OXIDATION REDUCTION REACTIONS

1
OXIDATION - REDUCTION REACTIONS

• Assigned reading Sparks Chapter 8, pp 245 - 255
  and Lindsay pp.23-30
• Additional reading McBride 1.2f, 7.1a, b and
  e, 7.2 Sposito Chap. 6 and Essington Chap. 9.

2
Introduction

• Importance of biological reactions
• Oxidation and reduction reactions in soils are to
  a great extent biologically mediated.
• Redox reactions require an oxidizing agent
  (electron acceptor) and reductant (electron
  donor).

3

• The scale of potential were developed assuming
• H e- 1/2 H2 Eo 0.00V
• The ?Go for this reaction is zero.

4
Electrode potential Measurements

• Electrode Potentials
• Standard Electrode Potential (EHo)
• Example Reduction of Fe3
• Fe3 e- Fe2 Eo 0.771 V
• oxidize reduced

5

• Defined vs. Standard Hydrogen Electrode
• H e- 1/2 H2 Eo 0.000
• Note This also defines the ? Gf 0 for H
• The overall reaction is
• Fe3 1/2H2 Fe2 H

6
Basic Cell for Measurement of Electrode Potentials
SHE Half Cell
7

• Half-cell Electrode Potentials (EH)
• Calculate the potential at other than standard
  conditions.
• Nernst equation
• Note the negative sign

8

• Also

9

• At equilibrium with respect to SHE , EH 0 and
  ?G 0 and
• and

10

• Rearrange
• and

11

• Then the relationship between ?Go and Eo is
• ?Go -nFEo
• also
• ?G -nFEH

12
Half-Cell Reactions of Common Electron Acceptors
in Soils

• Oxidized mH n electrons reduced H2O
• Nernst equation
• At 25 0C

RT
(reduced)

-
o
E
E
ln
H

m
nF
(oxidized)(H
)
13
In Volts
14
In millivolts
15
Eo values for reactions in soils (McBride, Table
7.1)
16
EH vs pH (Mcbride Figure 7.1)
17
Sparks Fig 8.1
18
Boundary reactions for electron acceptors that
limit EH in aqueous systems

• Upper boundary redox in water
• 1/4 O2 H e- 1/2 H2O Eo 1.23v
• Oxygen is an electron acceptor
• Lower boundary
• H e- 1/2 H2 Eo 0.00v
• Hydrogen is an electron acceptor

19
Half Cell Reaction involving Organic Electron
Donors e. g. Glucose

• Organic electron donors supply the energy to
  microbes in soils and sediments. The reaction
  below is a half-reaction that illustrates this
  process. In the soil the reaction goes from
  right to left.
• 1/4 CO2 H e- 1/24 C6H12O6 1/4 H2O
• EHo -0.014
• Carbon compounds are abundant electron donors in
  soils

20
Reaction in airSucrose donates ectrons to oxygen

• 1/4 O2 H e- 1/2 H2O Eo
  1.23V
• 1/24 C6H12O6 1/4 H2O 1/4 CO2 H e-
  0.014 V
• 1/24 C6H12O6 1/4 O2 1/4 CO2 1/4 H2O
  1.24 V.

21
"Electron Activity" (pe)

• We can imagine that electrons have a measurable
  activity and use -log (e) pe, instead of EH.
• For a half cell reaction.
• oxidized mH ne reduced

22
"Electron Activity" (pe) (cont.)

• at 25C
• Let pe -log(e-)
• then

(reduced)
-

logK
nlog(e
)
log
mlog(H
)

-
-

(oxidized)
23
"Electron Activity" (pe) (cont.)

• log K is sometimes called pe0
• Remember Electron activity is a convenient
  fiction, not a physical reality.

24
We also included protons in the Nernst equation
25
Comparison of the Nernst and pe equations
26
Equate the two equations
27
At 25 C
28
pe can easily be calculated from EH
29
Combining half cell reactions to describe a
complete reaction.

