Who is involved in the project?
This project would not be able to proceed without the collaboration between the Functional Materials experimental group at ANU and the theoretical work done at UNSW Canberra. We've been building this collaboration since 2012, to good effect.