1
Biogeochemistry of WetlandsS i d A li tiS i d A li ti
Institute of Food and Agricultural Sciences (IFAS)
Science and ApplicationsScience and Applications
Wetland Biogeochemistry LaboratorySoil and Water Science Department
Electrochemical Properties Electrochemical Properties
6/22/2008 16/22/2008 WBL 1
InstructorK. Ramesh [email protected]
Soil and Water Science DepartmentUniversity of Florida
Chemical Reactions in Natural Systems
Reactions in which neither protons nor electronsReactions in which neither protons nor electrons are exchanged
Fe2O3 + H2O = 2 FeOOHReactions involving protons
H2CO3 = H+ + HCO3-
Reactions involving electronsFe2+ = Fe3+ + e-
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Fe2+ = Fe3+ + e-
Reactions in which both protons and electrons are transferred
2Fe(OH)3 + 3H+ + e- = 2Fe2+ + 3H2O
2
Institute of Food and Agricultural Sciences (IFAS)
Electrons and Protons Electrons and Protons
e-
e-e-e-e-
e-e-e- e- H+
e-e- e-e-e-
e-e-e- e-e-
e-e- e-
e-e- e- H+H+H+
H+H+ H+
H+H+H+H+
H+
e- e-HH+e-
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H e e-e- ee- e-e-e- e- H+
H+H+ee-
H+H+
Topic OutlineIntroduction
Electrochemical Properties Electrochemical Properties
IntroductionOxidation-reduction reactionsNernst EquationEh - pH relationshipsBuffering of redox potentialMeasurement of redox potentials Soil and water column pHRedox couples in wetland soilsRedox gradients in wetland soils Walther Nernst
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edo g ad e ts et a d so sSpecific conductanceSoil oxygen demand
Walther NernstThe Nobel Prize in Chemistry 1920 http://www.corrosion-doctors.org/Biographies/Nernst.htm
3
Learning Objectives
Electrochemical Properties Electrochemical Properties
Basic concepts related to oxidation-reduction 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
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Source: D. R. Lovley, 2006. Nature Reviews 4:497-508
Diel changes in water column pHRedox couples and microbial metabolic
activities in wetlandsRedox gradients and aerobic/anaerobic
interfaces in wetlandsSoil oxygen demand and nutrient fluxes
Oxidation-ReductionReductant Oxidant + e-
Reductant = Electron donor
Oxidant + e- Reductant
Oxidant = Electron acceptor
[Organic matter, NH4+, Fe2+, Mn2+, S2-, CH4, H2, H2O]
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Oxidant = Electron acceptor
[O2, NO3-, MnO2, Fe(OH)3, SO4
2-, CO2, and some organic compounds]
4
Oxidation
OxidationOxidation--ReductionReduction[Aerobic Respiration]
CC66HH1212OO66 + 6H+ 6H22O = 6COO = 6CO22 + 24H+ 24H++ + 24 e+ 24 e--
6O6O22 + 24H+ 24H++ + 24 e+ 24 e-- = 12H= 12H22OOReduction
Reductant
Oxidant
Oxidant
Reductant
CC66HH1212OO66 + 6O+ 6O22 = 6CO= 6CO22 + 6H+ 6H22OO
Oxidation - Reduction
OxidationOxidation--ReductionReduction
Oxidation
[Nitrate Respiration – Dentrification]
5C5C66HH1212OO66 + 30H+ 30H22O = 30COO = 30CO22 + 120H+ 120H++ + 120 e+ 120 e--
24NO24NO33-- + 144H+ 144H++ + 120e+ 120e-- = 12N= 12N22 + 72H+ 72H22OO
Reduction
Reductant
Oxidant
Oxidant
Reductant
