Polymer type, environmental aging, and DOM compositions collectively control DOM fractionation on microplastics

Interactions between microplastics (MPs) and dissolved organic matter (DOM) are crucial in controlling MP surface properties, transport, and environmental behavior in aquatic systems. Although DOM adsorption to MPs is well-documented, it remains unclear how polymer type, environmental aging, and DOM composition collectively determine both the extent and selectivity of DOM adsorption on MPs. This study examines DOM adsorption on five common polymers (polyethylene [PE], polypropylene [PP], polyethylene terephthalate [PET], polystyrene [PS], and polyvinyl chloride [PVC]), subjected to controlled UV- and O3-aging, using Suwannee River fulvic acid (SRFA) and humic acid (SRHA) as representative DOM. For pristine polymers, SRFA exhibited similar distribution coefficients (Kd), whereas SRHA showed pronounced polymer-dependent variability in Kd. Oxidative aging altered DOM adsorption in a strongly polymer- and DOM-specific manner, producing divergent trends in both adsorption capacity and selectivity. The aging pathways (O3 vs UV) did not monotonically affect DOM fractionation, with O3-aging inducing a greater increase in the C1 component for PET, and a greater decrease in the C1 component for PS. Fluorescence component analysis and molecular-weight fractionation revealed that aging induced distinct, polymer-specific shifts in the composition and size distribution of DOM retained on MP surfaces. Machine-learning-assisted analysis identified specific surface area and surface carbon speciation (reflecting surface hydrophobicity/hydrophilicity) as the dominant predictors of DOM adsorption behavior. Together, these results demonstrate that DOM fractionation on MPs emerges from the coupled effects of polymer type, weathering-induced surface modification, and DOM characteristics, underscoring the need to account for these factors jointly when predicting MP behavior in real environments.