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Aspects of redox in trace metal systems, and implications for Hybrid Type metals Outline: Redox active metals Abiotic Reactions Biotic Reactions Examples.

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Presentation on theme: "Aspects of redox in trace metal systems, and implications for Hybrid Type metals Outline: Redox active metals Abiotic Reactions Biotic Reactions Examples."— Presentation transcript:

1 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 L06 1

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 MoO 4 2-, in sulfidic waters: MoS

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 =  pe o – log [Red] / [Ox] The Sulfate-Sulfide couple S(VI)/S(-II) 1/8 SO /8 H + + e - = 1/8 HS - +1/2 H 2 O pe o =4.25 pe =  pe o – log [HS - ] 1/8 / [SO 4 2- ][H + ] 9/8 = * pH =8, [HS - ] =1e-5, SO 4 2- ] = 1e-3 The Oxygen-water couple O(0)/O(-II) ¼ O 2 + H + + e - = ½ H 2 O pe o =20.75 pe =  pe o – log 1 / P O2 1/4 [H + ] = pH = 7 P O2 = atm 3

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

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,

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

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 pe o = -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 drummersSpinal Tap 7

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

9 9

10 Stumm and Morgan

11 Biotic Redox Processes “ Edibles”“Breathables” From Nealson, 1997;

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

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

14 14

15 The redox interface is a profitable place to live 15

16 Abiotic (and biotic) reactions, example #2: Mn photoreduction (and microbial oxidation) in surface waters result in diel cycle (Sunda and Huntsman, 1990) 16

17 Example: oxygen minimum zones 17

18 Rue and Bruland, DSR,

19 Rue and Bruland, DSR,

20 Rue and Bruland, DSR,

21 Rue and Bruland, DSR,

22 Manganese – well studied, many hypotheses Tebo and Nealson,

23 Emerson, Tebo, Nealson, 1982 Mn oxidation by bacteria 23

24 Martin: Lateral advection explains Mn maxima in North Pacific 24

25 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 25

26 Mn flux out is correlated with O2 sink (Johnson et al., 1992) 26

27 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.” 27

28 Cobalt oxidized by same Mn oxidation pathway 28

29 29

30 Also redox water column dynamics for cobalt Data by Noble,

31 31

32 Extends across low oxygen tongue of the North Pacific (data by Saito, South of Hawaii, 8N) 32

33 33

34 These redox processes in sediments as major sources of Fe, Mn, and Co? Or is it water column based redox recycling? 34

35 Speciation and coordination environment matters for redox properties 35

36 Photoreactivity of Marine Siderophore Complexes - Barbeau et al.,

37 Photoreactivity of Marine Siderophore Complexes - Barbeau et al.,

38 38

39 Photoreduction of iron chelates: Fe(II) 39

40 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


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