ANN ARBOR—The molecular culprit behind harmful algal blooms and some contamination in groundwater is the same thing that's kept civilization fed for millennia: a stable form of nitrogen called nitrate.
Nitrates are necessary components of fertilizer, but because they are so stable, the molecules are difficult to reduce and remove from the environment. Now, University of Michigan researchers have developed a method that can reduce nitrates into compounds that could be reused as fertilizer or otherwise recycled.
The study, led by U-M chemist Nathaniel Szymczak , is published in Nature Chemistry and was supported by the National Institutes of Health and the U.S. National Science Foundation.
For as long as plants have had the ability to turn sunlight into food, nitrogen has been a critical part of that cycle. As it exists in our atmosphere, nitrogen is inert: it doesn't typically react with other elements. But through a chemical process sparked by lightning in the atmosphere or through biological processes by bacteria in soil, nitrogen can be triggered to bond with hydrogen to become ammonia, or with oxygen to form nitrates, which then can be taken up by plants and other organisms.
"Now we tend to overfertilize crops, and a huge majority of the fertilizer we apply actually leaches off with runoff into streams, groundwater, lakes and oceans," Szymczak said. "Human impacts have basically caused an imbalance, and it's impossible for biological systems to compensate for as much nitrate as we're dumping into them."
To tackle the nitrate problem, Szymczak and his team looked at how nature deals with nitrates. They found that nitrate transporter proteins—proteins that help plants use nitrates—bind to nitrates using hydrogen bonds. These hydrogen bonds are found in a halo of surrounding molecules called the "secondary sphere."
"When we look at this problem of how we actually tackle nitrate reduction, we look to the enzymes, we look to biology, and what we've found is nature has provided cues about how to bind and reduce nitrate," he said. "We found that just by having well-positioned hydrogen bonds, you can actually change the bonding structure in nitrate and force a subsequent reduction step."
The research team started with an iron complex surrounded by a secondary sphere of hydrogen bonds. They then tuned the hydrogen bonds to selectively grab onto binding sites on nitrates, priming it for the next chemical reaction.
When the researchers used heat to drive the chemical reaction, the iron complex was able to grab oxygen atoms from nitrate, reducing it to nitric oxide. When the researchers used light, the iron complex was able to remove oxygen atoms from nitrate altogether, converting it to ammonia. Nitric oxide is used in medical therapies to reduce blood pressure, along with other applications, and ammonia can be reused as fertilizer.
Szymczak says their finding lays the foundation for scientists to develop methods of removing nitrates from the environment. Scientists need to understand how a molecule like nitrate behaves and how it can be reduced before they can develop devices that can be used to remediate them from sites like wastewater treatment plants.
"The timeframe for development of solutions to big picture problems has a large time horizon, and they require fundamental studies to develop principles and invent new ways to do molecular transformations that are societally important," Szymczak said. "We are giving people a roadmap of how to achieve the difficult reduction step that we hope could be translated to engineered systems that might be used down the road."
Nature Chemistry
Secondary-sphere hydrogen bonding promotes catalytic nitrate reduction at iron