aspects of aquatic redox chemistry. part - i redox conditions in natural waters redox conditions in...
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ASPECTS OF AQUATIC REDOX CHEMISTRY
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PART - IREDOX CONDITIONS IN
NATURAL WATERSRedox conditions in natural waters are controlled largely by photosynthesis and bacterial respiration
processes
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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
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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
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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)
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Redfield Ratio Concept
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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
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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)
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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
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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-
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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
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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).
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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
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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
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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
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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
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Example Redox Impact on the Aquatic Cycling of Iron
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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)
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Limitations of Thermodynamic Predictions Measured Eh Vs Calculated Eh in Groundwaters
From Lindberg and Runells, 1984 (Science)
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