Extraction, Fractional Structure and Physiological Activities of Fulvic Acid from Yunnan Xundian Lignite

Abstract The efficient conversion of lignite is important for environmental protection. Coal‐based fulvic acid (FA) was prepared by the oxidative degradation of lignite in a microwave field. The highest extraction rate of FA was 35.03 %. To reduce the heterogeneity of FA, it was divided into four fractions‐FA I , FA II , FA III and FA IV ‐with molecular weights of <500, 500–1000, 1000–3500, and >3500 Da, respectively. A variety of characterisation methods revealed that all the FA fractions contained carboxyl, phenolic hydroxyl, amino and sulfhydryl groups. The FA I contained the most oxygen‐containing functional groups. The benzene rings of the >1000 Da FA fractions were mostly tri‐substituted. FA could break dormancy and promote the germination of coreopsis seeds but this effect was weakened when the FA concentration was too high. FA I could effectively promote chlorophyll synthesis, FA II could effectively improve plant stress resistance and FA III promoted seedling longitudinal growth.