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11/17/2014

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11/17/2014

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  1. 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 krr@ufl.edu 11/17/2014 1 WBL

  2. 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 11/17/2014 WBL 2

  3. Institute of Food and Agricultural Sciences (IFAS) Electrons and Protons e- e- e- H+ e- e- e- e- e- e- H+ H+ H+ e- H+ e- e- H+ H+ e- e- e- H+ H+ H+ e- e- e- H+ e- e- e- e- e- e- e- e- e- H+ e- H+ e- e- e- H+ H+ e- H+ e- e- e- H+ e- WBL

  4. 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 Walther Nernst The Nobel Prize in Chemistry 1920 http://www.corrosion-doctors.org/Biographies/Nernst.htm WBL

  5. 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 • Soil oxygen demand and nutrient fluxes Source: D. R. Lovley, 2006. Nature Reviews 4:497-508 WBL

  6. Reductant Oxidant + e- Reductant = Electron donor Oxidation-Reduction [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] WBL

  7. Oxidation C6H12O6 + 6H2O = 6CO2 + 24H+ + 24 e- Oxidant Reductant Reduction 6O2 + 24H+ + 24 e- = 12H2O Oxidant Reductant C6H12O6 + 6O2 = 6CO2 + 6H2O Oxidation - Reduction Oxidation-Reduction [Aerobic Respiration]

  8. Oxidation 5C6H12O6 + 30H2O = 30CO2 + 120H+ + 120 e- Oxidant Reductant Reduction 24NO3- + 144H+ + 120e- = 12N2 + 72H2O Oxidant Reductant 5C6H12O6 + 24NO3- + 24H+ = 12N2 + 30CO2 + 42H2O Oxidation - Reduction Oxidation-Reduction [Nitrate Respiration – Dentrification]

  9. Oxidation C6H12O6 + 6H2O = 6CO2 + 24H+ + 24e- Oxidant Reductant Reduction 3SO42- + 24H+ + 24e- = 3S2- + 12H2O Oxidant Reductant C6H12O6 + 3SO42- = 3S2- + 6CO2 + 6H2O Oxidation - Reduction Oxidation-Reduction [Sulfate Respiration]

  10. UPLAND SOILS FLOODED SOILS H2O O2 N NH4+ NO3- 2 Mn2+ Mn4+ Reduction Fe2+ Fe3+ Reduced Oxidized S2- SO42- CH4 CO2 Oxidation PH3 PO43- H2 H2O Oxidation-Reduction 11/17/2014 WBL 10

  11. 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 11/17/2014 WBL 11

  12. Wetland Soil Drained Soil Anaerobic Aerobic Moderately Reduced Highly Reduced Reduced Oxidized -100 0 100 300 500 700 -300 Oxidation-Reduction Potential (mV) 11/17/2014 WBL 12

  13. Oxidation-Reduction Electron Pressure Strongly reduced Strongly oxidized -100 0 100 300 500 700 -300 Oxidation-Reduction Potential (mV) WBL

  14. e- O2 Electron acceptors NO3- Mn4+ Fe3+ Energy SO42- CO2 H2 O -400 300 200 100 0 -100 -300 600 Oxidation-Reduction Potential (mV) Electron donors [Organic matter, NH4+, Fe2+, Mn2+, S2-, CH4, H2, H2O] e- e- e- e- e- H+ e- e- e- e- e- H+ e- e- e- H+ H+ H+ e- H+ e- e- e- H+ H+ e- H+ H+ H+ e- e- e- H+ e- e- e- e- e- e- WBL

  15. [+] O2 N-Oxides Electrode Potentials Energy Yield Mn (IV) Fe (III) SO42- CO2 [-] Ease of Reduction How much energy is released during oxidation - reduction reactions? WBL

