Engineers at Rice University’s NSF-funded Nanotechnology Enabled Water Treatment (NEWT) Center find a catalyst that cleans toxic nitrates from drinking water by converting them into air and water. The research is available online in the American Chemical Society journal ACS Catalysis.
“Nitrates come mainly from agricultural runoff, which affects farming communities all over the world,” says Rice chemical engineer Michael Wong, the lead scientist on the study. “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, and when that happens, the flushed water just returns a concentrated dose of nitrates right back into the water supply.”
Wong’s lab specializes in developing nanoparticle-based catalysts, submicroscopic bits of metal that speed up chemical reactions. In 2013, his group showed that tiny gold spheres dotted with specks of palladium could break apart nitrites, the more toxic chemical cousins of nitrates.
“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,” Wong says. “More than 75% of Earth’s atmosphere is gaseous nitrogen, so we’re really turning nitrates into air and water.”
From their previous work, Wong’s team knew that gold-palladium nanoparticles were not good catalysts for breaking apart nitrates. Co-author Kim Heck, a research scientist in Wong’s lab, said a search of published scientific literature turned up another possibility: indium and palladium. In collaboration with chemical engineering colleagues Jeffrey Miller of Purdue University and Lars Grabow of the University of Houston, the Rice team found that the indium speeds up the breakdown of nitrates while the palladium apparently keeps the indium from being permanently oxidized.
“Indium likes to be oxidized,” Heck said. “From our in situ 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.”
Wong said his team will work with industrial partners and other researchers to turn the process into a commercially viable water-treatment system.
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