Mechanisms of Sb(III) oxidation mediated by low molecular weight phenolic acids

Abstract

Quinone-like moieties are active components of natural organic matter (NOM), whose redox reactions may exert substantial influence on the behavior of trace metals in natural waters. The interactions between NOM and antimony (Sb) influence the mobility and toxicity of Sb, but the mechanism of this process is poorly understood. In this study, batch oxidation experiments were conducted to evaluate the effects of model quinone-like moieties, i.e., 1,4-hydroquinone and low molecular weight phenolic acids (LMWPAs), including gallic acid, caffeic acid, and protocatechuic acid, on the oxidation of Sb(III) in the absence of light. The results indicated that Sb(III) was rapidly oxidized to Sb(V) in the presence of different phenolic acids in neutral and alkaline conditions. The oxidative reactions were pH dependent and the Sb(III) oxidation rate constant (kobs) increased by one or two orders of magnitude when the pH shifted from 7.0 to 9.0. The combination of electron paramagnetic resonance and free radical quenching studies elucidated that reactive oxygen species (hydrogen peroxide) and quinone species (the semiquinone radical and oxidized forms of LMWPAs) were the main contributors to the Sb(III) oxidation. According to density functional theory based calculations, the bond dissociation enthalpy of the phenolic hydroxyl group is well correlated with kobs, which is in line with its pro-oxidant property towards Sb(III) through the production of semiquinone radical. The findings of this study provided new insights into the mechanism of the oxidation of Sb(III) by NOM.