Journal
JOURNAL OF HAZARDOUS MATERIALS
Volume 401, Issue -, Pages -Publisher
ELSEVIER
DOI: 10.1016/j.jhazmat.2020.123884
Keywords
Organohalogen; Halogen radicals; Photosensitizer; Organic substances; Photolysis
Categories
Funding
- National Natural Science Foundation of China (NSFC) [41977313, 21677031, 41273108, 21007009]
- Foundation of Key Laboratory of Yangtze River Water Environment, Ministry of Education (Tongji University), China [YRWEF202003]
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Halogenated organic compounds, previously thought to be mainly anthropogenic, can also be formed through photochemical reactions in sunlit surface waters. Halide ions in natural waters may be oxidized to reactive halogen species by photochemistry, leading to reactions with organic substances. The chemical mechanism of halogenation remains complex and challenging to fully understand.
Halogenated organic compounds, also termed organohalogens, were initially regarded to be of almost exclusively anthropogenic origin. However, recent research has demonstrated that photochemical reactions are important abiotic sources of organohalogen compounds in sunlit surface waters. Halide ions (X-, X represents Cl, Br and I) are common anions in natural waters and might be oxidized by reactive species originated from photochemistry of dissolved organic matter (DOM) or inorganic photoactive species. The resulting reactive halogen species may react with organic substances with diverse bimolecular reaction rate constants, depending on the complexity and structure of organic substances. Therefore, the chemical mechanism of halogenation remains challenging to be fully elucidated. To better understand the trends in the existing data and to identify the knowledge gaps that may merit further investigation, this review gives an integrative summary on the sources of reactive oxygen species (ROS) and halogen radicals (X-center dot/X-2(center dot)-). Photochemical halogenation of phenolic compounds and formation of methyl halide and brominated organic pollutants are highlighted. By evaluating existing literature and identifying some uncertainties, this review emphasizes the environmental significance of sunlight-driven halogenation and proposes further research directions on mechanistic investigation and rational experimental design close to natural systems.
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