Layer-type MnOx (birnessite, vernadite) exhibit a range of particle sizes (nm to μm), vacancy site content (0 to 20%), Mn(III) content and location [0 to 30% present as layer and/or interlayer Mn(III)] and interlayer cation nature. While the presence of Mn(III) at vacancy sites is known to lower the sorption reactivity of Mn oxides, its role in the redox cycling of Mn in sunlit aquatic environments has not been well studied and remains unclear. In addition, since increasing vacancy sites content in birnessite structure lowers the mineral band gap, Mn oxides with higher vacancy content are more likely to display enhanced MnOx (photo)reduction processes. Our study aims to better understand how vacancy content, Mn(III) content/location and interlayer cation nature influence Mn(III)/Mn(II) photo-generation processes. To investigate the influence of mineral structure and composition on Mn(III)/Mn(II) photogeneration, we selected a range of manganese oxides with different vacancy content, Mn(III) content, and interlayer cation nature. Transient absorption spectroscopy was used to probe electron photoexcitation kinetics within the different oxides. Aqueous suspensions of these oxides were continuously irradiated over a four-day period using 400 nm LED lights. The suspensions were sampled three times per day and were characterized to determine the Mn oxidation state. Sodium pyrophosphate (PP) was used to extract solid-phase Mn(III) and Mn(III)-PP complexes were quantified spectrophotochemically (λ245 nm). Our measurements suggest that vacancy sites presence, which enhance electron photoexcitation, is critical for Mn(III) photo-generation in birnessite-like structures. However the initial Mn(III) content does not seem to affect the amount of photo-generated Mn(III) over a given period. Finally, the interlayer cation nature, which does not impact electron photoexcitation kinetics in MnOx, tends to affect MnOx photoreduction rates through aggregation processes. These findings strengthen our understanding of the role of manganese oxides in biogeochemical processes and provide insight for the design of materials with specific redox properties.

Investigation of the main mineral properties driving MnOx photoreduction

2019 Fall National Meeting

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