A new study led by Rice University researchers has taken steps toward producing fertilizer in a more sustainable way through the electrochemical conversion of nitrate ions to ammonia in wastewater.
The process converts wastewater nitrate levels of 2,000 parts per million into ammonia, followed by an efficient gas stripping process for ammonia product collection, explains Texas—based Rice University’s chemical and biomolecular engineer, and study co-lead, Haotian Wang.
Wang designed a catalyst of ruthenium atoms in a copper mesh to extract ammonia from nitrate-rich industrial wastewater and polluted groundwater, the study states. The copper suppresses the hydrogen evolution reaction, the researchers found.
“While we understood that converting nitrate wastes to ammonia may not be able to fully replace the existing ammonia industry in the short term, we believe this process could make significant contributions to decentralized ammonia production, especially in places with high nitrate sources,” Wang said in a statement about the findings.
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The remaining nitrogen contents after these wastewater treatments can be brought down to “drinkable” levels as defined by the World Health Organization, said the researchers, which also involved Arizona State University and the Pacific Northwest National Laboratory in Washington.
A prime benefit of the conversion process is the reduction of carbon dioxide emissions from traditional industrial production of ammonia, one of the most highly-produced inorganic chemicals. These are not insignificant, amounting to 1.4% of the world’s annual emissions, the researchers noted. Some 175 million tonnes of ammonia are produced at plants every year through intensive industrial processes.
China, for instance, produces more ammonia than any other country, and produces the majority of its ammonia from coal.
Some 50% of the world’s food production relies on ammonia fertilizer.
The research team used density functional theory calculations to explain why ruthenium atoms make the chemical path that connects nitrate and ammonia easier to cross, according to Christopher Muhich, an assistant professor of chemical engineering at Arizona State.
“When there is only ruthenium, the water gets in the way,” Muhich said in a statement. “When there is only copper, there isn’t enough water to provide hydrogen atoms. But on the single ruthenium sites water doesn’t compete as well, providing just enough hydrogen without taking up spots for nitrate to react,” he added.
This article appears in ES&E Magazine’s August 2022 issue: