One-step Catalyst Turns Nitrates into Water and Air

A team of engineers at Rice University's Nanotechnology Enabled Water Treatment (NEWT) Engineering Research Center published a paper detailing a method for removing toxic nitrates from drinking water.

The paper, available in the American Chemical Society journal ACS Catalysis, explains how the team developed a way to convert nitrates into air and water by use of a catalyst.

“Nitrates come mainly from agricultural runoff, which affects farming communities all over the world,” said Michael Wong, the study’s lead scientist. “Nitrates are both an environmental problem and health problem because they’re toxic. There are ion-exchange filters that can remove them from water, but these need to be flushed every few months to reuse them. When that happens, the flushed water just returns a concentrated dose of nitrates right back into the water supply.”

Nanoparticle catalysts may look like pepper, but they aren’t. (Image courtesy of Rice University.)

Wong and his team of researchers previously demonstrated that the use of gold and palladium nanoparticle catalysts could aid with nitrite removal. From principles learned in that research, the team discovered the use of nanoparticles to remove nitrates.

“Nitrates are molecules that have one nitrogen atom and three oxygen atoms,” Wong said. “Nitrates turn into nitrites if they lose an oxygen, but nitrites are even more toxic than nitrates, so you don't want to stop with nitrites. Moreover, nitrates are the more prevalent problem.

“Ultimately, the best way to remove nitrates is a catalytic process that breaks them completely apart into nitrogen and oxygen, or in our case, nitrogen and water because we add a little hydrogen—more than 75 percent of Earth’s atmosphere is gaseous nitrogen, so we’re really turning nitrates into air and water.”

While gold and palladium were suitable for nitrites, they are not effective on more prevalent nitrates. For nitrate removal the team created a different type of nanoparticle made from indium and palladium.

“We were able to optimize that, and we found that covering about 40 percent of a palladium sphere’s surface with indium gave us our most active catalyst,” said Kim Heck, research scientist and co-author of the paper. “It was about 50 percent more efficient than anything else we found in previously published studies. We could have stopped there, but we were really interested in understanding why it was better, and for that we had to explore the chemistry behind this reaction.”

In collaboration with a team from the University of Houston, the Rice researchers discovered that indium speeds up the breakdown of nitrates while palladium apparently keeps the indium from being permanently oxidized.

“Indium likes to be oxidized,” Heck said. “From our studies, we found that exposing the catalysts to solutions containing nitrate caused the indium to become oxidized. But when we added hydrogen-saturated water, the palladium prompted some of that oxygen to bond with the hydrogen and form water, and that resulted in the indium remaining in a reduced state where it’s free to break apart more nitrates.”

In the future the team will work with industrial partners to turn the process into a commercially viable water-treatment system and will make steps toward treating commercial agriculture areas affected by nitrate pollution such as the U.S. Corn Belt and California's Central Valley, where fertilizers are heavily used.

The research is available online in the ACS Catalysis journal here.