Fabrication of core-shell structured Fe3O4@SiO2@PPy-CTAB magnetic microspheres for the effective adsorption of Congo red

Congo red (CR) is a representative benzidine-based anionic azo dye with high toxicity, carcinogenicity, and poor biodegradability, posing substantial threats to human health and aquatic ecosystems. Therefore, its effective removal from wastewater remains a major environmental challenge. To address this, magnetic Fe 3 O 4 @SiO 2 @PPy-CTAB composite microspheres (PPy: polypyrrole; CTAB: cetyltrimethylammonium bromide) were fabricated using a hydrothermal technique in combination with surface modification and in-situ polymerization. The composite was systematically examined using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), vibrating sample magnetometry (VSM), and X-ray photoelectron spectroscopy (XPS), showing a well-defined core-shell framework, with a specific surface area (SSA) of 52.96 m 2 /g, a saturation magnetization of 42.8 emu·g − 1 , and a positively charged surface as a result of CTAB modification. Under optimal conditions (pH = 5, Fe 3 O 4 :CTAB mass ratio = 1:1.5), the CR adsorption capacity reached a maximum of 612.68 mg/g. Kinetic, isothermal, and thermodynamic analyses indicated that the adsorption process was best described by the pseudo-second-order kinetic model and the Langmuir isotherm model, suggesting a spontaneous, endothermic, and entropy-driven chemisorption process. The adsorption mechanism was primarily governed by electrostatic interactions, synergistic π-π stacking, and hydrogen bonding. After ten desorption-adsorption cycles, the adsorption capacity retained 87.1% of its initial adsorption capacity, demonstrating good reusability. The Fe 3 O 4 @SiO 2 @PPy-CTAB composite demonstrates high CR adsorption capacity (612.68 mg/g) and good reusability (87.1% retention after 10 cycles), making it a promising candidate for magnetic separation-based dye wastewater treatment. However, the use of CTAB and organic solvents limits the designation as “green”, and real-world validation is needed before practical application.