Journal Article

Halogen radicals contribute to photooxidation in coastal and estuarine waters

Parker, Kimberly M., Mitch, William A.

2016

Overview

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.g., excited-state triplet DOM (3DOM*), reactive oxygen species]. Computational kinetic modeling indicates that seawater halogen radical concentrations are two to three orders of magnitude greater than freshwater •OH concentrations and sufficient to account for the observed halide-specific increase in photodegradation. Dark •OH generation by gamma radiolysis demonstrates that halogen radical production via •OH scavenging by halides is insufficient to explain the observed effect. Using sensitizer models for DOM chromophores, we show that halogen radicals are formed predominantly by direct oxidation of Cl⁻ and Br⁻ by ³DOM*, an •OH-independent pathway. Our results indicate that halogen radicals significantly contribute to the phototransformation of algal products in coastal or estuarine surface waters.

• Publisher: National Academy of Sciences
• Subject: Algae / Algal toxins / Biodegradation / Carbonates / Carbonates - chemistry / Chemical analysis / Chemical synthesis / Dissolved organic matter / Earth, Atmospheric, and Planetary Sciences / Estuaries / Fresh Water - chemistry / Fresh Water - microbiology / Freshwater environments / Halides / Halogens - chemistry / Harmful Algal Bloom - radiation effects / Hydroxyl Radical - chemistry / Hydroxyl radicals / Iron - chemistry / Kainic Acid - analogs & derivatives / Kainic Acid - chemistry / Kinetics / Light / Marine Toxins - chemistry / Nitrates - chemistry / Organic chemistry / Organic compounds / Oxidation / Photochemistry / Photodegradation / Photolysis / Photooxidation / Physical Sciences / Seawater / Seawater - chemistry / Seawater - microbiology / Sulfides - chemistry / Surface water / Toxins / Water analysis / Water sampling