Surface physicochemical properties and dibutyl phthalate adsorption of microplastics naturally aged in seawater

Microplastics (MPs) are prevalent in marine environments and can adsorb contaminants from surrounding seawater, potentially transferring harmful chemicals through the food chain and raising ecological concerns. While the adsorption of aquatic pollutants by MPs has been intensively studied, research on phthalate esters (PAEs, common plasticizers frequently found in seawater) remains limited, primarily focusing on pristine MPs in artificial media. This study characterized the surface physicochemical properties of polyethylene (PE) and polystyrene (PS) MPs before and after natural aging for one week to three months off the coast, and investigated the adsorption isotherms of dibutyl phthalate (DBP, one of the most abundant PAEs in seawater) on these MPs in both natural seawater and ultrapure water. Surface characterization revealed significant alterations in MP surface characteristics due to natural aging, with morphologies and nanomechanical features varied by MP type and oxidation occurring after one-month aging. The best-performing Langmuir-Freundlich model suggested that DBP adsorption onto MPs involved multilayer processes on heterogeneous surfaces with varying adsorption energies. Further analysis indicated that PS had a higher DBP adsorption capacity than PE, attributed to its porous glassy structure and π-π interactions with DBP. The trivial impact of natural aging could relate to competing effects of increased roughness and the formation of polar oxygen-containing groups on aged MPs. The "salting-out" effect in natural seawater was likely impeded by free ion competition and MP aggregation under higher ionic strength. This study provides valuable insights into the interactions between MPs and their coexisting contaminants in marine environments.