Sustained production of superoxide radicals by manganese oxides under ambient dark conditions

Manganese (Mn) oxides are ubiquitous in the environment and have strong reactivity to induce the transformation of various contaminants. However, whether reactive oxygen species contribute to their surface reactivity remains unclear. Here, sustainable production of superoxide radicals (O-2(center dot-)) by various MnO2 polymorphs in the dark was quantified and the mechanisms involved were explored. The results confirm that O-2(center dot-) was produced through one-electron transfer from surface Mn(III) to adsorbed O-2. In contrast, no H2O2 was detected due to its decomposition by Mn oxides to form O-2(center dot-)- and Mn(III), leading to the sustained production of O-2(center dot-) on Mn oxide surfaces. In addition, the production of O-2(center dot-) was found to make a clear contribution (4 - 28%) to the transformation of a series of halophenols by MnO2, suggesting that the O-2(center dot-)-mediated surface reaction is an important supplement to the direct electron-transfer mechanism in the reactivity of Mn oxides. These findings advance our understanding of the surface reactivity of Mn oxides and also reveal an important but hitherto unrecognized abiotic source of O-2(center dot-) in the natural environment. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

This study demonstrates that various MnO2 polymorphs can sustainably produce superoxide radicals in the dark, mainly through a one-electron transfer mechanism. These superoxide radicals make significant contributions to the transformation of contaminants by MnO2.