Magnetic coagulation and flocculation of a kaolin suspension using Fe3O4 coated with SiO2

Abstract In order to improve dispersibility, chemical stability and magnetic flocculation performance, Fe3O4 was functionalized through SiO2 coating. The characterization of Fe3O4/SiO2 particles was analyzed, such as morphology, size, structure, chemical composition, zeta potential, magnetic and thermal properties. Magnetic flocculation of kaolin particles was investigated under different pH conditions, Fe3O4/SiO2 dosages and Ca2+ concentrations. Flocculation factors were further analyzed using response surface methodology. A modified Derjaguin-Landau-Verwey-Overbeek (MDLVO) model was applied, and the size and fractal dimension of magnetic flocs were analyzed to establish the mechanism of flocculation. Results showed that Fe3O4/SiO2 is a core-shell nanocomposite with a specific surface area of 65.14 m2/g and saturation magnetization of 46.54 emu/g. Coating with SiO2 greatly reduced the median equivalent-volume diameter (D50) of Fe3O4 from 10.48 to 2.88 µm. A kaolin removal efficiency of 93.8% was achieved under conditions of an initial turbidity of 200 NTU, pH 7.0, 1.0 g/L Fe3O4/SiO2, and 4.5 mmol/L Ca2+. Response surface analysis indicated that interactions occur between Fe3O4/SiO2 and Ca2+, as well as Ca2+ and pH. Fe3O4/SiO2 can be effectively detached from the aggregates and reused, maintaining a removal efficiency of more than 87.7% after ten cycles of use. The energy barrier between kaolin and Fe3O4/SiO2 was significantly reduced due to the electrostatic interaction of Ca2+. Furthermore, the MDLVO theory predicted that magnetic forces induce collision and movement between Fe3O4/SiO2 and kaolin-Ca2+. The floc structures formed in the magnetophoresis process significantly affect the flocculation of kaolin particles.