• Question What is the equilibrium constant for
  the oxidation of Fe2 by O2?
• log K
• Fe2 e- Fe3 -13.04
• H 1/4O2 e- 1/2H2O 20.78
• --------------------------------------------------
  -------------
• H 1/4O2 Fe2 1/2H2O Fe3 7.74
• This equation says that O2 can readily oxidize
  Fe2 at acid pH values

30
Combining half cell reactions (cont.)

• log K pH -1/4log Po2 log(Fe3)/(Fe2)

31
Combining half cell reactions (cont.)

• Find the pH at which (Fe3) (Fe2) at Po2
  0.21 atm

32
Answer

• 7.7 pH .17
• pH 7.57

33
Combining half cell reactions with solubility
reactions

• If (Fe3) is controlled by the solubility of
  Fe(OH)3 in a soil then under reducing (flooded
  soil) conditions Fe2 can be calculated as a
  function of pH and EH.
• Fe(OH)3 3H Fe3 3H2O log K 2.70
  (Lindsay)
• Predict Fe2 as a function of pe in soil

34
Combining half cell reactions with solubility
reactions(cont.)

• log K
• Fe3 e- Fe2 13.04
• Fe(OH)3 3H Fe3 3H2O 2.70
• Soil
• --------------------------------------------------
  --------------------
• Fe(OH)3 e- 3H Fe2 3H2O 15.74
• EH 0.93 V
• Similar to the value of pe in Sparks Table 1
• McBride Table 1 is different (is for freshly ppt
  Fe(OH)3)

35
Combining half cell reactions with solubility
reactions(cont.)
36
Combining half cell reactions with solubility
reactions(cont.)

• If measured pH 7.0 and
• (Fe2) 1.0 x 10-5 M, what is pe?

37
Answer

• pe 15.74 5 - 3(7)
• pe 15.74 - 16.0
• pe -0.26 and EH -0.014

38
Combining half cell reactions with solubility
reactions(cont.)

• Plot pe (or EH) vs. pH
• Example Fe(OH)3 3H e- Fe2 3H2O
• pe log K - 3pH - log(Fe2)
• at log (Fe2) -5
• pe 20.74 - 3pH

39
McBride Fig. 7.1
40
pe - pH plot for Fe
41
Consequences of flooding a soil

• 1. Oxygen is depleted.
• 2. Obligate and facultative anerobes utilize
  other electron acceptors.
• In order they are , nitrate, MnO2 , Fe(OH)3 and
  other oxides of FeIII, and sulfate.
• At very low EH methane is produced.

42
Compare reactions using conditional log K (log
cK7.0 or Log Kw) at pH 7.0

• Conditional constants for pH dependent reactions
  can be recalculated for pH 7.0 log(H) -7.
• This allows for ranking of redox reactions at
  near neutral pH values.
• For reaction (1)
• log K pe pH - 1/4 log PO2

43
Reactions in Flooded SoilsSee Sparks Table 8.7

• Note Sparks assumes soluble FeII and MnII
  10-4M (Lindsay uses 10-5
• Electron acceptors Electron donors
• pe at pH 7
• logK logcK7.0
• (1) 1/4 O2(g) H(aq) e- 1/2 H2O 20.8
  13.6
• (2) 1/4 NO3- 5/4 H e- 1/8 N2O(g) 5/8
  H2O 18.9 9.6

44

• (3) 1/2 MnO2 2H e 1/2 Mn2 H2O
• 20.7 8.8
• (4) Fe(OH)3 3H e- Fe2 3H2O
  15.8 -1.2
• (5) 1/8 SO42- 5/4 H e- 1/8 H2S(aq) 1/2
  H2O 5.2 -3.5

45

• (6) 1/8 CO2(g) e- H 1/8 CH4(g) 1/2
  H2O 2.9 -4.1
• (7) 1/4 CO2(g) e- H 1/24 C6H12O6
  1/4H2O -0.2 -7.2
• (8) H e- 1/2 H2 0.0 -7.0

