Study on the infiltration mechanism of molten urea and biochar for a novel fertilizer preparation

•The BUF were prepared by infiltration method of urea and RH-biochar.•The maximum N content in BUF reached 29.19%.•The reactions on the surface of the RH-biochar to form products with worse thermal stability than that of pure urea.•Urea combined with the RH-biochar through hydrogen bonding and intermolecular forces after Cooling and solidifying. Nitrogen fertilizer is an essential nutrient for the crops. However, conventional nitrogen fertilizer has efficiency limitation from runoff, volatilization, and leaching. In this study, a novel nitrogen fertilizer, biochar based urea fertilizer (BUF), was prepared by the infiltration process of biochar and molten urea. The BUF was characterized in terms of ultimate analysis, Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). The results showed that the maximum N content reached 29.19% in the BUF. Molten urea was evenly distributed in BUF by infiltrating into the pores of biochar and forming small uniform size particles on the biochar surface. The addition reaction between C=O and molten urea switched to the deamination reaction of molten urea and −OH / −COOH on biochar surface when the infiltration temperature exceeded 150 °C, which resulted in the formation of different kinds of amides. Therefore the thermal decomposition temperature of BUF was lower than that of pure urea.