Environmental manganese (Mn) oxides typically occur as layer-type minerals (birnessite) and contain varying amounts of vacancy sites and Mn(III). Mn oxides are among the most reactive minerals in soils and sediments and control the mobility and bioavailability of contaminants. However, the relationship between the redox reactivity of Mn oxides and their chemistry and mineralogy is still under debate. Our previous quick X-ray absorption spectroscopy (QXAS) study used Co(II) to probe the redox reactivity of Mn(III)-rich δ-MnO2 nanoparticles. We showed that Co(II) was first oxidized by Mn(III) sites at the particle edges, while the vacancy sites occupied by interlayer Mn(III) atoms were largely unreactive. However, the kinetics of Co(II) oxidation at edge versus vacancy sites for a pure Mn(IV) oxide (δ-MnIVO2), which possesses abundant free vacancy sites (∼11%), have not been characterized. In this study, we used QXAS analyses to probe the evolution of Co speciation during Co(II) oxidation by δ-MnIVO2 at pH 4, 6 and 8 with Co(II)/Mn(IV) molar ratios ranging from 0.1 to 0.4 over minute to hour time-scales. The results show that edge sites were quickly saturated by Co, while Co loading on vacancy sites increased slowly with Co(III) progressively filling the vacancy site. The adsorption and oxidation of Co(II) on the edges at pH 8 occurred to a much higer extent than at pH 4 and 6, whereas the rates of oxidation on the vacancies were similar at all pH values. This study demonstrates the distinct kinetics of metal adsorption and oxidation at different surface sites on δ-MnIVO2 over a broad range of pH values. Furthermore, our work indicates that not only Mn valence but also crystallographic location of Mn centers must be considered in determining the redox reactivity of Mn oxides minerals.

Using time-resolved X-ray absoprtion spectroscopy to probe the mechanism of contaminant oxidation by δ-MnO2

253rd National Meeting (2017)

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