Biogeochemistry of Wetlands 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 9/17/2018 9/17/2018 WBL 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 9/17/2018 WBL 2

Electrons and Protons e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- Institute of Food and Agricultural Sciences (IFAS) Electrons and Protons e- e- e- H+ e- e- e- e- e- e- H+ H+ e- H+ e- H+ e- H+ e- H+ e- H+ e- H+ e- e- H+ e- e- e- H+ e- e- e- e- e- e- H+ e- e- e- H+ e- e- H+ H+ e- e- e- H+ H+ e- e- 9/17/2018 WBL

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 9/17/2018 WBL

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 9/17/2018 WBL

Oxidation-Reduction Reductant Oxidant + e- 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] 9/17/2018 WBL

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

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

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

Oxidation-Reduction Reduced Oxidized UPLAND SOILS FLOODED SOILS N NH4+ 2 NH4+ Mn2+ Fe2+ S2- CH4 PH3 H2 H2O NO3- Mn4+ Fe3+ SO42- CO2 PO43- O2 Oxidized Reduced 9/17/2018 WBL 10 10

Eh = Eo - [0.059/n] log [Reductant/Oxidant] 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 9/17/2018 WBL 11

Oxidation-Reduction Potential (mV) Wetland Soil -300 700 300 500 100 -100 Oxidation-Reduction Potential (mV) Highly Reduced Moderately Oxidized Anaerobic Aerobic Wetland Soil Drained Soil 9/17/2018 WBL 12 12

Oxidation-Reduction Electron Pressure -300 -100 100 300 500 700 Strongly reduced Strongly oxidized -300 -100 100 300 500 700 Oxidation-Reduction Potential (mV) 9/17/2018 WBL

Electron donors e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- e- [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+ e- H+ e- H+ e- e- H+ H+ e- H+ H+ e- e- H+ e- e- e- H+ e- e- e- e- NO3- Mn4+ Fe3+ SO42- CO2 H2 O O2 e- 600 -300 200 -100 100 300 Oxidation-Reduction Potential (mV) -400 Energy Electron acceptors 9/17/2018 WBL

How much energy is released during oxidation - reduction reactions? Electrode Potentials Ease of Reduction CO2 SO42- Fe (III) Mn (IV) N-Oxides O2 Energy Yield [+] [-] 9/17/2018 WBL

Oxidation-Reduction Redox Potential, mV (at pH 7) Mn4+ Mn2+ CO2 CH4 SeO32- Se(0); Se2- SeO42- SeO32- SO42- S2- Fe3+ Fe2+ NO3- N2 O2 H2O -200 -100 +100 +200 +300 +400 Redox Potential, mV (at pH 7) 9/17/2018 WBL 16 16

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

Oxidation-Reduction Wetlands and Aquatic Systems Uplands Electron acceptor non-limiting Electron donor limiting Electron acceptor limiting Electron donor non-limiting 9/17/2018 WBL

Sequential Reduction of Electron Acceptors Organic Substrate [e- donor] Fe2+ S2- SO42- Relative Concentration CH4 NO3- Mn2+ O2 Oxygen Nitrate Iron Methanogenesis Manganese Sulfate Time or Soil Depth 9/17/2018 WBL 19 19

Redox Zones with Depth WATER Oxygen Reduction Zone Aerobic SOIL I Eh = > 300 mV 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 9/17/2018 WBL 20 20

Regulators of Eh Water-table fluctuations. Activities of electron acceptors. Activities of electron donors. Temperature pH 9/17/2018 WBL

Field Redox Electrodes Copper wire Volt meter Heat shrinking tube Calomel Reference Water Epoxy Soil Platinum wire Platinum electrodes 9/17/2018 WBL

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 9/17/2018 WBL

Okeechobee Basin Wetland Soils and Stream Sediments 9/17/2018 WBL

Flooded Organic Soils: Everglades Agricultural Area 9/17/2018 WBL

Flooded Paddy Soils: Louisiana Aerobic Anaerobic Redox Potential, mV Time, days 9/17/2018 WBL

Electron Acceptors - Redox Potential -300 -200 -100 100 200 300 400 500 600 700 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) 9/17/2018 WBL

Electron Donor [Organic Matter] – Redox Potential 300 200 Low Organic Matter Soil 100 Redox potential (mV) -100 High Organic Matter Soil -200 Time after flooding 9/17/2018 WBL

Redox Gradients in Sediments -20 Aerobic Layer -40 Anaerobic Layer -60 Depth (mm) -80 -100 -120 -140 -160 100 200 300 400 500 600 700 Redox Potential (mV) 9/17/2018 WBL

Sediment Microbial Fuel Cell Source: D. R. Lovley, 2006. Nature Reviews 4:497-508 9/17/2018 WBL

Redox Potential and pH Eh mv] pH 1000 800 600 400 200 -200 -400 -600 -200 -400 -600 0 2 4 6 8 10 12 pH 9/17/2018 WBL Baas Becking et al. 31

