title: Assessing the occurrence of the dibromide radical (Br₂⁻•) in natural waters: measures of triplet-sensitised formation, reactivity, and modelling.
abstract: The triplet state of anthraquinone-2-sulphonate (AQ2S) is able to oxidise bromide to Br(•)/Br(2)(-•), with rate constant (2-4)⋅10(9)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 ((3)CDOM*). The brominating agent Br(2)(-•) could thus be formed in natural waters upon oxidation of bromide by both (•)OH and (3)CDOM*. Br(2)(-•) 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⋅10(2)L(mg C)(-1)s(-1) was measured between Br(2)(-•) and DOM. It was thus possible to model the formation and reactivity of Br(2)(-•) in natural waters, assessing the steady-state [Br(2)(-•)]≈10(-13)-10(-12)M. It is concluded that bromide oxidation by (3)CDOM* would be significant compared to oxidation by (•)OH. The (3)CDOM*-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 Br(2)(-•) would be limited by the formation rate of the hydroxyl radical. In contrast, the formation rate of (3)CDOM* is much higher compared to that of (•)OH in most surface waters and would provide a large (3)CDOM* reservoir for bromide to react with. A further issue is that nitrite oxidation by Br(2)(-•) could be an important source of the nitrating agent (•)NO(2) 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.
authors.affiliation: Università degli Studi di Torino, Dipartimento di Chimica, Via P. Giuria 5, 10125 Torino, Italy. http://www.chimicadellambiente.unito.it
authors.forename: Elisa
authors.initials: E
authors.lastname: De Laurentiis
authors.forename: Marco
authors.initials: M
authors.lastname: Minella
authors.forename: Valter
authors.initials: V
authors.lastname: Maurino
authors.forename: Claudio
authors.initials: C
authors.lastname: Minero
authors.forename: Gilles
authors.initials: G
authors.lastname: Mailhot
authors.forename: Mohamed
authors.initials: M
authors.lastname: Sarakha
authors.forename: Marcello
authors.initials: M
authors.lastname: Brigante
authors.forename: Davide
authors.initials: D
authors.lastname: Vione
mesh_terms: D000880:Anthraquinones / Q000737:chemistry; D001965:Bromides / Q000032:analysis* / Q000737:chemistry / Q000528:radiation effects; D005618:Fresh Water / Q000737:chemistry*; D006812:Humic Substances / Q000032:analysis / Q000528:radiation effects; D017665:Hydroxyl Radical / Q000737:chemistry / Q000528:radiation effects; D008956:Models, Chemical; D009573:Nitrites / Q000737:chemistry / Q000528:radiation effects; D010084:Oxidation-Reduction; D019800:Phenol / Q000737:chemistry / Q000528:radiation effects; D010782:Photolysis; D012623:Seawater / Q000737:chemistry*; D013472:Sunlight; D014874:Water Pollutants, Chemical / Q000032:analysis* / Q000737:chemistry / Q000528:radiation effects
chemical_list: D000880:Anthraquinones; D001965:Bromides; D006812:Humic Substances; D009573:Nitrites; D014874:Water Pollutants, Chemical; C023554:anthraquinone sulfonate; D017665:Hydroxyl Radical; D019800:Phenol