New environmentally friendly way to turn wastewater into valuable ammonia.

In a potential boon for the enormous worldwide demand for ammonia, engineers have designed a new, high-performance catalyst that can produce ammonia from low levels of nitrates that are present in industrial wastewater and polluted groundwater.

Ammonia (NH₃) is used in everything from chemical synthesis and fertilisers to fuels and clean energy carriers. Today, global ammonia demand exceeds 150 million tons per year, but its production depends on an energy-intensive process that consumes 2% of the world’s energy and releases 1.4% of global carbon dioxide emissions.  

The catalyst is made from ruthenium atoms on a mesh of copper nanowires, and it enables a more than 99% nitrate conversion into solid and liquid ammonia from low-nitrate concentrations typically found in industrial wastewater. The remaining nitrogen content in the wastewater can be brought down to “drinkable” levels as defined by the World Health Organization (WHO). The process, which could represent a sustainable route for both ammonia generation and wastewater treatment, is reported in a new study in Nature Nanotechnology.

“We fulfilled a complete water denitrification process,” said first author Feng-Yang Chen, a PhD student in the Department of Chemical and Biomolecular Engineering at Rice University, US. “With further water treatment on other contaminants, we can potentially turn industrial wastewater back to drinking water.”

Nitrate (NO₃^–) – an ion consisting of one nitrogen atom and three oxygen atoms – can reach and pollute surface water and groundwater through industrial wastewater disposal and agricultural activity.

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Nitrate can be converted into ammonia through a chemical process called reduction that involves the addition of electrons.

Before now, researchers knew that it was possible to use the element ruthenium to catalyse the reduction of nitrate-rich wastewater into ammonia.

However, many practical nitrate resources, such as industrial wastewater, have low nitrate concentrations (from hundreds to thousands of parts per million) which makes it difficult to achieve high efficiencies due to the unwanted side-effect – hydrogen evolution – which produces hydrogen from water.

“We knew that ruthenium was a good metal candidate for nitrate reduction, but we also knew there was a big problem, that it could easily have a competing reaction, which is hydrogen evolution,” says Chen. “When we applied current, a lot of the electrons would just go to hydrogen, not the product we want.”

To overcome this phenomenon, they embedded ruthenium atoms within a mesh of copper nanowire.

“We borrowed a concept from other fields like carbon dioxide reduction, which uses copper to suppress hydrogen evolution,” adds senior author Haotian Wang, assistant professor of Chemical and Biomolecular Engineering at Rice University, US. “Then, we had to find a way to organically combine ruthenium and copper.

“It turns out that dispersing single ruthenium atoms into the copper matrix works the best.”

They were able to successfully obtain high purity solid and liquid ammonia products from an industrial wastewater nitrate level of 2000 ppm, after which nitrate concentration was decreased to a drinkable water level of less than 50 ppm.

The process works at room temperature, under ambient pressure, and using an “industrially relevant” current of 1 amp per square centimetre to maximise the rate of catalysis. Taken together, Chen says that this should make it easy to scale up for industrial use.  

“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 decentralised ammonia production, especially in places with high nitrate sources,” explains Wang.