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. 9/17/2018 WBL

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 9/17/2018 WBL

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 9/17/2018 WBL

Effect of Flooding on Soil pH 8 Clay loam [ pH = 8.7; OM = 2.2%; Fe = 0.63%] 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 9/17/2018 WBL

Effect of Flooding on Soil Porewater Ionic Strength Ca2+ Soil Fe2+ NH4+ Mn2+ K+ Solid Phase Soil Solution B Ionic Strength 9/17/2018 WBL Time after flooding

Redox Couples in Wetlands C6H12O6/CO2 and NO3-/N2 C6H12O6/CO2 and FeOOH/Fe2+ C6H12O6/CO2 and MnO2 /Mn2+ C6H12O6/CO2 and SO42- /H2S H2/H+ and CO2 /CH4 C6H12O6/CO2 and O2/H2O 9/17/2018 WBL

Aerobic Respiration and Energy Yield C6H12O6 + 6O2 = 6CO2 + 6H2O Gr = -686.4 kcals/mole ADP + Pi = ATP Gr = -7.7 kcals/mole 9/17/2018 WBL

Biogeochemistry of Wetlands 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 9/17/2018 9/17/2018 WBL 39

Oxygen Oxygen is an electron acceptor Reduction [Electron acceptor] O2 + 4H+ + 4e- = 2H2O : Oxidation [Electron donor] C6H12O6 + 6H2O = 6CO2 + 24H+ + 24e- Oxidant Reductant 9/17/2018 WBL

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+ 9/17/2018 WBL

Oxidation-Reduction Carbon Nitrogen O2 O2 Floodwater Aerobic soil Anaerobic soil Floodwater Aerobic soil Anaerobic soil CO2 O2 + CH4 NO3 O2 + NH4 OM CH4 OM NH4 9/17/2018 WBL

Oxidation-Reduction Iron Manganese O2 O2 Floodwater Aerobic soil Anaerobic soil Floodwater Aerobic soil Anaerobic soil Fe3+ O2 + Fe2+ Mn4+ O2 + Mn2+ FeOOH Fe2+ MnO2 Mn2+ 9/17/2018 WBL

Oxidation-Reduction Carbon Sulfur O2 O2 Floodwater Aerobic soil Anaerobic soil Floodwater CO2 O2 + OM SO4 O2 + H2S Aerobic soil Anaerobic soil SO4 H2S 9/17/2018 WBL

Oxygen Consumption Low organic matter soil [C/Co] during chemical oxidation Consumption during biological oxidation [C/Co] High organic matter soil Time (hours) 9/17/2018 WBL

Dissolved organic C, mg/kg Aerobic Respiration 100 200 300 400 500 600 700 Impacted Everglades, FL Unimpacted Everglades, FL Talladega, AL Houghton Lake Oxygen consumption, mg/kg day marsh, MI Salt marsh, LA Belhaven, NC y=-1036+200 ln(x) R2=0.84 Lake Apopka marsh, FL Prairie pothole, ND Crowley, LA 500 1,000 1,500 2,000 2,500 3,000 3,500 Dissolved organic C, mg/kg 9/17/2018 WBL

Impacted Unimpacted DEPTH (cm) DISSOLVED OXYGEN (% SATURATION) -2 -1 1 1 2 3 4 5 6 7 8 9 10 DEPTH (cm) DISSOLVED OXYGEN (% SATURATION) 12N 6P 12M 6A FILAMENTOUS ALGAE PERIPHYTON Peat Unconsolidated Peat WATER MACRO-LITTER AND ALGAE 20 40 60 80 100 Impacted Unimpacted 9/17/2018 WBL

Oxygen - Periphyton % O2 Saturation Depth (mm) Irradiance 50 100 150 200 250 19 36 68 192 306 593 - 1 2 Depth (mm) 4 6 8 10 Irradiance (μmol m-2 s-1) S. Hagerty, SFWMD unpublished results 9/17/2018 WBL

Lake Apopka Marsh Depth, cm Soluble P, mg L-1 Dissolved Fe, mg L-1 2 4 2 4 6 8 1 2 3 30 Phosphorus Iron 20 Depth, cm 10 Water Soil -10 -20 9/17/2018 WBL

Mobile and Immobile Iron Insoluble Fe Fe2+ Aerobic 2 4 Depth below soil surface Anaerobic 6 8 1 2 3 % Fe 9/17/2018 WBL

OXYGEN: Sources and Sinks Water Air Plants and Algae Release by Plant Roots Soil Oxygen Oxidation of Reductants Chemical oxidation Respiration Chemolithotrophic oxidation 9/17/2018 WBL

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 9/17/2018 WBL

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 9/17/2018 WBL

http://wetlands.ifas.ufl.edu http://soils.ifas.ufl.edu 9/17/2018 WBL