metals cycling iron and manganese cycling iron reducers iron oxidizers acid mine drainage manganese...
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Metals Cycling
Iron and Manganese CyclingIron ReducersIron Oxidizers
Acid Mine DrainageManganese Nodules
Fe+2
(ferrous)Fe+3
(ferric)
oxidation
reduction
Mn+2
(manganous)Mn+4
(manganic)
Feº(metalic)
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Iron Chemistry• Neutral to alkaline; all
insoluble.• Very acidic; Fe+2 and Fe+3
both soluble.• Anoxic and pH < 7; only
Fe+2 soluble.• Organics may chelate;
soluble. dept
hFe+3
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Iron Requirements• All life requires iron (cytochromes, heme groups, other proteins).
• Not very bioavailable in oxic environments.
• Some microbes produce siderophores (e.g. enterochelin).
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Iron Reduction
• Photochemical– Enhanced by hydroxyl radical formation from
organic mater such as humic acids.
• Biological– Anaerobic Respiration
– Requires absence of O2 and Nitrate
– Often important in aquatic sediments and water saturated soils (anoxic habitats).
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Aerobic respiration yields greatest energy due to very positive O2 redox potential.
Without O2, anaerobic respiration uses alternate terminal electron acceptors in the order of decreasing redox potential.
E = -240 mV
Methanogenesis
E = +820 mV
E = +420 mV
E = -200 mV
E = -180 mV
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Iron Reducing Bacteria in Anaerobic Decomposition
What’s Soil Gleying?
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0.5 μm
MagnetosomesGreigite (Fe3S4) or Magnetite (Fe304)
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Microaerophilic Magnetotactic(Need the Oxic Anoxic Transition Zone)
Dashed arrows are Earth’s inclined geomagnetic field lines.
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Metalic Iron OxidationCorrosion of Steel
• Abiotic Aerobic: rust!
2Feº + 1½ O2 + 3 H2O → 2Fe(OH)2
• Anaerobic with Sulfate Reducing Bacteria (SRB):
Fe + H2O → Fe(OH)2 + H2
4H2 + SO4-2 → H2S + 2OH- + 2H2O
H2S + Fe → FeS + H2
4Fe + 4H2O + SO4-2 → FeS +3Fe(OH)2 + 2OH-
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Desulfovibrio spp., and SRB
Microbial Influenced Corrosion (MIC)
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Ferrous Iron Oxidation
• Abiotic oxidation is low at pH < 4.
• Microbial catalysis 10-1000 faster.
• Different prokaryotes depending on:
- pH range - sulfide content;
-organic matter content
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There are four commonly accepted chemical reactions that represent the chemistry of pyrite weathering to form AMD. An
overall summary reaction is as follows:
4 FeS2 + 15 O2 + 14 H2O → 4 Fe(OH)3 ¯ + 8 H2SO4Pyrite + Oxygen + Water à "Yellowboy" + Sulfuric Acid
1) 2 FeS2 + 7 O2 + 2 H2O → 2 Fe2+ + 4 SO42- + 4 H+ Pyrite + Oxygen + Water → Ferrous Iron + Sulfate + Acidity
2) 4 Fe2+ + O2 + 4 H+ → 4 Fe3+ + 2 H2OFerrous Iron + Oxygen + Acidity → Ferric Iron + Water
{Thibacillus ferrooxidans; acidophilic pH < 3.5; consumes protons
intracellularly to create PMF for ATP synthesis; other bacteria and archaea}
3) 4 Fe3+ + 12 H2O → 4 Fe(OH)3 ¯ + 12 H+ Ferric Iron + Water → Ferric Hydroxide (yellowboy) + Acidity
4) FeS2 + 14 Fe3+ + 8 H2O → 15 Fe2+ + 2 SO42- + 16 H+Pyrite + Ferric Iron + Water → Ferrous Iron + Sulfate + Acidity
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PA Coal Field (Sources of AMD)
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Circumneutral Fe+2 Oxidizers• Microaerophiles• Heterotrophic
– No energy yield from ferrous ion– Morphology of iron oxides
• Ribbons (Gallionella)• Sheaths (Sphaerotilus-Leptothrix
Group)• Amorphous ppt coating (Siderocapsa)
– Selective pressures for Fe(OH)3 ppt covering or attached to the bacteria cell surface:
• Fe+2 toxicity• O2 toxicity• Protist predation• Viral attack
• Autotrophs – Some facultative autotrophic
Gallionella spp.– Some obligate lithoautotrophs
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Emerson et al., 2000