Preparation of N-doped biochar from sewage sludge and melamine for peroxymonosulfate activation: N-functionality and catalytic mechanisms

In this study, a group of nitrogen-doped sludge biochar were prepared by a single-step pyrolysis method and employed as catalysts for the activation of peroxymonosulfate (PMS). The N content of as-synthesized biochar composites was altered by adding a different ratio of melamine with the precursors. The sample characterization results indicated that the N content of sludge-melamine-blended biochar named SM-(0.5:1) significantly increased from 1.91 to 9.93% compared with that of raw sludge biochar. Consequently, the surface area and mesoporosity also enhanced. SM-(0.5:1) exhibited excellent degradation ability of reluctant organic pollutants in PMS/acidic media, which outperformed many previously reported carbocatalysts. Complete color removal of a mixture solution of seven cationic and anionic dyes (10 mg L−1 of each) was achieved within 50 min by using 200 mg L−1 of SM-(0.5:1) and 350 mg L−1 of PMS. A mechanism study indicated that the non-radical process performed by the pyridine N dominated the oxidative degradation of pollutants rather than that of SO4− and OH radical process governed by graphitic N, CO, and surface metal oxides. This study concurrently provides a facile route of enhancing N functionality of sludge-based carbocatalysts and an efficient way of sludge valorization.