Buffering of Redox Potential [Poise]
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Transcript Buffering of Redox Potential [Poise]
Institute of Food and Agricultural Sciences (IFAS)
Biogeochemistry of Wetlands
Science and Applications
Electrochemical Properties
Wetland Biogeochemistry Laboratory
Soil and Water Science Department
University of Florida
Instructor
K. Ramesh Reddy
[email protected]
7/16/2015
7/16/2015
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1 1
Chemical Reactions in
Natural Systems
Reactions in which neither protons nor electrons
are exchanged
Fe2O3 + H2O = 2 FeOOH
Reactions involving protons
H2CO3 = H+ + HCO3 Reactions involving electrons
Fe2+ = Fe3+ + e Reactions in which both protons and electrons
are transferred
2Fe(OH)3 + 3H+ + e- = 2Fe2+ + 3H2O
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Institute of Food and Agricultural Sciences (IFAS)
Electrons and Protons
e-
+
- e- e- eH
e
e
+
e H+
e
+
+
- H
+ e e- H - H+ H
e
+
H
e H+ H e- e- e- e- e- e- H+ H+e -e
- ee
e
e- + - + ee
- H
e
+
e
H
e
e H+
H
e
+
H+ e- e e- e- He-
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Electrochemical Properties
Topic Outline
Introduction
Oxidation-reduction reactions
Nernst Equation
Eh - pH relationships
Buffering of redox potential
Measurement of redox potentials
Soil and water column pH
Redox couples in wetland soils
Redox gradients in wetland soils
Specific conductance
Soil oxygen demand
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Walther Nernst
The Nobel Prize in Chemistry 1920
http://www.corrosion-doctors.org/Biographies/Nernst.htm
4
Electrochemical Properties
Learning Objectives
Basic concepts related to oxidationreduction reactions
Use of Nernst Equation to calculate redox
potential (Eh)
Relationship between redox potential (Eh)
and pH
Laboratory and field measurements of
redox potentials
Diel changes in water column pH
Redox couples and microbial metabolic
activities in wetlands
Redox gradients and aerobic/anaerobic
interfaces in wetlands
Source: D. R. Lovley, 2006. Nature Reviews 4:497-508
Soil oxygen demand and nutrient fluxes
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Oxidation-Reduction
Oxidant + e-
Reductant
Reductant = Electron donor
[Organic matter, NH4+, Fe2+, Mn2+, S2-, CH4, H2, H2O]
Reductant
Oxidant + e-
Oxidant = Electron acceptor
[O2, NO3-, MnO2, Fe(OH)3, SO42-, CO2, and
some organic compounds]
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Oxidation-Reduction
[Aerobic Respiration]
Oxidation
C6H12O6 + 6H2O = 6CO2 + 24H+ + 24 eReductant
Oxidant
Reduction
6O2 + 24H+ + 24 e- = 12H2O
Oxidant
Reductant
C6H12O6 + 6O2 = 6CO2 + 6H2O
Oxidation - Reduction
Oxidation-Reduction
[Nitrate Respiration – Dentrification]
Oxidation
5C6H12O6 + 30H2O = 30CO2 + 120H+ + 120 eReductant
Oxidant
Reduction
24NO3- + 144H+ + 120e- = 12N2 + 72H2O
Oxidant
Reductant
5C6H12O6 + 24NO3- + 24H+ = 12N2 + 30CO2 + 42H2O
Oxidation - Reduction
Oxidation-Reduction
[Sulfate Respiration]
Oxidation
C6H12O6 + 6H2O = 6CO2 + 24H+ + 24eReductant
Oxidant
Reduction
3SO42- + 24H+ + 24e- = 3S2- + 12H2O
Oxidant
Reductant
C6H12O6 + 3SO42- = 3S2- + 6CO2 + 6H2O
Oxidation - Reduction
Oxidation-Reduction
