Imagine a world where industrial waste isn’t just reduced, it’s turned into something useful. This kind of circular economy is already in the works for carbon. Now, researchers at Washington University in St. Louis have developed a promising pathway to convert harmful nitric oxide, a key component of acid rain, into valuable nitric acid, which is used in everyday applications from fertilizer production to metal processing.

Feng Jiao, the Lauren and Lee Fixel Distinguished Professor in the McKelvey School of Engineering at WashU, and collaborators developed a method to convert nitric oxide (NO) emissions into high-purity, concentrated nitric acid (HNO₃).

The new process operates at near-ambient conditions with minimal infrastructure, offering an economically viable solution to industrial nitrogen waste with economic and environmental benefits. The work is published in Nature Catalysis.

“We’ve developed an electrochemical approach to converting NO, a toxic waste gas, into valuable nitric acid,” Jiao said.

“Our primary motivation is to address NO waste gases from mining sites, where large amounts of nitric acid are used to dissolve metal ores, leading to significant emissions. Our technology enables on-site NO conversion back into nitric acid for immediate reuse, creating a more sustainable and circular process.”

The innovative electrochemical process uses a low-cost carbon-based catalyst for NO oxidation. When combined with a single-metal oxygen reduction catalyst developed by Gang Wu, Professor of Energy, Environmental & Chemical Engineering in McKelvey Engineering, the process operates with low energy consumption to convert NO into HNO₃ without the need for chemical additives or extra purification steps.

The electrochemical oxidation system is designed to be “plug and play,” Jiao says, constructed on-site without massive investments in infrastructure or expensive raw materials, such as precious metals. It is flexible and customizable for small- or medium-scale operations, and it works at near room temperature, significantly reducing energy use, cost and environmental impact compared with the most prevalent NO processing method that requires elevated operating temperatures.

The system achieves over 90% faradaic efficiency when using pure NO. Even at lower concentrations of NO, the system retains more than 70% faradaic efficiency, making it adaptable to a variety of industrial waste streams.

The direct synthesis of concentrated high-purity HNO₃—up to 32% by weight—from NO and water without electrolyte additives or downstream purification establishes an electrochemical route to valorize NO waste gases, advancing sustainable pollution mitigation and chemical manufacturing.

Beyond mining, Jiao noted that the approach may have broader industrial applications as well as strong commercial potential, which Jiao and his collaborators demonstrated in a detailed techno-economic analysis that showed their process boasts lower energy consumption and reduced costs compared with traditional HNO₃ manufacturing methods.

Turning industrial pollutants into valuable chemical products is just good business, as well as being good for the environment, Jiao said.

“The nitric acid output by our system can be directly used in mining applications or other chemical processes,” Jiao said.

“We’ve already achieved very impressive efficiency and purity in our output. Going forward, we’ll be working to improve those numbers even further while also scaling up for practical applications. We’re looking at how we can build this technology into a nitrogen circular economy that will open doors to more efficient and sustainable agriculture, manufacturing and many other things.”