Manganese oxide minerals are critical players in a number of biogeochemical processes and have the potential to be used for pollutant transformation in technical systems due to their high oxidative and adsorption capacities. The reactivity of layer-type manganese oxides (birnessite) is controlled by their structure and composition, including particle size, presence of structural defects in the form of vacancy sites, and the presence of adsorbed or structural MnIII. We investigated the redox properties for a suite of birnessite minerals that varied in terms of structure (nm - μm size) and composition (0 - 25 % MnIII) using solid-phase characterization and electrochemical methods with the overall goal of linking mineral structure and composition to its oxidation chemistry. We hypothesized that particle size and crystallographic location and amount of MnIII in the mineral govern the mineral redox properties and thus its reactivity. We applied two analytical methods to assess the redox state of the minerals. In the first method, sodium pyrophosphate (PP) was used to extract MnIII from the minerals and form MnIII-PP complexes. In the second method, the average manganese oxidation number (AMON) was determined by potentiometric titration: after using Mohr’s salt to reductively dissolve the mineral to Mn2+, permanganate was used to back titrate the Mn2+ to MnIII using excess PP. These results were also compared to MnIII/MnIV quantification using XANES spectroscopy. To determine the reduction potential of the minerals, we performed mediated potentiometric measurements. We selected 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) as the electron transfer mediator to facilitate redox equilibration between the mineral and the working electrode. Initial results suggest that ABTS may be appropriate to determine the reduction potential of birnessite minerals with significant MnIII contents, but showed irreversible oxidation of ABTS by pure MnIV oxides. Measurements of ABTS speciation also indicate that the oxidative capacity of the minerals correlates with MnIII content and specific surface area. A more fundamental understanding of the relationship between mineral structure and redox properties will improve our understanding of the role of Mn oxides in biogeochemical processes and pollutant dynamics. This work may also allow for control and design of Mn oxides with specific redox properties.

Redox properties of layer-type manganese oxide minerals

253rd National Meeting (2017)

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