Performance and Mechanism of Sulfathiazole Adsorption by Magnetic Biochar: Promoting Effect of Co-existing Polystyrene and Simultaneous Removal

<title>Abstract</title> Antibiotics and microplastics (MPs) combined pollution threatens aquatic ecosystems and human health. Polystyrene (PS, a typical MP), widely detected in water, interacts with sulfathiazole (STZ, a sulfonamide antibiotic), affecting wastewater treatment. In this study, a novel magnetic biochar (MBC) was synthesized via co-precipitation, and its performance in removing STZ under the co-existence of PS was systematically evaluated using experiments and theoretical calculations. The MBC exhibited excellent superparamagnetism (saturation magnetization: 11 emu·g⁻¹) and a high specific surface area (702.67 m²/g), enabling efficient separation and recovery, enabling efficient separation and recovery. In single-pollutant systems (only STZ), the MBC showed an adsorption capacity of 39.31 mg・g⁻¹ for STZ; notably, in co-existing systems (PS + STZ), the presence of PS exerted a promoting effect, boosting the STZ adsorption capacity to 42.49 mg・g⁻¹ (removal rate &gt; 98%), with optimal performance under neutral conditions (pH 5–7). Notably, while removing STZ, MBC still maintains high efficiency in PS removal, with PS removal rate reaching up to 98%. From a kinetic perspective, in the single STZ system, STZ adsorption relies on hydrogen bonding and π–π conjugation. However, in the system with co-existing PS and STZ, PS forms PS-STZ complexes. These complexes anchor to the hydrophobic regions of MBC, and at the same time, the polar groups of STZ bind to the polar sites of MBC. This further enhances the stability of the binding between PS and MBC. This study provides a feasible strategy for the synergistic removal of antibiotics and microplastics.