A Rice University study finds palladium catalysts destroy the carcinogen TCE up to a billion times faster than iron catalysts.

In the first side-by-side tests of a half-dozen palladium-
and iron-based catalysts for cleaning up the carcinogen TCE, Rice University
scientists have found that palladium destroys TCE far faster than iron—up to a
billion times faster in some cases.

The results will appear in a new study in Applied Catalysis B: Environmental.

TCE, or trichloroethene, is a widely used chemical degreaser
and solvent that’s found its way into groundwater supplies the world over. The
TCE molecule, which contains two carbon atoms and three chlorine atoms, is very
stable. That stability is a boon for industrial users, but it’s a bane for
environmental engineers.

“It’s difficult to break those bonds between chlorine and
carbon,” said study author Michael Wong, professor of chemical and biomolecular
engineering and of chemistry at Rice. “Breaking some of the bonds, instead of
breaking all the carbon-chlorine bonds, is a huge problem with some TCE
treatment methods. Why? Because you make byproducts that are more dangerous
than TCE, like vinyl chloride.”

“The popular approaches are, thus, those that do not break
these bonds. Instead, people use air-stripping or carbon adsorption to
physically remove TCE from contaminated groundwater,” Wong said. “These methods
are easy to implement but are expensive in the long run. So, reducing water
cleanup cost drives interest in new and possibly cheaper methods.”

In the U.S.,
TCE is found at more than half the contaminated waste sites on the
Environmental Protection Agency’s Superfund National Priorities List. At U.S. military
bases alone, the Pentagon has estimated the cost of removing TCE from
groundwater to be more than $5 billion.

In the search for new materials that can break down TCE into
nontoxic components, researchers have found success with pure iron and pure
palladium. In the former case, the metal degrades TCE as it corrodes in water,
though sometimes vinyl chloride is formed. In the latter case, the metal acts
as a catalyst; it doesn’t react with the TCE itself, but it spurs reactions
that break apart the troublesome carbon-chlorine bonds. Because iron is considerably
cheaper than palladium and easier to work with, it is already used in the
field. Palladium, on the other hand, is still limited to field trials.

Wong led the development of a gold-palladium nanoparticle
catalyst approach for TCE remediation in 2005. He found it was difficult to
accurately compare the new technology with other iron- and palladium-based
remediation schemes because no side-by-side tests had been published.

“People knew that iron was slower than palladium and
palladium-gold, but no one knew quantitatively how much slower,” he said.

In the new study, a team including Wong and lead author
Shujing Li, a former Rice visiting scholar from Nankai University, China,
ran a series of tests on various formulations of iron and palladium catalysts.
The six included two types of iron nanoparticles, two types of palladium
nanoparticles—including Wong’s palladium-gold particle—and powdered forms of
iron and palladium-aluminum oxide.

The researchers prepared a solution of water contaminated
with TCE and tested each of the six catalysts to see how long they took to
break down 90% of the TCE in the solution. This took less than 15 min for each
of the palladium catalysts and more than 25 hrs for the two iron nanoparticles.
For the iron powder, it took more than 10 days.

“We knew from previous studies that palladium was faster,
but I think everyone was a bit surprised to see how much faster in these
side-by-side tests,” Li said.

Wong said the new results should be helpful to those who are
trying to compare the costs of conducting large-scale tests on catalytic
remediation of TCE.

Source: Rice University