Aspects of redox in trace metal systems, and implications for Hybrid Type metals Outline: Redox active metals Abiotic Reactions Biotic Reactions Examples Examples: 1. Fe(II) Southern Ocean 2. Mn photoreduction 3. Fe, Co, Mn in OMZ systems 4. Redox reactions in OMZ sediments: major fluxes 5. Effects of ligands on redox reactions 12.755 L06 1
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Aspects of redox in trace metal systems, and implications for Hybrid Type metals
12.755 L06. Aspects of redox in trace metal systems, and implications for Hybrid Type metals. Outline: Redox active metals Abiotic Reactions Biotic Reactions Examples Examples: Fe(II) Southern Ocean Mn photoreduction 3. Fe, Co, Mn in OMZ systems - PowerPoint PPT Presentation
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Aspects of redox in trace metal systems, and implications for Hybrid Type metals
Outline:Redox active metals
Abiotic ReactionsBiotic Reactions
Examples
Examples: 1. Fe(II) Southern Ocean
2. Mn photoreduction3. Fe, Co, Mn in OMZ systems
4. Redox reactions in OMZ sediments: major fluxes5. Effects of ligands on redox reactions
12.755 L06
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Some redox active metals
• Fe (II/III)• Cu (I/II)• Co (II/III)• Mn (II/III/IV)• I (-I/V) (iodide, iodate)• Cr (III/VI) • Se (-II, 0, IV/VI) (selenite, selenate, Se –II is associated with
peptides, Cutter and Cutter 1995)• Mo, oxyanion MoO4
2-, in sulfidic waters: MoS42-
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pe’s of Dominant Redox Couples
From Morel and Hering: If one redox couple is present in much larger concentrations than the other, the corresponding free concentrations of oxidant and reductant are unaffected by the advancement of the complete redox reaction toward equilibrium. The equilibrium electron activity is then effectively that of the corresponding dominant redox couple.
pe =peo – log [Red] / [Ox]
The Sulfate-Sulfide couple S(VI)/S(-II)1/8 SO4
2- + 9/8 H+ + e- = 1/8 HS- +1/2 H2O peo=4.25
pe =peo – log [HS-]1/8 / [SO42-][H+]9/8 = - 4.5
* pH =8, [HS-] =1e-5, SO42-] = 1e-3
The Oxygen-water couple O(0)/O(-II)¼ O2 + H+ + e- = ½ H2O peo=20.75
pe =peo – log 1 / PO21/4[H+] = + 13.58
pH = 7 PO2=10-0.7atm3
Redox of an environment typically governed by the dominant redox couple
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Abiotic redox reactions • Fe has many reactions• Temperature is important• Numerous potential oxidants (reactive oxygen species: ROS)• From Voelker and Sedlak, 1995
• O2 oxidation of Fe(II) and Fe(II)L is most important • Underestimated due to superoxide Fe(III) reduction back reaction not being taken into account• From Rose and Waite, 2002
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Abiotic Rxns – Examples
Temperature allowsFe(II) to accumulate in
cold waters
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Yet redox equilibrium is often not achieved due to slow kinetics
• Obvious example: organic matter and oxygen (the disequilibria allows life to exist) (Organic matter/carbon dioxide couple has a peo = -6.25)
• Exceptions have been reported (drummers, in particular):
Dozens of people spontaneously combust each year; it's just not very widely reported”
~ David St. Hubbins on The unfortunate death of one of the former Spinal Tap drummers