A catalyst capable of cleaning toxic nitrates from drinking water by converting them into water and air has been discovered by engineers at Rice University’s Nanotechnology Enabled Water Treatment (NEWT) Center.
The research can be viewed online in the American Chemical Society journal ACS Catalysis.
Nitrates come mainly from agricultural runoff, which affects farming communities all over the world. 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.
Michael Wong, Lead Scientist
Wong’s lab focuses on developing nanoparticle-based catalysts, referring to submicroscopic bits of metal that accelerate chemical reactions. In 2013, his group demonstrated that small gold spheres dotted with specks of palladium have the potential to break apart nitrites, considered to be the more toxic chemical cousins of nitrates.
“Nitrates are molecules that have one nitrogen atom and three oxygen atoms,” Wong explained. “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,” he said. “More than 75 percent of Earth’s atmosphere is gaseous nitrogen, so we’re really turning nitrates into air and water.”
Nitrates are toxic to pregnant women and infants and may also be carcinogenic. Nitrate pollution is frequently observed in agricultural communities, particularly in the U.S. Corn Belt and California’s Central Valley, where fertilizers are greatly used, and a few studies have demonstrated that nitrate pollution is on the rise because of changing land-use patterns.
Nitrates are regulated by the Environmental Protection Agency, which sets acceptable limits for safe drinking water. In communities with polluted lakes and wells, that usually means pretreating drinking water with ion-exchange resins capable of trapping and removing nitrates without destroying them.
From their earlier work, Wong’s team found that gold-palladium nanoparticles were not good catalysts to help break apart nitrates. Co-author Kim Heck, a research scientist in Wong’s lab, stated that a search of published scientific literature gave rise to another possibility: palladium and indium.
“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,” Heck said. “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 association with chemical engineering colleagues Jeffrey Miller of Purdue University and Lars Grabow of the University of Houston, the Rice team discovered that the indium accelerates the breakdown of nitrates while the palladium actually prevents the indium from being oxidized permanently.
“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 explains his team will work with industrial partners and several other researchers in order to turn the process into a commercially viable water-treatment system.
“That’s where NEWT comes in,” he said. “NEWT is all about taking basic science discoveries and getting them deployed in real-world conditions. This is going to be an example within NEWT where we have the chemistry figured out, and the next step is to create a flow system to show proof of concept that the technology can be used in the field.”
NEWT is considered to be a multi-institutional engineering research center located at Rice that was established by the National Science Foundation in 2015 in order to produce compact, mobile, off-grid water-treatment systems capable of providing clean water to millions of people and making U.S. energy production more cost-effective and sustainable. NEWT is anticipated to leverage over $40 million in industrial and federal support by 2025 and is keen on applications for rural water systems, humanitarian emergency response and wastewater treatment and reuse at remote sites, including both offshore and onshore drilling platforms for gas and oil exploration.
The other co-authors include Sujin Guo, Huifeng Qian and Zhun Zhao, all of Rice, and Sashank Kasiraju of the University of Houston. The research was financially supported by funds from the National Science Foundation, the Department of Energy and the China Scholarship Council.