It has long been recognized that small organic acids cause the reductive dissolution of layered manganese oxide minerals. However, the persistence of manganese oxides in soils and aquatic sediments shows that in natural systems such reduction is incomplete. Recent work has shown that dissolved Mn(II), a product of manganese oxide reduction, may react with the mineral, forming Mn(III) sites that alter the mineral structure. In addition, Mn(II) alters how trace metals bind to manganese oxides, affecting how these important mineral sinks controls contaminant fate and micronutrient availability. We have thus set out to determine how partial reduction of manganese oxides by small organic acids affects the structure of these layered minerals as well as the binding of the trace metals nickel and zinc. Oxalate, citrate, and p-hydroxybenzoate were investigated to evaluate dicarboxylic, tricarboxylic, and aromatic acids. These acids caused subtle structural changes in manganese oxides, including lattice expansion, improved sheet stacking, and capping of vacancy sites with Mn atoms, similar to the effects of dissolved Mn(II). Each organic acid caused redistributions of trace metals among sites capping vacancies in the mineral sheets, on the sheet edges, and, for nickel, filling sheet vacancies. The specific behavior varied with the organic acid, pH, and initial Mn(III) content of the mineral and differed from the behavior seen for direct addition of dissolved Mn(II) to the system. These ligand-specific effects indicate that complexation of the metals, including possibly on the mineral surface, substantially alters their behavior beyond just site competition with the Mn(II) generated via reduction. This work shows that organic acid-manganese oxide interactions in soils and sediments likely preserves the overall layered manganese oxide structure while increasing Mn(III) content and altering the affinity of these minerals for contaminants and micronutrients.

Reduction of layered manganese oxides by organic acids: Effects on mineral structure and trace metal fate

255th National Meeting (2018)

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