Abstract
Halide ions are ubiquitous in natural waters and wastewaters. Halogens play an important and complex role in environmental photochemical processes and in reactions taking place during photochemical water treatment. While inert to solar wavelengths, halides can be converted into radical and non-radical reactive halogen species (RHS) by sensitized photolysis and by reactions with secondary reactive oxygen species (ROS) produced through sunlight-initiated reactions in water and atmospheric aerosols, such as hydroxyl radical, ozone, and nitrate radical. In photochemical advanced oxidation processes for water treatment, RHS can be generated by UV photolysis and by reactions of halides with hydroxyl radicals, sulfate radicals, ozone, and other ROS. RHS are reactive toward organic compounds, and some reactions lead to incorporation of halogen into byproducts. Recent studies indicate that halides, or the RHS derived from them, affect the concentrations of photogenerated reactive oxygen species (ROS) and other reactive species; influence the photobleaching of dissolved natural organic matter (DOM); alter the rates and products of pollutant transformations; lead to covalent incorporation of halogen into small natural molecules, DOM, and pollutants; and give rise to certain halogen oxides of concern as water contaminants. The complex and colorful chemistry of halogen in waters will be summarized in detail and the implications of this chemistry for global biogeochemical cycling of halogen, contaminant fate in natural waters, and water purification technologies will be discussed.
Highlights
Halide ions are ubiquitous in natural waters
Even though the halides themselves do not absorb light in the solar region, in nature they provide far more than just background electrolytes—they participate in a rich, aqueous-phase chemistry initiated by sunlight that has many implications for dissolved natural organic matter (DOM)
Since in most waters carbonates will be at millimolar concentrations, whereas H2 O2, NO2 −, and O3 will seldom exceed micromolar concentrations, scavenging of the rRHS by carbonates will usually predominate over the others
Summary
Halide ions are ubiquitous in natural waters. Ordinary levels of halides in seawater are 540 mM chloride, 0.8 mM bromide, and 100–200 nM iodide [1,2]. Surface fresh water and groundwater may contain up to 21 mM chloride and 0.05 mM bromide [1], with higher levels in some places. While generalizations are difficult, such waters often contain moderate-to-very-high halide ion concentrations, as well as Molecules 2017, 22, 1684; doi:10.3390/molecules22101684 www.mdpi.com/journal/molecules. This review aims to summarize the reactions of halides and their daughter products and offer insight into on on photochemical transformations takingtaking place in water. Can undergo intotheir theireffects effects photochemical transformations place in Halides water. DOM and anthropogenic including oxidation and incorporation of halogen. Compounds, including oxidation and incorporation of halogen. These reactions are described and Extensive rate constants for relevant or RHS generation and decay have been discussed.tabulations
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