46
Oxygen consumption in by organic carbon, when O2
is the electron acceptor

• Oxidation of organic carbon in soils generates
  energy for microorganisms.
• The most energy is gained by using electron
  acceptors with highest log K (E0) .
• Example, sucrose
• Combine reaction (1) with reaction (7)
• 1/24 C6H12O6 1/4 O2(g) 1/4 CO2(g) 1/4 H2O
  
• log K 21.0
• oxidation ---gt
• lt------ photosynthesis

47
Can calculate ?Go fromlog K or E0

• Energy units kJ/mole

48
Using nitrate as an electron acceptor

• Combine with reaction 7 and 2
• 1/24 C6H12O6(aq) 1/4 NO3- 1/4 H 1/8 N2O(g)
  3/8 H2O log K 19.1
• Can calculate Gibbs free energy
• ?G - 109.2 kJ/mole

49
When all electron acceptors are depleted then
fermentation

• Combine with reaction 7 and 6
• 1/24 C6H12O6(aq) 1/8 CH4(g) 1/8 CO2(g)
• log K 3.1
• eg. In methane digester or soil flooded for a
  long time.
• Low energy yield.

50
Review of consequences of flooding a soil

• With flooding O2 is used up because O2 diffusion
  in water is very slow.
• When O2 is depleted NO3- becomes the electron
  acceptor which yields the most energy,
• This followed followed by MnO2, Fe(OH)3, and then
  SO42-.
• Kinetic considerations can be important.
• Thus, Fe(OH)3 reduction can begin before all of
  the MnO2 is depleted.
• Because of pH dependence, the exact order of
  these reactions can vary slightly with pH,

51
McBride Figure 7.1
52
Chemical changes after flooding of a soil
(Sposito chapter 6)
53
Changes in N chemistry with flooding (Sposito
Chapter 6)
54
Changes in N chemistry with flooding (Sposito
Chapter 6)
55
Determination of EH

• Measurements
• Use a bright Pt electrode and a reference
  electrode. Essington suggests the use of a
  Calomel (Hg2Cl2/Hg) electrode. In fact, most
  people use an AgCl/Ag electrode.

56
Reference electrode

• log K
• AgCls e- Ags Cl- 3.75
• Inside the electrode is an Ag plated wire with
  AgCl and a fixed KCl concentration. The KCl is
  generally in the range of 3 M to saturated.
  Potential is about 0.2 v. (Compare to standard)

57
Use reference electrode

• Measure E vs. the above electrode then add 0.2 V
  to get EH
• In practice standardize meter and reference
  electrode vs. a solution like quihydrone
  quinone/hydroquinone 1.0
• (at pH 4.01)
• EH 0.461v

58
Reference standardQuinhydrone reaction

• 2H 2e- ?
  H2O
• quinone hydroquinone

59
What does a Pt electrode measure in soils?

• Measures EH only of electrode reactive reactions
  (electrons can be transferred to or from
  electrode).
• Fe3 gt Fe2 is electrode reactive.
• 1/4 O2 H e- 1/2 H2O is not
• Cant measure O2 in soils or waters using Pt
  electrodes
• Sucrose gtH2O and CO2 is not electrode reactive.

60
Electrode data allows for general statements
about redox status McBride (Fig.7-6)
61
Range of measured pe and pH in soils
62
Reactions in flooded soils vs. measured EH (Fig.
7.5)
63
Compare to EH - pH diagrams McBride Fig. 7.1
64
pH Changes Following Flooding

• In low pH soils the pH increases to near
  neutrality because H is consumed during the
  reduction of most electron acceptors. Most
  notably
• 3H Fe(OH)3 e- Fe2 3H2O
• In calcareous soils pH decreases because of the
  precipitation of carbonates at the high Pco2.
• Ca2 H2O CO2 CaCO3 2H

65
pH Changes Following Flooding (cont.)

