Visualizing surface chemical heterogeneity of aged microplastics via TOF-SIMS to elucidate their transport in porous...

The environmental fate of microplastics (MPs) is largely controlled by surface alterations that occur during aging. Conventional bulk spectroscopy has fostered an assumption of uniform surface oxidation, yet this cannot explain the high mobility of severely aged MPs. Here, time-of-flight secondary ion mass spectrometry (TOF-SIMS) was used to map the surface chemistry of four MPs, polyethylene, polypropylene, polystyrene, and poly(lactide), subjected to chemical, photo, and discharge plasma aging. Saturated column experiments with quartz sand showed that more intensive aging consistently enhanced MP transport. TOF-SIMS imaging revealed that aging produces localized oxidation hotspots enriched in oxygenated fragments (C₃H₃O⁺ and C₄H₇O₂⁺), rather than a uniform oxidized layer. These oxygen-containing functional groups include carbonyl species (CO) together with carboxyl (-COOH) and hydroxyl (-OH) moieties. Deprotonation of the carboxyl groups in aqueous media yields carboxylate anions (-COO^-), which combine with the polar hydroxyl groups to increase both surface hydrophilicity and negative charge density. XDLVO calculations show that the oxidation-driven increase in hydrophilicity and negative charge raises the interaction energy barrier and suppresses hydrophobic attraction toward quartz sand, consistent with the enhanced transport observed at alkaline pH (mass recovery of 63.49% for plasma-treated PLA at pH 9). The TOF-SIMS mapping provides direct spectroscopic evidence of nanoscale chemical heterogeneity on aged MP surfaces, complementing bulk-scale interaction models and contributing to a physical basis for predicting aged MP mobility.