Layered, nanocrystalline manganese oxides, the phyllomanganates, are common phases in soils and terrestrial and marine aquatic sediments that often control trace metal fate in the environment. A relatively unique property of these minerals is their capacity to undergo net reduction through reaction with diverse environmental species while maintaining their general structure. Notably, dissolved Mn(II) reacts with phyllomanganates via comproportionation to form structural Mn(III), while a similar increase in solid-phase Mn(III) occurs when these phases oxidize small organic acids. This talk will explore how similar extents of Mn(III) generation by Mn(II) and organic acids yield distinct changes in the binding of Ni and Zn to phyllomanganate minerals. Reaction of oxalate and low concentrations (&lt;1 mM) of dissolved Mn(II) produced similar average Mn oxidation state values. While Mn(II) had a negligible effect on Ni adsorption mechanisms, oxalate induces the incorporation of Ni into the mineral sheet. Zn binding was largely unperturbed by either oxalate or low Mn(II) concentrations. Citrate and 4-hydroxybenzoate induce much greater Mn(III) generation than oxalate, similar to exposure to higher concentrations of dissolved Mn(II). This drives Ni and Zn from vacancy sites to the edges of phyllomanganate sheets. Notably, these organic acids enhance the formation of tridentate edge sharing complexes, which form at Mn(III)-rich regions of sheet edges. Similar complexes are not observed when phyllomanganates are reacted directly with dissolved Mn(II). This suggests that organic acids may inject electrons at different structural locations than Mn(II) and that even after the 4 week aging periods of our studies electron equivalents do not redistribute in the phyllomanganate sheet. These observations demonstrate that reduction of phyllomanganate minerals by organic acids and dissolved Mn(II) generates distinct, pathway-dependent effects on trace metal binding.

Phyllomanganate reduction by organic acids and Mn(II) produce distinct effects on trace metal binding

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.