Surface Oxidation and Defect Formation Under Five Photoaging Regimes Enhance Cd2+ and Pb2+ Adsorption by Polyethylene and Polyvinyl Chloride Microplastics

Photoaging alters polymer surface chemistry and can thereby regulate the interfacial binding of metal ions by microplastics. In this study, polyethylene (PE) and polyvinyl chloride (PVC) microplastics were exposed for three weeks to five regimes comprising UV-air, UV-ultrapure water, UV-simulated seawater, UV/H2O2, and UV/Cl. Changes in surface morphology, functional groups, crystallinity, hydrophobicity, and specific surface area were characterized before Cd2+ and Pb2+ adsorption. UV/Cl and UV/H2O2 induced the strongest transformations in both polymers, although the rate of aging decreased with exposure time. Photoaging generated surface cracks and defects, increased the accessible surface area, reduced hydrophobicity, and introduced hydroxyl, carbonyl, and carboxyl groups. These changes significantly enhanced Cd2+ and Pb2+ adsorption. Adsorption involved physical and chemical contributions, with chemical interactions predominating. FTIR, elemental mapping, and surface analyses were consistent with coordination and surface complexation at oxygen-containing sites, electrostatic attraction, and nonspecific physical retention on roughened surfaces and within defects. Variations in crystallinity and surface area further modulated adsorption. The results show that aging medium and polymer structure jointly determine the extent of surface oxidation and defect formation, establishing a structure–property relationship between photoaging-induced surface evolution and the enhanced metal-binding capacity of PE and PVC microplastics.