Mn oxides commonly form in natural and metal polluted systems via microbial Mn(II) oxidation as nanocrystalline Mn(IV/III)(hydr)oxide minerals with large surface areas, high reactivities, and varying amounts of Mn(IV) vacancies and Mn(III). These Mn oxide minerals often serve as the dominant control on the concentration and speciation of trace metals like Ni and Zn in soils, sediments, and metal contaminated sites. Ni and Zn typically adsorb on or incorporate into vacancy sites. Thus, the Mn oxide mineral structure and composition largely control the extent of Mn oxide reactivity toward these trace metals, with high vacancy content (corresponding to low Mn(III) substitution) promoting Ni and Zn uptake. Upon microbially-mediated Mn oxide formation, residual dissolved Mn(II) coexists with these Mn oxide minerals. Mn(II) can also bind to vacancy sites and react with structural Mn(IV) via comproportionation reactions, producing Mn(III). As a result, the overall Mn oxide composition can be affected by Mn(II) as well as the Mn oxide’s affinity for trace metals. We investigated the effect of dissolved Mn(II) on a series of Mn oxide minerals with varying initial Mn(III) and vacancy content, and on Ni and Zn binding to these minerals at acidic and neutral pH. We find that under acidic conditions, Mn(II) causes substantial Mn oxide structural changes that directly correlate with Mn(II)-induced Ni and Zn ejection from vacancy sites. At neutral pH, Mn(II) has little to no effect on Mn oxide structures and trace metal binding behaviors unless reacted in the presence of MES buffer, a Good’s Buffer widely used in geochemical and biological research. Because the majority of studies on microbially-precipitated Mn oxides to date used Good’s Buffers, we also investigated the Mn oxides produced by three Mn(II)-oxidizing Ascomycete fungi unbuffered or with carbonate, HEPES, or MES buffer at circumneutral pH. We observed that Good’s Buffers alter fungal Mn oxide production, unrelated to any observed pH differences between buffered and unbuffered systems. For example, <i>Plectosphaerella cucumerina</i> DS2psM2a2 oxidizes substantially more Mn(II) when grown with HEPES and MES buffers than in unbuffered solutions. Conversely, <i>Paraconiothyrium sporulosum </i>AP3s5-JAC2a rapidly oxidizes Mn(II) in unbuffered solutions while Mn(II) oxidation occurs much more slowly with HEPES and MES buffers, ultimately producing a Mn(III)-rich oxide.

Interfacial reactions during Mn biogeochemical cycling: Impact on Mn oxide structures and reactivities towards trace metals

253rd National Meeting (2017)

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