Fe(VI) oxidation of synthetic phenolic antioxidants: Kinetics, influencing factors, transformation mechanism and toxicity

Synthetic phenolic antioxidants (SPAs) have been commonly used in industrial and domestic products to prevent the adverse effects of oxidants such as oxygen. However, attention to the removal of SPAs is scarcely attracted. In this paper, the removal kinetics, influencing factors, transformation mechanisms and toxicity in Fe(VI) oxidation of 18 SPA congeners were investigated in detail. Results showed that propyl gallate (PG), butylated hydroxyanisole (BHA), and tert-butylhydroquinone (TBHQ) had the highest reactivities with Fe(VI), while the reaction rate constant (k(app)) value of 2,4-di-tert-butylphenyl 3,5-di-tert-butyl-4-hydroxybenzoate (UV120) was the lowest (221.3 M-1 s(-1)). A robust quantitative structure-activity relationship (QSAR) model between k(app) and molecular structural descriptors of SPAs was established. The energy gap between the lowest unoccupied molecular orbital (E-LUMO) and the highest occupied molecular orbital (E-HOMO) was a vital parameter to determine the removal efficiencies of the SPA congeners. Furthermore, the numbers of oxygen atoms and alkyl groups in SPA structures also determined their reactivities with Fe(VI) to some extent. In addition, aqueous 2,6-di-tert-butyl-4-methylphenol (BHT, a representative of SPAs) could be efficiently removed with the increased amount of oxidant, and decreased substrate concentration and slightly alkaline conditions. Seventeen intermediates were identified, and oxidation routes including H-abstraction, single- and double-oxygen(s) transfer, and coupling were deduced. However, H-abstraction inevitably led to the generation of BHT-Q with higher toxicity than parent. These findings provide novel insights into the production and removal of SPA analogs.