Molecular Evolution of Dissolved Organic Nitrogen from Photoaged Nitrogen-Containing Microplastics Reveals Amine Structures as Key Nitrosamine Precursors

Although dissolved organic nitrogen (DON) is continuously released and transformed as nitrogen-containing microplastics (N-MPs) age, the evolution of this replenished pool and its reactivity during downstream chloramination remains unresolved. Here, we tracked the compositional evolution of DON molecules in polyamide (PA) and polyacrylonitrile (PAN) suspensions under UV irradiation using a nitrogen atom-based molecular classification strategy (N1–N5). After 7 d of irradiation-induced molecular reorganization, N1–N5 molecules accounted for 87% and 79% of the molecular signal intensity in PA-DOM and PAN-DOM, respectively. Eight annotated compounds followed increasing, decreasing, or transient accumulation trajectories, revealing simultaneous molecular accumulation and depletion. Reaction-network analysis further showed that N1–N5 molecules progressively became the dominant reactants, and these trajectories were dominated by oxidation and accompanied by increasingly frequent amine-related transformation links. Structural analysis revealed amide-rich N1–N2 and N-heterocycle-rich N3–N5 molecules, while secondary and tertiary amines occurred across all N1–N5 classes. As this molecular reorganization progressed, N-MP-DON level dropped, but substantially enhanced N-nitrosodimethylamine and N-nitrosodiethylamine formation, and potential precursor identification was strongly associated with amine structures, rather than nitrogen atom number. Overall, our study highlights N-MP-DON as a potential source of nitrosamine precursors, and the need to carefully consider amine-containing additives use in plastic manufacturing.