The oxidation of Mn2+(aq) is a kinetically-limited reaction. While biotic processes achieve fast oxidation of Mn2+(aq) to Mn(IV), comparably abundant abiotic sources are known to have much slow oxidation kinetics. Furthermore, the observed abiotic oxidation products in environmental systems are Mn(III) oxides, not Mn(IV). Thus, biotic processes are believed to be the dominant contributor to Mn oxidation in natural systems. In this study, using a nitrate photolysis pathway, we demonstrated the abiotic oxidation of Mn2+(aq) to Mn(IV) within hrs. Contrary to previously held expectations, the kinetics of this abiotic oxidation were fast, even comparable to those of biotic processes. We also found that superoxide, generated from nitrite photo-excitation, was the major contributor to the abiotic fast oxidation, with two one-electron transfers from Mn2+(aq) to Mn(IV), with Mn(III) as an intermediate product. Consequently, disordered birnessite (δ–MnO2) nanosheets formed, which are one of the most abundant Mn (hydr)oxides and are also useful catalysts for water oxidation. The findings suggest a new and previously unconsidered abiotic pathway for the occurrence of Mn(IV) oxide in environmental systems, and for the engineered production of highly reactive disordered layered δ–MnO2 nanosheets by mimicking a natural system.

Nitrate photolysis drives fast abiotic oxidation of Mn2+(aq) to Mn(IV) and promotes the formation of δ–MnO2 nanosheets

253rd National Meeting (2017)

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