Assessing the occurrence of the dibromide radical (Br2−radical dot) in natural waters: Measures of triplet-sensitised formation, reactivity, and modelling Original Research Article

The triplet state of anthraquinone-2-sulphonate (AQ2S) is able to oxidise bromide to Brradical dot/Br2−radical dot, with rate constant (2–4) ⋅ 109 M− 1 s− 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−radical dot could thus be formed in natural waters upon oxidation of bromide by both radical dotOH and 3CDOM*. Br2−radical dot would be consumed by disproportionation into bromide and bromine, as well as upon reaction with nitrite and most notably with 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 ⋅ 102 L (mg C)− 1 s− 1 was measured between Br2−radical dot and DOM. It was thus possible to model the formation and reactivity of Br2−radical dot in natural waters, assessing the steady-state [Br2−radical dot] ≈ 10− 13–10− 12 M. It is concluded that bromide oxidation by 3CDOM* would be significant compared to oxidation by radical dotOH. The 3CDOM*-mediated process would prevail in DOM-rich and bromide-rich environments, the latter because elevated bromide would completely scavenge radical dotOH. Under such conditions, radical dotOH-assisted formation of Br2−radical dot would be limited by the formation rate of the hydroxyl radical. In contrast, the formation rate of 3CDOM* is much higher compared to that of radical dotOH in most surface waters and would provide a large 3CDOM* reservoir for bromide to react with. A further issue is that nitrite oxidation by Br2−radical dot could be an important source of the nitrating agent radical dotNO2 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.