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Destroying PFAS, the 'forever chemicals'

SCIENCE

Perfluorinated and polyfluorinated alkyl substances, or PFASs for short, are pretty much indestructible chemicals. They don’t biodegrade and end up accumulating in humans and polluting the environment. Controlling them is largely confined to keeping these ‘forever chemicals’ out of the water supply. But not that well, it appears. Around the world, there are thousands of contaminated sites; more than 2800 of them are littered across the US and there are at least 90 in Australia. Suspected health effects include asthma, cancer and changes in the reproductive organs. And up until now, no one’s known how to dispose of the chemicals, let alone cleanup the sites already affected. Fortunately, reports Scientific American, a new approach is showing some promise — at least for disposing of the chemicals themselves.


Industry likes PFASs for what they can do: they repel both oil and water and resist damage from high temperatures and chemicals, making them ideal for many consumer products, as well for such things as firefighting foam. And, perhaps not surprisingly, as a result they can now be found almost everywhere, in the soil and groundwater.


The challenge is to break the carbon-fluorine bonds PFASs have. In a study published in the February 2022 issue of the Journal of Environmental Engineering, the US Environmental Protection Agency (EPA) found that applying a heat and pressure-based technique, known as supercritical water oxidation, destroyed almost all of the PFASs in a water sample. When oxidising substances were added to PFAS-contaminated water and heated above its critical temperature of 374°C at a pressure of more than 220 bars, the water became what’s called supercritical: neither a gas nor a liquid. In this state, even water-repellent substances such as PFASs dissolve much more readily.


Various versions of this technique had previously been developed to break down different types of chemicals, but this is the first time it’s been tested on PFASs in a peer-reviewed study. The researchers tried methods from three companies, each differing slightly in the chemicals and processes used. But all delivered: in each case, the quantity of PFASs in the water dropped by more than 99%.

“Given that supercritical water oxidation systems are already commercially available, this may be a technology that could soon be deployed for significantly impacted sites or waste waters,” says EPA researcher Max J. Krause. “We are currently evaluating air emissions to understand all of the pathways and to be certain we are destroying the PFAS.”


The new study also found that current techniques used to identify what chemicals are in the PFAS only show a fraction of them; the variety of industrially-used PFASs is now so large that many of them are barely known.


There is a caveat, though. It only potentially deals with part of the problem. While this new hot-water method does reduce the invisible pollution caused by the enormous number of known and obscure PFASs, it doesn’t entirely reduce the threat of these ‘forever chemicals’.


“The idea of a supercritical fluid being used to destroy PFAS seems like it could be a clever option, but I wonder about its practical application,” says Jamie DeWitt, an associate professor of pharmacology and toxicology at East Carolina University’s Brody School of Medicine, who wasn’t involved in the new study. “It may be a great idea on the benchtop, but can it be scaled up to a watershed or even a drinking water treatment facility?”


For one thing, the technology is relatively complex — and therefore expensive — because of the high temperatures and pressures involved. And for another, it is currently unrealistic to clean soils and groundwater already contaminated. That’s why some experts contend the use of PFASs should be limited to only the most necessary applications. “Some argue that the persistence of PFAS is so great that [they should be phased] out of production,” DeWitt says, “as they are not part of a sustainable world.”


Given that supercritical water oxidation systems are already commercially available, this may be a technology that could soon be deployed for significantly impacted sites or waste waters.

Max J. Krause


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