Valorization of biogas digestate through hydrothermal humification for nitrogen-enriched artificial humic acid production

Biogas digestate (BD), a nutrient-rich and low-cost carbon source, can be converted into artificial humic acid (AHA) through hydrothermal humification (HTH). However, the process is often constrained by low efficiency and inconsistent product quality. In this study, the effects of different reaction media, including biogas slurry (BS) and deionized water, on the properties of AHA derived from BD at various temperatures (180–260 °C) were investigated. The results showed that BS addition increased both AHA yield and nitrogen content, with a maximum AHA selectivity of 40.16% in the hydrochar at 220 °C. FT-ICR-MS, 3D-EEM, and solid-state 13C NMR analyses indicated that lignin in BD was the main precursor for aromatic AHA synthesis in the deionized water system. When BS was introduced into the HTH process, lignin depolymerization and nitrogen-involving Maillard reactions were likely enhanced, resulting in greater nitrogen incorporation into AHA, with a maximum of 26.02 mg N retained in AHA at 220 °C. This enhanced nitrogen retention was associated with the formation of reactive intermediates and subsequent incorporation of nitrogen into more stable heterocyclic structures. This study provides a sustainable pathway for the efficient conversion of biogas plant residues into nutrient- and humic-rich products with potential as soil amendments.