Modification of sludge biochar by MnO2 to degrade methylene blue: Synergistic catalysis and degradation mechanisms

In this study, sludge biochar (SBC) was prepared by pyrolysis method, MnO2 and MnO2@SBCs with different MnO2/SBC mass ratios (3%, 5%, 10% and 20%) were synthesized by co-precipitation method. The catalytic performance was examined by activating peroxymonosulfate (PMS) for methylene blue (MB) degradation. MB can be completely removed within 30 min when the mass ratio of MnO2/SBC is greater than 5% and the degradation kinetics of MB is more in line with the pseudo-first-order kinetic model in the MnO2@SBC/PMS system. Besides, the pseudo-first-order kinetic constants (k) in MnO2@SBC/PMS are significantly higher than that in SBC/PMS and MnO2/PMS, and the largest degradation rate of MB is in 5% MnO2@SBC/PMS system. MnO2@SBC exhibited excellent PMS activation ability in a wide range of pH (5–9) with complete degradation of MB in 30 min. Furthermore, MnO2@SBC has good stability after five usages still activating PMS for complete degradation of MB in 180 min. The quenching experiments and EPR spectrum indicated that both SO4−, OH and 1O2 were the main reactive oxygen species. Finally, the probable MB degradation pathway was proposed in the MnO2/PMS system. This study provides a reliable insight into sludge resource utilization, sludge biochar modification and its catalytic performance improvement.