Matthias Händel

Identification of Mn(II)-Oxidizing Bacteria from a Low-pH Contaminated Former Uranium Mine

ABSTRACT Biological Mn oxidation is responsible for producing highly reactive and abundant Mn oxide phases in the environment that can mitigate metal contamination. However, little is known about Mn oxidation in low-pH environments, where metal contamination often is a problem as the result of mining activities. We isolated two Mn(II)-oxidizing bacteria (MOB) at pH 5.5 (Duganella isolate AB_14 and Albidiferax isolate TB-2) and nine strains at pH 7 from a former uranium mining site. Isolate TB-2 may contribute to Mn oxidation in the acidic Mn-rich subsoil, as a closely related clone represented 16% of the total community. All isolates oxidized Mn over a small pH range, and isolates from low-pH samples only oxidized Mn below pH 6. Two strains with different pH optima differed in their Fe requirements for Mn oxidation, suggesting that Mn oxidation by the strain found at neutral pH was linked to Fe oxidation. Isolates tolerated Ni, Cu, and Cd and produced Mn oxides with similarities to todorokite and birnessite, with the latter being present in subsurface layers where metal enrichment was associated with Mn oxides. This demonstrates that MOB can be involved in the formation of biogenic Mn oxides in both moderately acidic and neutral pH environments.

Synthesis of cryptomelane- and birnessite-type manganese oxides at ambient pressure and temperature.

Cryptomelane-type octahedral molecular sieves (K-OMS-2) were successfully synthesized at ambient pressure and temperature by a simple one-step reaction pathway. We designed three synthesis mixes based on redox reactions of either MnSO4 or MnCl2 together with KMnO4 in aqueous solution. The synthesis products were characterized structurally (XRD, FTIR spectroscopy), morphologically (SEM, BET surface area), and chemically (SEM-EDX, ICP-OES). For all mixes, a precursor octahedral layered K-birnessite (K-OL-1) was formed after 1d that subsequently transformed into K-OMS-2. This transformation process depends on the pH of the reaction solution, the respective Mn(II) salt and time. We obtained K-OMS-2 materials with BET surface areas between 50.4 and 104.5 m(2) g(-1) and different crystallinities. The described method is reliable, reproducible, easy to handle and may be the basis to produce well defined Mn oxides that could be used for remediation and catalysis purposes.