Multispectral Structural Characterization of Humic Acid-Enhanced Urea

Humic acid-enhanced urea (HAU) can be produced by adding humic acid (HA) into melted urea during urea production. Field studies have proved that HAU showed a better urea hydrolysis rate, crop yield, and nitrogen use efficiency than normal urea (U). However, the main reaction between HA and U during the production of HAU has not been reported yet. In this study, HA, derived from weathered coal, was used to produce HAU, and the added amount of humic acid was 5%, 10%, and 20%, respectively (named HAU5, HAU10, HAU20). The paper collected and analyzed the infrared spectra and their second derivative infrared spectra of HAU5, HAU10, HAU20, and U. HAU20 and U were characterized using X-ray photoelectron spectroscopy (XPS), and oxygen is near-edge X-ray absorption fine structure (NEXAFS). The urea in HAU20 was removed by dissolving HAU20 with absolute ethanol, and FTIR and XPS characterized the residue (UHA). The result showed that: (1) FTIR spectra and the second derivative spectra showed that the vibration intensity of primary amine C-N in HAU was lower than that in U, and the vibration intensity decreased with the increase of the addition amount humic acid. There were more secondary amine nitrogen, and non-carbonyl oxygens in HAU20 were separated from the XPS N (1s) spectra and O (1s) NEXAFS spectra, respectively, and prominent amide characteristics were shown from the result of FTIR spectra for UHA, which indicated that HA reacted with urea during the HAU production. (2) the percentage of carboxyl carbon in HAU20 or UHA was lower than in HA. FTIR spectra showed that C-O-H in-plane bending vibration from carboxylic acid detected in HA did not exist in UHA, the C=O stretching vibration position from carboxyl groups in UHA was shifted, and the characteristics of primary amine nitrogen for UHA were obvious. The above indicated that the carboxyl groups of HA participated in the reaction of HA and urea. The structure for R-CO-NH-CO -NH2 in HAU will be produced after the dehydration reaction between the carboxyl group of HA and the amide group of urea. Therefore, the results from the spectral analysis used in this study clarified the main reaction modes of humic acid and urea during the production of HAU, which will provide basic information for the reveal of the synergistic mechanism of HAU and the development of value-added urea.