Microplastic-Derived Dissolved Organic Matter Regulates Reactive Bromine Species Formation by Modulating the Photochemistry of Seawater Dissolved Organic Matter

Microplastic-derived dissolved organic matter (MP-DOM) is emerging as a non-negligible organic input to marine systems, yet how its interactions with seawater DOM (S-DOM) regulate the photochemical formation of reactive bromine species (RBS) remains unclear. In this study, the RBS formation potential of poly(vinyl chloride) (PVC)-derived MP-DOM, S-DOM, and their mixed systems was systematically investigated under simulated solar irradiation. MP-DOM input exerted a nonlinear ratio-dependent regulatory effect on S-DOM-mediated RBS formation. At relatively low MP-DOM proportions (5–20%), the formation rate constant of 4-bromo-3,5-dimethyl-1H-pyrazole (Br-DMPZ), which represents the RBS formation potential, increased relative to S-DOM alone, reaching a maximum enhancement of 47.4% at 20% MP-DOM under seawater halide ion conditions (540 mM Cl– + 0.8 mM Br–). This enhancement was associated with increased quinone-/carbonyl-related redox-active structures, electron exchange capacity, and tannin-like components, which could facilitate photoinduced charge separation, electron transfer, and Br– oxidation. In contrast, higher MP-DOM proportions (40–50%) promoted hydrophobic lipid-like mixed assemblies, enhancing excited-state quenching and suppressing electron transfer and RBS photogeneration. These findings demonstrate that MP-DOM is not an inert byproduct of microplastic aging but rather an important regulatory factor in S-DOM-mediated halogen photochemistry in plastic-impacted marine waters.