  16. Oxidation-Reduction Mn4+ Mn2+ CO2 CH4 SeO32- Se(0); Se2- SeO42- SeO32- SO42- S2- Fe3+ Fe2+ NO3- N2 O2 H2O -200 -100 0 +100 +400 +200 +300 Redox Potential, mV (at pH 7) 11/17/2014 WBL 16

  17. Iron Redox Couple and Eh-pH 1200 Fe3+ 800 O2 Redox Potential (mV) Fe2O3 400 H2O Fe2+ FeCO3 0 H2O H2 FeS2 -400 Fe2+ FeCO3 Fe3O4 0 2 4 6 8 10 12 14 pH

  18. Electron acceptor non-limiting Electron donor limiting Electron acceptor limiting Electron donor non-limiting Oxidation-Reduction Wetlands and Aquatic Systems Uplands WBL

  19. Sequential Reduction of Electron Acceptors Organic Substrate [e- donor] S2- Fe2+ SO42- Relative Concentration CH4 NO3- Mn2+ O2 Iron Oxygen Nitrate Methanogenesis Manganese Sulfate Time or Soil Depth 11/17/2014 WBL 19

  20. Redox Zones with Depth WATER Oxygen Reduction Zone Oxygen Reduction Zone Aerobic Oxygen Reduction Zone SOIL Eh = > 300 mV I Nitrate Reduction Zone Facultative Mn4+ Reduction Zone II Eh = 100 to 300 mV Fe3+ Reduction Zone III Depth Eh = -100 to 100 mV Sulfate Reduction Zone Anaerobic IV Eh = -200 to -100 mV Methanogenesis V Eh = < -200 mV 11/17/2014 WBL 20

  21. Regulators of Eh • Water-table fluctuations. • Activities of electron acceptors. • Activities of electron donors. • Temperature • pH WBL

  22. Field Redox Electrodes Copper wire Volt meter Heat shrinking tube Calomel Reference Water Epoxy Soil Platinum wire Platinum electrodes WBL

  23. Laboratory Redox Electrodes Platinum Glass Electrode Copper wire Saturated KCl Heat shrinking tube Glass tube Glass tube Calomel + Mercury Mercury Platinum wire Salt bridge Epoxy Mercury Platinum wire Calomel Reference Electrode WBL

  24. Okeechobee Basin Wetland Soils and Stream Sediments WBL

  25. Flooded Organic Soils: Everglades Agricultural Area WBL

  26. Flooded Paddy Soils: Louisiana Aerobic Anaerobic Redox Potential, mV Time, days WBL

  27. 700 600 500 400 300 200 100 0 -100 -200 -300 Electron Acceptors - Redox Potential Oxygen Eh (mV) Nitrate Sulfate Bicarbonate 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Time (wk) WBL

  28. Electron Donor [Organic Matter] – Redox Potential 300 200 Low Organic Matter Soil 100 Redox potential (mV) 0 -100 High Organic Matter Soil -200 Time after flooding WBL

  29. Redox Gradients in Sediments 0 -20 Aerobic Layer -40 Anaerobic Layer -60 -80 Depth (mm) -100 -120 -140 -160 0 100 200 300 400 500 600 700 Redox Potential (mV) WBL

  30. Sediment Microbial Fuel Cell Source: D. R. Lovley, 2006. Nature Reviews 4:497-508 WBL

  31. Redox Potential and pH 1000 800 600 400 Eh mv] 200 0 -200 -400 -600 0 2 4 6 8 10 12 pH Baas Becking et al. 11/17/2014 WBL 31

  32. 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. WBL

  33. 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 WBL

  34. 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 WBL

  35. Effect of Flooding on Soil pH Clay loam [ pH = 8.7; OM = 2.2%; Fe = 0.63%] 8 7 6 pH Clay [ pH = 3.4; OM = 6.6%; Fe = 2.8%] 5 4 Clay [ pH = 3.8; OM = 7.2%; Fe = 0.1%] 3 0 2 4 6 8 10 12 14 Time after flooding WBL

