Preparation and structural characterisation of coal-based fulvic acid based on lignite

Coal-based fulvic acid (FA) is a scarce resource with unique efficacy in many fields such as modern agriculture, ecological restoration and life sciences. However, the efficient and environmentally friendly preparation of FA and its structural characterisation still need to be achieved. In this study, coal-based FA was prepared from Qujing lignite by hydrogen peroxide oxidation under microwave irradiation using CuO as the catalyst. According to the different molecular weights and polar solvent solubility, FA components were separated. Molecular structure models of the components were deduced and constructed through elemental analysis, infrared (IR) spectroscopy and 13C nuclear magnetic resonance (NMR) spectroscopy, combined with quantum chemical calculations and computational analysis. The results showed that each component contains many oxygen-containing functional groups and that carboxyl content > phenolic hydroxyl content > alcohol hydroxyl content > carbonyl content. The chemical formulas of the two components were C44H30O25N (Mr = 972) and C53H35O31N (Mr = 1181). The three-dimensional characteristics of the molecules were determined. The theoretical IR spectra of the models calculated by Gaussian software matched well with the actual spectra, confirming the established models to be reliable. This study provides experimental support and a scientific basis for the efficient preparation and structural characterisation of coal-based FA.