5C5C66HH1212OO66 + 24NO+ 24NO33-- + 24H+ 24H++ = 12N= 12N2 2 + 30CO+ 30CO22 + 42H+ 42H22OO
Oxidation - Reduction
5
Oxidation
OxidationOxidation--ReductionReduction[Sulfate Respiration]
CC66HH1212OO66 + 6H+ 6H22O = 6COO = 6CO22 + 24H+ 24H++ + 24e+ 24e--
3SO3SO4422-- + 24H+ 24H++ + 24e+ 24e-- = 3S= 3S22-- + 12H+ 12H22OO
Reduction
Reductant
Oxidant
Oxidant
Reductant
CC66HH1212OO66 + 3SO+ 3SO4422-- = 3S= 3S22-- + 6CO+ 6CO22 + 6H+ 6H22OO
Oxidation - Reduction
UPLAND SOILSUPLAND SOILS FLOODED SOILSFLOODED SOILS
OxidationOxidation--ReductionReduction
Reduction
N2 NH4+
Mn2+
Fe2+
S2-
CH
H2O
NO3-
Mn4+
Fe3+
SO42-
CO
O2
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OxidationOxidation CH4
PH3
H2
CO2
PO43-
H2O
6
Nernst EquationNernst Equationm (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
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96.50 kJ/volt moleR = Gas constant (0.001987 kcal/mole degree or
0.008314 kJ/mole degreeT = Temperature (298.15 K (273.15 + 25 oC))n = number of electrons involved in the reaction
Wetland Soil
Drained Soil
HighlyReduced
Reduced ModeratelyReduced
Oxidized
Anaerobic Aerobic
Drained Soil
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-300 7000 300 500100-100Oxidation-Reduction Potential (mV)
7
ure
OxidationOxidation--ReductionReduction
Elec
tron
Pre
ss
Stronglyreduced
Stronglyoxidized
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-300 7000 300 500100-100Oxidation-Reduction Potential (mV)
Electron donorsElectron donors[Organic matter, NH4
+, Fe2+, Mn2+, S2-, CH4, H2, H2O]e-
e-e-e-e- e-e-
e-e-e- e- H+
e-e- e-e- e-e-e-e- e-e-
e-e- e-
e-e- e- H+H+H+
H+
H+ H+H+
H+H+H+H+
e- ee
NO3-
Mn4+Fe3+
SO42-
CO2
O2
e
Ene
rgy
Electron acceptorsElectron acceptors
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CO2H2 O
600 -300200 0 -100100300OxidationOxidation--Reduction Potential (mV)Reduction Potential (mV)
-400
8
How much energy is released How much energy is released during oxidation during oxidation -- reduction reactions?reduction reactions?
ctro
de P
oten
tials
Fe (III)Fe (III)Mn (IV)Mn (IV)
NN--OxidesOxides
OO22
nerg
y Yi
eld
[+]
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Elec
Ease of Reduction
COCO22SOSO44
22--Fe (III)Fe (III) En
[-]
CO2 CH4 SeO32- Se(0); Se2-
Mn4+ Mn2+
SeO42- SeO3
2-
OxidationOxidation--ReductionReduction
2 4
SO42- S2-
3
Fe3+ Fe2+SeO4 SeO3NO3
- N2 O2 H2O
6/22/2008 WBL 16
-200 -100 0 +100 +200 +300 +400Redox Potential, mV (at pH 7)
9
1200
l (m
V)Fe3+
Iron Redox Couple and EhIron Redox Couple and Eh--pH pH
800
400
0
edox
Pot
entia
Fe2+
Fe2O3
FeS
FeCO3
O2
H2O
H2OH2
0 2 4 6 8 10 12 14
-400
pH
Re
Fe2+
Fe3O4
FeS2
FeCO3
OxidationOxidation--ReductionReductionWetlands and
Electron acceptor non-limiting
Electron acceptor limitingElectron donor
UplandsWetlands andAquatic Systems
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limitingElectron donor limiting
Electron donor non-limiting
10
Sequential Reduction of Sequential Reduction of Electron Acceptors Electron Acceptors
Organic Substrate[e- donor]
ratio
n
O2
NO3-
SO42-
Mn2+
Fe2+ S2-
CH4
elat
ive
Con
cent
r
6/22/2008 WBL 19
