Preparation and structural analysis of nitrohumic acids through two-step oxidation of Shengli lignite

A two-step oxidation method involving HNO 3 pretreatment and ozone oxidation was employed to effectively depolymerize Shengli lignite (SL) into nitrohumic acids. This approach achieved a high organic matter conversion of 82.48% and a nitrohumic acids yield of 72.75%. The high yield of nitrohumic acids is attributed to ozone oxidation of HNO 3 -treated SL (NASL), which introduced nitro (–NO 2 ) groups into the lignite. The chemical properties of NASL are characterized by an increase in O and N content, while C, H, and S content are reduced. Mechanistic analysis revealed that oxidative cleavage of ether bonds is the main pathway for the decomposition of lignite macromolecular networks into nitrohumic acids. This study provides a novel and efficient strategy for the production of nitrohumic acids, offering valuable insights into the comprehensive utilization of lignite.