Mechanochemical Synthesis of Nitrogen-Efficient Zn- and Cu-Sulfate Urea Cocrystals from Carbonate Minerals

Minerals are an important plant nutrient source, while nitrogen needs to be fixated from air via energy-intensive processes. Combining urea and mineral-derived zinc and copper carbonates into a cocrystal may result in novel fertilizer materials with decreased reactive nitrogen loss in moist soil. In this work, urea cocrystals were synthesized with Zn and Cu sulfates in a single crystalline cell (as opposed to state-of-the-art multicomponent fertilizers that retain their distinct crystalline phases). Mechanochemical synthesis was utilized whereby Zn and Cu carbonates were reacted with solid commercially available urea-sulfuric acid cocrystals to form CuSO4·3CO(NH2)2·H2O and ZnSO4·CO(NH2)2·2H2O cocrystals. The resulting crystalline structure was confirmed using powder X-ray diffraction, thermogravimetric analysis was used to investigate their thermal stability, while X-ray photoelectron spectroscopy measured their surface structure and composition. Sparx Romaine lettuce was cultivated in pots to investigate the initial efficacy of the materials as well as residual soil nitrate and soil respiration properties. Importantly, pot experiments showed that CuSO4·3CO(NH2)2·H2O and ZnSO4·CO(NH2)2·2H2O cocrystals not only efficiently provided nitrogen to the plants but also did so in a slow-release manner. The approach described here resulted in materials exhibiting reduced nitrogen losses and has the potential to unlock nutrients confined in low-solubility minerals and rocks for more sustainable development.