Engineering α-MnO2 on multifunctional composite derived from co-pyrolysis of waste red mud and spent coffee grounds for RhB degradation via peroxymonosulfate activation

Background Red mud is the world’s most extensively generated waste in the non-ferrous metal industry, posing significant environmental risks due to its strong alkalinity and lack of effective disposal methods. It is urgent to find a reasonable and effective strategy to transform red mud into a Heterogeneous catalyst for environmental remediation. Methods α-MnO 2 nanorods were anchored on biochar-modified red mud composite (RMBC) via the hydrothermal method and utilized as a catalyst to activate PMS for the degradation of Rhodamine B (RhB). Reactive oxygen species generated during the catalytic process was identified through EPR, XPS, and Raman spectroscopy measurements. Degradation intermediates of RhB were identified using LC-MS. Furthermore, the ecotoxicity of RhB and its transformation products was evaluated using the ecological structure-activity relationship (ECOSAR) prediction model. Significant findings The as-prepared α-MnO 2 /RMBC achieves a remarkable RhB degradation efficiency of 98.6 % within 30 min, significantly outperforming control samples including α-MnO 2 /RM, α-MnO 2 /BC, and the mixture of α-MnO 2 /RM and α-MnO 2 /BC, suggesting a pronounced synergistic interaction between α-MnO 2 and RMBC. Mechanistic analysis reveals that RhB degradation is predominantly governed by an activated surface-PMS inner-sphere complex, signified as Mn III (s) -(O)OSO– 3. Furthermore, the Fe II species inherent to RM facilitates the redox-mediated reduction of Mn IV to Mn III , thereby sustaining the catalytic activity of active Mn III sites and enabling robust recyclability and reusability of the α-MnO 2 /RMBC catalyst.