A new multimodal magnetic nanozyme and a reusable peroxymonosulfate oxidation catalyst: Manganese oxide coated-monodisperse-porous and magnetic core-shell microspheres

Monodisperse-porous, polydopamine and manganese oxide coated, core-shell type, magnetic SiO₂ (MagSiO₂@PDA@MnO₂) microspheres 6.4 μm in size were synthesized for the first time, using magnetic, monodisperse-porous SiO₂ (MagSiO₂) microspheres 6.2 μm in size as the starting material. MagSiO₂ microspheres were obtained by a recently developed method namely “staged shape templated hydrolysis and condensation protocol”. In the synthesis, MagSiO₂ microspheres were consecutively coated by polydopamine (PDA) and then by a MnO₂ layer in the aqueous medium. The pore volume and the specific surface area of monodisperse-porous MagSiO₂@PDA@MnO₂ microspheres were measured as 0.59 cm³ g⁻¹ and 154 m² g⁻¹, respectively. Their Mn and Fe contents were determined as 66 ± 1 mg g⁻¹ and 165 ± 5 mg g⁻¹ respectively. MagSiO₂@PDA@MnO₂ microspheres exhibited multimodal enzyme mimetic behavior with highly superior catalase-like, oxidase-like and peroxidase-like activities. The effective production of singlet oxygen (¹O₂) and superoxide anion (O₂⁻*) radicals in MagSiO₂@PDA@MnO₂-peroxymonosulfate (PMS) system was demonstrated by ESR spectroscopy. By evaluating this property, MagSiO₂@PDA@MnO₂ microspheres were tried as a reusable catalyst for dye removal via peroxymonosulfate (PMS) activation in batch experiments for the first time. The degradation runs were made with, rhodamine B (Rh B), methyl orange (MO) and methylene blue (MB) as the pollutant. The core-shell type design allowing the deposition of porous MnO₂ layer onto a large surface area provided very fast, instant removals with all dyes, via both physical adsorption and degradation via PMS activation. In the reusability experiments, the removal yields of MO and Rh B decreased 1.8% and 8.9% over five consecutive runs in batch fashion. MagSiO₂@PDA@MnO₂ microspheres exhibited very good functional and structural stability in consecutive dye degradations. No significant change was observed in Fe content of microspheres while Mn content exhibited a decrease of 7.4% w/w over 5 consecutive degradation runs.