Fabrication of Magnetic Core-Shell Fe3O4@MnO2@polydopamine Composite for High-Performance Adsorption of Methylene Blue and Malachite Green in Aqueous Solution

The composite material featuring a magnetic core-shell structure, designated as Fe₃O₄@MnO₂@polydopamine (PDA), has been synthesized for the purpose of eliminating methylene blue (MB) and malachite green (MG) dyes from aqueous solutions. This composite is characterized by the hydrothermal assembly of densely packed manganese dioxide (MnO₂) nanosheets onto Fe₃O₄ microspheres, resulting in a significant enhancement of the composite's specific surface area. The encapsulated PDA exhibits a significant presence of nitrogen and oxygen elements, which serve as active sites for dye adsorption. At a temperature of 298.15 K, the adsorption capacities for MB and MG are measured at 50.17 and 48.73 mg g⁻¹, respectively. Furthermore, at 308.15 K, the adsorption efficiencies for MB and MG are recorded at 98.55% and 97.84%, respectively. The test results indicate that the adsorption behavior is characterized as chemisorption. Furthermore, it adheres to the pseudo-second-order kinetic model and the Langmuir isotherm model across various temperatures. The overall process is identified as a spontaneous endothermic reaction, with the rate primarily influenced by micropore diffusion and the equilibrium adsorption associated with intraparticle diffusion. Following the desorption treatment, Fe₃O₄@MnO₂@PDA continues to exhibit effective adsorption capabilities for MB and MG dyes, indicating significant practical application potential.