Manganese oxides can transform a wide variety of organic contaminants and can oxidize the reduced forms of many trace and toxicant elements. The facile and rich redox reactivity of layer-type manganese oxides (birnessite) derives from the ability of Mn to cycle between the +2, +3 and + 4 redox states and from the presence of reactive sites at particle edges and interlayer vacancies. Previous spectroscopic studies have relied on the use of batch experiments to study contaminant oxidation by birnessite. However, such studies can obscure the reaction mechanism. Recently, we investigated the redox reactivity of MnOx through time-resolved measurements of surface speciation. Kinetic studies of Co oxidation by δ-MnO2 coupled to quick X-ray absorption spectroscopy provided the data necessary to address three key questions regarding the reactivity of of layer-type MnO2 : What is the reactivity of particle edge sites versus vacancy sites? How do the amount and crystallographic location of Mn(III) centers influence redox reactivity? How does the reactivity of abiotic phases to compare to biogenic phases? We found that Co sorption and oxidation occurs rapidly at particle edges (t &lt; 10 minutes; 0.10 - 0.30 mol Co mol-1 Mn), whereas reactions at vacancy sites occur over several hours of reaction (up to 0.12 mol Co mol-1 Mn). Dynamic equilibrium between adsorbed Co(II) species at the particle edges and trace aqueous Co(II) provides a pathway for the redistribution of Co from particle edges to vacancy sites, where more thermodynamically stable incorporated-Co(III) species form. In addition, suspension pH dictates the extent of relative importance of edge sites versus vacancy sites. Finally, when occupied by Mn(III), vacancy sites show a markedly diminished ability to oxidize Co, presumably because electron transfer from Mn to Co requires the formation of an inner-sphere surface complex. Overall, this research suggests that surface sites at the particle edges of Mn oxides will act as the first line of defense, but vacancy sites will serve as the long-term contaminant-scavengers unless previously passivated by Mn(III).

Redox reactivity of layer-type MnOx nanoparticles: Insights from kinetic and site-specific surface speciation studies

255th National Meeting (2018)

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