Mn (hydr)oxides play critical roles in contributing to electron transfer in natural and environmental systems, thus it is of great importance to understand the fate of Mn (hydr)oxides during redox cycling. Among more than 30 naturally occurring Mn (hydr)oxides, the most abundant phases are layer structured birnessite and tunnel structured todorokite with 3 × 3 tunnel size. Although the formation of birnessite is relatively well constrained, which can occur through both biotic and abiotic oxidation of dissolved Mn2+, the formation of todorokite under low temperature environments still remains poorly understood. Previous studies have pointed to the importance of redox reactions in the transformation of layered Mn oxides to todorokite, yet no studies have investigated Mn oxide phase transformation pathway and mechanisms during repeated redox cycling in environmentally relevant systems. Here, using a novel electrochemical method, we demonstrate the direct transformation from layered Mn oxide d-MnO2 to todorokite at room temperature and circumneutral pH conditions. Both reduction and oxidation processes involved a two-step one-electron transfer mechanism, and interlayer Mn(III) was found to be critical in facilitating the structure transformation. These findings will facilitate the understanding of the formation and transformation of diverse Mn oxides in natural systems.

Effect of redox cycles on the transformation of d-MnO2 nanosheets to tunnel structured Mn oxide

2019 Spring National Meeting

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