A magnetic-void-porous MnFe2O4/carbon microspheres nano-catalyst for catalytic ozonation: Preparation, performance and mechanism

Wastewater treatment is essential to guarantee human health and ecological security. Catalytic ozonation with nanocatalysts is a widely studied and efficient treatment technology. However, this method has always been limited by nanocatalysts disadvantages such as easily loss, difficult to separate and reuse, and catalytic ability decay caused by aggregation, which could cause severe resources waste and potential risk to human health and ecosystem. To remedy these challenges, a magnetic-void-porous MnFe2O4/carbon microsphere shell nanocatalyst (CMS-MnFe2O4) was successfully synthesized using renewable natural microalgae. The separation test showed CMS-MnFe2O4 was rapidly separated within 2 min under an external magnetic field. In catalytic ozonation of oxalic acid (OA), CMS-MnFe2O4 showed efficient and stable catalytic efficiency, reaching a maximum total organic carbon removal efficiency of 96.59 % and maintained a 93.88 % efficiency after 4 cycles. The stable catalytic efficiency was due to the supporting effects of the carbon microsphere shell, which significantly enhanced CMS-MnFe2O4 chemical stability and reduced the metal ions leaching to 10-20 % of MnFe2O4 through electron transfer. To explore the catalytic mechanism, radical experiments were conducted and a new degradation pathway of OA involving superoxide anions rather than hydroxyl radicals was proposed. Consequently, this study suggests that an efficient, recyclable, stable, and durable catalyst for catalytic ozonation could be prepared.