Salinity inhibits UV photoaging of thermoplastic polyurethane microplastics and enhances atrazine adsorption

Microplastics (MPs) in saline marine environments undergo UV photoaging, altering their pollutant vector potential. This study investigates how salinity modulates the UV-induced aging of thermoplastic polyurethane (TPU) MPs and its implications for pesticide adsorption, comparing freshwater (deionized water mimic) and simulated seawater (0.54 M NaCl). UV irradiation caused TPU fragmentation, chain scission, and surface oxidation, evidenced by increased surface area (from 0.16 to 0.20 m·g) and reduced particle size (from 90.07 to 49.65 μm), and elevated O/C ratios (from 0.14 to 0.30). High salinity inhibited photoaging by scavenging reactive oxygen species (ROS), converting HO to less reactive halogen radicals (e.g., Cl), resulting in slower probe degradation rates for O, HO, and O in saline versus freshwater systems. Paradoxically, this protective effect preserves a higher abundance of oxygen- and nitrogen-containing functional groups. As a result, saline-aged TPU exhibits dramatically enhanced atrazine adsorption capacity (Langmuir Qₘₐₓ = 87.12 mg·g⁻¹, a 331 % increase over pristine TPU) versus only 59.43 mg·g⁻¹ in freshwater-aged samples, despite slower adsorption kinetics (rapid-phase rate constants decreased 31-42 %). Biphasic kinetic and Freundlich isotherm models indicated heterogeneous, partitioning-dominated sorption. These findings demonstrate that marine environments may face greater long-term risk from polar MPs as persistent, high-capacity pesticide carriers, whereas freshwater systems experience faster aging and more rapid contaminant release. Overall, this research highlights the need for salinity-specific risk assessments of MP-pollutant interactions and provides new insights into the environmental fate of polar polymers in marine system.