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.