Bursting bubbles break down PFAS

The sea-dwelling mantis shrimp strikes its prey with enough force that even if it misses, it creates bubbles of gas that rapidly collapse, sending shockwaves that can stun or kill. Mantisonix – a spin-out from the University of Surrey, UK – is using a similar technique to break down persistent fluorochemicals with high-frequency sound waves.

Per- and polyfluoroalkyl substances (PFAS) are persistent, man-made pollutants, sometimes called ‘forever chemicals’. These materials are hydrophobic, oleophobic, durable and chemically inert, owing to their strong carbon–fluorine bonds. These same properties make PFAS extremely hard to break down. Such chemicals are associated with a variety of health and environmental effects.

Strategies such as pyrolysis or supercritical water oxidation can destroy PFAS compounds, but these methods often require high temperatures and pressures. Electrolysis, photolysis and plasma chemistry offer ways to break down PFAS at ambient conditions. However, these typically require catalysts and other additives. Sonolysis using high-frequency ultrasound could be a way to treat PFAS contamination in water without these issues.

‘The [sound] wave creates high- and low-pressure regions in the liquid… which cause any dissolved gas in the solution to expand and contract,’ explains co-founder Madeleine Bussemaker. She adds that these bubbles ‘expand a little bit more than they contract’, leading them to become unstable once they reach a certain size. The bubbles then collapse, generating local pressures and temperatures strong enough to break down PFAS on the bubbles’ surfaces. This creates fluoride, carbon oxides and other naturally occurring compounds, depending on the exact PFAS compounds present.

For PFAS degradation to occur, the team generally uses frequencies between 200 and 1000kHz; humans can typically hear up to 20kHz. The technology works without the need for catalysts or other chemicals, uses electricity and can operate in the field, reducing the need to transport contaminated materials.

‘It’s very difficult to measure and observe the process because it happens in microseconds,’ says Bussemaker. Measuring levels of fluoride and tracking PFAS concentrations helps determine the rate of PFAS destruction.

‘If we have parts per million of our initial PFAS, we will see the shorter chain PFAS at concentrations about 1000 times lower. So, we kind of hypothesise that this degradation process occurs via chain shortening,’ explains Bussemaker.

‘PFAS in the environment normally occurs or is legislated down to levels where we’re talking about grains of sand in a swimming pool, or parts per trillion,’ says Bussemaker. These extremely dilute concentrations can make it difficult to efficiently degrade PFAS, meaning that Mantisonix uses various strategies to concentrate these molecules. These include using ion exchange resins or foam fractionation, where PFAS stick to foam bubbles that can be siphoned off.

‘In an ideal world, we wouldn’t make these PFAS and put them into the environment [in the first place],’ says Bussemaker. She adds that this technology could couple with a PFAS waste stream at a manufacturing plant to destroy waste onsite. ‘But of course, at the end of the day, [PFAS are] always going to end up in our wastewater and water treatment works.’

The team is looking to do a large-scale pilot test with a wastewater treatment company, aiming to reach destruction rates of up to 140mg/hour . Mantisonix is currently able to break down around 30mg/hour of PFAS.

‘There are 1000s of different PFAS compounds. Every waste has a different PFAS composition profile, different co-contaminant profile and depending on what industry you’re working with, has a different goal,’ explains Bussemaker. ‘One of the challenges is to show that we can work with all these different solutions to meet different customer needs.’