07-15 tap water

[Image above] PFAS chemicals infiltrate water supplies, the environment, and our bodies as well—how can we get rid of them? Credit: Pixabay

When my grandmother moved from her trailer into a nursing home, she had to downsize considerably, which led to us grandchildren being gifted many of her belongings.

I inherited quite a few kitchen supplies, and I use many of the utensils on a regular basis. However, there is one item that I rarely use—her Teflon frying pan.

Teflon is a DuPont brand name for the chemical compound polytetrafluoroethylene (PTFE). PTFE belongs in a family of compounds known as PFAS, or per- and polyfluoroalkyl substances.

PFAS are a group of more than 4,700 manmade chemicals containing linked chains of carbon and fluorine. The first PFAS were invented in the late 1930s, and two in particular—perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS)—quickly gained widespread application in a variety of consumer products, including nonstick cookware like Teflon (which was produced using PFOA until 2013).

Yet as PFAS became a staple in many consumer products, studies showing the negative environmental and health impacts of these chemicals began to grow as well. In particular, 3M and DuPont, two companies that specialize in chemicals, started documenting the negative impacts as early as the 1950s and 1960s.

Instead of sharing this knowledge with the public and regulators, 3M and DuPont concealed and downplayed the dangers of PFAS for decades. It wasn’t until American environmental attorney Robert Bilott, on behalf of a West Virginia farmer, began investigating the situation in 1998 that the dangers of PFAS chemicals came to light—a story recently depicted in the 2019 film “Dark Waters.”

The United States Congress also recently doubled down on the dangers of PFAS. In December 2019, the House formally recognized that the Department of Defense is a significant contributor of nationwide PFAS contamination and successfully included numerous provisions in the fiscal year 2020 National Defense Authorization Act to limit use of PFAS, such as phasing out the use of DOD PFAS-containing firefighting foams.

Unfortunately, even if use of PFAS is limited in the future, there still exists the problem of PFAS already in the environment.

PFAS are known as “forever” chemicals because, once created, they are notoriously difficult to break down. This stability means these chemicals stick around far after PFAS-containing products are sent to the dump. And they do not stick around only in the environment—more than 97% of people living in the U.S. have a detectable amount of PFAS in their blood, as explained in the video below.

Credit: Bloomberg Law, YouTube

To date, incineration is considered the most effective way to dispose of PFAS waste. However, this method is not very cost effective, cannot be used to remove PFAS in our water supplies, and may actually do more to spread PFAS than to break it down.

In recent years, researchers started investigating the potential of photocatalytic degradation to remove PFAS from water supplies. In photocatalytic degradation, a material called a photocatalyst uses energy from light to accelerate a chemical reaction that breaks down the reacting substance.

Traditionally, photocatalytic degradation of PFAS was not widely pursued because of the limited ability of common semiconductor materials to break the carbon–fluorine bonds in aqueous systems. However, some materials—indium oxide (In2O3), gallium oxide (Ga2O3), and titanium dioxide (TiO2) in particular—degrade some PFAS quite well under ultraviolet light.

There is much room to improve on photocatalytic degradation of PFAS, and one researcher investigating that topic is Michael Wong, William M. McCardell Professor in Chemical Engineering at Rice University.

Wong specializes in catalysts, and about 18 months ago, he tasked his team with finding new photocatalysts that could degrade PFOA, one of the most common PFAS chemicals found in water supplies, the environment, and people today.

To their complete surprise, one material stood out above the rest in terms of photocatalytic ability—boron nitride.

Boron nitride: An unexpected photocatalyst

The reason the Rice University researchers were surprised by boron nitride’s success was because of the wavelength of light involved in the experiments.

As mentioned before, photocatalysts work by using energy from light to accelerate a reaction. However, not every light will trigger a photocatalyst—the incoming light must have enough energy to activate the band gap in a material, and energy is determined by wavelength. (The shorter the wavelength, the more energy.)

In their search for a photocatalyst, the researchers used ultraviolet light with a wavelength of 254 nanometers to investigate the photocatalytic properties of various materials. This wavelength should be too long (i.e., not have enough energy) to activate boron nitride, which is why they used boron nitride as a control group in experiments.

While none of the experimental groups performed well, boron nitride did.

“Here’s the observation,” Wong explains in a Rice University press release. “You take a flask of water that contains some PFOA, you throw in your [boron nitride] powder, and you seal it up. That’s it. You don’t need to add any hydrogen or purge it with oxygen. … You expose that to ultraviolet light, specifically to UV-C light with a wavelength of 254 nanometers, come back in four hours, and 99% of the PFOA has been transformed into fluoride, carbon dioxide and hydrogen.”

The researchers were confused and decided to hold off on publishing the results until they could offer a plausible explanation for the observations—an explanation that ended up centering around atomic defects.

“We concluded that our material does absorb the 254-nanometer light, and it’s because of atomic defects in our powder,” Wong says. “The defects change the bandgap. They shrink it enough [lower the energy requirement] for the powder to absorb just enough light to create the reactive oxidizing species that chew up the PFOA.”

An illustration of the boron nitride photocatalysis that destroys the pollutant PFOA in water. Credit: Michael Wong, Rice University

More experimentation will be needed to confirm the explanation. But one thing is for certain—boron nitride can destroy other PFAS as well, specifically GenX.

GenX is a PFAS that was widely used to replace PFOA when that chemical was banned. More studies are suggesting that GenX could be just as big an environmental problem as PFOA, but unfortunately, there so far has been no success in using catalysts to degrade GenX.

When Wong and colleagues tested the ability of boron nitride to degrade GenX, the results were not as good as with PFOA—two hours exposure only led to about 20% of the GenX being destroyed—but Wong says the team has ideas about how to improve the catalyst for GenX.

“The research has been fun, a true team effort,” Wong says. “We’ve filed patents on this, and [the Rice-based Nanosystems Engineering Research Center for Nanotechnology-Enabled Water Treatment] interest in further testing and development of the technology is a big vote of confidence.”

The paper, published in Environmental Science & Technology Letters, is “Efficient photocatalytic PFOA degradation over boron nitride” (DOI: 10.1021/acs.estlett.0c00434).

Update 07/16/2020 – Clarification of relationship between band gap, energy, and wavelength