Kinetics and mechanism of syringic acid degradation initiated by hydroxyl radical and sulphate radical in the aqueous phase

摘要

Syringic acid (Syr) is an abundant component in aerosol particles. Multiphase photo-oxidation in aerosol phase provides an important oxidation pathway for Syr in the atmosphere. In this work, we studied the multiphase degradation of Syr by measuring rate coefficients of its reactions with potential radical oxidants (OH and SO4-center dot) in aqueous solutions and by theoretical calculations, and degradation mechanisms by identifying the (intermediate) products. Rate coefficients, in 10(9) M-1 s(-1), were obtained as 32 +/- 2 (pH 3) and 25 +/- 2 (pH 6) for reactions with OH radical, and 1.7 +/- 0.1 (pH 3) and 0.9 +/- 0.02 (pH 6) for reactions with SO4-center dot. Reactions of Syr with OH and SO4-center dot were all in diffusion-control limit. Rate coefficients' difference under pH 6 and pH 3 in SO4-center dot reactionwas caused by Coulomb's force between negatively charged species. Theoretical calculations showed that the reaction of Syr with OH starts mainly by hydrogen atom transfer (HAT) from phenolic groups and secondly by OH addition to the aromatic ring. No product was identified in the reaction of Syr and OH radical at pH 3 due probably to the rapid mineralization of phenoxy radical formed from HAT, while products after OH additions were identified for a reaction at pH 6. On the other hand, reaction of Syr with SO4-center dot starts by single-electron transfer (SET), forming Syr(+), which can undergo hydrolysis, sulfation, and dimerization with Syr and other aromatic intermediates, etc. Dimerization products from the phenoxy-type radical were not found here. (C) 2020 Elsevier Ltd. All rights reserved.