Ultrasound cleans “forever chemicals” from polluted groundwater.

Scientific Frontline reports on new Ohio State University research that shows sound waves can shake apart toxic PFAS, molecules so durable they previously earned the nickname “forever chemicals”:

Invented nearly a century ago, per- and poly-fluoroalkyl substances, also known as “forever chemicals,” were once widely used to create products such as cookware, waterproof clothing and personal care items. Today, scientists understand that exposure to PFAS can cause a number of human health issues such as birth defects and cancer. But because the bonds inside these chemicals don’t break down easily, they’re notoriously difficult to remove from the environment.

By conducting experiments on lab-made mixtures containing three differently sized compounds of fluorotelomer sulfonates – PFAS compounds typically found in firefighting foams – their results showed that over a period of three hours, the smaller compounds degraded much faster than the larger ones. This is in contrast to many other PFAS treatment methods in which smaller PFAS are actually more challenging to treat.

“We showed that the challenging smaller compounds can be treated, and more effectively than the larger compounds,” said co-author of the study Linda Weavers, a professor of civil, environmental and geodetic engineering at The Ohio State University. “That’s what makes this technology potentially really valuable.”

One of only a few studies to probe into how ultrasound might be used to rid our surroundings of toxic PFAS chemicals, this paper is an extension of previous research of Weavers’ that determined that the same technology could also degrade pharmaceuticals in municipal tap and wastewater.

Unlike other traditional destruction methods that attempt to break down PFAS by reacting them with oxidizing chemicals, ultrasound works to purify these substances by emitting sound at a frequency much lower than typically used for medical imaging, said Weavers. Ultrasound’s low-pitched pressure wave compresses and pulls apart the solution, which then creates pockets of vapor called cavitation bubbles.

“As the bubbles collapse, they gain so much momentum and energy that it compresses and over-compresses, heating up the bubble,” said Weavers.

Much like powerful combustion chambers, the temperatures inside these tiny bubbles can reach up to 10,000 Kelvin, and it’s this heat that breaks down the stable carbon-fluorine bonds that PFAS are made of and renders the byproducts essentially harmless. Unfortunately, this degradation method can be costly and extremely energy intensive, but with few other options, it may be something the public needs to consider investing in to protect groundwater for drinking and other uses, said Weavers.

You can read more of Weavers’ research here, in The Journal of Physical Chemistry A.