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

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Aspects of redox in trace metal systems, and
implications for Hybrid Type metals
12.755 L06

• 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

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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 MoO42-, in sulfidic waters MoS42-

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pes 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 peo4.25 pe
peo log HS-1/8 / SO42-H9/8 - 4.5 pH
8, HS- 1e-5, SO42- 1e-3 The Oxygen-water
couple O(0)/O(-II) ¼ O2 H e- ½ H2O
peo20.75 pe peo log 1 / PO21/4H
13.58 pH 7 PO210-0.7atm
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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

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Abiotic Rxns Examples Temperature
allows Fe(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
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Biotic Redox ReactionsFroelich series
sequential redox cycles in vertical sedimentary
(or water column) profiles corresponding to
thermodynamic benefit
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• Stumm and Morgan 1996

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Biotic RedoxProcesses

• Edibles Breathables

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

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Photosynthesis obviously has a major role in
setting up Earths redox environment
From Stumm and Morgan, 1996
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The redox interface is a profitable place to live
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Abiotic (and biotic) reactions, example 2Mn
photoreduction (and microbial oxidation) in
surface waters result in diel cycle(Sunda and
Huntsman, 1990)
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Example oxygen minimum zones
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Rue and Bruland, DSR, 1997
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Rue and Bruland, DSR, 1997
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Rue and Bruland, DSR, 1997
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Rue and Bruland, DSR, 1997
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Manganese well studied, many hypotheses

• Tebo and Nealson, 1984

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Mn oxidation by bacteria
Emerson, Tebo, Nealson, 1982
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Martin Lateral advection explains Mn maxima in
North Pacific
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But Johnson et al.,1992 and 1996 revisits and has
a different storyThe smallest flux of Mn occurs
in the oxygen minima, therefore lateral advection
cannot be the source
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Mn flux out is correlated with O2 sink (Johnson
et al., 1992)
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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.
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Cobalt oxidized by same Mn oxidation pathway
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Also redox water column dynamics for cobaltData
by Noble, 2005
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Extends across low oxygen tongue of the North
Pacific (data by Saito, South of Hawaii, 8N)
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These redox processes in sediments as major
sources of Fe, Mn, and Co?Or is it water column
based redox recycling?
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• Speciation and coordination
• environment matters for redox
• properties

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Photoreactivity of Marine Siderophore Complexes -
Barbeau et al., 2003
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Photoreactivity of Marine Siderophore Complexes -
Barbeau et al., 2003
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Photoreduction of iron chelates Fe(II)
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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