Microwave-mediated red mud/biochar activation of percarbonates: synergistic optimization of oxygen vacancies and dual hotspot effect

Tailwater treatment from high-standard plastic-cleaning wastewater and red mud (RM) resource recycling pose major challenges. In this study, a heterogeneous microwave-responsive catalyst, RM/biochar (RM/BC), was successfully prepared to synergistically integrate magnetic and dielectric losses for activation of sodium percarbonate (SPC) to degrade the plastic additive diethyl phthalate (DEP). Characterization analyses revealed that under oxygen-limited conditions at 600 °C, poplar leaf powder was fully carbonized, while Fe₂O₃ in RM was reduced to FeO and further transformed into Fe₃O₄, forming a composite dominated by amorphous carbon and Fe₃O₄. In RM/BC-600, Fe₃O₄ and amorphous carbon collaboratively enabled dual hot-spot effects under microwave irradiation via magnetic and dielectric losses, accompanied by enriched oxygen vacancies. Hydroxyl radicals (•OH) were identified as the dominant reactive oxygen species (ROS) in SPC activation. Through single-factor and response surface optimization, under conditions of 40 μM DEP concentration, 4.03 mM SPC dosage, 6.24 g/L RM/BC-600 dosage, 87.2 °C reaction temperature, and 100 W microwave power, the optimal DEP removal efficiency reached 96.1 %. In-situ FTIR/Raman spectroscopy and EPR analysis demonstrated that RM/BC primarily facilitated SPC decomposition to hydrogen peroxide, with oxygen vacancies catalyzing hydrogen peroxide to generate •OH as the main ROS, supplemented by carbonate radicals (•CO₃⁻), superoxide radicals (•O₂⁻), and singlet oxygen (¹O₂), while dual hot-spot effects accelerated this process. HPLC-QTOF-MS identified 12 intermediate products, and plausible degradation pathways were proposed. ECOSAR software predicted reduced acute toxicity of degradation products, and seed germination assays confirmed minimal ecotoxicity of RM/BC to soil-plant systems. This study achieves RM resource recovery and hazard mitigation via a green strategy, enhancing plastic wastewater treatment through DEP detoxification and stability.