Magnetic sludge-based biochar derived from Fenton sludge as an efficient heterogeneous Fenton catalyst for degrading Methylene blue

Fenton sludge was converted into magnetic sludge-based biochar (SMBC) using a one-step pyrolysis process and employed as a novel heterogeneous catalyst to activate hydrogen peroxide (H2O2) for the oxidative removal of methylene blue (MB). The crystal structure and catalytic activity of SMBC are strongly affected by the pyrolysis temperature. X-ray powder diffraction, Mössbauer, and scanning electron microscope-energy dispersive spectrometer analyses were conducted to determine the crystal phase transfer of iron (Fe) oxides at different pyrolytic temperatures and retention times. The results revealed that the catalytic efficiency of SMBC was strongly related to the crystal phase of Fe oxides. The catalyst obtained at 800 °C for 1 h exhibited superior catalytic properties and degraded 98.56% of the MB (100 mg·L−1) in 3 min via the synergistic effect of Fe3O4, Fe0, and biochar. Free radical quenching experiments and electron paramagnetic resonance studies demonstrated that ·OH, O2·− and 1O2 participated in the H2O2 system, and·OH radicals were promptly generated owing to the catalysis of the SMBC. Aging tests were repeated for four cycles in three conditions to examine the stability of the material. The results showed that SMBC was relatively stable in non-aerated and aqueous conditions, but its activation performance was severely inhibited in an oxygen environment. Furthermore, the catalytic ability of SMBC was maintained at 88.13% after the 4th run. This study presents a cost-effective method of converting ferric sludge to an environmentally friendly catalyst that enables H2O2 activation for degrading refractory organic contaminants.