O2
Oxygen Nitrate Iron Methanogenesis
Sulfate Manganese
Time or Soil Depth
Re
Redox Zones with DepthOxygen Reduction ZoneOxygen Reduction Zone
WATERWATER
SOILSOIL AerobicAerobicOxygen Reduction ZoneOxygen Reduction Zone
Eh = > 300 mVNitrate Reduction Zone
Mn4+ Reduction ZoneEh = 100 to 300 mVFe3+ Reduction ZoneEh = -100 to 100 mV
SOILSOILI
II
III
AerobicAerobic
FacultativeFacultative
Dep
thD
epth
Sulfate Reduction ZoneEh = -200 to -100 mV
MethanogenesisEh = < -200 mV
IV
V
AnaerobicAnaerobic
DD
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11
Regulators of EhRegulators of Eh
Water table fluctuationsWater-table fluctuations.Activities of electron acceptors.Activities of electron donors.TemperaturepH
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pH
Field Field RedoxRedox ElectrodesElectrodes
Copper wire
Volt meter
Water
il
Heat shrinkingtube
Epoxy
CalomelReference
Volt meter
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Soilp y
Platinumwire Platinum
electrodes
12
Copper wire
Saturated KCl
Laboratory Laboratory RedoxRedox ElectrodesElectrodes
Platinum Glass Electrode
Heat shrinkingtube
Glass tubeGlass tube
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Mercury
Epoxy
Platinumwire
Platinumwire
Mercury
Calomel +Mercury
Salt bridge
Calomel Reference Electrode
Okeechobee Basin Wetland Soils Okeechobee Basin Wetland Soils and Stream Sedimentsand Stream Sediments
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13
Flooded Organic Soils: Flooded Organic Soils: Everglades Agricultural AreaEverglades Agricultural Area
6/22/2008 WBL 25
Flooded Paddy Soils: LouisianaFlooded Paddy Soils: Louisiana
mV Aerobic
Red
ox P
oten
tial,
m
Anaerobic
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Time, days
R
14
600700
O
Electron Acceptors Electron Acceptors -- Redox PotentialRedox PotentialE
h (m
V)
0100200300400500600 Oxygen
Nitrate
Sulfate
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-300-200-100
Time (wk)1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Bicarbonate
mV
) 300
Electron Donor [Organic Matter] Electron Donor [Organic Matter] ––Redox PotentialRedox Potential
Low Organic Matter Soil
High Organic Matter Soil
Red
ox p
oten
tial (
m
200
100
0-100200
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Time after flooding
R -200
15
0
-20 Aerobic Layer
RedoxRedox Gradients in SedimentsGradients in Sediments
-120
-100
-80
-60
-40
Dep
th (m
m)
y
Anaerobic Layer
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0-160
-140
120D
100 200 300 400 500 600 700
Redox Potential (mV)
Sediment Microbial Fuel CellSediment Microbial Fuel Cell
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Source: D. R. Lovley, 2006. Nature Reviews 4:497-508
16
1000
800
RedoxRedox Potential and pHPotential and pH
600
400200
0
-200
Eh m
v]
6/22/2008 WBL 31
0 2 4 6 8 10 12
-200
-400
-600
pH Baas Becking et al.
Limitations of Limitations of RedoxRedox PotentialsPotentials
Most of the redox couples are not in equilibrium except in highly reduced soilsequilibrium 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.
