Enhanced adsorption removal of anionic dyes via a facile preparation of amino-functionalized magnetic silica

A novel amino-functionalized magnetic silica (Fe₃O₄@SiO₂-NH₂) was easily prepared via a one-step method integrating the immobilization of 3-aminopropyltriethoxysilane with a sol-gel process of tetraethyl orthosilicate into a single process. This showed significant improvement in the adsorption capacity of anionic dyes. The product (Fe₃O₄@SiO₂-NH₂) was characterized with scanning electron microscopy, Fourier transform infrared spectroscopy, energy dispersive X-ray spectrometry, zeta potential and vibrating sample magnetometry. The adsorption performance of Fe₃O₄@SiO₂-NH₂ was then tested by removing acid orange 10 (AO10) and reactive black 5 (RB5) from the aqueous solutions under various experimental conditions including initial solution pH, initial dye concentrations, reaction time and temperature. The results indicated that the maximum adsorption capacity of AO10 and RB5 on Fe₃O₄@SiO₂-NH₂ was 621.9 and 919.1 mg g⁻¹ at pH 2, respectively. The sorption isotherms fit the Langmuir model nicely. Similarly, the sorption kinetic data were better fitted into the pseudo-second order kinetic model than the pseudo-first order model. In addition, the thermodynamic data demonstrated that the adsorption process was endothermic, spontaneous and physical. Furthermore, Fe₃O₄@SiO₂-NH₂ could be easily separated from aqueous solutions by an external magnetic field, and the preparation was reproducible.