Hydrothermal Humification for Producing Humic-like Acids and Nitrogen-Enriched Humic-like Acids from Recycled Wheat Straw CTMP Black Liquor

The high-value utilization of non-wood pulping black liquor is of great significance for the sustainable development of the pulp and paper industry. In this study, concentrated black liquor, obtained from the five-time recycling of KOH-pretreated wheat straw chemi-thermomechanical pulp (CTMP), was used as the feedstock for the preparation of humic-like acids (HLAs) through hydrothermal humification by utilizing the enriched organic components and residual alkalinity. Urea was further introduced to synthesize nitrogen-enriched humic-like acids (N-HLAs). The hydrothermal conditions and urea dosage were systematically optimized, and the products were characterized by elemental analysis (EA), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and particle size analysis (PSA). The results showed that the optimal hydrothermal condition was 180°C for 4 h, under which the HLA yield reached 15.75%. With the addition of 1 mol/L urea, the yield of 1N-HLA further increased to 16.81%. Structural analyses demonstrated that hydrothermal treatment promoted the transformation of small molecular organics into highly aromatic and condensed macromolecular structures, while nitrogen-containing functional groups were successfully incorporated into the HLA molecular framework through urea modification. Bioactivity assay results showed that 1N-HLA exhibited a promoting effect on radish seed germination and seedling growth at a concentration of 100 mg/L. This study provides theoretical and technical support for the valorization of pulping black liquor and the green synthesis of functional humic-like materials.