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

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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: MoS4

2-

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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

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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 199610

Biotic RedoxProcesses

“ Edibles” “Breathables”

From Nealson, 1997; 2003

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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 Earth’s redox environment

From Stumm and Morgan, 199613

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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, 199718

Rue and Bruland, DSR, 199719

Rue and Bruland, DSR, 199720

Rue and Bruland, DSR, 199721

Manganese – well studied, many hypotheses

Tebo and Nealson, 1984

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Emerson, Tebo, Nealson, 1982

Mn oxidation by bacteria

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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 cobaltData by Noble, 2005

Station 5 - Costa Rica Upwelling Dome

Nitrite (M)

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Dep

th (

m)

0

200

400

600

800Nitrite

Oxygen (mM)

0 5 10 15 20 25 30

Dep

th (

m)

0

200

400

600

800

Oxygen

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Co Concentration (pM)

0 50 100 150 200

Dep

th (

m)

0

200

400

600

800 Total Cobalt

Station 5 - Costa Rica Upwelling Dome

Nitrite (M)

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Dep

th (

m)

0

200

400

600

800Nitrite

Oxygen (mM)

0 5 10 15 20 25 30

Dep

th (

m)

0

200

400

600

800

Oxygen

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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

Dep

th (

m)

0

200

400

600

800

1000

1200

1400

1600

1800

N2ODiss. Cobalt

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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

Many 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

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

40