A greener strategy for ultra-fast adsorption-promoted ozonation of livestock-excreted pharmaceuticals by co-pyrolysis of steel converter slag and biogas residues: synergistic effects, environmental impacts and DFT study

Abstract

China is the biggest pork-producing country, consequently raising serious environmental threats, due to the direct irrigation of untreated swine urine containing emerging toxic antibiotic residues like ofloxacin. Similarly, China's steel and biogas industries generate steel converter slag (SCS) residue (120 million tons annually) and biogas residue (BR). The disposal of SCS and BR is not eco-friendly due to the potential risk of leaching of heavy metals and eutrophication. Herein, the eco-friendly, one-pot co-pyrolysis of BR and SCS successfully achieved synergistic integration at various temperatures under an N₂ atmosphere, as confirmed by characterization studies. Adsorption-promoted ultra-fast ozonation over an Fe-SCS@BR-700 °C catalyst eliminated OFL (20 mg L⁻¹) and toxic organic pollutants of swine urine. High-Performance Liquid Chromatography (HPLC) and Triple Quadrupole Liquid Chromatography-Mass Spectrometry (QTRAP ABI-3200) were employed successfully to trace and remove (<0.01 ppm) more than 16 toxic pharmaceuticals. ICP-MS and toxicity assessment revealed 0.2 g as a safer dosage. Wheat grains and Escherichia coli growth were seen in ozonated urine. Density functional theory and electron paramagnetic resonance revealed abundant active sites, exponential electron transfer, and ROS generation like hydroxyl radicals (˙OH), superoxide (O₂˙⁻), and singlet oxygen (¹O₂) via ozone decomposition. Magnetization without regeneration enables robust recovery and recyclability. Comparatively, Fe-SCS@BR-700 °C reduces emissions, resource use, and costs more effectively. This study successfully established a greener, economical, and efficient heterogeneous catalyst to eliminate toxic pollutants from real swine urine at a small pigsty for safer urine resource irrigation, showing a sustainable approach for treating waste by waste.