Molecular transformation and photochemical reactivity of microplastic-derived dissolved organic matter on goethite: Implications for persistence and reactive oxygen species dynamics.

Microplastics (MPs)-derived dissolved organic matter (MPs-DOM) is emerging as a significant contributor to environmental DOM pools. However, the molecular-scale processes governing its interactions with mineral and their effects on photoreactivity remain poorly understood. This study elucidates the structure-dependent molecular transformations and photochemical reactivity of DOM during its interaction with goethite, revealing distinct mechanisms driving reactive oxygen species (ROS) dynamics. Hydroxyl radical (•OH) production universally declined as goethite adsorbed electron-donating groups. Singlet oxygen (1O2) remained stable due to the presence of persistent aromatic photosensitizers. Triplet-excited DOM (3DOM*) exhibited divergent trends: humic acid (HA) and polybutylene adipate terephthalate (PBAT)-DOM showed decreases due to aromatic core adsorption, while fulvic acid (FA) and polystyrene (PS)-DOM exhibited increases driven by humification-stabilized quinoid systems and sulfonation-enhanced conjugation, respectively. The aliphatic inertia of polyethylene (PE)-DOM minimized ROS fluctuations. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) indicated that HA retained lignin-like components and stable sulfur/nitrogen heterocycles, while FA preserved branched quinones. PS-DOM underwent sulfonation, generating benzene sulfonates that enhanced 1O2 formation, whereas PBAT-DOM hydrolyzed to nitrogenous metabolites that suppressed •OH production. PE-DOM remained structurally resistant, consistent with its hydrophobic aliphatic chains. These findings emphasize that goethite's selective adsorption enriches redox-active aromatics in natural DOM but promotes the persistence of MPs-DOM through sulfurization and nitrogenization.