institute of food and agricultural sciences (ifas) … · 2008-06-22 · institute of food and...

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-

6/22/2008 WBL 2

Fe2+ = Fe3+ + e-

Reactions in which both protons and electrons are transferred

2Fe(OH)3 + 3H+ + e- = 2Fe2+ + 3H2O

2


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-

6/22/2008 WBL 3

H e e-e- ee- e-e-e- e- H+

H+H+ee-

H+H+

Topic OutlineIntroduction


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

6/22/2008 WBL 4

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


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

6/22/2008 WBL 5

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]

6/22/2008 WBL 6

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


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


UPLAND SOILSUPLAND SOILS FLOODED SOILSFLOODED SOILS


Reduction

N2 NH4+

Mn2+

Fe2+

S2-

CH

H2O

NO3-

Mn4+

Fe3+

SO42-

CO

O2

6/22/2008 WBL 10

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

6/22/2008 WBL 11

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

6/22/2008 WBL 12

-300 7000 300 500100-100Oxidation-Reduction Potential (mV)

7

ure


Elec

tron

Pre

ss

Stronglyreduced

Stronglyoxidized

6/22/2008 WBL 13

-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

6/22/2008 WBL 14

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

[+]

6/22/2008 WBL 15

Elec

Ease of Reduction

COCO22SOSO44

22--Fe (III)Fe (III) En

[-]

CO2 CH4 SeO32- Se(0); Se2-

Mn4+ Mn2+

SeO42- SeO3

2-


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

6/22/2008 WBL 18

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

6/22/2008 20WBL

11

Regulators of EhRegulators of Eh

Water table fluctuationsWater-table fluctuations.Activities of electron acceptors.Activities of electron donors.TemperaturepH

6/22/2008 WBL 21

pH

Field Field RedoxRedox ElectrodesElectrodes

Copper wire

Volt meter

Water

il

Heat shrinkingtube

Epoxy

CalomelReference

Volt meter

6/22/2008 WBL 22

Soilp y

Platinumwire Platinum

electrodes

12

Copper wire

Saturated KCl

Laboratory Laboratory RedoxRedox ElectrodesElectrodes

Platinum Glass Electrode

Heat shrinkingtube

Glass tubeGlass tube

6/22/2008 WBL 23

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

6/22/2008 WBL 24

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

6/22/2008 WBL 26

Time, days

R

14

600700

O

Electron Acceptors Electron Acceptors -- Redox PotentialRedox PotentialE

h (m

V)

0100200300400500600 Oxygen

Nitrate

Sulfate

6/22/2008 WBL 27

-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

6/22/2008 WBL 28

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

6/22/2008 WBL 29

0-160

-140

120D

100 200 300 400 500 600 700

Redox Potential (mV)

Sediment Microbial Fuel CellSediment Microbial Fuel Cell

6/22/2008 WBL 30

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.

6/22/2008 WBL 32

Redox potential is closely related to pH.Platinum electrode surface can be contaminated by coatings of oxides, sulfides and other impurities.

17

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

6/22/2008 WBL 33

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

6/22/2008 WBL 34

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%]

6/22/2008 WBL 35

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

6/22/2008 WBL 37

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

6/22/2008 WBL 38

Gr 7.7 kcals/mole

20

Biogeochemistry of WetlandsS i d A li tiS i d A li ti


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


NO3 O2 + NH4

Floodwater

Aerobic soilCO2 O2 + CH4

Floodwater

Aerobic soil

O2 O2

OM NH4

Aerobic soilAnaerobic soil

OM CH4


6/22/2008 42WBL

22

IronIron ManganeseManganese


Fe3+ O2 + Fe2+ Mn4+ O2 + Mn2+

Floodwater

Aerobic soil

Floodwater

Aerobic soil

O2 O2

FeOOH Fe2+ MnO2 Mn2+



6/22/2008 43WBL

CarbonCarbon SulfurSulfur


CO2 O2 + OM SO4 O2 + H2S

Floodwater Floodwater

O2 O2

CO2 O2 OM SO4 O2 H2S

H2S


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

institute of food and agricultural sciences (ifas) … · 2008-06-22 · institute of food and...

Documents