MYCOGENIC MN OXIDE STRUCTURAL CHANGES IN THE PRESENCE OF NI ALLOW FOR INCREASED NI INCORPORATION OVER TIME

Contaminant and micronutrient fate in environmental and metal-polluted systems are often controlled via interactions with manganese (Mn) oxide minerals. Nickel (Ni), an important micronutrient but also a contaminant at high concentrations, often binds to Mn oxides via adsorption and incorporation processes. In most modern-day systems, microbially mediated Mn oxidation is likely responsible for Mn oxide formation, primarily forming phyllomanganate type minerals. With fungi dominating the microbial Mn oxidizing community at metal-polluted sites, here we examine Ni uptake by fungal Mn oxides and their potential for long term Ni sequestration. Past abiotic studies found aqueous Mn(II) concentrations and timing of Ni addition both impact Ni binding to various Mn oxide surface and structural sites (e.g., Ni adsorption above or incorporation into structural vacancy sites and adsorption to sheet edges). Thus, the impact of the timing of Ni addition with fungal Mn oxidation (by the Mn-oxidizing fungi Stagonospora sp. SRC1lsM3a) on Ni partitioning and Mn oxide structures over time was here examined by investigating Ni and Mn coprecipitation as well as reacting Ni with preformed mycogenic Mn oxides. Samples were collected at 48 hours and 14 days and analyzed via Mn- and Ni-K edge X-ray absorption fine structure (XAFS) spectroscopy and scanning electron microscopy (SEM) paired with energy dispersive X-ray spectroscopy (EDS). These experiments produce phyllomanganates that differ initially, with Ni coprecipitated samples containing more structural Mn(IV) and higher average Mn oxidation states than those in which Ni is reacted with a preformed phyllomanganate. Upon aging for 14 days, however, all phyllomanganates are structurally similar, exhibiting increased vacancy content relative to those sampled after 48 hours. Ni binding also changes over time, with Ni becoming progressively incorporated into the mineral structure, regardless of the timing of addition scenario. Incorporated Ni may be more stable than adsorbed Ni, thus mycogenic Mn oxides may provide a long-term solution for Ni contaminated sites.