Effects of mineral composition on microbiological reductive leaching of manganese oxides

The present paper combines two fields of research: (a) the microbiological leaching of Mn-oxide ores, and (b) the normative quantification of Mn-oxide minerals in samples containing finely intergrown phases. Commonly, the difficulty of analyzing the concentration of various Mn-oxides within a sample hinders investigations on the reducing capability of microorganisms, because it is difficult to estimate how significantly the presence or absence of particular minerals influences the activity of the biota. The normalization technique overcomes this problem by providing a tool for the interpretation of experimental data. The examined ores were sampled from a geologically and mineralogically relatively uniform stratigraphic section at the Mn-oxide deposit at Groote Eylandt in the Northern Territory of Australia. Although the examined reduction system is highly specific (i.e. only Enterobacter sp. is used), the results of the laboratory tests illustrate how natural mobilization processes may operate. In principle, however, the findings can be applied to other organisms in comparable geochemical environments. The investigations also show that the microbially induced dissolution of the ores is largely controlled by the mineralogy. The thermodynamically most stable mineral, pyrolusite, is unaffected by the leaching procedure during the tests. Romanechite, cryptomelane and todorokite are reduced successively. This mineral destruction is strongly time-dependent (cryptomelane/todorokite break down earlier than romanechite), and the final remobilization of the respective mineral is generally marked by a good correlation of the pH and Mn²⁺ concentration. The data clearly demonstrate that only ores with a relatively low pyrolusite content, when compared with coexisting Mn-oxide phases, are likely to release larger quantities of dissolved Mn, and that these materials are more suitable for technological leaching processes. Most other Mn-oxides are reduced faster and more effectively than pyrolusite.