Dynamic formation characteristics and mechanism of molecular components of dissolved organic matter during photoaging of polyamide microplastic

Microplastic-derived dissolved organic matter (MP-DOM) has attracted widespread attention due to its adverse effects on ecological health. However, the dynamic formation of MP-DOM at the molecular level is not yet fully understood. Herein, the molecular level formation characteristics and mechanism of polyamide-MP-derived DOM (MPPA-DOM) during irradiation were explored using fluorescence spectroscopy. Fourier transform-ion cyclotron resonance mass spectrometry, and parallel factor analysis. The results showed that the time-dependent fluorescence signatures revealed a dominant tyrosine-like component, whose relative abundance increased from 49.63% to 89.62% during irradiation, suggesting a gradual accumulation of protein-related substances. Molecular element analyses of MPPA-DOM revealed the predominance of CHON molecules (78.82%-89.30%), which was attributed to the degradation of the C-N backbone structure. In contrast, CHO molecules exhibited a lower proportion (9.45%) under prolonged irradiation. Aliphatic/peptide-like compounds in MPPA-DOM remained the dominant component with a percentage range of 66.4% to 68.7%, whereas lignin-like compounds slightly increased with the increase of irradiation time. The reduced molecules were dominated in MPPA-DOM with a percentage range of 96.4% to 99.1%. As irradiation increased, the saturated compounds decreased from 91.53% to 82.45% and the unsaturated compounds increased from 7.6% to 14.1%. This study proposed a molecular-level formation mechanism of MPPA-DOM under irradiation. Nitrogen-rich molecules were persistent and highly stable during irradiation, indicating that they could play a more important role in the migration and transformation of MPPA-DOM. The findings in this study will provide support for assessing the potential ecological risks of MP-DOM in water systems.