Combining Phenolization Treatment with the Mannich Reaction for Modification of Kraft Lignin to Produce Highly...

In this study, the amination-based modification of kraft lignin (KL) was implemented through phenolization treatment combined with the Mannich reaction to synthesize the aminated lignin (APKL) with high nitrogen content. Afterward, the chemical structural changes and reaction mechanism of KL during the modification process were surveyed in depth using diverse analytical techniques. The results revealed that the phenolization treatment markedly raised the active site number in KL from 5.79 to 25.5 mmol/g, which led to a significant increase in the chemical reactivity of KL. Meanwhile, the amine group was successfully grafted onto the best phenolized kraft lignin (PKL) after the Mannich reaction. Furthermore, the effects of amination reagent, reactant mass ratio, temperature and time on the nitrogen content of APKL were systematically examined to optimize the reaction conditions for amination. Using FTIR, molecular weight and elemental analyses, the optimal amination conditions were determined as a reaction temperature of 75 °C, reaction time of 3 h and PKL₆/arginine/formaldehyde mass ratio of 3:21:28. Under these parameters, APKL₁₀ with a higher nitrogen content of 19.2% and lower C/N ratio of 2.46 was acquired. In addition, TG and SEM results revealed that the obtained APKL₁₀ possessed a flake-like structure and outstanding thermal stability, which was beneficial for its subsequent application as a slow-release soil fertilizer. More importantly, the soil column leaching test confirmed that the as-prepared APKL₁₀ had excellent nitrogen slow-release properties in the soil. As a result, this kraft lignin derivative generated by phenol treatment followed by amination-based modification could serve as an efficient nitrogen fertilizer, providing a long-term nitrogen source for plant growth in soil.