1 / 40

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

ismet
Download Presentation

Aspects of redox in trace metal systems, and implications for Hybrid Type metals

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. 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 4. Redox reactions in OMZ sediments: major fluxes 5. Effects of ligands on redox reactions

  2. 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 MoO42-, in sulfidic waters: MoS42-

  3. 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 SO42- + 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=10-0.7atm

  4. Redox of an environment typically governed by the dominant redox couple

  5. 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

  6. Abiotic Rxns – Examples Temperature allows Fe(II) to accumulate in cold waters

  7. 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

  8. Biotic Redox Reactions“Froelich series” – sequential redox cycles in vertical sedimentary (or water column) profiles corresponding to thermodynamic benefit

  9. Stumm and Morgan 1996

  10. Biotic RedoxProcesses “ Edibles” “Breathables” From Nealson, 1997; 2003

  11. Many ways to conceptualize microbially mediated redox reactions • From Stumm and Morgan, 1996

  12. Photosynthesis obviously has a major role in setting up Earth’s redox environment From Stumm and Morgan, 1996

  13. The redox interface is a profitable place to live

  14. Abiotic (and biotic) reactions, example #2:Mn photoreduction (and microbial oxidation) in surface waters result in diel cycle(Sunda and Huntsman, 1990)

  15. Example: oxygen minimum zones

  16. Rue and Bruland, DSR, 1997

  17. Rue and Bruland, DSR, 1997

  18. Rue and Bruland, DSR, 1997

  19. Rue and Bruland, DSR, 1997

  20. Manganese – well studied, many hypotheses Tebo and Nealson, 1984

  21. Mn oxidation by bacteria Emerson, Tebo, Nealson, 1982

  22. Martin: Lateral advection explains Mn maxima in North Pacific

  23. But Johnson et al.,1992 and 1996 revisits and has a different story:The smallest flux of Mn occurs in the oxygen minima, therefore lateral advection cannot be the source

  24. Mn flux out is correlated with O2 sink (Johnson et al., 1992)

  25. Martin flux equation with oxygen dependent scavenging “The Mn maxima can form because of a reduction in the pseudo-first order rate scavenging rate constant within the oxygen minimum.”

  26. Cobalt oxidized by same Mn oxidation pathway

  27. Also redox water column dynamics for cobaltData by Noble, 2005

  28. Extends across low oxygen tongue of the North Pacific (data by Saito, South of Hawaii, 8N)

  29. These redox processes in sediments as major sources of Fe, Mn, and Co?Or is it water column based redox recycling?

  30. Speciation and coordination environment matters for redox properties

  31. Photoreactivity of Marine Siderophore Complexes - Barbeau et al., 2003

  32. Photoreactivity of Marine Siderophore Complexes - Barbeau et al., 2003

  33. Photoreduction of iron chelates: Fe(II)

  34. Brief overview of redox in trace metal systemsMany of the processes are only somewhat elucidated. The microbes and the functional genes are also only beginning to be understood Outline: Redox active metals Abiotic Reactions Biotic Reactions Examples Fe(II) Southern Ocean Mn photoreduction 3. Fe, Co, Mn in OMZ systems 4. Redox reactions in OMZ sediments: major fluxes 5. Effects of ligands on redox reactions

More Related