Removal of Low‐Concentration Methylene Blue From Water by Lignite‐Loaded Humic Acid‐Coated Nano Ferric Oxide

ABSTRACT The composite material prepared in this study effectively removes low‐concentration methylene blue (MB) from dye wastewater through immobilization of humic acid (HA)‐coated Fe 3 O 4 nanoparticles onto lignite substrates. Under controlled experimental conditions, systematic investigation was conducted to examine the adsorption behavior of this composite material toward low‐concentration MB solutions. Under the optimized experimental conditions (MB concentration of 20 mg L −1 , pH = 7, and temperature of 298K), the maximum adsorption capacity reached 23.58 mg g −1 . The materials were comprehensively characterized using infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X‐ray photoelectron spectroscopy (XPS). The experimental data fitted well with the pseudo‐second‐order kinetic model and the Freundlich isothermal model. Thermodynamic analysis confirmed the spontaneity and endothermic nature of the adsorption process, with chemical adsorption being identified as the main mechanism. The study found that the adsorption process is regulated by cooperative effects, including electrostatic attraction, metal complexation, hydrogen bonds, and π‐π stacking. After five consecutive regeneration cycles were completed, the initial adsorption efficiency stayed at 85.97%, which indicated that the material had excellent regeneration capability and significant potential for practical applications in wastewater treatment.