Interfacial Enrichment of Microplastics at Microdroplet Air–Water Interface Facilitates Photoaging

Abstract

Microplastics (MPs) pollution has become a critical global environmental issue, posing potential threats to aquatic ecosystems and human health. However, existing studies on MPs photoaging have largely focused on bulk systems, while the role of microdroplet environments in natural processes (e.g., wave breaking and aerosol formation) remains underexplored. The role of the air–water interface (AWI) of microdroplets in regulating MPs photoaging was investigated. By comparing polystyrene (PS), polyethylene terephthalate (PET), and polycaprolactone (PCL) in microdroplet and bulk systems, microdroplets significantly enhance MPs-derived dissolved organic matter (MP-DOM) release, reaching 2.60–3.35 times that of the bulk phase (248.26 mg C/L in the microdroplet system and 74.17 mg C/L in the bulk system at 96 h), while altering the evolution of surface functional groups. Mechanistic analysis revealed that interfacial enrichment of MPs and a strong electric field together sustain higher steady-state concentrations of reactive oxygen species (ROS), particularly hydroxyl radicals. Molecular-level analysis shows that polystyrene-derived dissolved organic matter (PS-DOM) in microdroplets exhibits higher aromaticity, greater oxidation, and enhanced electron transfer capacity (dominated by electron-accepting). Highly oxidized PS-DOM can promote oxidation and drive a self-amplifying radical cycle, thereby continuously accelerating MPs photoaging. This study elucidates microdroplet regulation of MPs degradation and provides new insights into the fate of MPs in heterogeneous aquatic environments.