Different sorption behaviours of pyrene onto polyethylene microplastics in a binary system with water and a ternary system with water and sediment

Microplastics (MPs) and polycyclic aromatic hydrocarbons (PAHs) are two ubiquitous coexisting pollutants that pose potential threats to the environment and organisms and have attracted increasing attention in recent years. However, the interaction between these two pollutants in water, with and without sediment, and the associated mechanisms have not been fully understood. This study aims to explore differences in the sorption behaviours of pyrene at environmentally realistic concentrations by polyethylene MPs (PE-MPs) in binary (water-MPs) and ternary (water-MPs-sediment) systems and to elucidate the role of sediment in the sorption process. Kinetic and isothermal studies show that sediment acted as a "sponge" to adsorb the vast majority of pyrene at the beginning of the sorption experiment in the ternary system, which behaved differently than in the binary system. The sediment-sorbed pyrene was then gradually released but re-adsorbed by PE-MPs, while the concentration of pyrene in water remained at a steady low level. These findings reveal that PE-MPs exhibited a higher pyrene affinity than sediment and being potential pyrene carriers in aqueous systems. Through sorption kinetics and isotherm models study, it was found that pseudo-second-order (PSO) kinetics (0.9773), Elovich (0.9314) and Henry's (0.9850) models has the highest R2 value, indicating that chemisorption and hydrophobic interactions were attributed to the sorption process. The present study also demonstrates the competition between sediment and MPs in adsorption of pyrene, and highlights the impact of sediment and MPs in the partitioning of PAHs in aquatic environments, which may change the fate of pollution carried by MPs.