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A single atom of silver working in synergy with carbon and nitrogen atoms can efficiently convert polluting nitrogenous waste in water from industries such as agriculture and mining into ready-to-use liquid fertilizer.

Typically, the pathways and technologies in use today, such as biological remediation, target sources with high concentrations of nitrogenous waste that is converted into nitrogen, which has no value. The challenge has been to efficiently convert the low concentration of nitrogenous wastes such as nitrate and nitrite found in wastewater into high-value ammonia-based products. And that is where the synergy of silver, carbon and nitrogen weave their catalytic magic.

Researchers have precisely woven silver atoms into a carbon and nitrogen support matrix that, with perfect synergy, choreograph a series of complex catalytic steps to convert polluting nitrate into ammonium that can be used directly as a fertilizer.

Closing the N-waste loop

Removing the nitrogenous waste at source and converting it into a valuable product helps close the NO_x (air pollutant, nitrogen-based gases), nitrate and nitrite loop and prevents the waste from entering the environment where it has adverse effects on aquatic life and human health.

Researchers from the Center of Excellence for Carbon Science and Innovation, and the University of New South Wales School of Chemical Engineering and School of Minerals and Energy Resources Engineering conducted the research and it in Applied Catalysis B: Environment and Energy.

"Our work showcases how carbon-based materials can be engineered at the atomic level to turn waste nitrate into valuable ammonia using extremely low amounts of silver atoms," says Dr. Thanh Son Bui, UNSW School of Chemical Engineering and lead author on the paper.

Center Chief Investigator, Dr. Rahman Daiyan says that, "Because we are creating that circular economy, this technology is not just about the valued fertilizer end product. We are abating the nitrate and nitrite for which there is also a market value. That is, there is an environmental penalty and cost associated with not abating these waste forms of nitrogen.

"For example, the nitrate, nitrite concentration in a mine's tailings dam, which gets there from the use of explosives, can be high and tailings dams can be thousands of kilometers square—an area the size of a large city.

"If left untreated then, alongside the risk of escape into waterways, nitrate and nitrite can convert into some of the more potent greenhouse gases, such as nitrous oxide (laughing gas), which is 290 times more potent than carbon dioxide.

"Abating the nitrogen and nitrite and solving the environmental issue is a today problem. It is not a tomorrow problem and that is where the economics of our technology start making sense," he says.

The trial and error before the Goldilocks moment

For decades, researchers have used catalytic systems to make ammonia from nitrogen gas, but nitrogen is stable and insoluble, making it difficult to convert to ammonia. Further, yields are typically low and unreliable.

It is only in the last few years that researchers have turned their attention to the potential of nitrates and nitrites as an alternative pathway to ammonia because these forms of nitrogen are more reactive and soluble.

"The unique aspect of our research is the atomic design of our catalyst that enables us to target the low concentrations of nitrate in wastewater. We start with a carbon-nitrogen support structure that we tune by removing some of the nitrogen and replacing them with single silver atoms," says Center Chief Investigator, Dr. Emma Lovell.

"We looked at how we could minimize the cost and maximize performance of our catalyst. Carbon is cheap and abundant. The amount of silver in the catalyst makes up only 0.1% of the catalyst, but those few silver atoms, working in synergy with the carbon and nitrogen, make the catalyst highly selective, allowing the complete conversion of nitrate that would otherwise enter the environment in runoff from fertilizers, urban waste and mining activities," says Dr. Lovell.

The tricky part was finding the right amount of silver: Too much silver and the silver starts to form multi-atom clumps and you make hydrogen. Too little and you lose the ability to convert nitrate all the way through to ammonium. There is a Goldilocks amount and achieving that required precise control of the system to ensure we got the right amount of silver in the precise locations in the carbon-nitrogen support.

"Despite the precise nature of the manufacture, we have an easily made and scalable catalyst," says Dr. Lovell.

"One challenge, though, that needs to be tackled in parallel with catalytic design is in the broader systems engineering and being able to apply the technology to a specific industry," says Dr. Daiyan.

"We need to look at potential commercial companies that are out there to find ways to scale up technologies specific to those companies and de-risk a bit further, and run some techno-economic models to understand the economic viability of each pathway. We have identified a few commercial pathways with potential."