Preparation of Magnetic Response Sludge Fly Ash-Based N-TiO ₂ Composite Materials and Their Synergistic Photocatalytic Mechanism for the Degradation of Rhodamine B

The conventional TiO 2 photocatalyst suffers from several limitations, including poor separation efficiency, high dosage requirements, and low resource utilization efficiency of solid waste sludge fly ash. This study developed a magnetic-responsive N-TiO 2 /SFA composite by immobilizing demulsified nano-TiO 2 sol and urea onto SFA via an emulsion-breaking combined with a hydrothermal method. The resulting composite was systematically evaluated for its performance in degrading Rhodamine B dye wastewater under visible light irradiation. Results demonstrated that the optimal sample, N-TiO 2 /SFA-1.0 (with a TiO 2 : SFA mass ratio of 1:1), achieved a remarkable degradation efficiency of 98.55% within 180 min in a 15 mg/L RhB solution. The synergistic effects of nitrogen doping and the SFA support effectively narrowed the band gap to 2.21 eV, significantly enhancing the separation efficiency of photogenerated electron-hole pairs. Moreover, this composite reduced the required TiO 2 content by 50% compared to pristine N-TiO 2 , thereby improving material economy. Importantly, N-TiO 2 /SFA-1.0 exhibited excellent structural stability and reusability, maintaining over 96.78% degradation efficiency after six consecutive cycles. The inherent Fe 3 O 4 content in SFA endowed the composite with magnetic responsiveness, enabling rapid and convenient recovery using an external magnet. This study presents a technically feasible and economically advantageous strategy for the high-value utilization of solid waste and the efficient degradation of organic pollutants in waste water.