The 3D molecular structure of perfluorobutanoic acid, a type of PFAS “forever chemical”
Shutterstock/Sergei Shimanovich
The US military is testing a method of destroying compounds known as “forever chemicals” by mixing water and hot air under pressure. The tests will be carried out in contaminated groundwater on two Air Force bases and a Navy base as part of a wider search for technologies that can break down the long-lasting molecules.
Perfluoroalkyl and polyfluoroalkyl substances (PFAS) are a class of synthetic chemicals used since the 1940s that can be toxic to humans at very low concentrations. They are used to create coatings and foams that resist heat, oil and water, and have been found in air, soil and water all over the world, prompting cleanup efforts. But PFAS don’t break down easily and are difficult to destroy.
PFAS contamination is particularly extensive at US military bases, where foam containing PFAS has been widely used to extinguish fires. The US Department of Defense (DOD) has put billions of dollars towards finding alternatives to the foam, as well as developing better ways to destroy PFAS to prevent the chemicals going to landfills where they can leach out into soil and water again.
Existing methods of incineration may be able to destroy some types of PFAS but can also lead to further contamination. This has made the practice controversial, and the DOD temporarily suspended it in 2022.
The DOD is now planning to test a newly developed PFAS-destroying system designed by a company called 374Water. The system will be tested on two Air Force bases and one Navy base, treating PFAS from contaminated groundwater as soon as fall 2023. The DOD is still deciding which locations will host the tests.
Doug Hatler, 374Water’s chief revenue officer, says the company’s system works by mixing a slurry of water and contaminated material with hot air within a high-pressure reaction chamber. At temperatures above 374°C (705°F), the slurry reaches a “supercritical” state where it behaves both like a liquid and a gas. This technique is called supercritical water oxidation, or “SCWO”.
“The water becomes dissociated,” says Igor Novosselov at the University of Washington in Seattle. It mixes with the air to generate loose molecules made up of a single hydrogen and oxygen atom. These molecules can then cleave the carbon-fluorine bonds that make PFAS so tough. “[The chamber] is a very aggressive environment,” he says.
At the military bases, the system will treat a concentrated mixture of water and absorbent materials used to filter PFAS out of the groundwater.
The current system can fit inside a shipping container and process slurry at around the same rate as a low-pressure garden hose, or over 5600 litres per day. A larger unit in development will be able to process five times as much waste, but Hatler says that is still a relatively small volume, and the system is best suited for concentrated waste streams.
The smaller system uses 300 kilowatt hours of electricity per day, but Hatler says the larger system may instead be able to generate 300 kilowatt hours daily by recycling the heat produced when breaking down the waste. “The waste actually becomes the fuel,” he says.
Novosselov, who is not involved with 374Water, says lab tests on SCWO show it can destroy even the toughest PFAS, but reactions may be less efficient with more complex, real-world waste at larger scales.
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