Enhanced ballasted magnetic coagulation process based on one-step pyrolysis synthesis of sludge-derived AC@MNPs for advanced purification of secondary effluent from wastewater treatment plants: specific removal of low molecular weight organic matter

The removal of dissolved organic matter from secondary effluent in sewage treatment is a significant challenge, necessitating the development of green and efficient composite magnetic materials for coagulation. In this study, activated carbon-modified magnetite nanoparticles (AC@MNPs) were synthesized via one-step activation pyrolysis using iron sludge as a precursor. The ballast coagulation process employed AC@MNPs as both magnetic species and adsorbent, with polymeric aluminum chloride (PACl) as a coagulant, for advanced treatment of secondary effluent. A response surface methodology identified optimal pyrolysis conditions: an activation ratio (KOH/ST) of 1.2, a pyrolysis time of 1 h, and a temperature of 640 °C. The influence of pyrolysis temperature (600 ∼ 700 °C) on the morphology and pore characteristics of AC@MNPs was investigated, highlighting its significance in coagulation performance. Characterization through XRD, VSM, and FTIR confirmed the favorable settling properties of the composite magnetic flocs and recyclability of AC@MNPs. Coagulation experiments revealed a synergistic effect of AC@MNPs and PACl, enhancing the removal of UV 254 and COD from 33.4 % and 36.2 % (with PACl alone) to 74.02 % and 76.53 %, respectively. The overall performance of the ballast coagulation process was evaluated, demonstrating effective removal of organics of various molecular weights. Finally, the coagulation mechanism was systematically discussed. Overall, this research offers insights into the application of magnetic coagulation technology for wastewater treatment advancements.