Effects of Environmental Aging Pathways on the Formation, Composition and Settling Behavior of Microplastic–Oil Agglomerates

Marine oil spills and microplastic (MP) pollution frequently coexist, but the effects of microplastic aging on microplastic–oil agglomerates’ (MOAs) formation and transport remain poorly resolved. In this study, polybutylene terephthalate and polyvinyl chloride microplastics were subjected to acid, UV, and sequential UV→Acid aging and then interacted with three crude oils under controlled mixing conditions. Surface properties were related to oil dispersion efficiency, MOA size, composition, and settling behavior. Aging generated pathway-specific surface states rather than a single monotonic weathering trend. Acid aging increased negative surface charge and wettability, coinciding with reduced oil capture and smaller MP-enriched aggregates. UV aging caused surface roughening and was associated with larger, more MP-rich aggregates, consistent with a greater contribution of MP-MP contacts despite reduced hydrophobic affinity. Sequential UV→Acid aging combined more negative charge, increased roughness and elevated surface free energy. These MPs formed large, MP-rich MOAs with relatively high settling velocities despite reduced oil affinity. Aggregate size became decoupled from captured oil mass after aging, while relatively high oil dispersion could coexist with rapid MOA settling. These findings indicate the potential for partial separation of petroleum-hydrocarbon and MP transport and highlight the importance of aging pathway, polymer chemistry, and aggregate composition.