Redox Transformations of Iron at Extremely Low pH: Fundamental and Applied Aspects

Abstract

Many different species of acidophilic prokaryotes, widely distributed within the domains Bacteria and Archaea, can catalyze the dissimilatory oxidation of ferrous iron or reduction of ferric iron, or can do both. Microbially mediated cycling of iron in extremely acidic environments (pH < 3) is strongly influenced by the enhanced chemical stability of ferrous iron and far greater solubility of ferric iron under such conditions. Cycling of iron has been demonstrated in vitro using both pure and mixed cultures of acidophiles, and there is considerable evidence that active cycling of iron occurs in acid mine drainage streams, pit lakes, and iron-rich acidic rivers, such as the Rio Tinto. Measurements of specific rates of iron oxidation and reduction by acidophilic microorganisms show that different species vary in their capacities for iron oxido-reduction, and that this is influenced by the electron donor provided and growth conditions used. These measurements, and comparison with corresponding data for oxidation of reduced sulfur compounds, also help explain why ferrous iron is usually used preferentially as an electron donor by acidophiles that can oxidize both iron and sulfur, even though the energy yield from oxidizing iron is much smaller than that available from sulfur oxidation. Iron-oxidizing acidophiles have been used in biomining (a technology that harness their abilities to accelerate the oxidative dissolution of sulfidic minerals and thereby facilitate the extraction of precious and base metals) for several decades. More recently they have also been used to simultaneously remediate iron-contaminated surface and ground waters and produce a useful mineral by-product (schwertmannite). Bioprocessing of oxidized mineral ores using acidophiles that catalyze the reductive dissolution of ferric iron minerals such as goethite has also recently been demonstrated, and new biomining technologies based on this approach are being developed.

Anaerobic Sulfur Metabolism Coupled to Dissimilatory Iron Reduction in the Extremophile Acidithiobacillus ferrooxidans

“…In either case, it would be important to understand how electron flow is regulated. An important consideration is that some strains of iron-oxidizing acidithiobacilli can grow by coupling the oxidation of hydrogen to the reduction of Fe 3ϩ (7,12). In such cases, S 0 reduction to H 2 S would not be expected to occur and an alternative mechanism for iron reduction other than the indirect mechanism must exist.…”

“…Dissimilatory Fe 3ϩ reduction is widespread among moderately acidophilic and extremely acidophilic bacteria (reviewed in reference 7). Acidophilic iron reducers are unrelated to their neutrophilic counterparts and display considerable phylogenetic diversity (7). Different strategies to reduce Fe(III) have been described in phylogenetically diverse Fe(III) reducers, suggesting that these mechanisms evolved independently several times (8).…”

Geochemical Niches of Iron-Oxidizing Acidophiles in Acidic Coal Mine Drainage

“…Specific iron oxidation rates for acidophilic isolates range from 192 to 532 mg min Ϫ1 g protein Ϫ1 for autotrophic strains and from 191 to 449 mg min Ϫ1 g protein Ϫ1 for heterotrophic strains (9). When different isolates were used to treat real and synthetic AMD in packed-bed bioreactors, a betaproteobacterial isolate (later named "Ferrovum myxofaciens" [10]) achieved the fastest iron oxidation rates while Acidithiobacillus ferrooxidans proceeded the slowest (11).…”

Review of Biohydrometallurgical Metals Extraction from Polymetallic Mineral Resources

“…arsenitoxidans, which obtains energy from the oxidation of arsenic(III) to arsenic(V) [104]. In their comprehensive review on redox reactions of iron in acidic environments, Johnson et al [105] discussed the diverse metabolic characteristics of acidophiles that catalyse iron redox transformations at low pH and the mechanisms employed by acidophiles engaged in iron oxidation and reduction, and described examples of iron cycling in acidic environments, including the degradation of iron(III) compounds under microaerobic or anaerobic conditions. The manganese(IV) mineral asbolane can be solubilised, releasing cobalt, and chromium is released from chromite mineral as the less toxic chromium(III) species [106].…”