Effects of manganese oxide mineralogy on microbial and chemical manganese reduction

Abstract

In this study we examine the effects of manganese oxide mineralogy and crystal structure on two types of manganese reduction: microbial manganese reduction coupled to the oxidation of organic matter, and chemical reduction of manganese oxides by Fe²⁺. Three synthetic manganese minerals were used: δ‐MnO₂ (or vernadite), Mg‐bimessite, and pyrolusite. The δ‐MnO₂ and Mg‐birnessite are relatively high‐surface‐area amorphous materials, while the pyrolusite is a low‐surface‐area crystalline solid. Studies of microbial manganese reduction were carried out using an enrichment culture isolated from sediments in the mesohaline region of the Chesapeake Bay, and with a pure culture of the organism Shewanella (formerly Alteromonas,) putrefaciens strain MR‐1. With the Chesapeake Bay enrichment culture, identical rates of manganese reduction were observed with equivalent initial suspension concentrations of all three manganese minerals. This occurred in spite of ∼ 200‐fold differences in the available surface area of either of the amorphous phases relative to the crystalline pyrolusite. In contrast, the reduction of manganese oxides by strain MR‐1 showed a strong dependence on the particular oxide being reduced. Here, the highly crystalline pyrolusite was reduced at a slower rate than either the amorphous δ‐MnO₂ or birnessite phase. The reduction of manganese oxides by Fe²⁺ also showed a strong dependence on the particular oxide being reduced. The effects observed here were similar to those observed with microbial manganese reduction by strain MR‐1. These results indicate that the controls on manganese reduction may differ depending on the rates and mechanisms of the process. Both manganese reduction by Fe²⁺ and microbial manganese reduction appear to be controlled by reactions occurring on the oxide surface. However, for microbial manganese reduction, the thermodynamic factors that can lead to differences in the reactivity of a manganese oxide mineral phase may also be overcome by kinetic factors that ultimately control the overall rate of the process.