Some microbes float freely in the solution around the minerals and some attach themselves to the mineral particles forming a biofilm. The microbes, whether they are freely floating or whether they are in the biofilm, continuously devour their food sources—iron (chemically represented as Fe2+) and sulfur. The product of the microbial conversion of iron is ‘‘ferric iron’’, chemically represented as ‘‘Fe3+’’. Ferric iron is a powerful oxidizing agent, corroding metal sulfide minerals (for example, pyrite, arsenopyrite, chalcocite, and sphalerite) and degrading them into a dissolved metal, such as copper, zinc, and more iron—the latter is the food source for the microbes.

The reaction of the biological oxidation involved in leaching of a mineral sulfide is

MS + 2O2 ! MSO4 where, M is a bivalent metal.

There are two major mechanisms involved in microbial metal solubilization of sulfide minerals. One is a direct mechanism that involves physical contact of the organism with the insoluble sulfide.

Microorganisms oxidize the metal sulfides obtaining electrons directly from the reduced minerals. Another, indirect mechanism, involves the ferric-ferrous cycle. The oxidation of reduced metals is mediated by the ferric (III) ion and this is formed by microbial oxidation of ferrous (II) ion present in the minerals. Ferric (III) ion acts as an oxidant and oxidizes metal sulfides and is reduced to ferrous (II) ion that, in turn, can be microbially oxidized. Both direct and indirect mechanisms of bacterial leaching are shown schematically in Fig. 14.9.