Institute of Food and Agricultural Sciences (IFAS) Biogeochemistry of Wetlands SiScience an dAd App litilications
Electrochemical Properties
Wetland Biogeochemistry Laboratory Soil and Water Science Department University of Florida
Instructor K. Ramesh Reddy [email protected]
6/22/20086/22/2008 WBL 1 1
Chemical Reactions in Natural Systems ¾ Reactions in which neither protons nor electrons are exchanged
¾ Fe2O3 + H2O = 2 FeOOH ¾ Reactions involving protons + - ¾ H2CO3 = H + HCO3 ¾ Reactions involving electrons ¾ Fe2+ =Fe= Fe3+ +e+ e- ¾ Reactions in which both protons and electrons are transferred + - 2+ ¾ 2Fe(OH)3 + 3H + e = 2Fe + 3H2O
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1 Institute of Food and Agricultural Sciences (IFAS)
Electrons and Protons
- - - + - - - - e e e - -H + e e e e H+ H+ - H+ e - + e - H + - + H+ e e H e + - H - e H - e- - e- - H+ H e e - e e e- e - - e- e + - - - e e - He H+ e - + e - e H+ - e H e + e- e - e- H+ H e e-
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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 g rad ie nts in w etla nd so il s Walther Nernst Specific conductance The Nobel Prize in Chemistry 1920 Soil oxygen demand http://www.corrosion-doctors.org/Biographies/Nernst.htm
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2 Electrochemical Properties
Learning Objectives
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 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 Reductant Oxidant + e- Reductant = Electron donor
+ 2+ 2+ 2- [Organic matter, NH4 , Fe , Mn , S , CH4, H2, H2O]
Oxidant + e- Reductant Oxidant = Electron acceptor
- 2- [O2, NO3 , MnO2, Fe(OH)3, SO4 , CO2, and some organic compounds]
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3 OxidationOxidation--Reduction [Aerobic Respiration] Oxidation + - C6H12O6 + 6H2O = 6CO2 + 24H + 24 e Reductant Oxidant Reduction + - 6O2 + 24H + 24 e = 12H2O Oxidant Reductant
C6H12O6 + 6O2 = 6CO2 + 6H2O Oxidation - Reduction
OxidationOxidation--Reduction [Nitrate Respiration – Dentrification] Oxidation + - 5C6H12O6 + 30H2O = 30CO2 + 120H + 120 e Reductant Oxidant Reduction - + - 24NO3 + 144H + 120e = 12N2 + 72H2O Oxidant Reductant
- + 5C6H12O6 + 24NO3 + 24H = 12N2 + 30CO2 + 42H2O Oxidation - Reduction
4 OxidationOxidation--Reduction [Sulfate Respiration] Oxidation + - C6H12O6 + 6H2O = 6CO2 + 24H + 24e Reductant Oxidant Reduction 22-- + - 22-- 3SO4 + 24H + 24e = 3S + 12H2O Oxidant Reductant
22-- 22-- C6H12O6 + 3SO4 = 3S + 6CO2 + 6H2O
Oxidation - Reduction
OxidationOxidation--Reduction
UPLAND SOILS FLOODED SOILS
O2 H2O - + NO3 N2 NH4 Mn4+ Mn2+ Reduction Fe3+ Fe2+
2- 2- SO4 S
CO2 Oxidation CH4 3- PO4 PH3
H2O H2
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5 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 Oxidized Reduced Reduced
-300 -100 0100 300 500 700 Oxidation-Reduction Potential (mV)
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6 OxidationOxidation--ReductionReduction ure
Strongly Strongly
Electron Press reduced oxidized
-300 -100 0100 300 500 700 Oxidation-Reduction Potential (mV)
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Electron donors + 2+ 2+ 2- [Organic matter, NH4 , Fe , Mn , S , CH4, H2, H2O] - - - - - e e + - - + - - e e e - -H + e e - H - e e H+ H+ - H+ e - + e - H + e - e - + H+ e e H e + H e e H - e- - e- - H+ H - - e e e- e e- e e- e O2 - NO3 Electron acceptors Mn4+ Fe3+ 2- SO4 Energy CO2 H2 O
600 300 200100 0 -100 -300 -400 OxidationOxidation--ReductionReduction Potential (mV) 6/22/2008 WBL 14
7 How much energy is released during oxidation -reduction reactions?
[+]
O2 NN--OxidesOxides Mn (IV) ergy Yield trode Potentials Fe (III) n c 22-- E SO4 [-] Ele CO2 Ease of Reduction
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OxidationOxidation--Reduction
Mn4+ Mn2+ CO CH SeO 2- Se(0); Se2- 2- 2- 2 4 3 SeO4 SeO3 2- 2- 3+ 2+ - SO4 S Fe Fe NO3 N2 O2 H2O
-200 -100 0 +100 +200 +300 +400 Redox Potential, mV (at pH 7)
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8 Iron Redox Couple and EhEh--pHpH
1200 Fe3+ l (mV) 800
O2 Fe2O3 400 H O Fe2+ 2
FeCO 0 H2O 3
dox Potentia H2
e FeS2 R -400 Fe2+ FeCO3 Fe3O4 0 2 4 6 8 10 12 14 pH
OxidationOxidation--ReductionReduction
Wetlands and Uplands Aquatic Systems ¾Electron ¾Electron acceptor non- acceptor limiting limiting ¾Electron donor ¾Electron donor non-limiting limiting
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9 Sequential Reduction of Electron Acceptors Organic Substrate [e- donor] ation r 2+ S2- 2- Fe SO4
- CH NO3 4 Mn2+ lative Concent
e O2 R
Oxygen Nitrate Iron Methanogenesis
Manganese Sulfate Time or Soil Depth 6/22/2008 WBL 19
Redox Zones with Depth
WATER OxygenOxygen ReductionReduction ZoneZone SOIL Oxygen Reduction Zone Aerobic I Eh = > 300 mV Nitrate Reduction Zone Facultative II Mn4+ Reduction Zone Eh = 100 to 300 mV Fe3+ Reduction Zone III
epth Eh = -100 to 100 mV D Sulfate Reduction Zone Anaerobic IV Eh = -200 to -100 mV
V Methanogenesis Eh = < -200 mV 6/22/2008 WBL 20
10 Regulators of Eh
¾Water-table fluctuations . ¾Activities of electron acceptors. ¾Activities of electron donors. ¾Temperature ¾pH
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Field Redox Electrodes
Copper wire VoltVo l t meter
Heat shrinking tube
Calomel Reference
Water Eppyoxy Soil Platinum wire Platinum electrodes
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11 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 Mercury Platinum wire Calomel Reference Electrode 6/22/2008 WBL 23
Okeechobee Basin Wetland Soils and Stream Sediments
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12 Flooded Organic Soils: Everglades Agricultural Area
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Flooded Paddy Soils: Louisiana
Aerobic V m
Anaerobic edox Potential, R
Time, days
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13 Electron Acceptors - Redox Potential
700 600 Oxygen 500 400 300 Nitrate 200
Eh (mV) 100 Sulfate 0 -100 -200 Bicarbonate -300 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Time (wk)
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Electron Donor [Organic Matter] – Redox Potential
V) 300 m 200 Low Organic Matter Soil 100 0
-100 High Organic Matter Soil edox potential ( edox potential
R -200
Time after flooding
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14 Redox Gradients in Sediments
0 -20 Aerobic Layer -40 Anaerobic Layer -60 -80 -100
epth (mm) -120 D -140 -160 0 100 200 300 400 500 600 700 Redox Potential (mV) 6/22/2008 WBL 29
Sediment Microbial Fuel Cell
Source: D. R. Lovley, 2006. Nature Reviews 4:497-508
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15 Redox Potential and pH
1000 800 600 400 200 Eh mv] 0 -200 -400 -600 0 2 4 6 8 10 12
Baas Becking et al. 6/22/2008 WBLpH 31
Limitations of Redox Potentials ¾Most of the redox couples are not in equilibrium except in highly reduced soils. ¾In biological systems, electrons are added and removed continuously. ¾Platinum electrodes respond favorably to reversible redox couples. ¾Redox potential is closely related to pH. ¾Platinum electrode surface can be contaminated by coatings of oxides, sulfides and other impurities.
