Synthesis of peroxymonosulfate composite catalyst (Fe0/Fe3O4/biochar) using waterworks sludge and walnut shell for degrading methylene blue

Waste waterworks sludge and walnut shell were employed to synthesize a composite catalyst consisting of Fe-0/Fe3O4/biochar using a one-step pyrolysis process. The synthesized catalyst was applied to active peroxymonosulfate (PMS) for studying the degradation of methylene blue (MB). Various characterization results confirmed that Fe0 and Fe3O4 were distributed on the surface of composite catalyst, which had a porous structure and large specific surface area of 109.87 m(2)/g. Owning to these structural properties, the composite catalyst presented desirable catalytic capability and reusability. When 0.4 g/L PMS and 0.3 g/L composite catalyst were added into the MB solution (pH 7), 99.9% MB could be degraded within 60 min at a high reaction rate of 0.1550 min(-1), which was much better than that of zero valent iron (ZVI)/PMS system. After the three cycles, the catalyst could still maintain an MB degradation of over 68.0%. The free radical quenching experiments and electron paramagnetic resonance experiments demonstrated that SO4-center dot and HO center dot were generated during the activation of PMS. The X-ray photoelectron spectrometer analysis revealed that Fe3O4 and various functional groups, such as C=O were formed on the composite catalyst, and the Fe2+ was oxidized to Fe3+. Furthermore, the iron leaching experiments indicated that both the homogeneous and heterogeneous reactions occurred during the catalysis of PMS using the synthesized composite catalyst. Acidity and higher temperature were conducive to the catalytic reaction. This study offers a green and sustainable route for simultaneously degrading contaminants and reusing wastes.