UPLAND SOILS
FLOODED SOILS
H2O
O2
NO3-
N2 NH4+
Mn4+
Mn2+
Fe3+
Reduction
S2-
SO42CO2
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Fe2+
Oxidation
CH4
PO43-
PH3
H2O
H2
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Nernst Equation
m (OXIDANT) + m H+ + n e- = m (REDUCTANT)
Eh = Eo - [0.059/n] log [Reductant/Oxidant]
- 0.059 [m/n] pH
E = Electrode potential (volts)
Eo= Standard electrode potential (volts)
F = Faraday’s constant (23.061 kcal/volt mole or
96.50 kJ/volt mole
R = Gas constant (0.001987 kcal/mole degree or
0.008314 kJ/mole degree
T = Temperature (298.15 K (273.15 + 25 oC))
n = number of electrons involved in the reaction
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Wetland Soil
Drained Soil
Anaerobic
Aerobic
Highly
Reduced Moderately
Reduced
Reduced
-300
-100 0 100
300
Oxidized
500
700
Oxidation-Reduction Potential (mV)
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Electron Pressure
Oxidation-Reduction
Strongly
reduced
Strongly
oxidized
-300 -100 0 100
300
500
700
Oxidation-Reduction Potential (mV)
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Electron donors
[Organic matter, NH4+, Fe2+, Mn2+, S2-, CH4, H2, H2O]
+
- ee
e
+
H
e
e
e
e
H
+
e e H+
e
+
+
- H
- H
- ee
+
+
e
H
e
e
+
e
e- H+ H e- e- e- e- e- He- H+ H+ e-H- e-
e
e-
e
Energy
O2
NO3Mn4+
Electron acceptors
Fe3+
SO42CO2
H2 O
600
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300 200 100 0
-100 -300 -400
Oxidation-Reduction Potential (mV)
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[-]
O2
N-Oxides
Mn (IV)
Fe (III)
SO42-
Energy Yield
[+]
Electrode Potentials
How much energy is released
during oxidation - reduction reactions?
CO2
Ease of Reduction
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Oxidation-Reduction
Mn4+
Mn2+
Se(0); Se2SeO42SeO32Fe3+
Fe2+
NO3- N2 O2
H2O
CH4 SeO32-
CO2
SO42-
-200
S2-
-100
0
+100
+200
+300 +400
Redox Potential, mV (at pH 7)
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Redox Potential (mV)
Iron Redox Couple and Eh-pH
1200
Fe3+
800
O2
Fe2O3
400
H2O
Fe2+
0
FeCO3
H2O
H2
-400
FeS2
Fe2+
0
2
FeCO3
4
6
8
pH
Fe3O4
10
12
14
Oxidation-Reduction
Wetlands and
Aquatic Systems
Uplands
Electron
acceptor nonlimiting
Electron donor
limiting
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Electron
acceptor limiting
Electron donor
non-limiting
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Relative Concentration
Sequential Reduction of
Electron Acceptors
Organic Substrate
[e- donor]
Fe2+
SO42NO3-
S2CH4
Mn2+
O2
Oxygen
Nitrate
Iron
Manganese
Time or Soil DepthWBL
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Methanogenesis
Sulfate
19
Redox Zones with Depth
WATER
Oxygen
Oxygen Reduction
Reduction Zone
Zone
SOIL
Oxygen Reduction Zone
Eh = > 300 mV
Nitrate Reduction Zone
Mn4+ Reduction Zone
Eh = 100 to 300 mV
Fe3+ Reduction Zone
Eh = -100 to 100 mV
I
Depth
II
III
IV
V
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Sulfate Reduction Zone
Eh = -200 to -100 mV
Aerobic
Facultative
Anaerobic
Methanogenesis
Eh = < -200 mV
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Regulators of Eh
Water-table fluctuations.
Activities of electron acceptors.
Activities of electron donors.
