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:
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
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Some redox active metals
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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-
2
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
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Redox of an environment typically governed by the dominant redox couple
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Abiotic redox reactions
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Fe has many reactions
Temperature is important
Numerous potential oxidants (reactive oxygen species: ROS)
From Voelker and Sedlak, 1995
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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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5
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
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Obvious example: organic matter and oxygen (the disequilibria allows life to
exist) (Organic matter/carbon dioxide couple has a peo = -6.25)
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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 Reactions
“Froelich 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 Redox
Processes
“ Edibles”
“Breathables”
From Nealson, 1997; 2003
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Many ways to conceptualize microbially
mediated redox reactions
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From Stumm and Morgan, 1996
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Photosynthesis obviously has a major role in setting up Earth’s 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 #2:
Mn 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
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 story:
The 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 cobalt
Data by Noble, 2005
0
0
200
200
Depth (m)
Depth (m)
Station 5 - Costa Rica Upwelling Dome
400
400
600
600
800
800
Nitrite
Oxygen
0
5
10
15
20
Oxygen (mM)
25
30
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Nitrite (M)
30
0
0
200
200
Depth (m)
Depth (m)
Station 5 - Costa Rica Upwelling Dome
400
400
600
600
800
800
Total Cobalt
Nitrite
Oxygen
0
5
10
15
20
25
30
0.0
0.5
1.0
Oxygen (mM)
1.5
2.0
2.5
3.0
Nitrite (M)
0
50
100
150
200
Co Concentration (pM)
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Extends across low oxygen tongue of the North Pacific
(data by Saito, South of Hawaii, 8N)
N2O and Total Dissolved Cobalt
0
20
40
60
80
100
0
200
400
Depth (m)
600
800
1000
1200
1400
1600
N2O
Diss. Cobalt
1800
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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 systems
Outline:
Redox active metals
Abiotic Reactions
Biotic Reactions
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
Many of the processes are only somewhat elucidated. The microbes and
the functional genes are also only beginning to be understood
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