RICE (US) — In the first side-by-side tests of palladium- and iron-based catalysts, palladium destroyed the carcinogen TCE up to a billion times faster than iron, a new study shows.
TCE, or trichloroethene, is a widely used chemical degreaser and solvent that has found its way into groundwater supplies the world over. The TCE molecule, which contains two carbon atoms and three chlorine atoms, is very stable—a boon for industrial users, but a bane for environmental engineers.
“It’s difficult to break those bonds between chlorine and carbon,” says study author Michael Wong, professor of chemical and biomolecular engineering and of chemistry at Rice University.
“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 says. “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 US, TCE is found at more than half the contaminated waste sites on the Environmental Protection Agency’s Superfund National Priorities List. At US 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.
With palladium, 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 says.
In the new study published in the journal Applied Catalysis B: Environmental, 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 percent of the TCE in the solution. This took less than 15 minutes for each of the palladium catalysts. The reaction rate was so much slower for both the iron nanoparticles and the iron powder that extra material had be added. With the additional material, it took the iron nanoparticles more than 25 hours to break down 90 percent of the TCE. 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 says.
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.
The research was supported by the National Science Foundation, the Welch Foundation, and the China Scholarship Council.
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