| Manganese oxides (MnOx) are very reactive species in natural and engineered systems. Many bacterial species catalyze the formation of MnOx, but their structure and formation kinetics is poorly understood. This work uses a multi-method approach, including transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS), and kinetic experiments, to study the MnOx formed by the sheathed Leptothrix discophora SP-6 (SP6-MnOx). TEM of unstained thin sections of SP6-MnOx embedded in hydrophilic resin show that they are primarily formed by layers associated to fibrillar structures (50–100 nm size). X-ray absorption near edge structure (XANES) at the MnK-edge shows octahedral Mn coordination, with an oxidation state of 3.8 ± 0.3, similar to the family of MnO2 polymorphs. The inspection of the radial distance function of the extended X-ray absorption fine structure (EXAFS) suggests similarity with todorokite (3 x 3 tunnel structure) and chalcophanite (layer structure coordinated to extra-layer Zn). A mathematical model of the EXAFS contribution of the first three coordination shells around Mn was calibrated and validated using big tunnel (2 x 2, 2 x 3, and 3 x 3) and layered MnO 2 minerals. The estimated radial distances and coordination numbers suggest that the SP6-MnOx are formed by layers with fewer vacancies than chalcophanite, which are coordinated to extra-layer Mn2+ or Mn3+. This finding is consistent with the layer morphology observed in the TEM micrographs. Kinetic experiments show that at circumneutral pH the formation of the SP6-MnOx is governed by a purely biological process. The rate of autocatalytic MnOx formation was estimated to be lower than 5% of the biological rate. XANES shows that the SP6-MnOx oxidation state increases during the oxidation process, suggesting a preliminary sorption of Mn(II) on the active sites, followed by the oxidation step. The MnOx formation rate is inhibited by 25 μM Zn2+, Co2+, or Zn2+. For pH > 7.2, the MnOx formation is faster, but it is controlled abiotically, resulting in reduced forms of oxy-hydroxides with different morphology compared to the SP6-MnOx formed at circumneutral pH. Besides pH, other environmental variables do not affect significantly the SP6-MnOx structure. |
