Polystyrene photoaging properties, molecular changes and potential health risk under the effect of humic acid

Understanding the coexistence of humic acid (HA) and polystyrene (PS) is practically important for elucidating their combined environmental behavior and potential risks under natural sunlight. This study evaluated the aging behavior of PS in various environments, with a particular focus on the effects of a representative HA on the physicochemical properties, molecular transformation, and toxicity of the aged products. Although the direct photoaging of long-chain PS was slow, the presence of HA markedly accelerated its photoaging in a dose-dependent manner. Physicochemical analyses revealed substantial morphological deterioration, particle size reduction, and an increase in typical aging-related functional groups (e.g., C=O) in PS. PS photosensitization and reactive oxygen species (ROS) were critically involved in the photoaging process. The strong light-absorbing properties of HA promoted ROS-mediated chain reactions. Fourier-transform Ion Cyclotron Resonance Mass Spectrometry analysis identified 4142 products from direct PS aging, compared to 3737 products in HA-mediated system. These products were mainly aliphatic, lignin, lipid, and tannin-like substances. Moreover, toxicity assessment revealed that high concentrations HA did not generate directly toxic products, whereas the products formed in the low HA group exhibited bioaccumulation potential. In summary, this study elucidates the environmental behavior and potential risks of PS, providing a scientific basis for the sustainable management of aquatic ecosystems.