Communities across the US are desperate to rid their environments of toxic per- and polyfluoroalkyl substances (PFAS), especially when these chemicals are in their drinking water. But even when PFAS are successfully filtered out of water, disposing of the extracted material remains a challenge. Now Congress is starting to examine technologies to destroy these widely used synthetic chemicals.
Strengthened by carbon-fluorine bonds rarely found in nature, PFAS are used in applications such as cosmetics, water- and fire-resistant clothing, and aerospace equipment. But their durability also makes them environmentally persistent. Some forms of PFAS are toxic and linked to health problems including immune system dysfunction, endocrine disruption, and cancer. In hundreds of communities nationwide, water supplies are tainted with hazardous PFAS from manufacturing facilities and from the use of firefighting foams at commercial airports and military airfields.
At a Dec. 7 hearing held jointly by two subcommittees of the US House of Representatives Committee on Science, Space, and Technology, lawmakers learned about one emerging approach to PFAS destruction, supercritical water oxidation. Battelle, a nonprofit research and development organization that does contract work mainly for the US government, presented its work developing the technology.
Battelle’s PFAS Annihilator technology relies on supercritical water plus an oxidizer to break the carbon-fluorine bonds in these persistent compounds, Amy Dindal, the organization’s director of environmental research and development, said at the hearing.
The technology uses water above its critical point of 374 °C and 22 MPa and breaks PFAS into smaller molecules including hydrofluoric acid. Sodium hydroxide is added to neutralize the acid and form sodium fluoride, the organization says in an email. Sodium sulfate also forms if the PFAS contained sulfonate functional groups. A separator removes some of the salts, the rest are released at low levels in treated water, Battelle says.
Battelle’s website says its technology works on PFAS regardless of their sizes or specific structures and lowers PFAS concentrations in water to nondetectable levels within seconds.
In a recently published case study, researchers from the US Environmental Protection Agency examined supercritical water oxidation technology from Battelle, 374Water of Durham, North Carolina, and Denmark’s Aquarden Technologies. The researchers specifically looked at the potential of the technologies to destroy PFAS in water-diluted firefighting foams containing perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) (J. Environ. Eng. 2021, DOI: 10.1061/(ASCE)EE.1943-7870.0001957). They found that supercritical water oxidation technologies could destroy more than 99% of the PFAS run through them. The EPA team says more information is needed on operation and maintenance costs of supercritical water oxidation and on the technology’s potential for emitting PFAS into air or creating hazardous by-products.
While the congressional hearing gave attention only to supercritical water oxidation, this method is just one of the processes researchers are developing for PFAS destruction.
“Thermal technologies in general are one of the most promising approaches for actually achieving PFAS destruction,” Timothy J. Strathmann, a professor of civil and environmental engineering at the Colorado School of Mines, tells C&EN. Thermal technologies include incineration and pyrolysis as well as supercritical or subcritical water with oxidation, he says.
Strathmann’s group is working on subcritical hydrothermal technology that uses an alkali, like sodium hydroxide, as a reactant. The team, like many research groups, is also working on other approaches.
At the House hearing, Battelle’s Dindal told lawmakers that its PFAS Annihilator is an “economically viable solution.”
Battelle has been testing its PFAS destruction technology in the laboratory for more than 2 years, she said. In January 2022, the organization intends to field-test a mobile unit that can treat up to 1,900 L per day of liquids contaminated with PFAS, she said. The results of this work will help researchers estimate the costs of applying it commercially.
Strathmann tells C&EN that each technology under development for PFAS destruction, including the ones his group is working on, faces practical hurdles that will affect their cost. For instance, reactor systems for super- or subcritical water require relatively expensive materials to sustain pressures and temperatures and withstand corrosive fluids. New materials or coatings will be needed to maintain the equipment over extended operation times, he says.
Despite the lack of information on the economic viability of PFAS destruction, one key legislator suggested at the hearing that Battelle’s technology provides a path for continued production and use of PFAS. This counters arguments from a growing number of scientists that PFAS should be restricted to essential uses.
“This type of scalable technology should give us all comfort that economically feasible, safe, complete, and reliable destruction of PFAS is within our grasp, thus solving the most fundamental issue that comes with using these chemicals,” said Rep. Stephanie Bice (R-OK), the top Republican on the Subcommittee on Environment. “Using certain PFAS in a controlled, responsible manner is safe and effective.”
A version of this story first appeared on December 8, 2021.