Supercritical CO₂ extraction of lignite humic acid and its synergistic mechanism

Supercritical CO₂ (ScCO₂) extraction technology is one of the effective ways to extract soluble organic matter from coal. Its strong dissolution and extraction ability can change the morphology and pore structure of coal samples. The experiment of extracting humic substances from lignite by ScCO₂ was carried out with lignite from Inner Mongolia and lignite pretreated with 10% hydrogen peroxide solution as raw materials and ethanol as cosolvent. The humic products were analyzed by ICP-MS inductively coupled plasma mass spectrometer, Fourier transform infrared spectrometer, scanning electron microscope and ultraviolet-visible spectrophotometer. Scanning electron microscopy, specific surface area and pore size analyzer were used to explore the morphological characteristics and pore changes of coal samples during ScCO₂ extraction, and to reveal the mechanism of ScCO₂ extraction technology on the extraction of humic substances. The results show that the coal sample pretreated with hydrogen peroxide solution will make ScCO₂ co-dissolved with ethanol more effectively penetrate into the pores of lignite, and interact with fulvic acid functional groups through hydrogen bonding and other interactions, so that fulvic acid is dissolved in a large amount from lignite matrix to supercritical fluid. The overall loose pores of the extracted coal sample are more conducive to the subsequent extraction of humic acid. The ion concentrations of Mg, P, K and Fe in the extracted fulvic acid solution were significantly increased, which were 21432.77 μg/L, 148.83 μg/L, 8823.84 μg/L and 415.12 μg/L, respectively. The maximum total pore volume and specific surface area of the coal sample after extraction are 0.0179 cm³/g and 3.3743 m²/g, respectively. The humic acid prepared from it has high yield (49.38%), high O/C (0.74), high N/C (0.12), low sulfur content (1.11%), rich in carbonyl and carboxyl groups, low aromaticity, small molecular weight and rough surface. This indicates that ScCO₂ extraction technology has important implications for the effective extraction and quality improvement of fulvic acid and humic acid.