Preparation and application of a novel biochar-supported red mud catalystActive sites and catalytic mechanism

Highlights: • RM was not only loaded the outside surface of BC, but also inside the pores. • Excellent degradation performance was achieved for several dyes and antibiotics. • The Fe leaching in RM-BC_(HP)/persulfate system was negligible. • ¹O₂ was the predominant radical species responsible for AO7 degradation. • Fe⁰ inside the RM-BC_(HP) was the major active site. A novel catalyst RM-BC_(HP) was synthesized by hydrothermal treatment and pyrolysis (800 ℃) using red mud and coconut shells. Influence of different preparation conditions on catalyst performance was explored. SEM showed that RM-BC_(HP) was porous and RM was successfully loaded on the outside surface and inside the pores of BC. XRD revealed that Fe₂O₃ in RM was reduced to Fe⁰ and Fe₃O₄ in the pyrolysis process, in which pyrolysis temperature and addition ratio of coconut shells were critical. TGA-MS, FT-IR and XPS were also applied to character the catalyst. 100% of AO7 was removed within 30 min with conditions of 2 mM PS, 50 mg/L AO7 and 0.5 g/L RM-BC_(HP), and the Fe leaching was negligible. High removal rate was obtained in tap, river, and lake water. RM-BC_(HP)/PS system also exhibited excellent degradation performance for other dyes (MB, MG and RhB) and antibiotics (TC, OTC and CTC). The mechanism studies demonstrated that PS was mainly activated by Fe⁰ and Fe²⁺ in RM-BC_(HP) to produce different radicals, then ¹O₂ was generated by the reactions among these radicals to degrade AO7. Finally, nine intermediate products of AO7 were identified by FT-ICR-MS and a probable degradation pathway was proposed.