Supercritical water: this is how environmental toxins can be quickly destroyed

Science supercritical water

Easy destruction of long-lived environmental toxins

Organic pollutants can now be detected in every drop of rain Organic pollutants can now be detected in every drop of rain

Organic pollutants can now be detected in every drop of rain

Source: Getty Images/Oliver Rossic

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Organic matter from industrial wastewater is an environmental problem. Scientists from the University of Washington present a simple method with which the hazardous substances can be destroyed.

lIn times when there is a lot of talk about critical infrastructures, the word “supercritical” especially needs explanation. Physicists coined this term to describe a special state of matter.

In 1822, 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 distinguishable whether it is a liquid or a gas. A supercritical fluid is as dense as a liquid yet not as viscous as a gas. So it has a small viscosity.

For example, to bring water into its supercritical state, a temperature of at least 374.12 degrees Celsius and a pressure of at least 221 bar is required. Supercritical water has significantly different properties than normal water. These are specifically utilized in various technical applications. There are coal-fired power stations that use supercritical water in the steam process because this increases efficiency. Supercritical water is also used as a solvent. Substances can also be broken down in this way without the need for strong acids or alkalis.

Reactor destroys problem substances

Now scientists at the University of Washington are reporting the development of a reactor that can use supercritical water to destroy hazardous organics that are very difficult to destroy by other methods. In concrete terms, this concerns the chemically very stable per- and polyfluoroalkyl substances (PFAS), which are used in many industrial processes due to their technical properties and also occur in commercial products.

The compounds perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS) play a special role. If they get into the environment, they don’t decompose there, even after a very long 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.

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There are indications that these substances affect the immune system. In any case, in September 2020 the European Food Safety Authority (EFSA) reassessed the health risks of PFAS in food. People are therefore allowed to ingest a maximum of 4.4 nanograms (ng) of these substances per week and kilograms of body weight.

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

The researchers have designed the reactor in such a way that the substances to be destroyed can be supplied continuously. After just 30 seconds, the problem substances are completely broken down into water, carbon dioxide and fluoride salts – salts that are also found in toothpaste. The PFOS molecules were found to be more stable and persistent than the PFOA molecules. Although these can be destroyed at just below 400 degrees Celsius, the controller for the PFOS must be set to a minimum of 610 degrees Celsius.

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The researchers initially focused on PFOS and PFOA because the US Environmental Protection Agency specifically wants to support measures to remove these substances from the environment. But in principle their reactor with supercritical water is also suitable for destroying other problem organic substances, according to the researchers. Wherever they have accumulated in the environment, they can in principle be destroyed with this technology. But of course everything has its limits. “We won’t be able to rid the entire ocean of pollutants with this,” Novosselov says.

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