Photochemical Generation of Methyl Chloride from Humic Aicd: Impacts of Precursor Concentration, Solution pH, Solution Salinity and Ferric Ion

Alkyl halide formation from degradation of carboxylic acids in the presence of Fe(III) and halides under light irradiation

Advanced oxidation processes (AOPs) have been widely used in water and wastewater treatment and have shown excellent performance in remediating contaminated water. However, their oxidation byproducts, including halogenated organics, have recently attracted increasing attention. Alkyl halides are among the most important environmental pollutants in nature. Here, we report a Fenton-like reaction in which alkyl halides can form during the photodegradation of aliphatic carboxylic acids in the presence of Fe(III) and halides. Chloromethane, chloroethane, and 1-chloropropane were produced from the degradation of acetic acid, propionic acid and n-butyric acid, respectively. CH₃Cl, CH₂Cl₂ and CHCl₃ were all identified as the products of acetic acid with the yields of approximately 5.1%, 0.2% and 0.005%, respectively. It was demonstrated that hydroxyl radicals, halogen radicals and alkyl radicals were involved in the formation of alkyl halides. A possible mechanism of chloromethane formation was proposed based on the results. In real samples of saline water, the addition of carboxylic acid and Fe(III) significantly promoted the generation of CH₃Cl under xenon lamp irradiation. The results indicated that the coexistence of Fe(III), halides and carboxylic acids enhanced the photochemical release of alkyl halides. The reactions described in this paper may contribute to knowledge on the mechanism of halogenated byproduct formation during AOPs.

Application of copper(II)-based chemicals induces CH3Br and CH3Cl emissions from soil and seawater

Methyl bromide (CH₃Br) and methyl chloride (CH₃Cl) are major carriers of atmospheric bromine and chlorine, respectively, which can catalyze stratospheric ozone depletion. However, in our current understanding, there are missing sources associated with these two species. Here we investigate the effect of copper(II) on CH₃Br and CH₃Cl production from soil, seawater and model organic compounds: catechol (benzene-1,2-diol) and guaiacol (2-methoxyphenol). We show that copper sulfate (CuSO₄) enhances CH₃Br and CH₃Cl production from soil and seawater, and it may be further amplified in conjunction with hydrogen peroxide (H₂O₂) or solar radiation. This represents an abiotic production pathway of CH₃Br and CH₃Cl perturbed by anthropogenic application of copper(II)-based chemicals. Hence, we suggest that the widespread application of copper(II) pesticides in agriculture and the discharge of anthropogenic copper(II) to the oceans may account for part of the missing sources of CH₃Br and CH₃Cl, and thereby contribute to stratospheric halogen load.

Halogenated compounds impact stratospheric ozone. This study suggests agricultural application of Cu(II) chemicals induces abiotic production of methyl bromide and methyl chloride from soil and seawater, contributing to the atmospheric halogen load.

Photochemical origin of reactive radicals and halogenated organic substances in natural waters: A review

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/X₂^-). 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.

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

Methyl bromide (CH3Br) is one of the largest natural sources of bromine in the stratosphere, where it leads to ozone depletion. This paper reported the photochemical production of CH3Br from syringic acid (SA) that has been used as an environmentally relevant model compound for terrestrially-derived dissolved organic matter. The formation of CH3Br increased with the increase of bromide ion concentration ranging from 0.8 to 80 mmol L-1. Ferric ions (Fe(III)) enhanced CH3Br production, while chloride inhibited it, with or without Fe(III). Meanwhile, methyl chloride (CH3Cl) was generated in the presence of chloride and was inhibited by Fe(III). The different effects of Fe(III) on the formation of CH3Cl and CH3Br indicate their diverse formation paths. Based on the intermediates identified by liquid chromatography-mass spectrometry and the confirmation of the formation of Fe(III)-SA complexes, it was proposed that there were two formation paths of CH3Br from SA in the bromide-enriched water under simulated sunlight irradiation. One path was via nucleophilic attack of Br- on the excited state protonation of SA; the other was via the combination of methyl radical and bromine radical when Fe(III) was present. This work suggests that the photochemical formation of CH3Br may act as a potential natural source of CH3Br in the bromide-enriched environmental matrix, and helps in better understanding the formation mechanism of CH3Br.