Sinking behavior of polystyrene microplastics after disinfection

Abstract

Disinfection in wastewater treatment plants has potential effects on the sinking behavior of microplastics (MPs). Investigating its intrinsic mechanisms would be significant for predicting migration and transport patterns of MPs in aquatic ecosystem. In this study, polystyrene (PS) microspheres were selected as the model MP, and lab-scale ultraviolet-C (UV-C) irradiation, ozonation oxidation and chlorination were applied to simulate the disinfection processes. Furthermore, the sinking ratio and velocity experiments were performed in ultrapure water, river water and seawater. The apparent sinking velocity of PS microspheres (780 μm, ρs = 1.05 g cm−3) was 0.849 cm s−1, which was closed to its theoretical sinking velocity (0.897 cm s−1) in ultrapure water. In UV-C and ozonation treatments, photolysis and oxidation, which generated a smoother surface containing oxygen-bonds, were the dominating decomposition mechanisms. As a result, the decreasing surface roughness and hydrophobicity contributed to enhanced sinking ratio (from 49.3% to 93.0% and 62.3%, respectively) and velocity (from 0.849 cm s−1 to 0.908 cm s−1 and 0.885 cm s−1, respectively) of PS microspheres. On the contrary, PS tended to float on the water surface after chlorination (sinking ratio decreased to < 20%), which can be due to the generation of nanoscale holes corroded by chlorine on surface. The air trapped in the nanoscale holes would increase the hydrophobicity and roughness, resulting in a surface against water wetting. In general, the sinking performance of PS microplastics was altered with different patterns depending on disinfection methods.