Mdmw-iron25

FORMATION AND OCCURRENCE OF BIOGENIC IRON-RICH MINERALS

BY Sulochana Sahoo Admn. 612991 2nd Semester M.Tech(Mineral Exploration)

INTRODUCTION Iron, which represents the fourth most abundant element in the earth's crust, has been the focus of several studies in recent years, because of its importance in biogeochemical redox reactions in marine and freshwater environments .

 In the presence of ‘oxygen and at circumneutral pH conditions, ferrous iron is quickly oxidized to Fe(III) and precipitates as iron oxides.

Under the same pH conditions, but also at low pH, several microorganisms can oxidize Fe(II) to Fe(III) and gain energy from the reaction.

CONTD…  Role of bacteria in Fe(II) oxidation under anoxic

conditions is of greater importance in biogenic formation of iron rich minerals.  Under anoxic conditions, some iron oxides undergo

reduction as a result of a biotic reactions or microbial anaerobic respiration .  Oxidation and reduction reactions take place in marine

and freshwater environments, such as lake sediments, aquifers, soils, wetlands, deep-sea vent environments and in man-made settings, like mine tailings impoundments.

ROLE OF BACTERIA IN FORMATION OF EXTRACELLULAR IRON-OXIDE MINERALS  In natural sediments, iron oxide particulates often

occur in the close vicinity of bacterial cell walls (Fig. 1A) and bacterial exopolymers (Fig. 1B). These minerals are referred to as extracellular biogenic iron oxides.  Most common iron oxides include oxides (e.g., hematite,

magnetite), oxyhydroxides (e.g., goethite, lepidocrocite, akaganeite)

Fig. 1. Transmission electron micrographs of a mineralized bacterium in oxidized and acidic Cu–Zn mine tailings, showing fine Fe-rich minerals (see arrows) on the cell wall (A) and Fe-rich filaments (F) and nodules (N) in neutral-pH freshwater lake sediments (B).

OCCURRENCE AND MINERALOGY OF EXTRACELLULAR BIOGENIC IRON OXIDES  Akaganeite and ferrihydrite formed on the surface of

bacterial exopolymers in the water of a submerged mine and showed that the exopolymers acted as a template for Fe-oxide nucleation.  Banfield et al. (2000) identified nanocrystals of two-line

ferrihydrite on the sheath and stalk of neutrophilic ironoxidizing bacteria.

Fig. 2. Neutrophilic Fe-oxidizing bacteria closely associated with iron oxides in an Fe(II)-rich discharge area (scanning electron micrograph).

ROLE OF BACTERIA IN BIOGENIC IRON OXIDE FORMATION  Biotic reactions  Biotic reactions

responsible for the formation of biogenic iron oxides include the microbial oxidation of Fe(II) to Fe(III) by a wide range of microorganisms under both acidic and neutral pH and oxic and anoxic conditions.  Iron oxide formation also occurs as a result of passive reactions  Chemical and microbial oxidation of Fe(II) to Fe(III) depends on the pH and the O2 concentration .  Gallionella spp. and Leptothrix spp. are the most commonly observed bacteria in association with biogenic iron oxides in neutral pH environments.

 Abiotic reaction  Electrostatic attraction between negatively charged

bacteria and positively charged minerals along with the hydrophobicity of either substrate, often lead to the sorption of minerals onto the cell wall .  Rapid abiotic oxidation of Fe(II) at pH 7 in the

presence of various bacteria promotes the formation of lepidocrocite and ferrihydrite near and on the bacterial cell wall,which is shown in figure 3.

Fig. 3. Transmission electron micrographs showing the presence of nanocrystals of lepidocrocite on and away from the cell wall of Bacillus subtilis, during the oxidation of Fe(II) in the presence of SO4 (A) and ferrihydrite, in the presence of Si (B).

ROLE OF BACTERIA IN FORMATION OF INTRACELLULAR IRON-RICH MINERALS  Magnetotactic bacteria  “Magnetotactic

bacteria” encompasses a range of prokaryotes including anaerobic, sulfate-reducing bacteria such as Desulfovibrio magneticus. They are all motile by means of flagella, with Gram-negative cell wall architecture.

 The first indication that bacteria may be capable of

producing intracellular minerals came with the discovery of magnetotaxis by Blakemore (1975) .

 Magnetosomes-mineralogy and morphology  Most single magnetosome crystals are approximately 

 



35–120 nm long Two types of magnetosome mineral have been reported. First is the iron oxide, magnetite (Fe3O4)and second is greigite Magnetosome formation Within most magnetotactic cells, magnetite production is strictly confined to the lumens of magnetosome vesicles. Gorby et al. (1988) found evidence of some empty magnetosome vesicles, and some which were only partially filled with an amorphous iron phase, suggesting that the vesicle is formed first, and is later filled in as iron is deposited within it.

POTENTIAL ROLES FOR MICROORGANISMS IN THE FORMATION OF BIFS  Photosynthesis and the production of molecular

oxygen  Iron-oxidizing bacteria.  Iron-reducing bacteria.

CONCLUSION  Iron oxides formed in close association with bacteria

form as a result of several different metabolic activities, passive reactions and internal biomineralization processes.  Despite the fact that these oxides have been widely studied and that they possess unique features in terms of size, morphology and mineralogy .  Recent work by Daughney et al. (2004) showed that marine viruses (i.e., bacteriophage), which are often more abundant than bacteria in aquatic and terrestrial systems, possess surface binding sites which can bind soluble Fe and act as nucleation surfaces for Fe-oxide formation (Fig. 4).

Fig. 4. Transmission electron micrograph showing individual lepidocrocite crystals (see arrows) attached to the surface of viruses (marine bacteriophage PWH3a-P1) during the oxidation of Fe(II) at pH 7.