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

Combined pollution by antibiotics and microplastics poses a significant threat to aquatic ecosystems and human health. Polystyrene (PS), a typical microplastic widely detected in aquatic environments, interacts with the sulfonamide antibiotic sulfathiazole (STZ) and thereby impacts wastewater treatment processes. In this study, a novel magnetic biochar (MBC) was synthesized via co-precipitation. Its performance in removing STZ in the presence of PS was systematically evaluated using experimental and theoretical calculation methods. The MBC exhibited excellent superparamagnetism (saturation magnetization: 11 emu·g−1) and a high specific surface area (703 m2/g). In a single-pollutant system (STZ only), the MBC achieved an STZ adsorption capacity of 39.3 mg·g−1. In contrast, the coexistence of PS in the STZ-containing system exerted a promotional effect, increasing the STZ adsorption capacity to 42.5 mg∙g−1 (removal rate > 98%), with optimal performance under neutral conditions (pH 5.00–7.00). Notably, the MBC also maintained high efficiency in PS removal (up to 98%) while eliminating STZ. Kinetic analysis revealed that STZ adsorption in the single-pollutant system relied on hydrogen bonding and π-π conjugation. However, in the co-pollutant system (PS + STZ), PS formed complexes with STZ. These complexes anchored to the hydrophobic regions of the MBC, while the polar groups of STZ bound to the polar sites of the MBC, further enhancing the stability of PS-MBC binding. This is the first study to clarify that PS promotes STZ adsorption on MBC through complex formation, while also enabling efficient PS removal. It bridges the existing knowledge gap in the mechanism of co-pollutant removal and provides a feasible strategy for the synergistic elimination of antibiotics and microplastics.