Adsorption mechanism of methylene blue from water using core-shell structured magnetic Mn0.6Zn0.4Fe2O4@SiO2 as efficient recyclable adsorbent

To improve the performances of solid-liquid separation and in situ regeneration of adsorbents for the removal of azo dye methylene blue (MB) in water, core-shell structured Mn0.6Zn0.4Fe2O4@SiO2 (MZFS) magnetic nanocomposite (MNC) adsorbents were prepared and characterized. Adsorption kinetics, isotherms and thermodynamics were investigated. Then, the in situ regeneration of MB-binding MZFS-MNCs by advanced oxidation processes (AOPs) was conducted, and the reusability of MZFS-MNCs was evaluated. The adsorption mechanism was revealed based on the results of Fourier transform-infrared spectroscopy and X-ray photoelectron spectroscopy. The results indicated that Mn0.6Zn0.4Fe2O4 magnetic nanoparticles were encapsulated within silica shells. The MZFS-MNC surface was rough and porous, and the specific surface area was 242 m2 g-1. The saturation magnetization of MZFS-MNCs was 18.6 emu.g- 1. The kinetics and isotherms of the MB adsorption process can be described by the pseudo-second-order kinetic model and the Langmuir model, respectively. The results of adsorption thermodynamics indicated that MB adsorption on MZFS-MNCs was spontaneous and endothermic, and physisorption was the dominant process. The adsorption process was driven by the electrostatic interaction, electron-donor-acceptor interaction, and intermolecular hydrogen bonding. Only one dimethylamino group in the MB molecule participated in the MB adsorption. The hydroxyl oxygen MZFS-MNCs were involved in MB adsorption, while the lattice oxygen did not participate in adsorbing MB. The oxygen vacancies were favorable for the adsorption of MB and in situ regeneration of MB-binding MZFS-MNCs by AOPs.