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Molecular-level insights into the heterogeneous variations and dynamic formation mechanism of leached dissolved organic matter during the photoaging of polystyrene microplastics
Summary
Researchers investigated the molecular-level changes that occur when polystyrene microplastics break down under light exposure and release dissolved organic matter into water. They found that the released molecules were highly diverse and changed dynamically over the course of aging, with different chemical classes appearing at different stages. The study provides new insight into how degrading microplastics introduce complex mixtures of organic chemicals into aquatic environments.
Microplastics (MPs) and their derivatives have received worldwide attention owing to their adverse effects on ecosystems. However, molecular diversity and dynamic formation of dissolved organic matter (DOM) during the photoaging of MPs remain unclear. Herein, we explored a molecular‒level formation mechanism for polystyrene MP (MP)‒derived DOM (PSDOM) during the photoaging of MPs to explain the evolution, heterogeneity, and sequential response of molecules to irradiation. Two‒dimensional correlation spectroscopy was applied to correlate the variations of PSDOM molecules detected by Fourier transform-ion cyclotron resonance mass spectrometry with those of MP functional groups detected by Fourier transform infrared spectroscopy. Irradiation‒induced PSDOM contained the most highly unsaturated structures with oxygen, but showed fewer aromatic structures than natural aquatic DOM. Photochemical transformations occurred between saturated‒reduced and oxidized molecules during PSDOM leaching, with the low‒oxidized and high‒oxidized molecules undergoing considerable changes in the normal carbon oxidation state and molecular number, respectively. The primary sequential response of PSDOM molecules to increasing irradiation time [low‒oxidized/high‒weight (450<m/z<800) → high‒oxidized → saturated/low‒weight molecules (m/z<450)] corresponded to the response of MP functional groups (aromatic C‒H → carbonyl → aromatic ring, CH bend → C‒H groups), demonstrating well synergistic relationships between them. These novel findings will contribute to the understanding of underappreciated behaviors or risks of MPDOM in aquatic ecosystems.