A purely mechanical method can produce a novel, more sustainable and less polluting fertilizer. That’s the result of an optimized method in DESY’s PETRA III light source.
by German Electron Synchrotron
An international team used PETRA III to optimize the production method that is an adaptation of an ancient technique: by grinding two common ingredients, urea and gypsum, scientists produce a new solid compound that slowly releases two critical chemical elements for 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 fouls water systems and contributes to climate change. The scientists also discovered that their process is scalable; therefore, it could potentially be implemented industrially. The results of the DESY scientists; the Ruđer Bošković Institute (IRB) in Zagreb, Croatia; and Lehigh University in the USA have been 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 to initiate 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 on beamline P02.1 at PETRA III. “For thousands of years, we have been grinding things, for example, grains for bread. We are only now starting to look at these mechanochemical processes more closely using X-rays and seeing how we can use those 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 X-rays from a synchrotron. Etter has spent years developing the beamline and working with users to refine methods for analyzing and optimizing mechanochemical reactions. The result has been a world-renowned experiment setup that has been used to study many types of reactions important to materials science, industrial catalysis, and green chemistry.
“Actually, the mechanochemical setup of DESY is probably the best in the world,” says Krunoslav Užarević from IRB in Zagreb. “Few places can monitor the progress of mechanochemical reactions as well as here at DESY. It would have been virtually impossible to achieve this result without Martin Etter’s expertise and this PETRA III setup.”
For this result, the mechanochemistry collaboration partnered with Jonas Baltrusaitis, a professor of chemical engineering at Lehigh University. The team used the P02.1 configuration to obtain information on the parameters that govern the milling process and to optimize the reaction conditions to prepare the target fertilizer. The configuration in PETRA III allows a direct view of 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. In this way, the researchers were able to determine the exact reaction pathways and analyze the result and the purity of the product, which helped them to refine the mechanical procedure on the fly. They found a procedure that allowed 100% conversion of the starting materials to the target fertilizer.
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That end product is known as a “cocrystal,” a solid with a crystalline structure comprising two different chemicals that is stabilized by weaker intermolecular interactions in repeating patterns. “Co-crystals can be seen as LEGO structures,” says Etter. “You have sets of two kinds of two bricks, and with these two bricks you make a repeating pattern.” In this case, the “bricks” are calcium sulfate derived from gypsum and urea. Through the grinding process, urea and calcium sulfate bind with each other.
“On its own, urea forms a very loosely bound crystal that crumbles easily and releases its nitrogen too easily,” says Baltrusaitis. “But with calcium sulfate through this mechanochemical process, you get a much more robust co-crystal 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 being released at the same time.
That method of release is the great advantage of the fertilizer. For one thing, they have avoided one of the main drawbacks of the nitrogenous fertilizers in use since the 1960s. “The status quo for fertilizers, for food safety reasons, is to dump as much nitrogen and phosphorous into crops as possible. » says Baltrusaitis. More than 200 million tons of fertilizer are produced through the century-old Haber-Bosch process, which traps atmospheric nitrogen in urea crystals. Of this, only about 47 percent is actually absorbed into the ground, with the rest washing away and causing potentially massive disruptions to water systems. In the North Sea and the Gulf of Mexico, massive “dead zones” are growing, where algae blooms fed by excess fertilizer suck up all the available oxygen in the water, thereby killing marine life. .
In addition, the production of common fertilizers is energy intensive, consuming four percent of the world’s natural gas supply each year through the Haber-Bosch process. The new method provides an opportunity to reduce that dependency. “If you increase the efficiency of those urea materials by 50 percent, you need to produce less urea through Haber-Bosch, with all the issues related to energy consumption, such as natural gas demand,” says Baltrusaitis. The grinding procedure is fast and very efficient, resulting in a pure fertilizer with no waste by-products except water. “We are not just proposing a fertilizer that works better,” says Baltrusaitis, “we are also demonstrating an environmentally friendly method of synthesis.”
Although the analysis of PETRA III involved milligrams of fertilizer, the research team led by Baltrusaitis and Užarević managed to extend their procedures with the help of the data taken in 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 do a real industrial proof-of-principle version of the process. Baltrusaitis is already working on a scale-up and testing of co-crystal fertilizer for application in real-world conditions.
‘Beyond the product, the mechanochemical process generates virtually no unwanted by-products or waste,’ says IRB’s Užarević. “We are optimistic that there is strong application potential around the world.”
More information: Ivana Brekalo et al, Extension of Agrochemical Gypsum-Urea Cocrystal Synthesis by Thermally Controlled Mechanochemistry, ACS Sustainable Chemistry and Engineering (2022). DOI: 10.1021/acssuschemeng.2c00914
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