Simple destruction of long-lived environmental toxins

In times when there is a lot of talk about critical infrastructures, the word "supercritical" is particularly in need of explanation. Physicists coined this term to describe a special state of matter.

In 1822, the French scientist Baron Charles Cagniard de la Tour discovered that liquids sealed in a pressure vessel become “supercritical fluids” above a minimum temperature and pressure. It is then no longer possible to distinguish whether it is a liquid or a gas. A supercritical fluid is as dense as a liquid and yet as inviscid as a gas. So it has a small viscosity.

For example, bringing water to its supercritical state requires a temperature of at least 374.12 degrees Celsius and a pressure of at least 221 bar. Supercritical water has significantly different properties than normal water. These are specifically exploited in various technical applications. There are coal-fired power plants that work with supercritical water in the steam process because this increases efficiency. Supercritical water is also used as a solvent. Substances can also be decomposed in this way without having to use strong acids or alkalis.

Now scientists at the University of Washington report the development of a reactor that can use supercritical water to destroy dangerous organic substances that are very difficult to destroy with other methods. Specifically, it is about the chemically very stable per- and polyfluorinated alkyl substances (PFAS), which are used in many industrial processes due to their technical properties and are also contained in commercial products.

The compounds perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) play a special role. If they get into the environment, they do not decompose there even after very long periods of time. They are now detectable in the food chain and also in humans. The extent of the health risks they pose is a matter of debate.

There is evidence that these substances impair the immune system. In any case, the European Food Safety Authority (EFSA) reassessed the health risks of PFAS in food in September 2020. Accordingly, people should take in a maximum of 4.4 nanograms (ng) of these substances per week and kilogram of body weight.

Of course, it would be better not to let them get into the environment in the first place. The Washington researchers headed by Igor Novosselov report that their technology can be used to destroy PFOA and PFOS molecules in industrial wastewater. "First of all, our reactor can heat water very quickly," explains Novosselov, "but unlike in an ordinary saucepan, it can also be heated to temperatures well above 100 degrees Celsius." Together with high pressure, this creates supercritical water. "This is chemically so aggressive that organic molecules cannot survive in this environment."

The researchers designed the reactor in such a way that the substances to be destroyed can be fed in continuously. After just 30 seconds, the problem substances have completely broken down into water, carbon dioxide and fluoride salts - salts that are also contained in toothpaste. The PFOS molecules turned out to be more stable and persistent than the PFOA molecules. While these can already be destroyed at just under 400 degrees Celsius, the controller must be turned up to at least 610 degrees Celsius for the PFOS.

Researchers initially focused on PFOS and PFOA because the U.S. Environmental Protection Agency wants to give concrete support to measures to remove these substances from the environment. But in principle, according to the researchers, their reactor with supercritical water is also suitable for destroying other problem organic substances. Wherever they have accumulated in the environment, they could, in principle, be destroyed with this technology. But of course everything has its limits. "We will not be able to rid the entire ocean of pollutants with this," says Novosselov.

"Aha! Ten minutes of everyday knowledge" is WELT's knowledge podcast. Every Tuesday and Thursday we answer everyday questions from the field of science. Subscribe to the podcast on Spotify, Apple Podcasts, Deezer, Amazon Music, among others, or directly via RSS feed.