STRUCTURAL CONTROLS ON THE REDOX REACTIVITY OF MYCOGENIC MANGANESE OXIDES

Manganese (Mn) is an abundant element in the marine and terrestrial environments that is essential for life but toxic above certain threshold. Microorganisms such as fungi oxidize dissolved Mn²⁺_aq to its trivalent or tetravalent state, precipitating biogenic Mn oxides, which are strong electron acceptors that may in turn oxidize Cr, Co, As, as well as organic compounds. Consequently, it is important to understand the structure and reactivity of fungally produced Mn oxides, which are known to form in contaminated environments. It is generally accepted that geochemical kinetics of redox reactions are influenced by the electron transfer kinetics (Ket) of the minerals, which can vary in terms of their Mn³⁺ content, their density of vacancy sites, as well as their mineral structure (phyllo- vs tecto-manganates). We present data that explores relationships between electrochemically derived Ket values and the mineral chemistry (Mn³⁺ content), particle size and structure of the minerals, by employing voltammetric, wet-chemical, microscopic (TEM), and X-ray absorption spectroscopic (XANES, EXAFS, low angle scattering) techniques. Results may impact our understanding of Mn biogeochemical cycling as well as its contributions to the retention, mobilization and transformation of environmental contaminants.