The idea originates from a unique geological event observed in Mali, West Africa, during the 1980s. Locals discovered a hydrogen-rich well, which scientists later linked to a chemical reaction between water and rock beneath the Earth's surface.
"It was an 'aha' moment," said senior author Iwnetim Abate of the Massachusetts Institute of Technology (MIT). "We may be able to use Earth as a factory, harnessing its heat and pressure to produce valuable chemicals like ammonia in a cleaner manner."
Ammonia is vital for fertilizers and holds potential as a clean fuel. However, current industrial production is energy-intensive, consuming about 2% of global energy and releasing approximately 2.4 tons (5,291 lb) of CO2 per ton (2,204 lb) of ammonia, making it a major CO2 emitter in the chemical industry.
To test their concept, Abate and his team created a rock-water reaction system mimicking Earth's subsurface conditions. By exposing synthetic iron-rich minerals to nitrogen-laced water, they triggered a chemical reaction that oxidized the rock and yielded ammonia, which they named "geological ammonia." This process required no external energy, emitted no CO2, and even worked under ambient conditions.
The researchers then used olivine, a natural iron-rich rock, to simulate real-world scenarios. Adding a copper catalyst and increasing the temperature to 300 C (572 F), they produced about 1.8 kg (4 lb) of ammonia per ton of olivine within 21 hours, showcasing the method's feasibility and sustainability.
"These rocks are all over the world, so the method could be adapted very widely across the globe," said Abate. However, he noted that implementation presents challenges, such as drilling into iron-rich rocks, injecting nitrogen-laced water, and managing the complexities of how rocks interact with gases and liquids.
The economic outlook for this method is promising. Producing geological ammonia costs approximately $0.55 per kilogram (2.2 lb), comparable to conventional methods priced at $0.40-$0.80. Additionally, this approach could address wastewater pollution.
"Nitrogen sources are considered as pollution in wastewater, and removing them costs money and energy," said first author Yifan Gao of MIT. "But we may be able to use the wastewater to produce ammonia. It's a win-win strategy." Integrating ammonia production with wastewater treatment could yield an additional profit of $3.82 per kilogram of ammonia.
"Ammonia is pretty important for life," said senior author Ju Li of MIT. Aside from microbial processes, the only other natural way ammonia is produced on Earth is through lightning striking nitrogen gas. "That's why the geological production of ammonia is quite interesting when you think about where life came from."
Research Report:Geological Ammonia: Stimulated NH3 production from rocks
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