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

Abstract

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, ¹H 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 ¹H 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.