Review of reductive leaching of iron by anaerobic bacteria

Leaching of iron, either as an impurity to be removed or as a metal to be recovered, requires a different approach than that of the oxidative leaching that dominates biohydrometallurgy of other metals. In particular, the significant increase in solubility that results from reducing Fe3+ to Fe2+ suggests that a reductive leaching process is most suitable. A wide range of anaerobic iron-reducing bacteria have been demonstrated to exist in the last few decades, which reduce Fe3+ to Fe2+ as part of their respiration. More of these organisms are continually being discovered, and many of these have promise for reductive bioleaching of iron. This review examines the many types of bacteria that have been demonstrated to reduce and solubilize iron, and the environments where such organisms can be found. The cultivation requirements of these organisms are also discussed, along with descriptions of the work that has been done to apply them to specific applications. These applications include decolorization of kaolin and silica, iron removal from bauxite, recovery of iron from low-grade or difficult-to-process ores, and promoting breakdown of iron-rich rocks to liberate other metals. Reductive iron leaching in these applications has been shown to be most effective for dissolving the more hydrated and amorphous iron oxides, with low dissolution rates for highly crystalline oxides such as hematite. It has also been shown that, the given sufficient adaptation and leaching time, these microorganisms can produce iron-bearing solutions containing as much as 1800 mg of Fe2+ per liter. © 2014 Copyright Taylor and Francis Group, LLC