PFAS : Water Treatment: Perfectly filtered

The challenge is not entirely new. Ever since perfluorinated and polyfluorinated chemicals, or PFAS for short, were detected in wastewater, operators of wastewater treatment plants have been faced with the question of how to deal with these pollutants. There are several thousand such compounds. Due to their water, grease and dirt-repellent properties, they are used in countless products. PFAS can be found as components of textiles as well as in coated pots, in cosmetics or in extinguishing agents - just to name a few applications. If they end up in drinking water, they are considered a risk factor.

As toxicologist Marike Kolossa-Gehring from the German Federal Environment Agency explains, PFAS are suspected of damaging the thyroid gland and liver, causing cancer and weakening the immune system PFAS can also favour obesity, high blood pressure and metabolic disorders. The situation is exacerbated by the fact that there is no way to break down PFAS naturally, which is why they are sometimes referred to as "eternal chemicals". Due to their indestructibility, they accumulate indefinitely in water, soil and the human organism. The potential for damage is correspondingly high.

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The current common method of removing PFAS from water is to filter them with membranes or treat them with activated carbon. However, this approach means that although the PFAS are no longer present in the treated water, they are certainly present in the sewage sludge or in the remaining carbon. Isolating PFAS compounds in such a way that they could then be disposed of or rendered harmless has so far been impossible. However, a group of researchers from Germany and the USA have now found a way to do just that. They used an electrochemical method that exploits the behaviour of a specific group of substances, namely metal-containing polymers. "With these so-called metallocenes, we have found materials that are able to accept and accumulate PFAS compounds," reports Markus Gallei, Professor of Polymer Chemistry at Saarland University, who heads the research group.

But what is even more important: Unlike carbon or the membranes used to date, metallocenes also allow the PFAS bound in them to be specifically removed from them again - by applying an electrical voltage. While the researchers initially used ferrocene, an iron-based compound, for this purpose, they later switched to cobaltocene, which works even better at removing PFAS. "Unlike the activated carbon filter, which I have to destroy after the PFAS molecules get stuck in it, I can use metallocenes a thousand times in succession if I want to," says Gallei, explaining the breakthrough his group has achieved.

The researchers believe that their invention is definitely suitable for commercial use. "It could be scaled up," says Frank Hartmann, one of Gallei's colleagues. "By coating a membrane with the metal-containing polymers and then letting the water through. Then you would have water without PFAS."

Of course, such a solution would not be easy. After all, sewage treatment plants are rather complicated structures. Nevertheless, Markus Gallei, head of the research group, is also convinced that the use of metallocenes in an industrial wastewater treatment plant can work: "You could do it at the end of the chain, when the coarse dirt has already been removed." There are certainly interested parties from the waste management industry who would like to bring the idea to market maturity.

However, Gallei emphasises that the possibility of filtering PFAS out of water is also a milestone on the way to better understanding these substances. If it is possible to isolate them, even if they occur in very low concentrations somewhere, then this could be an important means of preventing pollution: "Then we can investigate which PFAS are involved? Where do they come from? Who is the polluter?"

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One way of minimising the risk posed by PFAS in the future could be to filter them with metallocenes, isolate them and render them harmless, but another could be to identify the sources of PFAS more precisely than before and then develop appropriate avoidance strategies. This much is clear: there are many different ways in which PFAS can end up in nature: PFAS enter the air and water via exhaust gases and wastewater. They get into the soil via rain, snow and irrigation and subsequently into food and drinking water. They have now been detected in plants and animals as well as in human blood and breast milk.

Finding out more about the substances, especially as there are so many of them, would therefore be important in any case, notes Gallei: "They can adhere to polar surfaces and non-polar surfaces and, in principle, they can be found anywhere and everywhere. Unfortunately, even in very low concentrations, these chemicals can accumulate in the human body over the years and cause damage. That's the problem." Keeping them out of drinking water is therefore a really important task.