Adsorption behaviour and mechanism of polycyclic aromatic hydrocarbons onto typical microplastics in a soil solution

Microplastics (MPs) have a tremendous potential to sorb hydrophobic organic contaminants in the soil environment. The effects of MPs in the soil systems, however, remain largely unexplored. The objective of this study was to investigate the impact of polyethylene and polystyrene MPs on the transport of the benzo[a]anthracene (BaA) and benzo[a]pyrene (BaP) in soil matrices. In this study, we investigated the sorption process of BaA and BaP in batch sorption experiments in a laboratory and systematically evaluated the particle sizes of MPs (0.8, 50, 200, and 500 μm) and the environmental factors, such as pH, ionic strength, and humic acid (HA) concentration on adsorption behaviour. The results showed that the adsorption of PAHs onto MPs reached equilibrium within 24 h. The adsorption kinetics of PAHs onto MPs were fitted well with the pseudo-first-order kinetic model. The adsorption isotherm modelling showed a better fit of the adsorption results to the Freundlich model. The thermodynamic research indicated the adsorption of BaA and BaP as spontaneous and endothermic processes. Additionally, the adsorption capacity of BaA and BaP reached the highest when polyethylene and polystyrene MPs with a particle size of 0.8 μm and 50 μm, respectively. The sorption of BaA and BaP on PE and PS MPs was significantly influenced by pH and the peaking at pH 5.0 and 7.0, respectively. Furthermore, the adsorption mechanisms of the BaA and BaP on MPs were dominated by electrostatic interaction, π–π interactions, and hydrophobic interaction.