Massive intercalation of urea in montmorillonite

Urea intercalated in montmorillonite (MMT) leads to considerable improvement of N efficiency in soils; however, there are no systematic approaches to elucidate the properties of urea intercalates within the interlayer space of swelling clays. This study focused on both the optimization of massive urea intercalation into MMT and the systematic characterization of urea-intercalated MMT (UCM). Anhydrous metal salts, which could allow optimal urea intercalation, were used for the fi rst time to synthesize metal-urea complexes in a solid state. Massive intercalation of metal-urea complex into MMT was performed by simply mixing dehydrated MMT with a molten metal-urea complex containing excess urea at 105 ± 5°C. Successful complexation was confi rmed by both the unique infrared spectra absorbance bands and the thermal behaviors of the coordinated urea molecules. The intercalation of metal-urea complexes into MMT was clearly demonstrated by a significant expansion of d ₀₀₁ spacing from 1.52 to 1.74 nm in Ca-UCM, as revealed by X-ray diff raction patterns. Infrared spectra also revealed that the intercalation of urea in the interlayer led to a distinct red shift in stretching vibration energy of the structural hydroxyl group of MMT from 3446 to 3483 cm ^-1 and from 3360 to 3400 cm ^-1. Experimental urea loading capacity was estimated to be 33.7% urea. All the intercalated urea molecules were found to be quickly and completely released by water molecules through ligand exchange, maintaining their effectiveness as a N source for crop cultivation.