Fertilizers bring yields, but lead to over-fertilization of water bodies and pollute the groundwater with nitrate. A new form of nitrogen fertilizer, which is more economical and environmentally friendly, could now help. To make it, gypsum and urea are ground together, forming crystals that dissolve more slowly than common fertilizers. As a result, only as much nitrogen is released as the plants can absorb – this protects soil and water and saves urea.
In order to be able to grow optimally, plants need certain nutrients, especially nitrogen and phosphorus. The use of fertilizers therefore helps modern agriculture to achieve high yields. But nitrogen fertilizers in particular are increasingly becoming an environmental problem: Because common fertilizers release their nitrogen faster than the plants can absorb it, an excess remains in the soil. On average, the plants only use half of the introduced nitrogen, the rest is washed out by the rain and ends up in lakes, rivers and the groundwater. This leads to over-fertilization of water bodies, low-oxygen dead zones in coastal seas and high nitrate levels in drinking water.
Mechanochemistry instead of test tubes
But there is another way, as a new type of nitrogen fertilizer now gives hope. Ivana Brekalo from the Ruder Boskovic Institute in Zagreb and her colleagues have developed and tested an alternative production method. To date, the nitrogen components of common fertilizers – ammonia or urea – have been produced using the Haber-Bosch process, an extremely energy-intensive method. In a further chemical process, these basic materials are then combined with calcium, potassium or other mineral components. When pure urea is used as fertilizer, its microbial degradation releases large amounts of ammonia gas and carbon dioxide. Its use is therefore regulated unless it is mixed with an inhibitor.
Brekalo and her team have now looked for a way to produce a urea-containing but environmentally friendly fertilizer – and have resorted to an old technique: mechanochemistry. “We have been grinding things, such as grain for bread, for thousands of years,” explains co-author Martin Etter from the German electron synchrotron DESY in Hamburg. What is less well known is that chemical reactions can also be triggered by mechanical processes such as grinding, vibration or compression. The scientists have now used this for their new fertilizer. To do this, gypsum and urea were finely ground together until they formed a solid compound.
Slower release, less excess
The team used the X-ray source PETRA III at DESY to investigate what the resulting powder consists of. The analyzes revealed that the mechanochemical processing produces a co-crystal – a crystal in which the two starting materials form alternating structural units. “You can think of co-crystals as something made out of LEGO bricks,” explains Etter. “You have two types of rocks and you use those two rocks to form a repeating pattern.” The result is a crystalline nitrogen fertilizer that comes from grinding alone and contains no chemical by-products. At the same time, it has particularly favorable properties: the cocrystal is 20 times less soluble in water than urea and is therefore released almost evenly over at least 90 days under soil conditions.
For use in agriculture, this means that the co-crystal fertilizer releases its nitrogen more slowly and in doses than conventional urea fertilizer. The plants only get as much fertilizer as they can absorb and metabolize. While up to now almost 50 percent of the nitrogen from urea fertilizers and other nitrogen fertilizers remains unused and is washed out, the new type of fertilizer is completely utilised. You therefore only need half as much to achieve the same fertilizing effect – this protects bodies of water and groundwater, but also saves raw materials. “Increasing the efficiency of the urea materials by 50 percent reduces the need to produce urea via the Haber-Bosch process and reduces the associated problems such as the need for natural gas,” explains co-author Jonas Baltrusaitis from Lehigh University in the USA . As a next step, the researchers plan to scale up their process to an industrial scale.
Source: German Electron Synchrotron DESY; Specialist article: Green Chemistry, doi: 10.1021/acssuschemeng.2c00914