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

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.