Co-present Pb(II) accelerates the oxidation of organic contaminants by permanganate: Role of Pb(III)

The permanganate (Mn(VII)) oxidation has emerged as a promising technology for the remediation and treatment of the groundwater and surface water contaminated with the organic compounds. Nonetheless, only a few studies have been conducted to explore the role of the heavy metals (especially the redox-active ones) during the Mn(VII) oxidation process. In this study, taking Pb(II) as an example, its influence on the Mn(VII) decontamination performance has been extensively investigated. It was found that, with the presence of Pb(II), Mn(VII) could degrade diclofenac (DCF), 2,4-dichlorophenol, and aniline more effectively than without. For instance, over a wide pH range of 4.5–8.0, the dosing of 10 µmol/L Pb(II) accelerated the DCF removal rate from 0.006–0.25 min−1 to 0.05–0.46 min−1 with a promotion factor of 1.9–9.4. Although the UV-vis spectroscopic and high resolution transmission electron microscopy analyses suggested that Mn(VII) could react with Pb(II) to produce Mn(IV) and Pb(IV) at pH 6.0–8.0, further experiments revealed that Pb(II) did not exert its enhancing effect through promoting the generation of MnO2, as the reactivity of MnO2 was poor under the employed pH range. At pH below 5.0, it was interesting to find that, a negligible amount of MnO2 was formed in the Mn(VII)/Pb(II) system in the absence of contaminants, while once MnO2 was generated in the presence of contaminants, it could catalyze the Pb(II) oxidation to Pb(IV) by Mn(VII). Collectively, by highlighting the conversion process of Pb(II) to Pb(IV) by either Mn(VII) or MnO2, the reactive Pb(III) intermediates were proposed to account for the Pb(II) enhancement effect.