ASPECTS OF AQUATIC REDOX CHEMISTRY

PART - IREDOX CONDITIONS IN

NATURAL WATERSRedox conditions in natural waters are controlled largely by photosynthesis and bacterial respiration

processes

Oxidation – Reduction Reactions

• Oxidation - a process involving loss of electrons.• Reduction - a process involving gain of electrons.• Reductant - a species that loses electrons.• Oxidant - a species that gains electrons.

• Free electrons do not accumulate in solution. Electrons lost from one species in solution must be immediately gained by another.

Ox1 + Red2 Red1 + Ox2

Reduction-Oxidation Potential

• The potential that is generated between an oxidation or reduction half cell and the standard hydrogen electrode

• In aqueous solutions, the reduction potential is the tendency of the solution to either gain or lose electrons

• The potential abundance of electrons or the electron activity

PHOTOSYNTHESIS

• Synthesis of organic matter by photosynthesis

)g(22light

)aq(2)g(2 O)OH(COHCO

)g(2116110263106light2

43)aq(2)g(2 O138PNOHCH18HPONO16OH122CO106

1:16:106P:N:C

The average composition of the organic matter in plankton is approximately:

C106H263O110N16P1. Therefore, photosynthesis reaction can be

represented by the following and more complex reaction

algae

Redfield Ratio (Redfield et al., 1963)

Redfield Ratio Concept

RESPIRATION

• In general, respiration involves the decomposition of organic matter produced through photosynthesis

• During respiration, the organic matter is oxidized and an electron acceptor is reduced

• Example electron acceptors:

• Respiration can occur under oxygenated (or aerobic) conditions or in the absence of molecular oxygen (anaerobic respiration).

OHCOO)OH(C 22nrespiratio

22

2243232 COSO)OH(FeMnONOO

RESPIRATION (cont’d)

• In water containing excessive biomass (e.g. during algal blooms), dead organic matter (OM) is mineralized via microbial respiration in the presence of terminal electron acceptors (TEA) as illustrated in the following general reaction

OHCOTEAOM 22nrespiratio

Using CH2O as a general formula for OM and different TEA types, one obtains (see next slide)

Progressive Microbial Respiration of OM in Natural Waters and Thermodynamics

2.23.........................................................CH8

1CO

4

1OCH

4

1CO

8

1

)sismethanogen(formationMethane)6(

0.26..............................OH4

1HS

8

1CO

4

1H

8

1OCH

4

1SO

8

1

reductionSulfate)5(

8.26................................OH4

7FeCO

4

1H2OCH

4

1FeOOH

oxides)hydr()III(Feofreductionby)II(FelelubsoofFormation)4(

0.85.............................OH4

3Mn

2

1CO

4

1HOCH

4

1MnO

2

1

oxides)IV(Mnofreductionby)II(MnlelubsoofFormation)3(

0.119..............................OH20

7N

10

1CO

4

1HOCH

4

1NO

5

1

ationDenitrific)2(

0.125.........................................................OH4

1CO

4

1OCH

4

1O

4

1

nconsumptioOxygen)1(

)equiv/kJ(G.........................................................................................actionRe

2222

22224

22

22

22

222

22223

2222

0w

Redox Couples

• For any half reaction, the oxidized/reduced pair is the redox couple:– Fe2+ Fe3+ + e-– Couple: Fe2+/Fe3+

– H2S + 4 H2O SO42- + 10 H+ + 8 e-

– Couple: H2S/SO42-

Redox Ladder1

0.5

0

-0.5Eh (V)

O2 H2O

NO3- NO2

-

NO2

- NH4+

Mn+4 Mn+2

FeOOH Fe+2

SO4-2 HS-

CO2 CH4

H+ H2HCOO-

CH2O

pE = Eh / 0.0591

Oxidized species (TEAs) Reduced species

Another Representation of The Redox Ladder

H2O

H2

O2

H2O

NO3-

N2 MnO2

Mn2+

Fe(OH)3

Fe2+SO4

2-

H2S CO2

CH4

Oxic

Sub-oxicanaerobic

Sulfidic

Methanic

Aerobes

Denitrifiers

Manganese reducers

Sulfate reducers

Methanogens

Iron reducers

The redox-couples are shown on each stair-step, where the most energy is gained at the top step and the least at the bottom step (i.e. the Gibb’s free energy of reaction becomes more positive going down the steps).

Half Reactions• Often split redox reactions in two:

– oxidation half rxn e- leaves left, goes right• Fe2+ Fe3+ + e-

– Reduction half rxn e- leaves left, goes right• O2 + 4 e- 2 H2O

• SUM of the half reactions yields the total redox reaction

4 Fe2+ 4 Fe3+ + 4 e-

O2 + 4 e- 2 H2O

4 Fe2+ + O2 4 Fe3+ + 2 H2O

Steps for Balancing Redox Reactions

1. Indentify principle reactants and products

2. Balance atoms other than Hydrogen and Oxygen

3. Balance oxygen using H2O

4. Balance H using H+

5. Balance Charge with electrons

7. Multiply each half cell by an integer so that both half cells contain same number of electrons

8. Add two balanced half cells

9. H+ may be present as product of reaction. If the reaction is known to take place in an alkaline solution, then add the reaction for the dissociation of water to eliminate the H+ form the overall redox reaction

Examples

Write the half reactions corresponding to each of these 2 reactions and show the

balanced overall redox reactions

Mn(IV) + H2S Mn2+ + S0 + H+

H2S + O2 S8 + H2O

Example Redox Impact on the Aquatic Cycling of Iron

Efficiency of Thermodynamic Predictions

Measured Eh Vs Calculated Eh in Acid Mine Waters

ACS, 1979

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95

Eh (mVolts)

Eh

(vol

ts)

Limitations of Thermodynamic Predictions Measured Eh Vs Calculated Eh in Groundwaters

From Lindberg and Runells, 1984 (Science)