Magnetic MnFe2O4/RHB Composite as an Efficient Catalyst to Activate Peroxymonosulfate for Methylene Blue Removal in Wastewater

This study developed a MnFe2O4/RHB composite catalyst through a straightforward one-step hydrothermal method, in which spinel-type manganese ferrite (MnFe2O4) was uniformly supported on rice husk biochar (RHB), effectively mitigating the agglomeration and limited active site exposure of unsupported MnFe2O4. Characterization results indicated that the composite exhibits a markedly increased specific surface area, a more developed porous structure, and a greater abundance of surface functional groups compared to pure MnFe2O4, providing enhanced active interfaces for activating peroxymonosulfate (PMS) to remove methylene blue (MB). The MnFe2O4/RHB+PMS system achieved complete MB removal within 30 min, while maintaining high catalytic activity and strong anti-interference capacity across a broad pH range and in the presence of organic or inorganic ions. The material demonstrated excellent stability, with MB removal rates exceeding 98.5% after four consecutive cycles. In a fixed-bed setup, the granular catalyst sustained over 98.8% MB removal throughout 450 min of continuous operation, indicating strong potential for practical application. Mechanistic investigation revealed that the removal process involves multiple reactive oxygen species, which jointly remove pollutants through radical (·OH, SO4−·) and nonradical (O12) pathways. Environmental safety assessments, conducted via pea seed toxicity tests, confirmed the low environmental risk of the catalyst, supporting its use as a sustainable and efficient material strategy for advanced water treatment technologies.