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16 Soil and Water Column -pH- pH
¾ Reactions involving protons
¾ CO2 + H2O = H2CO3 + - ¾ H2CO3 = H + HCO3 - + 2- ¾ HCO3 = H + CO3
¾ Reactions in which both protons and electrons are transferred + - 2+ ¾ 2Fe(OH)3 + 3H + e = 2Fe + 3H2O
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Water Column pH: Experimental Ponds – Lake Apopka Basin
10
Algae 9
pH 8 Cattails and Egeria
7 Water hyacinth 6 8 12 16 20 24 4 8 Time, hundred hours 6/22/2008 WBL 34
17 Effect of Flooding on Soil pH
8 Clay loam [ pH = 8.7; OM = 2.2%; Fe = 0.63%] 7 6 Clay [ pH = 3.4; OM = 6.6%; Fe = 2.8%] pH 5
4 Clay [ pH = 3.8; OM = 7.2%; Fe = 0.1%] 3
0 2 4 6 8 10 12 14 Time after flooding
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Effect of Flooding on Soil Porewater Ionic Strength
Fe2+ Fe2+ A
Fe2+ Ca2+ Soil 2+ Fe + NH4 Mn2+ K+
Solid Phase Soil Solution th
g B Ionic Stren
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18 Redox Couples in Wetlands
C6H12O6/CO2 and O2/H2O - C6H12O6/CO2 and NO3 /N2 2+ C6H12O6/CO2 and MnO2 /Mn 2+ C6H12O6/CO2 and FeOOH/Fe 2- C6H12O6/CO2 and SO4 /H2S + H2/H and CO2 /CH4
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Aerobic Respiration and Energy Yield
C6H12O6 + 6O2 = 6CO2 + 6H2O
Gr = -686.4 kcals/mole
ADP + Pii = ATP
Gr =- 7.7 kcals/mole
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19 Institute of Food and Agricultural Sciences (IFAS) Biogeochemistry of Wetlands SiScience an dAd App litilications
Soil Oxygen Demand
Wetland Biogeochemistry Laboratory Soil and Water Science Department University of Florida
Instructor K. Ramesh Reddy [email protected]
6/22/20086/22/2008 WBL 39 39
Oxygen
Oxygen is an electron acceptor Reduction [Electron acceptor] + - O2 + 4H + 4e = 2H2O : Oxidant Oxidation [Electron donor] + - C6H12O6 + 6H2O = 6CO2 + 24H + 24e Reductant
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20 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 2+ + 4Fe + 10H2O + O2 = 4Fe(OH)3 + 8H
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OxidationOxidation--ReductionReduction Carbon Nitrogen
O2 O2
Floodwater Floodwater
CO2 O2 + CH4 NO3 O2 + NH4 Aerobic soil Aerobic soil Anaerobic soil Anaerobic soil
OM CH4 OM NH4
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21 OxidationOxidation--ReductionReduction Iron Manganese
O2 O2
Floodwater Floodwater
3+ 2+ 4+ 2+ Fe O2 + Fe Mn O2 + Mn Aerobic soil Aerobic soil Anaerobic soil Anaerobic soil 2+ 2+ FeOOH Fe MnO2 Mn
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OxidationOxidation--ReductionReduction Carbon Sulfur
O2 O2
Floodwater Floodwater
CO2 O2 + OM SO4 O2 + H2S Aerobic soil Aerobic soil Anaerobic soil Anaerobic soil
SO4 H2S
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22 Oxygen Consumption
Low organic matter soil ]
o Consumption during chemical Consumption oxidation during biological [C/C oxidation
High organic matter soil Time (hours)
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Aerobic Respiration
700 Impacted Unimpacted ion, Everglades, FL t 600 Everglades, FL 500 Talladega, AL Houghton Lake 400 marsh, MI 300 Salt marsh, LA Belhaven, NC 200 mg/kg day mg/kg Lake Apopka marsh, FL y=-1036+200 ln(x) 2 ygen consump 100 Prairie pp,othole, ND R =0.84
x Crowley, LA
O 0 0 500 1,000 1,500 2,000 2,500 3,000 3,500 Dissolved organic C, mg/kg
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23 Impacted Unimpacted 10 12N 9 6P 8 12M WATER 7 6A
) 6 m FILAMENTOUS WATER 5 ALGAE 4 3 MACRO-LITTER
DEPTH (c 2 AND ALGAE 1 0 PERIPHYTON -1 Unconsolidated Peat Peat -2
0 20 40 60 80 100 0 20 40 60 80 100 DISSOLVED OXYGEN (% SATURATION) 6/22/2008 WBL 47
Oxygen - Periphyton
% O2 Saturation 0 50 100 150 200 250 -1 0 0 19 2 36 4 68 6 192
Depth (mm) 306 8 593 10 Irradiance (μmol m-2 s-1) S. Hagerty, SFWMD unpublished results
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24 Lake Apopka Marsh
Soluble P, mg L-1 Dissolved Fe, mg L-1 0 2 4 6 8 0 1 2 3
30 Phosphorus Iron 20 10 Water
pth, cm 0 e
D -10 Soil -20
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Mobile and Immobile Iron
0 Insoluble Fe 2+ Aerobic
rface Fe 2
4
Anaerobic 6 th below soil su th below p
De 8 0 123 % Fe
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25 OXYGEN: Sources and Sinks
Water Air Plants and Algae Release by Plant Roots Soil Oxygen
Oxidation of Chemical Reductants Respiration 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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26 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
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