Temperature
pH
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Field Redox Electrodes
Copper wire
Volt meter
Heat
shrinking
tube
Calomel
Reference
Water
Epoxy
Platinum
wire
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Soil
Platinum
electrodes
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Laboratory Redox Electrodes
Copper wire
Platinum Glass Electrode
Saturated KCl
Heat shrinking
tube
Glass tube
Glass tube
Calomel +
Mercury
Mercury
Platinum
wire
Salt bridge
Epoxy
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Platinum
wire
Mercury
Calomel Reference Electrode
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Okeechobee Basin Wetland Soils
and Stream Sediments
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Flooded Organic Soils:
Everglades Agricultural Area
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Redox Potential, mV
Flooded Paddy Soils: Louisiana
Aerobic
Anaerobic
Time, days
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Eh (mV)
Electron Acceptors - Redox Potential
700
600
500
400
300
200
100
0
-100
-200
-300
1 2 3 4 5 6 7 8 9
Oxygen
Nitrate
Sulfate
Bicarbonate
10 11 12 13 14 15 16 17 18
Time (wk)
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Redox potential (mV)
Electron Donor [Organic Matter] –
Redox Potential
300
200
100
Low Organic Matter Soil
0
-100
High Organic Matter Soil
-200
Time after flooding
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Redox Gradients in Sediments
0
Depth (mm)
-20
Aerobic Layer
-40
Anaerobic Layer
-60
-80
-100
-120
-140
-160 0
100
200
300
400
500
600
700
Redox Potential (mV)
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Sediment Microbial Fuel Cell
Source: D. R. Lovley, 2006. Nature Reviews 4:497-508
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Redox Potential and pH
Eh mv]
1000
800
600
400
200
0
-200
-400
-600
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0
2
4
pH
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6
8
10 12
Baas Becking et al.
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Limitations of
Redox Potentials
Most of the redox couples are not in
equilibrium except in highly reduced soils.
In biological systems, electrons are added
and removed continuously.
Platinum electrodes respond favorably to
reversible redox couples.
Redox potential is closely related to pH.
Platinum electrode surface can be
contaminated by coatings of oxides, sulfides
and other impurities.
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Soil and Water Column - pH
Reactions involving protons
CO2 + H2O = H2CO3
H2CO3 = H+ + HCO3 HCO3- = H+ + CO32 Reactions in which both protons and electrons
are transferred
2Fe(OH)3 + 3H+ + e- = 2Fe2+ + 3H2O
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Water Column pH: Experimental Ponds
– Lake Apopka Basin
10
Algae
pH
9
8
Cattails and Egeria
7
Water hyacinth
6
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8
12
16 20
24
4
Time, hundred hours
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8
34
Effect of Flooding on Soil pH
Clay loam [ pH = 8.7; OM = 2.2%; Fe = 0.63%]
8
pH
7
6
Clay [ pH = 3.4; OM = 6.6%; Fe = 2.8%]
5
4
Clay [ pH = 3.8; OM = 7.2%; Fe = 0.1%]
3
0
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2
4
6
8
10
Time after flooding
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12
14
35
Effect of Flooding on
Soil Porewater Ionic Strength
Fe2+
Fe2+
Fe2+
A
Ca2+
Soil
Fe2+
Mn2+
K+
Soil Solution
Ionic Strength
Solid Phase
NH4+
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B
WBLflooding
Time after
36
Redox Couples in Wetlands
C6H12O6/CO2 and O2/H2O
C6H12O6/CO2 and NO3-/N2
C6H12O6/CO2
C6H12O6/CO2 and FeOOH/Fe2+
C6H12O6/CO2 and SO42- /H2S
H2/H+ and CO2 /CH4
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and MnO2 /Mn2+
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Aerobic Respiration and Energy Yield
C6H12O6 + 6O2 = 6CO2 + 6H2O
Gr = -686.4 kcals/mole
ADP + Pi = ATP
Gr = -7.7 kcals/mole
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Institute of Food and Agricultural Sciences (IFAS)
Biogeochemistry of Wetlands
Science and Applications
Soil Oxygen Demand
Wetland Biogeochemistry Laboratory
Soil and Water Science Department
University of Florida
Instructor
K. Ramesh Reddy
[email protected]
7/16/2015
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Oxygen
Oxygen is an electron acceptor
Reduction [Electron acceptor]
O2 + 4H+ + 4e- = 2H2O :
