Kinetics and mechanism of Thallium(I) oxidation by Permanganate: Role of bromide

The oxidation of thallium(I) (Tl (I)) to Tl (III) is referred to as an efficient means for Tl removal. Bromide (Br‾) inevitably occurs in nearly all water sources at concentrations of 0.01–67 mg/L (0.14–960 μM). The effect of Br‾ remains largely unclear but likely of critical importance on the redox fate and thus the removal potential of Tl (I) during typical oxidation treatment processes. Here, we investigate the kinetics and tackle the mechanism of Tl (I) oxidation by permanganate (KMnO₄) under the influence of Br‾. The results demonstrated that Br‾ at environmental levels exhibited significant catalytic effect on Tl (I) oxidation kinetics by KMnO₄ at acidic pH of 4.0–7.0, while no significant effect of Br‾ was observed for Tl(I) oxidation under alkaline conditions of pH 8.0 and 9.0. It was found that the enhanced oxidation kinetics under acidic conditions was driven by the combined effect of and autocatalysis mediated by MnO₂ and a fast oxidation kinetics served by in-situ formed bromine species. Through quantifying the relative contributions of those bromine species to the homogenous oxidation of Tl(I), HOBr, Br₂ and Br₂O were found to play roles in catalyzing the oxidation of Tl(I) by KMnO₄. The results discussed herein highlight the critical role of Br‾ on the Tl(I) complex oxidation process by KMnO₄ and may have implications for evaluating the redox cycle and removal potential of Tl in bromide-containing water treatment.