PFAS, or per- and polyfluoroalkyl substances, have become known as forever chemicals. They do not break down easily, which is both their chief advantage and hazard. They contaminate soil and water and bioaccumulate in our bodies. And science does not know much about them.

And so PFAS remediation has become an urgent need. Science is only beginning to understand how to destroy forever chemicals.

PFAS comprise a family of several thousand fluorinated chemicals. Broadly speaking, their carbon atoms are either partially or fully bonded to fluorine atoms.

Beyond that, their chemical and physical properties vary widely. Many remain largely unknown. In fact, existing analytical techniques cannot even identify all of them.

They repel both oil and water and resist heat. This rare combination makes them very useful for coatings, flame retardants, and firefighting foam. PFAS have also appeared in a wide variety of consumer products, including non-stick cookware and stain-resistant carpets. They’re also part of the compressed paper packaging you get from fast food restaurants or microwave popcorn.

Some PFAS, such as those used in cookware like Teflon, have been banned in the US. Manufacturers elsewhere still make them, so they can still turn up in imported consumer goods.

PFAS still made and used still last forever in the environment.

Although PFAS have been used since the 1950s, they have only recently come to the attention of environmental and public health investigators. Not until the 2010s did the analytical methods of detecting low levels of PFAS become widely available.

They are among a number of chemicals that have been introduced with great fanfare to solve problems and later turn out to be dangerous.

In the case of forever chemicals, they have been linked to such health problems as liver damage, immune system suppression, asthma, changes in reproductive organs, and some cancers.

This article will describe two experimental PFAS remediation techniques that have recently made headlines.

Currently available PFAS remediation techniques

The chemistry of forever chemicals makes many standard decontamination techniques ineffective. Those that work or can work can be divided into three categories:

• Field-implemented technologies currently used commercially at full scale by multiple companies
• Limited application technologies that have been field tested but not yet demonstrated workable at scale
• Developing technologies that work well under laboratory conditions but have not been field tested

Currently available field-implemented technologies require removing contaminated material from its site. That is, treating groundwater requires pumping it and treating soil requires excavation. No currently available field-implemented technology treats PFAS in situ.

They use techniques that sequester PFAS, such as activated charcoal or ion exchange media. In other words, they remove PFAS from soil or water but do not change them chemically. They result in clean soil or water plus a waste that needs proper disposal. Disposal approaches include landfilling and incineration.

Some limited application technologies likewise sequester PFAS. About a dozen others explore various processes that chemically destroy these forever chemicals. These are technologies proven to work on other contaminants—some, at least, in situ—but not yet proven both effective and economical against PFAS. A few limited application technologies explore using ultraviolet rays, microbes, or fungi to degrade PFAS.

Here are two limited application technologies, one that sequesters PFAS relatively cheaply and another that appears to destroy PFAS, although it is energy intensive and therefore expensive.

Phytoremediation of PFAS using hemp

Plants remove substances from the soil, including substances they have no use for. Phytoremediation, then, means using plants to remove soil pollutants.

The pollutants bioaccumulate in the plant tissues and most likely do not undergo any chemical transformation. Those plants, being polluted, cannot be used for much and become a disposal issue.

A pilot partnership in hemp phytoremediation

Loring Air Force Base near Limestone, Maine closed in 1994 and became a Superfund site. During its years of operation, it used a lot of PFAS-containing firefighting foam. The Environmental Protection Agency (EPA) started to clean it up. In 2007, the EPA declared it ready for reuse, but it remains unhealthy.

The Aroostook Band of Micmac Nation acquired 600 acres of it in 2009 and therefore the responsibility of cleaning it up. Most of this native American nation lives in Canada. The Aroostook reservation is the only one in the US.

The Micmacs have a long history of receiving poor land and a long-time commitment to protecting the land.

When the environmental activist organization Upland Grassroots approached them to test whether hemp could accomplish PFAS remediation, the tribe immediately agreed. They enlisted scientific help from the Connecticut Agricultural Experimental Station (CAES).

The former base has several other pollutants besides PFAS. The team decided to concentrate on PFAS remediation because it best suited the needs of scientists, activists, and tribal members.