Oxidant
Oxidation [Electron donor]
C6H12O6 + 6H2O = 6CO2 + 24H+ + 24eReductant
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Oxygen Consumption
Heterotrophic microbial respiration
C6H12O6 + 6O2 = 6CO2 + 6H2O
Chemolithotrophic oxidation of reduced
inorganic compounds
NH4+ + 2O2 = NO3- + H2O + 2H+
Chemical oxidation of reduced inorganic
compounds
4Fe2+ + 10H2O + O2 = 4Fe(OH)3 + 8H+
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Oxidation-Reduction
Carbon
Nitrogen
O2
O2
Floodwater
Floodwater
CO2
O2 + CH4
Aerobic soil
Anaerobic soil
OM
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NO3
O2 + NH4
Aerobic soil
Anaerobic soil
CH4
OM
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NH4
42
Oxidation-Reduction
Iron
Manganese
O2
O2
Floodwater
Floodwater
Fe3+
O2 + Fe2+
Aerobic soil
Anaerobic soil
FeOOH
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Mn4+
O2 + Mn2+
Aerobic soil
Anaerobic soil
Fe2+
MnO2
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Mn2+
43
Oxidation-Reduction
Carbon
Sulfur
O2
O2
Floodwater
Floodwater
CO2
O2 + OM
Aerobic soil
Anaerobic soil
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SO4
O2 + H2S
Aerobic soil
Anaerobic soil
SO4
H2S
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Oxygen Consumption
[C/Co]
Low organic matter soil
Consumption
during chemical
oxidation
Consumption
during biological
oxidation
High organic matter soil
Time (hours)
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Oxygen consumption,
mg/kg day
Aerobic Respiration
700
600
500
Talladega, AL
400
Salt marsh, LA
300
Houghton Lake
marsh,
MI
Belhaven, NC
200
y=-1036+200 ln(x)
R2=0.84
Lake Apopka marsh, FL
Prairie pothole, ND
Crowley, LA
100
0
Impacted
Everglades, FL
Unimpacted
Everglades, FL
0
500
1,000
1,500
2,000
2,500
3,000
3,500
Dissolved organic C, mg/kg
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Impacted
DEPTH (cm)
10
9
Unimpacted
12N
6P
8
7
6
12M
WATER
6A
WATER
FILAMENTOUS
ALGAE
5
4
3
MACRO-LITTER
AND ALGAE
2
1
0
PERIPHYTON
-1
-2
Peat
Unconsolidated Peat
0
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20
40
60
80
100
0
20
40
60
80
100
DISSOLVED OXYGEN
(% SATURATION)47
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Oxygen - Periphyton
% O2 Saturation
Depth (mm)
0
50
100
150
250
0
-1
0
19
36
68
2
4
192
306
6
8
10
200
593
Irradiance (μmol m-2 s-1)
S. Hagerty, SFWMD unpublished results
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Lake Apopka Marsh
Depth, cm
Soluble P, mg L-1 Dissolved Fe, mg L-1
1
0
2
3
0 2 4 6 8
30
Phosphorus
Iron
20
Water
10
0
-10
Soil
-20
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Depth below soil surface
Mobile and Immobile Iron
0
Fe2+
Aerobic
Insoluble Fe
2
4
Anaerobic
6
8
0
1
2
3
% Fe
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OXYGEN: Sources and Sinks
Water
Plants and
Algae
Air
Release by
Plant Roots
Soil Oxygen
Respiration
Oxidation of
Reductants
Chemical
oxidation
Chemolithotrophic
oxidation
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Electrochemical Properties &
Soil Oxygen Demand
Summary
Oxidation-reduction reactions regulate several elemental cycles
Wetland soils are limited by electron acceptors and have abundant
supply of electron donors.
Upland soils are usually limited by electron donors, and have
abundant supply of electron acceptors (primarily oxygen).
Nernst Equation is used to calculate redox potential (Eh)
Redox potentials (Eh) are inversely related to pH (59 mV/pH unit)
Redox potential of soils are affected by (i) activities of electron donors
(ii) activities of electron acceptors and (iii) temperature
Laboratory and field electrodes can be used to measure redox
potentials of soils
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Electrochemical Properties &
Soil Oxygen Demand
Summary
Distinct Eh gradients are present at (i) the soil-floodwater interface,
(ii) root-zone of wetland plants, and (iii) around soil aggregates in
drained portions of wetlands during low water-table depths.
Water column pH is affected by photosynthesis
Soil pH is affected by reduction of electron acceptors
The rate of oxygen consumption is related to the concentration of
reductants
Oxygen consumption at the soil-floodwater interface regulates
nutrient fluxes
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http://wetlands.ifas.ufl.edu
http://soils.ifas.ufl.edu
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