Mechanistic insights into heavy metal adsorption and co-transport by UV-aged microplastics: A combined experimental and DFT study

Environmental aging transforms discarded plastics into microplastics (MPs) with altered surface properties, enabling them to adsorb heavy metals and co-transport through soil and groundwater. This study examines the effects of ultraviolet (UV) aging on MPs and their interactions with cadmium (Cd), lead (Pb), and copper (Cu). We combined experimental characterization with theoretical calculations. This integrated approach aims to systematically elucidate the metal-specific complexation mechanisms governing co-migration dynamics. Aging significantly increases the specific surface area (1.40 to 5.24 m 2 /g), oxygen content (4.64 % to 21.57 %), and abundance of oxygen-containing functional groups on MPs. These changes markedly enhance heavy metal adsorption, with capacities following the order: Pb (20.95 mg/g) > Cu (7.33 mg/g) > Cd (2.32 mg/g). The adsorption capacity of MPs for heavy metals is influenced by the ionic hydration radius, electronegativity, and affinity between metal ions and surface functional groups. Quantum chemistry calculations further reveal that Pb shows the strongest binding affinity to oxygen-containing functional groups, particularly to carboxyl group (−2.02 eV), followed by Cu (−1.89 eV) and Cd (−1.65 eV). Divalent cations (Ca 2+ ) significantly suppress heavy metal adsorption on aged MPs. Notably, aged MPs maintain substantial adsorption ability even after desorption, suggesting their potential role as long-term environmental carriers. Aged MPs facilitate the transport of heavy metals in porous media, while heavy metals inhibit the transport of MPs; both effects intensify with increasing metal adsorption. Our findings uncover the underlying reaction mechanisms, providing novel insights and a solid foundation for the synergistic remediation of MPs and heavy metal pollution.