Removal characteristics and mechanism of microplastics and tetracycline composite pollutants by coagulation process

Water composite pollution is an increasingly severe challenge in the field of water treatment. Especially, microplastic is an emerging concerned pollutant in recent years for its persistent eco-environmental influence. Therefore, investigating the composite pollution effect and developing effective removal technology surrounding microplastic is an urgent task for future need. In this aspect, the removal of composite pollutants, microplastics (PET/weathered PET) and tetracycline (TC), was procedurally studied by coagulation with AlCl3. Based on the analysis of variance (ANOVA) and correlation of experimental data, the characterization of fourier transform infrared spectroscopy (FTIR), solid-state 27Al nuclear magnetic resonance (27Al NMR), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) computation, the removal mechanism for coagulation was well discussed and proposed. It shows that the removal performance of individual PET and TC were mainly influenced by coordination reaction in forming Al-PET and Al-TC, corresponding to the removal rate of 100% and 28%, respectively. Charge neutralization and sweeping flocculation played an auxiliary role for PET removal. However, the weathered PET (WPET) exhibited worse removability (92%) than PET due to a sharp decrease in the hydrodynamic diameter of WPET. The poor removal of TC was induced by the formation of Al-TC which was difficult to transform into solid flocs. In regard to composite pollutants, the removal rate of PET was significantly decreased (91%) due to the low activity of TC in forming flocs and the complexation competition, while the removal of TC was greatly enhanced (41%). In this process of multilayered reaction among PET, TC and Al3+, Al3+ preferentially interacted with TC to form Al-TC, which was subsequently combined to PET to form the ternary complexes PET-Al-TC, thus realizing the transformation from dissolved TC complexes to solid TC composites. This work is of great significance in understanding the interaction mechanism of composite pollutants in coagulation process.