Mechanism of the improved Fe(III)/persulfate reaction by gallic acid for ibuprofen degradation

Abstract

Gallic acid (GA), a natural plant polyphenol, was applied as amendment of Fe(III)/persulfate (PS) system for ibuprofen (IBP) degradation in this study. The impacts of all agentia (GA, Fe(III), PS) concentration and initial pH values on IBP removal efficiency were investigated, and their corresponding observed pseudo-first-order rate constants (k_obs) were calculated. The addition of GA has significantly improved elimination efficiency of IBP due to the enhanced Fe(III)/Fe(II) cycle. Electron paramagnetic resonance (EPR) results confirmed that SO₄^•-, HO^• and O₂^•- were involved in GA/Fe(III)/PS system. However, quenching experiments further affirmed the impact of SO₄^•- and HO^• towards IBP decomposition instead of O₂^•-, with contribution ratio to IBP removal was 69.12% and 30.88%, respectively. SO₄^•- was the main radicals formed by directly activation of PS with Fe(II), while HO^• was the transformation product of SO₄^•-. Based on instrumental analysis (stopped-flow UV-vis spectrum and MS) and theoretical calculation, the potential reaction mechanism between GA and Fe(III) in the presence of PS was further proposed. GA complexed with Fe(III) firstly and the Fe(III)-GA complex was then converted into quinone substance, accompanied by the generation of Fe(II). Furthermore, the application of GA extended the optimal pH range to neutral as well, which made it a promising treatment in practical application.

Keywords: Fe(III)/Fe(II) cycle; Ibuprofen; Persulfate; Polyphenol; Reaction mechanism.