Efficient production of lignin-based slow-release nitrogen fertilizer via microwave heating

Microwave heating was first introduced into the Mannich reaction to prepare lignin-based organic slow-release nitrogen fertilizers. The response surface method was applied to acquire optimal conditions (amount of ethylenediamine, microwave heating temperature and time) for lignin with high organic nitrogen content. FT-IR spectra result revealed that the organic nitrogen was successfully grafted into the macromolecular structure of lignin. Elemental analysis showed that the organic nitrogen content increased (4.11–7.26 %) in lignin with the ethylenediamine content (4.0–8.0 g) or temperature (60–80 °C) or time (10–40 min) of microwave heating increased. By response surface analysis, the highest organic nitrogen content (7.84 %) was obtained under the condition (8.0 g ethylenediamine, 80 °C, 40 min). Contact angle, scanning electron microscopy, particle size, zeta potential, and thermal analysis results showed that lignin-based nitrogen fertilizers exhibited good slow-release properties of nitrogen. In short, lignin could be efficiently converted into organic slow-release nitrogen fertilizers by microwave heating. The approach with short processing time and simple operation was a potential method for preparing lignin-based fertilizer. The experiment will help to promote the value-added application of TL in the agricultural field.