Quantitative probe detection of photosensitized dibromide radical anions generated from dissolved organic matter in aquatic systems

Dibromide radical anions (Br₂^•−) are prevalent in marine and estuarine waters, significantly influencing the photochemical transformation of pollutants. Despite their importance, the lack of probes to quantitatively detect Br₂^•− greatly hampers the understanding of their environmental behavior. In this study, we demonstrated that 3,5-dimethyl-1-H-pyrazole (DMPZ) can detect Br₂^•− in aqueous solutions containing dissolved organic matter (DOM) and halide ions under simulated sunlight irradiation. The formation of Br-DMPZ through the reaction of DMPZ with Br₂^•− serves as a reliable indicator of Br₂^•− presence. The secondary reaction rate constant of DMPZ with Br₂^•− was determined to be 1.68 × 10⁷ M⁻¹ s⁻¹. The steady-state concentration of Br₂^•− ([Br₂^•−]ss) produced by DOM in water with seawater-level concentrations of halides (540 mM Cl⁻ + 0.8 mM Br⁻) under simulated sunlight irradiation was determined to be at the level of 10⁻¹⁸ M using DMPZ as a probe. Meanwhile, the [Br₂^•−]ss in natural seawater was measured to be at the level of 10⁻²⁰ M. Furthermore, preliminary exploration of Br₂^•− generation using DOM model compounds suggested that aromatic ketones and electron-rich chromophores may contribute to Br₂^•− formation. These findings confirm the feasibility of using DMPZ as a quantitative probe for Br₂^•− in seawater under light conditions, thereby enhancing our understanding of bromine radical formation in water and the complex environmental processes involved.