Ultrasonic-coupled ozone oxidation of Shengli lignite for efficient preparation of high-quality humic acid

Abstract

Humic acid (HA) is a natural organic substance with significant applications in modern agriculture, ecological restoration, and life sciences. However, the oxidative dissociation of organic matter fragments from lignite leads to polymerization, resulting in low soluble yields. The mechanical and cavitation effects of ultrasonic effectively reduces coal particle size, facilitating their oxidation in the presence of ozone. Additionally, the reactive free radicals generated by the ultrasonic-coupled ozone enhances the oxidizability of the reaction system. The organic matter conversion and HA yield without ash from Shengli lignite (SL) oxidized by ultrasonic-coupled ozone are 82.51% and 49.88%, respectively. The characterization results reveal that the oxidation process primarily breaks the C–O and C–C bridge bonds in SL, and HA has more polar functional groups (–COOH). This study provides a novel approach for efficiently preparing HA from lignite through ultrasonic-coupled ozone oxidation.