Effective removal of methylene blue from water using magnetic lignite-based composite hydrogel beads

Abstract

To mitigate environmental pollution caused by methylene blue (MB) dye wastewater discharge, a novel type of reusable and easily recyclable magnetic coal-based hydrogel bead, designated as PAM/SA/HA/Fe₃O₄@Lignite, was developed. This material was synthesized by incorporating humic acid (HA)-modified Fe₃O₄-loaded lignite into a pre-gel system of polyacrylamide (PAM) and sodium alginate (SA) via free radical polymerization. The adsorption performance of PAM/SA/HA/Fe₃O₄@Lignite toward MB was investigated under various experimental conditions. The material was characterized using EDS, XPS, SEM, FTIR, TGA and VSM. Based on experimental data fitting and parameter optimization, the adsorption of MB onto PAM/SA/HA/Fe₃O₄@Lignite was found to follow pseudo-second-order kinetics and the Freundlich isotherm model. At 313 K and pH = 7, the maximum adsorption capacity reached 183.50 mg g⁻¹. The adsorption process was determined to be spontaneous and endothermic, with mechanisms primarily involving ion exchange, electrostatic attraction, π–π stacking, and hydrogen bonding. Compared with similar materials, the magnetic PAM/SA/HA/Fe₃O₄@Lignite hydrogel beads exhibit excellent adsorption performance and are expected to address the research gap in the removal of MB using magnetic coal-based hydrogel materials.