  36. Fe2+ Fe2+ Fe2+ Mn2+ Effect of Flooding on Soil Porewater Ionic Strength A Fe2+ Ca2+ Soil NH4+ K+ Soil Solution Solid Phase B Ionic Strength WBL Time after flooding

  37. C6H12O6/CO2 and O2/H2O • C6H12O6/CO2 and NO3-/N2 • C6H12O6/CO2 and MnO2 /Mn2+ • C6H12O6/CO2 and FeOOH/Fe2+ • C6H12O6/CO2 and SO42- /H2S • H2/H+ and CO2 /CH4 Redox Couples in Wetlands WBL

  38. Gr = -7.7 kcals/mole Gr = -686.4 kcals/mole Aerobic Respiration and Energy Yield C6H12O6 + 6O2 = 6CO2 + 6H2O ADP + Pi = ATP WBL

  39. 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 krr@ufl.edu 11/17/2014 39 WBL

  40. Oxygen • Oxygen is an electron acceptor • Reduction [Electron acceptor] • O2 + 4H+ + 4e- = 2H2O : • Oxidation [Electron donor] • C6H12O6 + 6H2O = 6CO2 + 24H+ + 24e- Oxidant Reductant WBL

  41. 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+ WBL

  42. O2 O2 Floodwater Floodwater Aerobic soil Aerobic soil Anaerobic soil Anaerobic soil Oxidation-Reduction Carbon Nitrogen CO2 O2 + CH4 NO3 O2 + NH4 OM CH4 OM NH4 WBL

  43. O2 O2 Floodwater Floodwater Aerobic soil Aerobic soil Anaerobic soil Anaerobic soil Oxidation-Reduction Iron Manganese Fe3+ O2 + Fe2+ Mn4+ O2 + Mn2+ FeOOH Fe2+ MnO2 Mn2+ WBL

  44. O2 O2 Floodwater Aerobic soil Anaerobic soil Oxidation-Reduction Carbon Sulfur Floodwater CO2 O2 + OM SO4 O2 + H2S Aerobic soil Anaerobic soil SO4 H2S WBL

  45. Oxygen Consumption Low organic matter soil Consumption during chemical oxidation Consumption during biological oxidation [C/Co] High organic matter soil Time (hours) WBL

  46. 700 600 500 400 300 200 y=-1036+200 ln(x) 100 R2=0.84 0 Aerobic Respiration Impacted Everglades, FL Unimpacted Everglades, FL Talladega, AL Houghton Lake Oxygen consumption, mg/kg day marsh, MI Salt marsh, LA Belhaven, NC Lake Apopka marsh, FL Prairie pothole, ND Crowley, LA 3,500 0 500 1,000 1,500 2,000 2,500 3,000 Dissolved organic C, mg/kg WBL

  47. Impacted Unimpacted 10 12N 9 6P 8 12M WATER 7 6A 6 WATER FILAMENTOUS 5 ALGAE DEPTH (cm) 4 3 MACRO-LITTER 2 AND ALGAE 1 PERIPHYTON 0 -1 Peat Unconsolidated Peat -2 0 20 40 60 80 100 0 20 40 60 80 100 DISSOLVED OXYGEN (% SATURATION) WBL

  48. 0 19 36 68 192 306 593 Irradiance (μmol m-2 s-1) Oxygen - Periphyton % O2 Saturation 0 50 100 150 200 250 - 1 0 2 Depth (mm) 4 6 8 10 S. Hagerty, SFWMD unpublished results WBL

  49. Lake Apopka Marsh Soluble P, mg L-1 Dissolved Fe, mg L-1 1 6 8 0 2 3 4 2 0 Phosphorus Iron 30 20 Water Depth, cm 10 0 Soil -10 -20 WBL

  50. Mobile and Immobile Iron 0 Insoluble Fe Aerobic Fe2+ 2 4 Depth below soil surface Anaerobic 6 8 0 1 2 3 % Fe WBL