Aging of microplastics exposed to UV irradiation and its effect on the sorption of organic contaminants in freshwater

Microplastics (MPs) are recognised as vectors of hydrophobic organic contaminants in aquatic environments, yet their sorption behaviour can be strongly influenced by environmental aging processes. In this study, the effect of UV-induced photooxidative aging on the physicochemical properties and sorption capacity of common MPs was systematically investigated. Polymeric materials of different chemical nature, including polyolefins (HDPE, LDPE, PP), polyamides, PET, and PLA, were subjected to controlled UV irradiation to induce accelerated photooxidative aging under laboratory conditions. Photoaging induced significant surface modifications including increased roughness, microcrack formation, and changes in specific surface area, together with chemical oxidation evidenced by the formation of oxygen-containing functional groups. These changes resulted in altered surface polarity and reactivity. The sorption behaviour of aged MPs towards two representative polycyclic aromatic hydrocarbons (PAHs), pyrene (PYR) and fluoranthene (FLU), was evaluated under freshwater conditions and compared with pristine materials. Results showed that UV aging generally increased the sorption capacity of these organic contaminants, although the extent of this effect depended on both polymer type and the specific PAH considered, being most consistently observed for pyrene sorption onto polyolefins. At environmentally relevant low concentrations, PAHs sorption followed a Henry linear isotherm. Kinetic analysis revealed fast sorption, reaching equilibrium within 20 h. Mechanistically, retention appeared to be governed by molecular diffusion of PAHs through the aqueous boundary layer surrounding the particles, while empirically, data were described by a pseudo-second order model, consistent with strong surface-mediated interactions rather than proof of a specific chemisorption mechanism. These findings demonstrate that photooxidative aging plays a key role in determining the environmental behaviour of MPs and their capacity to retain organic contaminants.