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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 Soil and Water Column -- pHpH
Reactions involving protonsReactions involving protonsCO2 + H2O = H2CO3H2CO3 = H+ + HCO3
-
HCO3- = H+ + CO3
2-
Reactions in which both protons and electrons are transferred
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are transferred2Fe(OH)3 + 3H+ + e- = 2Fe2+ + 3H2O
Water Column pH: Experimental Ponds Water Column pH: Experimental Ponds –– Lake Apopka BasinLake Apopka Basin
10
9
8
pH
Algae
Cattails and Egeria
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7
68 12 16 20 24 4 8
Time, hundred hours
Water hyacinth
18
8 Clay loam [ pH = 8.7; OM = 2.2%; Fe = 0.63%]
Effect of Flooding on Soil pHEffect of Flooding on Soil pH
7
6
5
4
3
pH Clay [ pH = 3.4; OM = 6.6%; Fe = 2.8%]
Clay [ pH = 3.8; OM = 7.2%; Fe = 0.1%]
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3
0 2 4 6 8 10 12 14Time after flooding
Fe2+
2
Fe2+ A
Effect of Flooding on Effect of Flooding on Soil Soil PorewaterPorewater Ionic StrengthIonic Strength
Fe2+
Fe2+
Mn2+
Ca2+
NH4+
K+
Soil
Solid Phase Soil Solution
gth B
6/22/2008 WBL 36
Ioni
c St
reng
Time after flooding
B
19
RedoxRedox Couples in WetlandsCouples in Wetlands
C6H12O6/CO2 and NO3-/N2
C6H12O6/CO2 and FeOOH/Fe2+
C6H12O6/CO2 and MnO2 /Mn2+
C6H12O6/CO2 and SO42- /H2S
C6H12O6/CO2 and O2/H2O
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H2/H+ and CO2 /CH4
Aerobic Respiration and Energy Yield Aerobic Respiration and Energy Yield
CC66HH1212OO66 + 6O+ 6O22 = 6CO= 6CO22 + 6H+ 6H22OO
Gr = -686.4 kcals/mole
ADP + PADP + Pii = ATP= ATP
Gr = -7.7 kcals/mole
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Gr 7.7 kcals/mole
20
Biogeochemistry of WetlandsS i d A li tiS i d A li ti
Institute of Food and Agricultural Sciences (IFAS)
Science and ApplicationsScience and Applications
Wetland Biogeochemistry LaboratorySoil and Water Science Department
Soil Oxygen Demand Soil Oxygen Demand
6/22/2008 396/22/2008 WBL 39
InstructorK. Ramesh [email protected]
Soil and Water Science DepartmentUniversity of Florida
OxygenOxygenOxygen is an electron acceptorOxygen is an electron acceptorReduction [Electron acceptor]
O2 + 4H+ + 4e- = 2H2O :
Oxidation [Electron donor]Oxidant
Oxidation [Electron donor]C6H12O6 + 6H2O = 6CO2 + 24H+ + 24e-
Reductant
6/22/2008 40WBL
21
Oxygen ConsumptionOxygen ConsumptionHeterotrophic microbial respiration
C6H12O6 + 6O2 = 6CO2 + 6H2OChemolithotrophic oxidation of reduced inorganic compounds
NH4+ + 2O2 = NO3
- + H2O + 2H+
Chemical oxidation of reduced inorganic compounds
4Fe2+ + 10H2O + O2 = 4Fe(OH)3 + 8H+
6/22/2008 41WBL
CarbonCarbon NitrogenNitrogen
OxidationOxidation--ReductionReduction
NO3 O2 + NH4
Floodwater
Aerobic soilCO2 O2 + CH4
Floodwater
Aerobic soil
O2 O2
OM NH4
Aerobic soilAnaerobic soil
OM CH4
Aerobic soilAnaerobic soil
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22
IronIron ManganeseManganese
