Manganese (Mn) oxides are ubiquitous in aquatic and terrestrial environments, and the rich redox reactivity of Mn is of interest in both natural and engineered systems. Abiotic, inorganic oxidation of Mn has been considered less important than bacteria-mediated processes, because Mn2+ (aq) oxidation is thought to be very slow and kinetically-controlled in abiotic systems. Recently, we discovered direct, fast, photochemically-assisted abiotic inorganic oxidation of Mn2+ (aq) to Mn(IV) (layer structured birnessite (δ-MnO2)) in the presence of nitrate and without microorganisms. The formation rate of layered birnessite was even comparable to that of biotic processes. However, we still poorly understand the effects of pH and the existence of other abundant cations, such as Ca2+, on photochemically-driven oxidation of Mn2+ (aq) to Mn(IV) phases. In this study, in the presence of Ca2+, we found a new crystalline phase of todorokite was produced via nitrate photolysis under circumneutral pH. Notably, this reaction occurred within several hours, even at ambient temperature and pressure. Both XPS and SEM-EDX data revealed Ca2+ was incorporated into the MnO2 structures. Our findings could provide new insights about the formation mechanism of todorokite and help design environmentally-friendly processes for Mn oxide materials synthesis.

Photochemically-facilitated formation of todorokite with Ca2+ incorporation

2019 Fall National Meeting

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