Advanced milling technique produces slow-release soil nutrient crystals

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A purely mechanical method can produce a new, more sustainable fertilizer in a less polluting way. This is the result of a method optimized with DESY’s PETRA III light source. An international team used PETRA III to optimize the production method which is an adaptation of an old technique: by grinding two common ingredients, urea and gypsum, scientists produce a new solid compound which slowly releases two essential chemical elements to soil fertilization, nitrogen, and calcium. The grinding method is fast, efficient and clean, as is the fertilizer product, which has the potential to reduce nitrogen pollution that clogs water systems and contributes to climate change. Scientists have also discovered that their process is evolutionary; therefore, it could potentially be implemented industrially. The results of DESY scientists; the Ruđer Bošković Institute (IRB) in Zagreb, Croatia; and Lehigh University in the United States were published in the journal Green chemistry. The new fertilizer still needs to be tested in the field.

For several years, DESY and IRB scientists have been collaborating to explore the fundamentals of mechanical methods for initiating chemical reactions. This processing method, called mechanochemistry, uses various mechanical inputs, such as compression, vibration or, in this case, grinding, to achieve the chemical transformation. “Mechanochemistry is a fairly old technique,” ​​says Martin Etter, beamline scientist P02.1 at PETRA III. “For thousands of years we have been grinding things, for example, cereals for bread. It is only now that we are beginning to examine these mechanochemical processes more intensively using X-rays and to see how we we can use these processes to initiate chemical reactions.”

The Etter beamline is one of the few in the world where mechanochemistry can be routinely performed and analyzed using synchrotron X-rays. Etter has spent years developing the beamline and working with users to refine methods for analyzing and optimizing mechanochemical reactions. The result was an experimental setup that has been used to study many types of reactions important to materials science, industrial catalysis, and green chemistry.

“In fact, the mechanochemical configuration of DESY is probably the best in the world,” says Krunoslav Užarević from IRB in Zagreb. “In few places can you follow the evolution of mechanochemical reactions as well as here at DESY. It would have been practically impossible to obtain this result without the expertise of Martin Etter and this PETRA III setup.”

For this result, the mechanochemistry collaboration partnered with Jonas Baltrusaitis, professor of chemical engineering at Lehigh University. The team used the P02.1 configuration to better understand the parameters governing the grinding process, in order to optimize the reaction conditions for the preparation of the target fertilizer. The configuration of PETRA III allows direct insight into the evolution of the reaction mixture by applying synchrotron radiation to the grinding vessel. This means that the reaction can be observed without stopping the procedure. This allowed researchers to determine the exact reaction pathways and analyze product yield and purity, which helped them fine-tune the mechanical procedure on the fly. They found a procedure for 100% conversion of raw materials into target fertilizer.

This end product is known as a “cocrystal”, a solid with a crystal structure comprising two different chemicals that is stabilized by weaker intermolecular interactions in repeating patterns. “Co-crystals can be seen as LEGO structures,” Etter explains. “You have sets of two types of two bricks, and with those two bricks you create a repeating pattern.” In this case, the “bricks” are calcium sulfate derived from gypsum and urea. During the grinding process, urea and calcium sulfate bind together.

“On its own, urea forms a very loosely bound crystal that breaks down easily and releases its nitrogen too easily,” says Baltrusaitis. “But with calcium sulfate through this mechanochemical process, you get a much more robust cocrystal with a slow release.” The advantage of this co-crystal is that its chemical bonds are weak enough to release nitrogen and calcium but strong enough to prevent the two elements from going wild at the same time.

This method of release is the great advantage of the fertilizer. For one thing, they avoided one of the major drawbacks of nitrogen fertilizers that have been used since the 1960s. “The status quo in fertilizer, for food safety reasons, is to dump as much nitrogen and phosphorus as possible on the crops,” says Baltrusaitis. More than 200 million tons of fertilizer are produced through the more than a century-old Haber-Bosch process, which traps atmospheric nitrogen in urea crystals. Of this volume, only about 47% is actually absorbed by the ground, with the rest being washed away and causing potentially massive disruptions in water supply systems. In the North Sea and Gulf of Mexico, massive “dead zones” are developing, in which algal blooms fueled by excess fertilizers suck up all the available oxygen in the water and thereby kill marine life. .

In addition, the production of common fertilizers is energy-intensive, consuming 4% of the world’s natural gas supply each year via the Haber-Bosch process. The new method offers the possibility of reducing this dependency. “If you increase the efficiency of these urea materials by 50%, you have to produce less urea through Haber-Bosch, with all the associated energy consumption issues such as natural gas demand,” explains Baltrusaitis. The grinding procedure is fast and very efficient, resulting in pure fertilizer with no residual by-products except water. “Not only are we providing a fertilizer that works better,” says Baltrusaitis, “we are also demonstrating a method of green synthesis.”

While the PETRA III analysis involved milligrams of fertilizer, the research team led by Baltrusaitis and Užarević succeeded in extending their procedures using data collected at PETRA. So far they can, with the same procedure and efficiency, produce hundreds of grams of fertilizer. As a next step, the team plans to continue scaling up, to create a true industrial proof-of-principle version of the process. Baltrusaitis is already working on such scale-up and co-crystalline fertilizer testing for application under real-world conditions.

“Beyond the product, the mechanochemical process generates virtually no unwanted by-products or wastes,” says IRB’s Užarević. “We are optimistic about its strong potential for worldwide application.”

The Ruđer Bošković Institute in Zagreb, Croatia, Lehigh University in Bethlehem (Pennsylvania) in the United States, the chemical company ICL Group, the University of Zagreb and DESY participated in this research.

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