Recent research shows that δ-MnO2 photoreduction in water occurs through a single electron transfer leading to irreversible Mn(III) formation. This process involves the formation of a transient layer-Mn(III) atom that leaves the MnO6 octahedral sheet and creates a vacancy site onto which the photogenerated Mn(III) adsorbs. Photoreduction experiments coupled with XAS measurements confirm the mechanism of Mn(III) photogeneration on a Ni-doped δ-MnO2, where Mn(III) generation leads to Ni release as Ni(aq) and re-adsorption from vacancy sites to MnOx edge sites. However, these observations were made using a single mineral phase (δ-MnO2) and one set of reaction conditions. We expect that changes in experimental conditions may lead to different rates of Mn(III) photogeneration. Layer-type MnOx (birnessite, vernadite) exhibit a range of particle sizes (nm to μm), Mn(III) content and location [0 to 30% present as layer and/or interlayer Mn(III)], and vacancy site density (0 to 20%). While the presence of interlayer Mn(III) 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, we expect that Mn oxides with higher vacancy content will display enhanced MnOx (photo)reduction processes. Our study aims to better understand how vacancy content and Mn(III) content/location influence Mn(III)/Mn(II) photo-generation processes. To investigate the influence of mineral structure on Mn(III)/Mn(II) photogeneration, we selected three manganese oxides with different vacancy and Mn(III) contents. Aqueous suspensions of these oxides were continuously irradiated over a four-day period using 400 nm LED lights. The suspension was sampled three times per day and was characterized to determine the Mn oxidation state. Sodium pyrophosphate (PP) was used to extract solid-phase Mn(III). Mn(III)-PP complexes were quantified spectrophotochemically (λ245 nm). Our measurements suggest that vacancy sites presence 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. 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.

Influence of vacancy sites and Mn(III) atoms on MnOx photoreduction

256th National Meeting (2018)

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