Halogen radicals contribute to photooxidation in coastal and estuarine waters

Abstract

Although halogen radicals are recognized to form as products of hydroxyl radical (• OH) scavenging by halides, their contribution to the phototransformation of marine organic compounds has received little attention. We demonstrate that, relative to freshwater conditions, seawater halides can increase photodegradation rates of domoic acid, a marine algal toxin, and dimethyl sulfide, a volatile precursor to cloud condensation nuclei, up to fivefold. Using synthetic seawater solutions, we show that the increased photodegradation is specific to dissolved organic matter (DOM) and halides, rather than other seawater salt constituents (e.g., carbonates) or photoactive species (e.g., iron and nitrate). Experiments in synthetic and natural coastal and estuarine water samples demonstrate that the halide-specific increase in photodegradation could be attributed to photochemically generated halogen radicals rather than other photoproduced reactive intermediates [e. OH-independent pathway. Our results indicate that halogen radicals significantly contribute to the phototransformation of algal products in coastal or estuarine surface waters.halogen radicals | photochemistry | domoic acid | dimethyl sulfide | dissolved organic matter R esearch on the photochemical transformation of organic compounds in seawater has focused on freshwater-relevant pathways, including direct photolysis and dissolved organic matter (DOM)-sensitized indirect photodegradation by excited triplet state DOM ( 3 DOM*) or reactive oxygen species [e.g., hydroxyl radical ( • OH)] (1). High halide concentrations, the key characteristic distinguishing seawater from freshwater, are the predominant sink for• OH in seawater, with Br − scavenging ∼93% (Eq. 1) (2), resulting in an ∼40-fold reduction in• OH concentrations.• OH + BrThe role of the resultant Br• , as well as other radical reactive halogen species (RHS) (e.g., Cl ), in seawater photochemistry has received little attention. Here we evaluate the contribution of RHS, a family of photooxidants specific to seawater, to the photodegradation of important marine products for two reasons. Unlike diffusion-limited• OH rate constants, RHS rate constants with organic molecules span orders of magnitude (3), such that conversion of • OH to RHS focuses oxidation on more reactive molecules relative to bulk DOM. Secondly, direct halide oxidation by 3 DOM*, previously demonstrated using model triplet sensitizers (4, 5), may provide a seawater-specific,• OH-independent RHS production pathway that could increase overall radical generation rates.We investigated the importance of RHS for photooxidation of dienes and thioethers. Dienes occur in biomolecules (e.g., fatty acids) and in the marine algal toxin (6, 7), domoic acid (Fig. 1A, ii), where the diene contributes to toxicity (8). During summer 2015, a large bloom of the marine diatom Pseudo-nitzschia, extending from central California to Alaska, led to significant concern regarding the production of domoic acid, a potent neurotoxin (9). Consumption of mollusks and fish...

Multiple linear regression models to predict the formation efficiency of triplet excited states of dissolved organic matter in temperate wetlands

“…Average watershed slope (reported as rise/run), average watershed flow path length, and the ratio of the average flow path length to the average slope (L/G; a proxy for the average watershed water residence time) are reported for each watershed (McGuire et al 2005). Watershed land cover was determined by clipping watershed areas to 15-m resolution land cover data for Minnesota from 2013 (Rampi et al 2016). Relative land cover for six characteristics are reported:…”

“…3 CDOM* are particularly important in biogeochemical processes because they are potent oxidants and they form secondary reactive oxidants, such as radicals and reactive oxygen species (Zepp et al ; Parker and Mitch ). 3 CDOM* are involved in the photodegradation of soluble organic pollutants (Remucal ), pathogen inactivation (Kohn and Nelson ), and the phototransformation of lignin phenols and free amino acids (McNally et al ; Boreen et al ; Waggoner et al ).…”

Molecular Composition and Photochemical Reactivity of Size-Fractionated Dissolved Organic Matter

“…6,7 3 DOM are long-lived species resulting from photon absorption by DOM to form excited, singlet DOM and subsequent partial relaxation via intersystem crossing. 8,9 3 DOM degrades contaminants by direct reaction through energy 10 or electron transfer 11 or through the production of reactive species, such as singlet oxygen ( 1 O 2 ). 12 …”

Insight Into the Formation Paths of Methyl Bromide From Syringic Acid in Aqueous Bromide Solutions Under Simulated Sunlight Irradiation

“…HBrO As discussed above, Cl • hardly existed in the reaction system and most of the HClO •− was prone to reform • OH and Cl − , so Equation (5) and Equation (7) [37,41]. Consequently, BrCl •− would react with • CH 3 forming CH 3 Br or CH 3 Cl (Equation (9)).…”

“…Although Cl − occurs at 670-fold higher concentrations than Br − , Br − oxidation by • OH drives halogen radical production [43]. Even, Br 2 •− concentration may exceed those of BrCl •− bỹ 2.5-fold, since Br 2 •− arises from further reactions of BrCl •− with Br − [37,41]. Consequently, bromide oxidation by • OH and the further production of bromine radicals play an important role for the generation of CH 3 Br.…”