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

Abstract

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.

Keywords: methyl bromide; photochemical production; reaction paths; syringic acid.

Figure 1

Formation of methyl bromide (CH₃Br) in aqueous solutions containing syringic acid (50 μmol L⁻¹) and bromide ions (0.8~80 mmol L⁻¹). Error bars represent one standard deviation.

Figure 2

Formation of CH₃Br (triangle and star) and methyl chloride (CH₃Cl; circle) in the solutions containing 50 μmol L⁻¹ SA, 8 mmol L ⁻¹ Br⁻ (blue) and 0.8 mmol L⁻¹ Br⁻ (black) in the presence or in the absence of 0.5 mol L⁻¹ Cl⁻. There was no obvious difference for CH₃Cl in the presence of 0.8 or 8 mmol L⁻¹ Br⁻. Dashed line indicates the detection limit of CH₃Br. Error bars represent one standard deviation.

Figure 3

A possible formation pathway of CH₃Br from syringic acid (SA) under irradiation.

Figure 4

Effect of Fe(III) on the formation of CH₃Br from 50 μmol L⁻¹ SA with 8 mmol L⁻¹ NaBr. Error bars represent one standard deviation.

Figure 5

Effect of Fe(III) on the formation of CH₃Cl from 50 μmol L⁻¹ SA with 0.5 mol L⁻¹ NaCl. Dashed line indicates the detection limit of CH₃Cl. Error bars represent one standard deviation.

Figure 6

UV–Vis spectra of 50 μmol L⁻¹ SA, Fe(III) and Fe(III)-SA complexes with different concentration of Fe(III), and the emission spectrum of the Xenon lamp.

Figure 7

A possible formation pathway of CH₃Br in the Fe(III)-SA system.

Figure 8

Formation of CH₃Br (triangle) and CH₃Cl (square) in the solutions containing 50 μmol L⁻¹ SA, 600 μmol L⁻¹ Fe(III) and 8 mmol L⁻¹ Br⁻ in the presence (black) and in the absence (blue) of 0.5 mol L⁻¹ Cl⁻. Error bars represent one standard deviation.