New method breaks down dangerous chemicals into harmless substances. The findings show that ultrasonic technology can effectively treat per- and polyfluoroalkyl substances (PFAS), commonly known as "forever chemicals," to clean up contaminated groundwater. Developed nearly a century ago, these chemicals are used to make a wide range of products, such as non-stick pans, waterproof clothing and personal care products. Today, scientists know that exposure to PFAS can cause a range of human health problems, such as birth defects and cancer. But because the bonds within these chemicals do not break down easily, they are difficult to remove from the environment.
New research shows that ultrasonic technology can effectively treat the harmful chemical perfluorooctane sulfonate (PFAS) in contaminated groundwater. This method differs from traditional treatment methods in that it is more effective at degrading smaller PFAS compounds. This study extends previous drug degradation work by demonstrating that ultrasound can break down the stable carbon-fluorine bonds in PFAS. While this approach is costly and energy-intensive, it could be critical to protecting water sources and points to a potential direction for future water treatment technologies.
These difficulties prompted Ohio State University researchers to study how sonication - a process that uses sound to degrade substances by splitting the molecules they are made of - works on different types and concentrations of these chemicals.
Sonication: a potential solution
By experimenting with a lab-made mixture containing three different sizes of fluorobenzenesulfonic acid compounds, PFAS compounds commonly found in firefighting foam, their results showed that within three hours, the smaller compounds degraded much faster than the larger compounds. This is in contrast to many other PFOS treatment methods, where smaller PFOS are actually more difficult to treat.
"Our study shows that challenging smaller compounds can be treated more effectively than larger compounds," said study co-author Linda Weavers, a professor of civil, environmental and geotechnical engineering at The Ohio State University. "That's where the potential value of this technology lies."
The research was published in the Journal of Physical Chemistry A.
One of only a few studies to explore how ultrasound can be used to remove the toxic perfluorooctane sulfonate (PFAS) chemical from our surroundings, the paper is an extension of Weavers' previous research, which found that the same technology could also degrade pharmaceuticals in municipal tap water and wastewater.
Ultrasound versus traditional methods
"PFAS compounds are unique in that many of the destruction techniques we use in environmental engineering for other difficult-to-remove compounds don't work on them," Weavers said. "So we really need to develop a range of techniques to identify which ones might be useful in different applications."
Unlike other traditional destruction methods that attempt to break down PFOS by reacting with oxidizing chemicals, ultrasound decontaminates these substances by emitting sounds at frequencies much lower than those typically used in medical imaging. Ultrasound's low-frequency pressure waves compress and pull apart the solution, creating air pockets called cavitation bubbles. As bubbles collapse, they gain tremendous momentum and energy, compressing and overcompressing them, heating the bubbles.
"Like a powerful combustion chamber, the temperature inside these tiny bubbles can reach up to 10,000 Kelvin, and it is this heat that breaks down the stable carbon-fluorine bonds that PFOS is made of and renders the byproducts essentially harmless," Wevers said. "Unfortunately, this degradation method can be costly and extremely energy-intensive, but with few other options, the public may want to consider investing in this method to protect groundwater for drinking water and other uses."
Regulatory responses and public awareness
At the same time that manufacturing is moving away from the use of PFAS, regulators are also working to increase public awareness of how to avoid using PFAS. Earlier this year, the EPA proposed the National Primary Drinking Water Regulation (NPDWR), which would require public water systems to monitor certain PFAS, notify the public of levels of these substances and take steps to reduce their levels if levels exceed certain limits.
Because ultrasound is effective in removing PFOS from solutions, the study concluded that scientists and government agencies should consider using ultrasound in future treatment technology development and in conjunction with other integrated treatment methods.
While Weavers' research cannot yet be generalized to larger anti-pollution efforts, the study does suggest that their work could be the beginning of the creation of small, high-energy water filtration devices for the public to use in their homes.
"Our research revolves around how to scale this up and what is needed to make it work," Wevers said. "These compounds are found everywhere, so as we learn more about them, understanding how they degrade and break down is important to further the science."