The cycling of iron consists largely of oxidation-reduction reactions that reduce ferric iron to ferrous iron and oxidise ferrous iron to ferric iron. Ferric iron precipitates in alkaline environments as ferric hydroxide. Ferric iron may be reduced under anaerobic conditions to the more soluble ferrous form. Under some anaerobic conditions, however, sufficient H2S may be evolved to precipitate iron as ferrous sulphide. Flooding of soil, which creates anaerobic conditions, favours the accumulation of ferrous iron. In aerobic habitats such as well-drained soil, most of the iron exists in the ferric state.
Microbial growth often is limited by the availability of iron. Various bacteria produce siderophores which bind iron and facilitate its cellular uptake. Some chemolithotrophs oxidise iron to generate cellular energy. These iron-oxidising bacteria can lead to substantial iron deposits.
Iron is the fourth most abundant element in the earth's crust. Microbial metal transformations are essential for the production of metallic ores. They are important in extracting metals from low-grade ores.
Microbial iron transformations include:
.Iron scavenging and uptake
. Iron oxidation and precipitation
. Iron reduction and solubilisation
In aerobic environment, microbial iron oxidation dominates in acidic condition and chelation dominates in neutral environment. In the anaerobic environment, iron cycle is dominated by iron reduction and precipitation of iron sulphides.
Iron exists as metallic ion (ferric and ferrous ion). Metallic iron spontaneously oxidises in acidic condition:
FeO Fe2+ at pH < 5
Fe2+ + (O) Fe3+ at pH > 5
spontaneous reactions are slow Under aerated conditions, bacteria obtain energy from oxidation of ferrous ions (Fe2+) (e.g. Thiobacillus ferrooxidans and Sulfolobus acidocaldarius) are iron oxidising bacteria.
2Fe2+ +1/2 O2 + 2H+ Iron cycle 2Fe3+ + H2O
T. ferrooxidans is a motile chemolithotrophic bacillus, which grows at pH 2-4. It oxidises the iron in ferrous sulphate to ferric sulphate.
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ARD is the product formed by the atmospheric (i.e. by water, oxygen and carbon dioxide) oxidation of the relatively common iron-sulphur minerals pyrite and pyrrhotite in the presence of (catalysed by) bacteria (Thiobacillus ferrooxidans), and any other products generated as a consequence of these oxidation reactions.
An important reaction involving T. ferrooxidans is the oxidation of ferrous to ferric iron (Fe++ to Fe+++)
4Fe++ + O2 + 4H+ = Fe+++ + 2H2O
Ferric iron is a powerful oxidizing agent. Even at a Fe+++/Fe++ ratio of 1:1,000,000, a Redox potential of greater than +0.4 V is generated which is sufficient for the attack of most base metal sulphides (Dutrizac & MacDonald, 1974). The general equation for the ferric ion reaction with base metal sulphides is:
MS + nFe+++ = Mn+ + S + nFe++
Consequently T. ferrooxidans, in generating Fe+++, is indirectly responsible for the dissolution of base metal sulphide minerals and the mobilization of metallic cations such as Cu++, Zn++, Pb++ and Cd++. Base metal sulphides react only very slowly with sulphuric acid in the absence of ferric iron (Roman & Benner, 1973).
The importance of Redox potential in determining metal solubility and transport can be clearly seen for copper in the Eh-pH diagram for the Cu-H2O-O2-S-CO2 system (Figure 2, Garrels & Christ, 1965). The effects of bacteria upon the rate of dissolution of copper from chalcopyrite are highly pronounced, as demonstrated by Malouf & Prater (1961), Figure 3.