OxidationOxidation--ReductionReduction
Fe3+ O2 + Fe2+ Mn4+ O2 + Mn2+
Floodwater
Aerobic soil
Floodwater
Aerobic soil
O2 O2
FeOOH Fe2+ MnO2 Mn2+
Aerobic soilAnaerobic soil
Aerobic soilAnaerobic soil
6/22/2008 43WBL
CarbonCarbon SulfurSulfur
OxidationOxidation--ReductionReduction
CO2 O2 + OM SO4 O2 + H2S
Floodwater Floodwater
O2 O2
CO2 O2 OM SO4 O2 H2S
H2S
Aerobic soilAnaerobic soil
Aerobic soil
Anaerobic soilSO4
6/22/2008 44WBL
23
Low organic matter soil
Oxygen ConsumptionOxygen Consumption
Low organic matter soil[C
/Co] Consumption
during chemicaloxidation
Consumption during biological
oxidation
High organic matter soil
Time (hours)
6/22/2008 45WBL
600700
tion,
ImpactedEverglades, FLEverglades FL
Unimpacted
Aerobic RespirationAerobic Respiration
100
200
300
400
500
600
xyge
n co
nsum
ptm
g/kg
day
y=-1036+200 ln(x)R2=0.84
g
Houghton Lakemarsh, MI
Everglades, FL
Salt marsh, LA
Lake Apopka marsh, FLPrairie pothole, ND
l
Talladega, AL
Belhaven, NC
0 500 1,000 1,500 2,000 2,500 3,0000
Dissolved organic C, mg/kg
Ox p ,
Crowley, LA3,500
6/22/2008 46WBL
24
6789
10
m)
12N
6P
12M
6AWATER
Impacted Unimpacted
0123456
DE
PTH
(cm FILAMENTOUS
ALGAE
PERIPHYTON
WATER
MACRO-LITTERAND ALGAE
-2-10
DISSOLVED OXYGEN (% SATURATION)
PeatUnconsolidated Peat
0 20 40 60 80 100 0 20 40 60 80 100
6/22/2008 47WBL
0 50 100 150 200 250
% O2 Saturation
Oxygen Oxygen -- PeriphytonPeriphyton
-102
4
6
0 50 100 150 200 250
Dep
th (m
m)
0193668192306
8
10
306593
Irradiance (μmol m-2 s-1)S. Hagerty, SFWMD unpublished results
6/22/2008 48WBL
25
Soluble P, mg L-1
2 4 6 8 1 2Dissolved Fe, mg L-1
0 0 3
Lake Apopka Marsh
Water
epth
, cm
0102030
2 4 6 8 1 20 0 3
Phosphorus Iron
De 0
-20-10 Soil
6/22/2008 49WBL
0Fe2+ Insoluble Fe
rfac
e
Aerobic
Mobile and Immobile IronMobile and Immobile Iron
2
4
6
pth
belo
w s
oil s
u
Anaerobic
80 1 2 3
% Fe
Dep
6/22/2008 50WBL
26
Plants and Al
Water Air
OXYGEN: Sources and SinksOXYGEN: Sources and Sinks
Soil OxygenSoil Oxygen
Algae Release byPlant Roots
Oxidation ofOxidation of Reductants
Respiration
Chemolithotrophicoxidation
Chemicaloxidation
6/22/2008 51WBL
Summary
Electrochemical Properties &Electrochemical Properties &Soil Oxygen Demand Soil Oxygen Demand
SummaryOxidation-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)
6/22/2008 WBL 52
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) temperatureLaboratory and field electrodes can be used to measure redox
potentials of soils
27
Summary
Electrochemical Properties &Electrochemical Properties &Soil Oxygen Demand Soil Oxygen Demand
SummaryDistinct 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 photosynthesisSoil pH is affected by reduction of electron acceptorsThe rate of oxygen consumption is related to the concentration of
reductants
6/22/2008 WBL 53
reductantsOxygen consumption at the soil-floodwater interface regulates
nutrient fluxes
6/22/2008 WBL 54
http://wetlands.ifas.ufl.eduhttp://wetlands.ifas.ufl.eduhttp://soils.ifas.ufl.eduhttp://soils.ifas.ufl.edu