Assessing the occurrence of the dibromide radical (Br2°-) in natural waters: measures of triplet-sensitised formation, reactivity and modeling.
Résumé
The triplet state of anthraquinone-2-sulphonate (AQ2S) is able to oxidise bromide to Br*/Br2 −*,with rate constant (2-4)⋅109M−1s−1 that depends on the pH. Similar processes are expected to take place between bromide and the triplet states of naturally occurring chromophoric dissolved organic matter (3CDOM*). The brominating agent Br2−* could thus be formed in natural waters upon oxidation of bromide by both *OH and 3CDOM*. Br2−* would be consumed by disproportionation into bromide and bromine, as well as upon reaction with nitrite andmost notablywith dissolved organic matter (DOM). By using the laser flash photolysis technique, and phenol as model organic molecule, a second-order reaction rate constant of ~3⋅102L(mg C)−1s−1 was measured between Br2−* and DOM. It was thus possible to model the formation and reactivity of Br2 −* in natural waters, assessing the steady-state [Br2 −*]≈10−13-10−12M. It is concluded that bromide oxidation by 3CDOM* would be significant compared to oxidation by *OH. The 3CDOM*-mediated process would prevail in DOM-rich and bromide-rich environments, the latter because elevated bromide would completely scavenge *OH. Under such conditions, *OH-assisted formation of Br2 −* would be limited by the formation rate of the hydroxyl radical. In contrast, the formation rate of 3CDOM* ismuch higher compared to that of *OHinmost surfacewaters andwould provide a large 3CDOM* reservoir for bromide to react with. A further issue is that nitrite oxidation by Br2 −* could be an important source of the nitrating agent *NO2 in bromide-rich, nitrite-rich and DOM-poor environments. Such a process could possibly account for significant aromatic photonitration observed in irradiated seawater and in sunlit brackish lagoons.