Scientists rarely have opportunity to become involved in a community-based project. CAES found the Loring project attractive for that reason. Funding issues made travel unfeasible for CAES even before the pandemic made it impossible. Tribal members took on much of the physical labor. The activists took charge of communication and coordination.

This kind of cooperation among activists, community members, and scientists is rare but beneficial to all. The Loring project can be a model for similar partnerships in the future.

The experiment

The hemp phytoremediation project at Loring began in 2019 by planting several small plots of hemp. Hemp has proven successful in removing heavy metals from polluted soil. In fact, it was part of the effort to clean up Chernobyl after the meltdown.

The experiment aimed to find out what hemp does with the PFAS it takes up. How does it store them? Does it alter their chemistry in the process?

Three plots survived a drought. The soil is so polluted that anyone working in them must wear heavy protective gear. The water is likewise so polluted that the EPA does not allow using it. Therefore, irrigating the hemp required trucking in water.

After two years, the hemp had reduced the PFAS concentration in the soil.

These plots provide proof of concept, but using hemp for phytoremediation is still early in the experimental stage. Here are some unknowns:

• The experiment has not been large enough or long enough to show how much PFAS hemp can remove from soil.
• The EPA has not yet issued national standards to define a safe baseline for PFAS in soil or water.
• No land completely free of PFAS has been available to provide control data.
• Without a clear target for PFAS remediation, no one can know how many rounds of hemp planting can achieve it.
• Hemp removes PFAS from polluted soil and takes them into its own tissue. So what constitutes a safe way to dispose of it?

Potential advantages and disadvantages of hemp phytoremediation

The current standard approach to PFAS remediation is to excavate the soil, which can cost more than $25 million to clean up a 100-acre farm. Hemp phytoremediation, if it proves feasible at all, will probably cost a quarter of that amount.

It will also take a lot longer to complete the process. Chances are hemp phytoremediation of PFAS will mostly benefit farmers and other relatively small projects. Larger projects with larger budgets will prefer the speed of excavation for PFAS remediation technologies.

Destroying PFAS with supercritical water oxidation

Destroying PFAS means breaking the extremely strong bond between carbon and fluorine. Yet PFAS resist chemical processes that destroy other molecules. They also resist heat. But they can withstand only so much heat. Supercritical water exceeds that temperature.

Most substances can exist in two or more phases of solid, liquid, or vapor. The point at which a substance changes from one phase to another is called a critical temperature.

The critical temperature of water’s liquid/vapor boundary is about 374º C or 705º F. The critical temperature of some other fluid, such as PFAS dissolved in water, may be different. Above the critical temperature, the fluid is neither gas nor liquid but somehow both, a state called supercritical.

Supercritical water oxidation entails adding oxidizing substances such as ozone or catalyzed hydrogen peroxide to water contaminated with PFAS and heating the mixture above its critical temperature at a pressure of more than 3,200 psi.

A new study from the EPA tested supercritical water oxidation methods developed by three different companies. They all differ slightly in the chemicals and procedures they use, but they all reduced PFAS in the water more than 99%.

The EPA studied commercially available technologies. They can be deployed quickly for this new task, but more study is needed first. For one thing, the EPA is evaluating air emissions to make sure the process truly destroys PFAS instead of simply moving them out of the water.

But what works well in the laboratory might not work at a larger scale such as a water treatment plant. Complex technologies such as supercritical water oxidation are very expensive. In any case, the technique is not practical for remediating soil or groundwater.

In answer to the question posed in the title, we can destroy forever chemicals under laboratory conditions now. Practical methods will come eventually but probably not soon.

Sources:

A community based PFAS phytoremediation project at the former Loring Airforce Base / Sara L. Nason et al., iScience 24 (July 23, 2021)
How to destroy “forever chemicals” / Lars Fischer, Scientific American. January 31, 2022
PFAS—per- and polyfluroalkyl substances: treatment technologies / Interstate Technology and Regulatory Council. December 2021
A tribe in Maine is using hemp to remove “forever chemicals” from the soil / Lina Tran, Grist. February 22, 2022