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.

Modelling photochemical transformation of emerging organic pollutants in surface waters: effect of water level fluctuations following outflow or evaporation, relevant to arid and semi-arid environments

“…With increasing f, the filter effect by CDOM becomes more important and decreases the absorption of radiation by nitrate and nitrite. ) are the product of absorbed photon fluxes times depth-independent quantum yields [23,24], and they are ∝ f −1 as well. When considering the overall results shown in Figures 1-3, one sees that a decreasing water depth would be generally favourable to photochemical processes.…”

“…The time unit adopted by our model is the time taken by sunlight in cloudless sky to deliver 7.5⋅10 5 J m −2 UV energy to the ground (e.g., 9.5 hours of light at an average UV irradiance of 22 W m −2 ) [23,24]. Hereafter, this will be the meaning of the term "day" as far as the model output is concerned (note that modelled pseudo-first order degradation rate constants have units of day −1 ).…”

“…• OH, CO 3 −• , 1 O 2 and 3 CDOM*) as a function of variable water chemistry and depth [23,24]. The model can also predict pollutant transformation kinetics.…”

The role of humic and fulvic acids in the phototransformation of phenolic compounds in seawater

“…Singlet oxygen is poorly reactive toward Fe(II) (Wilkinson and Brummer, 1981). In contrast, 3 HS* in ASW would be significantly scavenged by chloride and bromide to produce Cl 2 −• and Br 2 −• , respectively (De Laurentiis et al, 2012;Brigante et al, 2014), which are involved in Fe(II) oxidation (Neta et al, 1988).…”

Phototransformation of the sunlight filter benzophenone-3 (2-hydroxy-4-methoxybenzophenone) under conditions relevant to surface waters

“…The model takes into account sunlight absorption by water (mostly accounted for by CDOM) and the wavelength-dependent differences of light penetration into the water column. Competition for sunlight irradiance between the main photosensitisers (nitrate, nitrite and CDOM) is also taken into account by the model (Vione et al, 2010b;De Laurentiis et al, 2012a). An important issue is that steady irradiation is usual in laboratory experiments, but one should take into account the variable outdoor irradiance and the day-night cycle.…”