Size-selective attachment of polyvinyl chloride microplastics on iron oxides in aqueous environments

The interaction of microplastics (MPs) with mineral particles is a crucial process determining the fate of MPs in aquatic environments, but potential size-selective interaction of MPs with minerals is unknown. In this work, strong and multi-layered attachment of a mix of heterogeneously size of polyvinyl chloride (PVC) MPs was observed on all the four types of iron oxides, following an order of Fe3O4-i > Fe3O4-s > γ-Fe2O3 > Fe3O4-n. Electrostatic attraction dominated their strong attachment, while hydrogen bonding and halogen bonding also contributed to MPs-mineral interaction. Importantly, both attachment rates and capacities were more favorable for large-sized PVC MPs (>1.0 μm) than small-sized MPs (0.6–1.0 μm), due to more available attachment sites and stronger electrostatic attraction. Large-sized MPs preferred to detach from iron oxides before small-sized MPs, confirming by configuration analysis. The attachment accelerated the sedimentation of PVC MPs, while a fraction (i.e., 26.7 %, PVC/iron oxide ratio at 0.2:1) of heteroaggregates floated at the air–water interface associated with bubbles. Natural organic matter decreased MPs-mineral interaction due to competitive adsorption, electrostatic repulsion, and steric hindrance, while the attachment was enhanced with increasing ionic strengths (0–200 mM). These findings contribute to a better understanding of the transport and fate of MPs in aquatic systems.