Adsorptive-photocatalytic removal of tetracycline from wastewater by Fe3O4- and SnO2-containing biochar nanocomposites

Tetracycline poses a major threat to the aquatic environment and human health, and there is an urgent need for a green, efficient and recyclable technological means to remove its negative effects. To address this need, three Fe3O4@SnO2 modified wheat straw biochar nanocomposites with different Fe/Sn content ratios (Fe/Sn = 0.5, Fe/Sn = 1, and Fe/Sn = 2) were prepared by pyrolysis. The adsorption and degradation properties and related mechanisms of Fe3O4@SnO2 modified wheat straw biochar nanocomposites were investigated by adsorption, degradation and free radical quenching experiments, as well as EPR analysis. The removal of tetracycline by Fe3O4@SnO2 modified wheat straw biochar nanocomposites (Fe/Sn = 0.5) at pH 7.0 within 3 h was up to 91.80%, which is in accordance with the quasi-primary kinetic model. Under the interference of inorganic anions and cations, the removal of tetracycline by the composites was not affected by the coexisting ions except Ca2+, Mg2+, HCO3 − and CO3 2−. The composites were able to efficiently remove tetracycline from pharmaceutical wastewater with a removal rate of 82.33%. It was found that the composites could be reused at least five times. FTIR, XPS combined with DFT calculations elucidated the mechanism of tetracycline removal by the composites, with •OH and •O2 − being the main substances involved in the photodegradation of tetracycline, the incorporation of manganese dioxide quantum dots enhancing the response to visible light, and the incorporation of magnetic nanoparticles resulting in rapid charge transfer and improved spatial separation of photogenerated carriers. The modification measures significantly increased the specific surface area of the biochar and enhanced the adsorption and photocatalytic properties. Based on these studies, the developed nanocomposites provide a new idea for the removal of antibiotics from pharmaceutical wastewater. Graphical Abstract