Open AccessRing-Cleavage Products Produced during the Initial Phase of Oxidative Treatment of Alkyl-Substituted Aromatic Compounds
Author(s) -
Jean Van Buren,
Carsten Prasse,
Emily L. Marron,
Brighton A. Skeel,
David L. Sedlak
Publication year - 2020
Publication title -
environmental science and technology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.851
H-Index - 397
eISSN - 1520-5851
pISSN - 0013-936X
DOI - 10.1021/acs.est.0c00432
Subject(s) - chemistry , hydroxylation , toluene , radical , substituent , ethylbenzene , hydrogen atom abstraction , medicinal chemistry , alkyl , organic chemistry , photochemistry , enzyme
Chemical oxidation with hydroxyl radical (HO • ) and sulfate radical (SO 4 •- ) is often used to treat water contaminated with aromatic compounds. Although oxidation of aromatics by these radicals has been studied for decades, the commonly accepted transformation pathway-sequential hydroxylation of the ring followed by ring cleavage and mineralization of the resulting products-does not account for the loss of the parent compound observed during the initial phase of the process. To assess the importance of pathways for aromatic compound oxidation that do not result in ring hydroxylation, we identified products formed after the initial reaction between HO • or SO 4 •- and benzene, toluene, ethylbenzene, and (BTEX) xylene isomers. We quantified products of ring hydroxylation and oxidation of alkyl substituents as well as a suite of ring-cleavage products, including acetaldehyde, formic acid, 6-, 7-, or 8-carbon oxoenals and oxodials. Other ring-cleavage products, which were most likely aldehydes and organic acids, were observed but not quantified. When SO 4 •- was used as the oxidant, aromatic organosulfates also were formed. Our results indicated that the initial phase of the oxidation process involves radical addition, hydrogen abstraction, or one-electron transfer to the ring followed by reaction with O 2 . The hydroxycyclohexadienylperoxy radical produced in this reaction can eliminate hydroperoxyl radical (HO 2 • ) to produce a phenolic compound or it can rearrange to form a bicyclic peroxy intermediate that subsequently undergoes ring cleavage. Hydroxylation of the ring and oxidation of the alkyl substituent accounted for approximately 15-40% of the reacted mass of the parent compound. Ring-cleavage products for which quantification was possible accounted for approximately 2 to 10% of the reacted mass. Our results raise concerns about the formation of toxic ring-cleavage products during the initial stage of oxidation whenever HO • or SO 4 •- is used for the treatment of water containing benzene or alkylbenzenes.