Interfacial Characteristics and Morphological Evolution of Microplastic-Oil Agglomerates (MOAs): Roles of Microplastic Type, Weathering State and Oil Properties.

The co-existence of microplastics (MPs) and spilled oil in marine environments leads to the formation of microplastic-oil agglomerates (MOAs), significantly altering the environmental fate of both pollutants. This study investigates the interactions between three types of MPs and three crude oils under turbulent conditions. Using a custom-developed in situ non-contact microscopic observation system, we characterized the oil-water interface and the morphological evolution of MOAs. Results demonstrate that pristine MPs, particularly PVC and PET, significantly reduce oil dispersion efficiency by promoting droplet refloating and coalescence, increasing median droplet sizes from <400 μm to over 600 μm. UV-induced aging of MPs increased surface polarity and roughness, which, while slightly mitigating oil coalescence compared to pristine MPs, promoted the formation of larger, structurally densified MOAs. Heavy crude oil consistently formed the largest MOAs up to 4.2 mm with the highest trapped oil mass due to its high viscosity and asphaltene content. Conversely, for lighter oils, the aging state of the MPs emerged as the primary driver of MOA morphology through enhanced physical entanglement. These findings provide morphological evidence and mechanistic understanding into MOA formation, offering critical insights into the sedimentation and transport of composite plastic-oil pollution in coastal ecosystems.