Synergistic Extraction and Characterization of Fulvic Acid by Microwave and Hydrogen Peroxide–Glacial Acetic Acid to Oxidize Low-Rank Lignite

[Image: see text] Fulvic acid (FA) has important properties and is used widely in agriculture, industry, medicine, and other fields. However, there is a lack of environmentally friendly and efficient extraction methods for coal-based FA and its molecular structural characterization. In this study, FA was extracted cleanly and efficiently from low-rank lignite via the innovative method of microwave combined with hydrogen peroxide and glacial acetic acid, followed by purification by the sulfuric acid–acetone method. The molecular structures of FA were precisely characterized by UV–vis spectroscopy, infrared (IR) spectroscopy, (1)H nuclear magnetic resonance (NMR) spectroscopy, and gas chromatography–mass spectrometry (GC–MS). The results showed that the microwave combined with hydrogen peroxide–glacial acetic acid method had stronger oxidative degradation ability compared with the conventional method. Under optimized conditions, the yield of FA reached 60.97%. During the oxidation process, the macromolecular network structure of coal was destroyed, resulting in the production of many oxygen-containing functional groups. According to the IR and UV–vis spectra, there were abundant oxygen-containing functional groups such as hydroxyl, carboxyl, carbonyl, and quinone groups in the molecular structure of FA. Determination of the total acid group content in the oxygen-containing functional groups of FA showed that the content of carboxyl groups was much higher than that of phenolic hydroxyl groups. The (1)H NMR showed that there were hydrogen atoms present as part of carboxyl, aromatic, phenolic hydroxyl, and aliphatic groups in FA. The (GC–MS) results suggested that FA is a mixture of dozens of complex compounds, including alkanes, alcohols, esters, etc.