This research investigates the extraction of fulvic acid (FA) from Hami lignite from Xinjiang, China, by microwave-hydrogen peroxide and microwave-hydrogen peroxide-formic acid methods. The effects of reaction time, oxidant concentration, oxygen-coal ratio and microwave power on FA yield are determined. Sulfuric acid-acetone was applied to purify the primary FA products. The results indicate that the oxidant concentration is a sensitive factor affecting the yield of FA, while reaction time has little effect. Formic acid can promote the extraction of FA from lignite by hydrogen peroxide, and under optimal experimental conditions, the yields of FA were 5.34% and 9.13%, respectively. UV-Vis analysis shows that FA has a lower aromaticity and degree of carbonyl conjugation in the presence of formic acid. Additionally, Fourier transform infrared spectroscopy was used to characterize the purified FA. The results show that the benzene rings of FA have one, two, three and four substitutions, oxygen-containing functional groups containing -COOH, C=O, -OCH 3 , CÀ OÀ C, Ar-OH, and R-OH and, furthermore, primary oxygen-containing functional groups consisting of C=O and C-OH. The aliphatic functional groups contain symmetrical and anti-symmetrical stretching vibrations of -CH 2 and -CH 3 . Finally, the hydrogen bonds are mainly composed of phenolic hydroxyls and alcoholic hydroxyls, while some hydrogen carboxylate is present.

Fulvic acid (FA) is composed of many molecular units with similar characteristic structures. The characterization and molecular model construction of coal-based FA is the key for the scientific basis and applied science of FA.

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