Figure 2: Eh-ph Diagram for Cu-H2O-O2-S-CO2 System (from Garrels and Christ, 1965)
Figure 3: Effects of Bacteria upon the Rate of Disolution of Copper from Chalcopyrite (from Malouf and Prater, 1961)
In general, for substantial metal mobilization from base metal sulphides the following conditions must be met:
Ferric iron for sulphide oxidation
T. ferrooxidans and oxygen for ferrous to ferric oxidation
pH compatible with T. ferrooxidans habitat requirements, typically pH 1.5-3.5 (Roman & Benner, 1973)
The typical habitat pH of T. ferrooxidans of 1.5 to 3.5 is not one that develops spontaneously. It is currently believed (Béchard, 1996) that these conditions are produced by a consortium of bacteria acting in succession. Such a succession may include T. thioparus at neutral pH, giving way to dominance by metallogenium bacteria under mildly acid conditions (pH 3.5 to 4.5) (Walsh & Mitchell, 1972), and finally T. ferrooxidans dominance at low pH.
The metabolic activity of T. ferrooxidans is temperature dependent, peaking at about 30-35 degrees Celsius, and falling with both increasing and decreasing temperature (Roman & Benner, 1973).
Leduc & Ferroni (1994) have demonstrated that T. ferrooxidans strains are site-specific.
From the above discussion it is clear that consideration of bacterial behaviour is most important in understanding the process of ARD generation. This is particularly so when "kinetic" tests are used to predict the rate of generation of ARD in the field. Only if the bacterial conditions of testwork are identical to those in the field, can rates of ARD generation and/or metal solubilization be taken from laboratory kinetic testwork and used to predict field behaviour with any degree of confidence.
Scientists Chart Iron Cycle in Ocean
Sunlight plays role in aquatic food chain
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Scientists at the University of California have found that sunlight plays an important role in cycling iron in the ocean and making it available to marine life.
Iron, which is necessary for the sustenance of life, is scarce in the ocean. National Science Foundation (NSF)-supported researchers found that light helps transform the mineral into a form that can be easily taken up by phytoplankton and other microorganisms. They report their findings in the September 27 issue of the journal Nature.
"This discovery helps us better understand one of the essential links in the ocean's food chain," said Donald Burland, acting director of NSF's Chemistry Division. "It may also have implications for global climate change, since living organisms are important in the absorption and release of carbon dioxide from the oceans."
Iron and other trace metals are important biochemical ingredients in the production of plankton, the most abundant organisms in seawater, which are at the bottom of the aquatic food chain. But iron is rare in surface seawater, and scientists believe it occurs almost entirely in complex molecules in which the iron is strongly bound by organic ligands presumed to be of biological origin. Bacteria produce small molecules called siderophores to help them obtain iron from their environments, and this process may contribute to the pool of tightly bound iron complexes.
"We determined that iron bound to the oceanic siderophores react to light," said chemist Alison Butler of the University of California at Santa Barbara. "This photochemical reaction helps transform the iron complexes into a form that enables marine organisms to more easily acquire the essential iron."
The sun's energy turns the molecules into more loosely bound configurations of iron and oxygen atoms, Butler explained. This enables bacteria, plankton and other microorganisms to grab and use the iron.
The team's research grew partly from early studies indicating that fertilizing the oceans with iron could stimulate the growth of plant life that consume carbon dioxide, and thereby counteract global warming. In those studies, iron dropped from ships encouraged phytoplankton to bloom profusely, but only for a short time.
"Understanding the uptake of this scarce micronutrient will help provide more insight into how these microscopic plants and bacteria cope in these oceanic environments," said oceanographer Ken Bruland of the University of California at Santa Cruz.
Butler believes the findings on how the complex iron molecules are broken down by sunlight could also contribute to research on alternate drug delivery systems, possibly providing a nanoscale vessel that reacts upon exposure to light.
The Center for Environmental Bio-Inorganic Chemistry in Princeton, N.J., established by NSF and the Department of Energy to study environmental issues at the molecular level, funded the research. This center and others established by NSF and DOE conduct interdisciplinary research aimed at understanding the natural environment and addressing global environmental challenges.