Photocatalytic oxidation of dissolved Mn(II) on natural iron oxide minerals

Abstract

The energy conversion in Earth’s surface environments over geological history heavily involves the redox reactions of natural minerals. Natural Mn(III/IV) oxides and Mn2+(aq) is one of the most important redox couples that significantly dictate the electron flow in nature. Elucidating their formation and redox reactions is of critical importance for understanding numerous elemental cycles and the evolution of redox environments on Earth and other planets. Previous studies generally consider biotic processes as the dominant oxidation pathway for Mn2+(aq) in natural environments. In this study, we demonstrate rapid abiotic oxidation of Mn2+(aq) through photocatalytic reactions in the presence of natural Fe oxide minerals at rates comparable to those of currently known biotic/abiotic oxidation processes. Fe oxide minerals not only facilitate the electron transfer, but also serve as templates for the heterogeneous nucleation of tunnel structured Mn oxides. This finding highlights an important yet previously overlooked abiotic process on the formation of Mn oxides, which offers a new pathway for explaining the commonly observed co-occurrence of Fe and Mn oxide minerals in nature and the origin of diverse structures of Mn oxides.