Reaction characteristics of glucose-modified urea and its effects on the urea conversion rate

Objectives Glucose-modified urea (GMurea) has high fertilizer use efficiency but low decomposition. We investigated the mechanism responsible for decreasing the conversion rate of glucose-modified urea from its structural perspective under various glucose addition rate.

Methods Glucose (10%) was added to molten urea to produce glucose-modified urea. Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and liquid chromatography-mass spectrometry (LC-MS) were employed to characterize the chemical structure, material composition and relative molecular mass of the glucose-modified urea. Glucose-modified urea containing 0.2%, 0.5% and 1.0% of glucose were prepared, then urease solution (1 U/mg) was added and incubated at (25±2)℃ for 30 min. The remaining urea was tested using spectrophotometer.

Results 1) In the mixture of glucose and melted urea, the FTIR at 1599 cm^–1, the primary amide NH₂ variable angle vibration disappeared, and the primary amine NH₂ at 3441 cm^–1 weakened the asymmetric stretching vibration intensity. It was speculated that glucose reacted with the urea amine group. In addition, XPS C 1s and N 1s spectra showed unknown carbon structure (―C_X) and nitrogen structure (―N_X), and aldehyde-based carbon (―CHO) disappeared, and the O 1s spectrum showed that the C=O chemical bond of the aldehyde group was broken, indicating that the aldehyde group of glucose reacted with the amine group of urea to form a new product. The aldehyde group in glucose underwent a nucleophilic addition reaction with the amine group of urea to produce a substance containing a C=N structure through LC-MS analysis. 2) All the three ratios of glucose-modified urea reduced the decomposition rate of urea. The decomposition rate of urea and GMurea containing 0.2%, 0.5% and 1.0% glucose were significantly different vis – 20.16%, 15.5%, 3.3%, and 11.0%, respectively. Reduction in the rate of urea decomposition was markedly higher by 0.5% modification treatment, with 16.9 percentage points lower than that of ordinary urea.

Conclusions The formation of C=N bond between glucose and urea slowed down the decomposition of urea by urease. The glucose addition ratio has significant impact on the decomposition of glucose-modified urea, with an optimal glucose addition of 0.5%.