11.4.1: Redox mechanism

Microorganisms can weather minerals by altering the redox state of a mineral component. The microbial reaction can reduce the mineral component, causing reductive dissolution of the mineral, or they can oxidize it, causing oxidative dissolution.

As an example of reductive dissolution, microbial reduction of oxidized manganese can cause dissolution of manganese oxides, as illustrated by the following example reaction, which includes the mineral pyrolusite \(\left(\text{MnO}_{2}\right)\) as the source of oxidized manganese and acetate as the source of electrons: \[\text{CH}_{3} \text{COO}^{-} + 7 \ \text{H}^{+} + 4 \ \text{MnO}_{2} \longleftrightarrow 2 \ \text{HCO}_{3}^{-} + 4 \ \text{Mn}^{2+} + 4 \ \text{H}_{2}\text{O}\]

As an example of oxidative dissolution, microbial oxidation of reduced iron and sulfur in pyrite can cause the mineral to dissolve. Although mechanisms vary with conditions (Baker and Banfield, 2003; Chen et al., 2014), the overall reaction of pyrite weathering can be represented by the following reaction: \[\text{FeS}_{2} + 3.5 \ \text{O}_{2} + \text{H}_{2} \text{O} \longleftrightarrow \text{Fe}^{2+} + 2 \ \text{SO}_{4}^{2-} + 2 \ \text{H}^{+}\]

Pyrite weathering causes significant impacts to water quality, which are discussed in more detail in Section 14.4.3. Among the impacts, a substantial amount of acid can be generated by pyrite oxidation, which in turn can react with other minerals and cause their dissolution. We consider this acid mechanism in the next section.

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