• Also Mn(II) and Fe(II) carbonates precipitate
• Siderite, FeCO3
• Rhodochrosite, MnCO3,
• These minerals are important in controlling the
  pH in many flooded soils.
• These minerals both have a calcite type
  structure with very similar unit cell sizes.
  They form a solid solution series.

66
Precipitation of Siderite

• log K
• Fe2 CO32- FeCO3 10.8
• H2O CO2 H HCO3- - 7.81
• HCO3- H CO32- -10.33
• --------------------------------------------------
  -----
• Fe2 H2O CO2 2H FeCO3 - 7.34

67
FeCO3 (cont.)

• From earlier slide
• Fe(OH)3 e- 3H Fe2 3H2O 15.74
• soil
• Fe2 H2O CO2 2H FeCO3 - 7.34
• --------------------------------------------------
  ------------------
• Fe(OH)3 e- CO2 H FeCO3 2H2O 8.40

68
Example, FeCO3 (cont.)

• log K pH pe - log Pco2
• Thus at equilibrium pe is a function of pH and
  Pco2, only.
• Set the PC02 then calc. pH
• At pH 7.0 and Pco2 0.10
• pe log K - pH log Pco2
• pe 8.40 - 7.0 - 1.0
• pe 0.4
• EH (0.059)( 0.4) - 0.024 V

69
pe - pH plot for Fe
70
Sulfide precipitation in reduced soils

• Sulfate is reduced to S2- and can precipitate
  with Fe2, Mn2, Zn2, Cd2 etc.
• FeS is the predominant form of sulfide.
• Sulfide formation can reduce the bioavailablity
  of heavy metals.

71
Formation of sulfidic coastal swamps
72
Formation of acid sulfate soils by draining
sulfidic swamps
73
Formation of surface oxic layer in flooded soils

• Slow O2 diffusion can result in an oxic surface.
• The thickness of the layer is result of the O2
  diffusion rate and O2 consumption rate.
• In low OM mineral soils this layer may be gt 1 cm
• In high OM soil it may be only a few mm thick.

74
Oxic surface in a rice paddy soil
75
N transformations in the surface of a rice paddy
soil
76
Phosphate is generally released by flooding

• Flooding releases P by
• 1. Raising pH of acid soils.
• 2. Lowering the pH of alkaline soils.
• 3. Reduction Fe(III) oxides that sorb P.

77
Reduction Fe(III) phosphate (strengite)

• Fe3 e- Fe2 13.04
• FePO42H2O 2H Fe3 H2PO4- - 6.85
• --------------------------------------------------
  -------------
• FePO42H2O e- 2H Fe2 H2PO43- 6.19

78
Redox reaction of strengite (cont.)

• pe 6.2 - log(Fe2) - log(H2PO4) - 2pH
• Let pH 7.0, (Fe2) 10-5 M, and
• (H2PO43-) 10-5 M
• pe 6.2 5 5 -14 2.2
• EH 0.129v

79
Redox reaction of strengite (cont.)

• Comparison with previous computations shows
  strengite is reduced at a similar pe as Fe(OH)3
• Reduction of iron oxides releases adsorbed P.
• Drainage and restoration of oxic conditions can
  tie up P.
• Following draining of rice soils, phosphate can
  be tied up causing P deficiency for rotation
  crops.

80
Short Summary

• The log K (E0) of half cell reactions can be used
  to calculate the log K of oxidation reduction
  reactions.
• EH can be converted to pe.
• The pe values do not represent real activities
  of electrons.
• Pt electrodes can be use to measure EH in flooded
  soils.
• The measured EH can only be approximately
  predicted from measured concentrations and half
  cell reactions.

81

• In flooded soils consumption of O2 and other
  electron acceptors by oxidation of reduced C
  (organic C) lowers pe (EH)
• Flooding increases the pH of acid soils and
  decreases the pH of calcareous soils.
• At very low EH sulfides can precipitate.
• A thin layer on the surface of flooded soils can
  be oxic.
• Flooding causes the release of bound P.

82
(No Transcript)
83
(No Transcript)