Boron nitride destroys PFAS 'forever' chemicals PFOA, GenX
Pollutant-destroying properties surprise engineer: 'It's not supposed to
work'

Date:
July 7, 2020
Source:
Rice University
Summary:
Chemical engineers have discovered a photocatalyst that can destroy
99% of the 'forever' chemical PFOA in laboratory tests on polluted
water.

Researchers showed the boron nitride catalyst also destroys GenX,
a PFOA replacement that's also an environmental problem.



FULL STORY ==========================================================================
Rice University chemical engineers found an efficient catalyst for
destroying PFAS "forever" chemicals where they least expected.


==========================================================================
"It was the control," said Rice Professor Michael Wong, referring to
the part of a scientific experiment where researchers don't expect
surprises. The control group is the yardstick of experimental science,
the baseline by which variables are measured.

"We haven't yet tested this at a full scale, but in our benchtop tests
in the lab, we could get rid of 99% of PFOA in four hours," Wong said of
boron nitride, the light-activated catalyst he and his students stumbled
upon and spent more than a year testing.

Their study, which is available online in the American Chemical Society
journal Environmental Science and Technology Letters, found boron
nitride destroyed PFOA (perfluorooctanoic acid) at a faster clip than
any previously reported photocatalyst. PFOA is one of the most prevalent
PFAS (perfluoroalkyl and polyfluoroalkyl substances), a family of more
than 4,000 compounds developed in the 20th century to make coatings for waterproof clothing, food packaging, nonstick pans and countless other
uses. PFAS have been dubbed forever chemicals for their tendency to
linger in the environment, and scientists have found them in the blood
of virtually all Americans, including newborns.

Catalysts are Wong's specialty. They are compounds that bring about
chemical reactions without taking part or being consumed in those
reactions. His lab has created catalysts for destroying a number of
pollutants, including TCE and nitrates, and he said he tasked his team
with finding new catalysts to address PFAS about 18 months ago.

"We tried a lot of things," said Wong, chair of the Department of Chemical
and Biomolecular Engineering in Rice's Brown School of Engineering. "We
tried several materials that I thought were going to work. None of them
did. This wasn't supposed to work, and it did." The catalyst, boron
nitride powder, or BN, is a commercially available synthetic mineral
that's widely used in makeup, skin care products, thermal pastes that
cool computer chips and other consumer and industrial products.



==========================================================================
The discovery began with dozens of failed experiments on more likely
PFAS catalysts. Wong said he asked two members of his lab, visiting
graduate student Lijie Duan of China's Tsinghua University and Rice
graduate student Bo Wang, to do final experiments on one set of candidate compounds before moving on to others.

"There was literature that suggested one of them might be a photocatalyst, meaning it would be activated by light of a particular wavelength,"
Wong said.

"We don't use light very often in our group, but I said, 'Let's go
ahead and doodle around with it.' The sun is free energy. Let's see
what we can do with light." As before, none of the experimental groups performed well, but Duan noticed something unusual with the boron nitride control. She and Wang repeated the experiments numerous times to rule out unexpected errors, problems with sample preparation and other explanations
for the strange result. They kept seeing the same thing.

"Here's the observation," Wong said. "You take a flask of water that
contains some PFOA, you throw in your BN powder, and you seal it
up. That's it. You don't need to add any hydrogen or purge it with
oxygen. It's just the air we breathe, the contaminated water and the
BN powder. 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 problem was the light. The 254-nanometer wavelength,
which is commonly used in germicidal lamps, is too small to activate
the bandgap in boron nitride.

While that was unquestionably true, the experiments suggested it could
not be.



==========================================================================
"If you take away the light, you don't get catalysis," Wong said. "If you
leave out the BN powder and only use the light, you don't get a reaction."
So boron nitride was clearly absorbing the light and catalyzing a reaction
that destroyed PFOA, despite that fact that it should have been optically impossible for boron nitride to absorb 254-nanometer UV-C light.

"It's not supposed to work," Wong said. "That's why no one ever thought
to look for this, and that's why it took so long for us to publish the
results. We needed some sort of explanation for this contradiction."
Wong said he, Duan, Wang and co-authors offered a plausible explanation
in the study.

"We concluded that our material does absorb the 254-nanometer light,
and it's because of atomic defects in our powder," he said. "The defects
change the bandgap. They shrink it enough for the powder to absorb
just enough light to create the reactive oxidizing species that chew
up the PFOA." Wong said more experimental evidence will be needed to
confirm the explanation.

But in light of the results with PFOA, he wondered if the boron nitride catalyst might also work on other PFAS compounds.

"So I asked my students to do one more thing," Wong said. "I had
them replace PFOA in the tests with GenX." GenX is also a forever
chemical. When PFOA was banned, GenX was one of the most widely used
chemicals to replace it. And a growing body of evidence suggests that
GenX could be just as big an environmental problem as its predecessor.

"It's a similar story to PFOA," Wong said. "They're finding GenX
everywhere now. But one difference between the two is that people have previously reported some success with catalysts for degrading PFOA. They haven't for GenX." Wong and colleagues found that boron nitride powder
also destroys GenX. The results weren't as good as with PFOA: With two
hours exposure to 254-nanometer light, BN destroyed about 20% of the
GenX in water samples. But Wong said the team has ideas about how to
improve the catalyst for GenX.

He said the project has already attracted the attention of several
industrial partners in the Rice-based Nanosystems Engineering Research
Center for Nanotechnology-Enabled Water Treatment (NEWT). NEWT is an interdisciplinary engineering research center funded by the National
Science Foundation to develop off-grid water treatment systems that both protect human lives and support sustainable economic development.

"The research has been fun, a true team effort," Wong said. "We've filed patents on this, and NEWT's interest in further testing and development of
the technology is a big vote of confidence." The research was supported
by the National Science Foundation (EEC-1449500) and the China Scholarship Council.


========================================================================== Story Source: Materials provided by Rice_University. Note: Content may
be edited for style and length.


========================================================================== Journal Reference:
1. Lijie Duan, Bo Wang, Kimberly Heck, Sujin Guo, Chelsea A. Clark,
Jacob
Arredondo, Minghao Wang, Thomas P. Senftle, Paul Westerhoff,
Xianghua Wen, Yonghui Song, Michael S. Wong. Efficient
Photocatalytic PFOA Degradation over Boron Nitride. Environmental
Science & Technology Letters, 2020; DOI: 10.1021/acs.estlett.0c00434 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/07/200707160153.htm

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