Single atoms of silver and earth-abundant carbon turn pollutants into fertilizer

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

Typically, the pathways and technologies used today, such as biological remediation, target sources with high concentrations of nitrogen waste that are converted to nitrogen, which has no value. The challenge has been to efficiently convert the low concentration of nitrogenous wastes such as nitrates and nitrites found in wastewater into high-value ammonia products. And this is where the synergy of silver, carbon and nitrogen weaves its catalytic magic.

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

Closing the waste loop N

Removing nitrogen waste at source and converting it into a valuable product helps shut down NO._x (air pollutant, nitrogen-based gas), nitrates and nitrites loop and prevent waste from entering the environment where it would have adverse effects on aquatic life and human health.

Researchers from the Center of Excellence for Carbon Science and Innovation and the School of Chemical Engineering and School of Mineral and Energy Resources Engineering at the University of New South Wales carried out the research and published it in Applied catalysis B: Environment and energy.

“Our work shows how carbon-based materials can be modified at the atomic level to transform waste nitrate into valuable ammonia using extremely small amounts of silver atoms,” says Dr Thanh Son Bui, from the UNSW School of Chemical Engineering and lead author of the paper.

The Centre’s chief investigator, Dr Rahman Daiyan, says: “Because we are creating this circular economy, this technology is not just about the end product of upcycled fertilizer. We reduce nitrates and nitrites for which there is also a market value.

“For example, the concentration of nitrates and nitrites in a mine’s tailings dam, which is achieved through the use of explosives, can be high and tailings dams can extend over thousands of square kilometers, an area the size of a large city.

“If left untreated, aside from the risk of leaking into waterways, nitrates and nitrites can turn into some of the most potent greenhouse gases, such as nitrous oxide (laughing gas), which is 290 times more potent than carbon dioxide.

“Reducing nitrogen and nitrites and solving the environmental problem is a problem of today. It is not a problem of tomorrow and this is where the economic aspect of our technology starts to make sense,” he says.

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. Additionally, returns are generally low and unreliable.

Only in recent years have researchers become interested in the potential of nitrates and nitrites as an alternative route 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 which allows us to target low nitrate concentrations in wastewater. We start with a carbon-nitrogen support structure which we adjust by removing some of the nitrogen and replacing it with single silver atoms,” explains the centre’s chief researcher, Dr Emma Lovell.

“We investigated how to minimize the cost and maximize the performance of our catalyst. Carbon is cheap and abundant. The amount of silver in the catalyst is only 0.1% of the catalyst, but these few silver atoms, working in synergy with the carbon and nitrogen, make the catalyst very selective, allowing the complete conversion of nitrate that would otherwise enter the environment as fertilizer runoff, municipal waste and mining activities,” explains Dr Lovell.

The tricky part was finding the right amount of silver: too much silver and the silver starts to form clusters of several atoms and you produce hydrogen. Too little and you lose the ability to convert nitrate to ammonium. There is a Goldilocks quantity and getting there required precise control of the system to ensure we got the right amount of silver at the precise locations of the carbon-nitrogen carrier.

“Despite the precise nature of manufacturing, we have a catalyst that is easy to make and scalable,” says Dr. Lovell.

“One challenge, however, that needs to be addressed alongside catalytic design concerns broader systems engineering and the ability to apply the technology to a specific industry,” says Dr Daiyan.

“We need to look at existing potential commercial ventures to find ways to scale up the technologies specific to those ventures and reduce risk a little more, and run techno-economic models to understand the economic viability of each pathway. We have identified a few commercial avenues with potential.”