Cobalt-doped red mud composite as a porous catalyst for activating peroxymonosulfate to degrade methylene blue

A porous composite catalyst, designated as 0.1Co/RM@G, is successfully fabricated with red mud (RM). This catalyst, incorporating CoFe₂O₄, Fe₃O₄ and Na₄CoO₃ as active species, is employed to activate peroxymonosulfate (PMS) for the methylene blue (MB) degradation in aqueous solutions. The results demonstrate that under optimal conditions: 1 g/L of 0.1Co/RM@G and 10 mM PMS, the removal efficiency of MB (initial concentration: 100 mg/L) achieves up to 99.7 % within 15 min. Radical scavenging experiments combined with electron paramagnetic resonance (EPR) analyses confirm that singlet oxygen (¹O₂) is the primary reactive oxygen species (ROS) responsible for the MB degradation. A plausible mechanism for PMS activation by 0.1Co/RM@G is proposed, where the synergistic effect between Co³⁺/Co²⁺ and Fe³⁺/Fe²⁺ redox cycles plays a crucial role in sustaining the high catalytic performance. Furthermore, recycling experiments reveal that the catalyst retains over a 71.3 % MB removal efficiency after four cycles, highlighting its strong reusability. Density functional theory (DFT) calculations suggest that the N sites (30, 21) and C sites (13, 6, 15, 11, 2, 4) on the MB molecule are particularly susceptible to attack by ¹O₂ during the oxidation process. The potential pathways for the MB degradation are proposed through intermediates analysis using LC-MS technology. Thanks to its robust catalytic efficiency and excellent recyclability, the 0.1Co/RM@G composite shows a great potential for practical applications in the treatment of MB wastewater.