Cation–π Interaction and Salinity Regulate the Bubble-Mediated Transport of Microplastics in the Presence of Aromatic Dissolved Organic Matter

Bubble-mediated ejection is a critical vector for the global transport of microplastics; yet, the interfacial physicochemical rules governing this process in complex aquatic environments remain unclear. Here, we combine single-molecule force spectroscopy, nanoscale colloidal probe measurements, and macroscopic transport experiments to resolve how aromatic dissolved organic matter, modeled by phenylalanine, regulates air-bubble interactions with nonpolar polystyrene (PS) and polar polylactic acid (PLA). Single-molecule force measurements demonstrated that cation-π interactions promote specific adsorption of aromatic organics onto PS to form a robust eco-corona, stabilizing the interfacial bond by strongly suppressing dissociation kinetics (koff: 0.01 s-1 to 3.73 s-1). This eco-corona suppresses nanoscale PS hydrophobicity and weakens bubble-particle adhesion despite minimal changes in macroscopic wettability. Crucially, we further identify a salinity-dependent regime shift in transport mechanics, where nanoscale adhesion dominates transport in freshwater, whereas colloidal stability governs fate in seawater. High salinity induces extensive aggregation of eco-corona-coated PS, causing a benthic shunt as its hydrodynamic diameter increases from 4.63 to 15.56 μm, whereas PLA remains colloidally stable and thus more amenable to vertical transport. These findings demonstrate that predictive fate models should integrate aggregation kinetics and interfacial chemistry to resolve atmospheric ejection versus sedimentation of microplastics.