Understanding and characteristics of coagulation removal of composite pollution of microplastic and norfloxacin during water treatment

Water composite pollution is still a great challenge in the field of water treatment. Especially for microplastic (MP), as an emerging pollutant, its wide distribution in water and persistent eco-environmental influence have received great concerns in recent years. Nevertheless, the removal characteristics and mechanism of conventional coagulation on MP composite pollution is quite insufficient. In this study, the coagulation removal performance and mechanisms of MP (polyethylene, PE) and norfloxacin (NOR) was investigated by polyaluminium chloride (PAC) and anionic polyacrylamide (APAM). Compared with single system, the removal efficiency of PE was significantly improved (>99.0%) under plateau stage in composite system, while the removal efficiency of NOR was slightly decreased to around 42% regardless of the addition of APAM. The scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), zeta potential and One-way analysis of variance (ANOVA) of experimental data were used to explore the coagulation mechanisms. The results demonstrated that the removal of individual PE and NOR was mainly controlled by charge neutralization and sweep flocculation by PAC and APAM, and adsorption by formation of Al-NOR complex, respectively. Importantly, in composite system, the removal of PE was enhanced not only by the stronger charge neutralization but also the adsorption via the formation of PE NOR-Al complex. Furthermore, the removal efficiency of PE and NOR in neutral and weak alkaline conditions was higher than that in weak acidic or strong alkaline conditions. The presence of metal ions and humic acid had obvious inhibition and promoting effects on the removal efficiency of PE and NOR. This study can provide a new perspective on fundamental understanding in characteristics and mechanisms of MP composite pollutants removed by coagulation.