Using common reagents in heated water, chemists can 'behead' and break down PFAS, leaving only harmless compounds
From: University of California Los Angeles
August 18, 2022 -- If
you're despairing at recent reports that Earth's water sources have been thoroughly
infested with hazardous human-made chemicals called PFAS that can last for
thousands of years, making even rainwater unsafe to drink, there's a spot of
good news. Chemists have developed a simple way to break down almost a dozen
types of these nearly indestructible 'forever chemicals' at relatively low
temperatures with no harmful byproducts.
In a paper published
today in the journal Science, the researchers show that in water
heated to just 176 to 248 degrees Fahrenheit, common, inexpensive solvents and
reagents severed molecular bonds in PFAS that are among the strongest known and
initiated a chemical reaction that "gradually nibbled away at the
molecule" until it was gone, said UCLA distinguished research professor
and co-corresponding author Kendall Houk.
The simple technology,
the comparatively low temperatures and the lack of harmful byproducts mean
there is no limit to how much water can be processed at once, Houk added. The
technology could eventually make it easier for water treatment plants to remove
PFAS from drinking water.
Per- and
polyfluoroalkyl substances -- PFAS for short -- are a class of around 12,000
synthetic chemicals that have been used since the 1940s in nonstick cookware,
waterproof makeup, shampoos, electronics, food packaging and countless other
products. They contain a bond between carbon and fluorine atoms that nothing in
nature can break.
When these chemicals
leach into the environment through manufacturing or everyday product use, they
become part of the Earth's water cycle. Over the past 70 years, PFAS have
contaminated virtually every drop of water on the planet, and their strong
carbon-fluorine bond allows them to pass through most water treatment systems
completely unharmed. They can accumulate in the tissues of people and animals
over time and cause harm in ways that scientists are just beginning to
understand. Certain cancers and thyroid diseases, for example, are associated
with PFAS.
For these reasons,
finding ways to remove PFAS from water has become particularly urgent.
Scientists are experimenting with many remediation technologies, but most of
them require extremely high temperatures, special chemicals or ultraviolet
light and sometimes produce byproducts that are also harmful and require
additional steps to remove.
Leading PFAS to the
guillotine
Northwestern chemistry
professor William Dichtel and doctoral student Brittany Trang noticed that
while PFAS molecules contain a long "tail" of stubborn
carbon-fluorine bonds, their "head" group often contains charged
oxygen atoms, which react strongly with other molecules. Dichtel's team built a
chemical guillotine by heating the PFAS in water with dimethyl sulfoxide, also
known as DMSO, and sodium hydroxide, or lye, which lopped off the head and left
behind an exposed, reactive tail.
"That triggered
all these reactions, and it started spitting out fluorine atoms from these
compounds to form fluoride, which is the safest form of fluorine," Dichtel
said. "Although carbon-fluorine bonds are super-strong, that charged head
group is the Achilles' heel."
But the experiments
revealed another surprise: The molecules didn't seem to be falling apart the
way conventional wisdom said they should.
To solve this mystery,
Dichtel and Trang shared their data with collaborators Houk and Tianjin
University student Yuli Li, who was working in Houk's group remotely from China
during the pandemic. The researchers had expected the PFAS molecules would
disintegrate one carbon atom at a time, but Li and Houk ran computer
simulations that showed two or three carbon molecules peeled off the molecules
simultaneously, just as Dichtel and Tang had observed experimentally.
The simulations also
showed the only byproducts should be fluoride -- often added to drinking water
to prevent tooth decay -- carbon dioxide and formic acid, which is not harmful.
Dichtel and Trang confirmed these predicted byproducts in further experiments.
"This proved to be
a very complex set of calculations that challenged the most modern quantum
mechanical methods and fastest computers available to us," Houk said.
"Quantum mechanics is the mathematical method that simulates all of
chemistry, but only in the last decade have we been able to take on large
mechanistic problems like this, evaluating all the possibilities and determining
which one can happen at the observed rate."
Li, Houk said, has
mastered these computational methods, and he worked long distance with Trang to
solve the fundamental but practically significant problem.
The current work
degraded 10 types of perfluoroalkyl carboxylic acids (PFCAs) and perfluoroalkyl
ether carboxylic acids (PFECAs), including perfluorooctanoic acid (PFOA). The
researchers believe their method will work for most PFAS that contain
carboxylic acids and hope it will help identify weak spots in other classes of
PFAS. They hope these encouraging results will lead to further research that
tests methods for eradicating the thousands of other types of PFAS.
The study,
"Low-temperature mineralization of perfluorocarboxylic acids," was
supported by the National Science Foundation.
https://www.sciencedaily.com/releases/2022/08/220818163721.htm
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