Manganese oxides are common in terrestrial and aquatic environments, occurring as lamellar, nanocrystalline phases with high surface areas and high reactivity. These minerals are often major controls on the fate of trace metals. Many metals, including Ni and Zn, are important micronutrients but are also toxic at higher concentrations. It is important to identify the processes controlling the fate and availability of trace metals in the environment and this requires understanding the behavior and stability of Mn oxides. Prior studies have shown that Mn oxides undergo redox and structural changes when exposed to Mn(II); this process weakens trace metal binding. The conditions studied to date are most similar to those found at redox interfaces, which are limited in spatial extent in nature. Small carboxylic acids, produced as root exudates or by degradation of organic matter in aerated soils, have been observed to reductively dissolve Mn oxides and produce Mn(II). However, the impact of such reactions on mineral structure and trace metal fate is largely unknown. The reaction of oxalate and citrate on δ-MnO<sub>2 </sub>and hexagonal birnessite at pH 4 and 7 was investigated. Aging of the Mn oxides with oxalate and citrate caused partial Mn reduction, with citrate having a greater effect, but neither organic acid substantially altered the overall phyllomanganate structure. Ni adsorption transitioned from above vacancy sites to the sheet edges of the hexagonal birnessite in the presence of the organic acids. This also occurs for δ-MnO<sub>2</sub> reacted with citrate, but oxalate promoted Ni incorporation into the phyllomanganate sheet at pH 7 and had no effect on Ni binding at pH 4.  The effects of these acids on Zn adsorption will also be discussed. This work improves our understanding of processes that control Mn oxide structure and reactivity, and their impact on micronutrient availability and contaminant fate.

Structural transformations of manganese oxides by small organic acids and the fate of trace metals

253rd National Meeting (2017)

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