Extraction and characterization of coal-based fulvic acid from Inner Mongolia lignite by hydrogen peroxide-glacial acetic acid

Fulvic acid (FA) has many unique functions, and it has important applications in agriculture, industry, medicine and other fields. An innovative method for efficient extraction of FA, using hydrogen peroxide-glacial acetic acid to synergistically oxidize lignite was proposed, and the molecular microstructure of FA was characterized. The results show that the synergistic system of hydrogen peroxide-glacial acetic acid has strong oxidizing ability, and can deeply oxidize and degrade the lignite macromolecule structure. The optimal experimental parameters for FA extraction are: mass ratio of glacial acetic acid to lignite, 1.12, mass concentration of hydrogen peroxide, 27.5%, mass ratio of hydrogen peroxide to lignite, 4.5, reaction temperature, 65 °C, and reaction time, 150 min. The extracted FA is a simple molecular aggregate with particular structures and characteristics, which mainly contains carboxyl, hydroxyl, other oxygen-containing functional groups and anhydrides, with carboxyl groups making up the highest content and the phenolic hydroxyl group comprising the major component in the hydroxyl group. FA has a low degree of conjugation and short fatty chains. The substituent benzene ring is dominated by tri-substituted benzene, and the content of methylene is high in aliphatics. The extraction of FA by hydrogen peroxide-glacial acetic acid was compared with that by hydrogen peroxide alone, and by hydrogen peroxide-microwave. The results show that the yield of FA extraction by hydrogen peroxide-glacial acetic acid was the highest, which reached 57.0%, and the content of total acidic groups and carboxyl groups were the most.