Efficient extraction and formation mechanism of fulvic acid from lignite: Experimental and DFT studies

Enhancing the coal-based fulvic acid (FA) yield through the effect of oxidation methods was of great importance. However, the realization of an efficient and environmentally friendly method for the preparation of FA, along with understanding of its formation mechanism, remains imperative. Herein, coal-based FA was prepared by oxidizing lignite with H₂O₂ and NaOH/KOH. The experimental data showed that ML lignite was pickled with HCl, metal ions such as iron, aluminum, and calcium can be removed, and this lignite is used as raw material, the reaction time was 150 min, the reaction temperature was 50 °C, and the volume ratio of H₂O₂ (30%) to KOH (3 mol/L) was 1:1, the effect of H₂O₂ and KOH on FA extraction was the best. The coal-based FA yield could reach 60.49%. The addition of silicone defoaming agent during the experiment resulted in a significant diminished the presence of bubbles and prevent the production of CO₂. A decrease in N₂ content was detected by GC. The FTIR, XPS, Py-GC/MS and other characterization results showed that FA has more polar functional groups (-COOH, -OH), and it contains more O-CO structure. Consequently, a greater quantity of FA molecules is generated during the reaction process. Moreover, the partial Gibbs free energies during the formation process of coal-based FA were calculated by density-functional theory (DFT). The highest energy required for free radicals was found to be between 1.3 and 1.7 eV. This study would provide theoretical support for exploring the FA formation process and the promotion of lignite humification by adding H₂O₂ or alkali to lignite.