Redox of Natural Waters Redox largely controlled by quantity
and quality (e.g. reactivity) of organic matter Organic matter generated with
photosynthesis Organic matter decomposes
(remineralized) during respiration
Photosynthesis CO2 plus nutrients (N, P, other
micronutrients) to organic matter and oxygen
This equation controls atmospheric oxygen If not driven to right by primary
production, all O2 would be consumed
CO2 + N + P + other = Corganic + O2
Redfield Ratio Organic matter is approximately
constant composition
Redfield ratio is thus 106C:16N:1P (molar ratio)
C106H263O110N16P1
More complex reaction better reflection of photosynthesis
106CO2 + 16NO3- + HPO4
2- + 122H20 + 18H+ + trace elements = C106H263O110N16P1 + 138O2
This reaction reflects the importance of P in the reaction: 106 moles C consumed/ mole of P 16 moles of N consumed / mole of P 138 moles of O2 consumed / mole of P
Reverse reaction (remineralization: respiration/decay) equally important
Products include Nitrate Phosphate CO2 – decrease pH
Much respiration results from microbes (bacteria, archea etc).
Oxidation of organic carbon also generates electrons:
Because free electrons are prohibited, there must be a corresponding half reaction to consume them
Corg + 2H2O = CO2 + 4H+ + 4e-
For example – reduction of oxygen to water:
Here oxygen is the terminal electron acceptor.
O2 + 4H+ + 4e- = 2H2O
There are multiple terminal electron acceptors:
2NO3- + 12H+ + 10e- = N2 + 6H2O
FeOOH + 3H+ + e- = Fe2+ + 2H2OSO4
2- + 10H+ + 8e- = H2S + 4H2O
MnO2/Mn2+
FeOOH/Fe2+
Rare
Decreasing amount of energy derived per mole of electrons transferred
Terminal electron acceptor controlled by microbes and by concentration of acceptor
Nitrate Reduction Denitrification (dissimilatory nitrate
reduction)
Final product is molecular nitrogen Conversion of nutrient to inert gas
5Corganic + 4NO3- + 4H+ = 2N2 + 5CO2 + 2H20
7e-
Other nitrate reduction pathways Reduction to nitrite:
Reduction to ammoniaCorg + 2NO3
- = CO2 + 2NO2-
2Corg + NO3- + H2O + H+ = 2CO2 + NH3
2e-
10e-
Ammonia also derived from decomposition of amino acids in proteins
Ammonia raises pH by formation of ammonium ion
NH3 + H2O = NH4+ + OH-
(now an acid-base reaction)
Why concern with NO3? Haber Process (early 20th century)
N2 fixation to NH3 with Fe catalyst NH3 oxidized to NO3 and NO2
Prior to this fertilizers required mining fixed N (guano) N fixing plants (legumes)
Ferric iron (and Mn) reduction
Common in groundwater where metal oxides concentrated. Rare in surface water
Fe2+ commonly precipitates as carbonate or sulfide depending on solution chemistry
Corg + 4Fe(OH)3 + 8H+ = CO2 + 4Fe2+ + 10H2O e-
Sulfate reduction
Commonly driven by microbes Products are H2S or HS- and H2CO3 or
HCO3- depending on pH
Microbes require simple carbon (e.g. < 20 C chains Formate HCOO-
Acetate CH3COO-
Lactate C3H5O3
Corg + SO42- + 2H2O = H2S + 2HCO3
-
8e-
Sulfate common seawater ion Sulfide and bisulfide highly toxic Used by oxidizing bacteria for
chemosynthesis Oxide to sulfides change sediment
color Metal chemistry
P and some metals adsorb to oxides Other metals soluble in oxidizing
solution (Cu, Zn, Mo, Pb, Hg) Other metals precipitate as sulfides
Fermentation and methanogenesis
Breakdown of complex carbohydrates to simpler molecules
Products can be used by sulfate reducing bacteria
Don’t require terminal electron acceptors
Fermentation
Methanogenesis
CH3COOH = CH4 + CO2
CO2 + 4H2 = CH4 + 2H2O
Each terminal electron acceptor requires specific bacteria
Bacteria derive energy from reactions Essentially catalyze breakdown of
unstable to stable system Reactions occur in approximate
succession with depth in the sediment
Dept
h in
se
dim
ent MnO2/Mn2+
FeOOH/Fe2+
Rare
Sediment-water interface
Oxygen depleted
Nitrate depletedN, P, CO2 (alkalinity) increaseMn2+ increase
Fe2+ increaseSO4
2- decrease Sulfide increase
Methane increase
Depth variations depend on:(1)Sedimentation
rate(2)Diffusion rate(3)Amount of
electron acceptor
(4)Amount of organic carbon
Sediments The range of reactions are very
common in marine sediments Controls
Amount of organic matter Sedimentation rate – controls diffusion
Eastern equatorial Atlantic:
Slow sed ratelow OC
contentCoastal salt marshHigh sed ratehigh OC
content
Example IRL
Redox Buffering pe can be buffered just like pH
Depends on the electron receptor present
Example of surface water, contains oxygen and SO4
2- (no nitrate, metals etc).
With oxygen present, pe remains fairly constant at around 13
Once oxygen reduced, sulfate becomes terminal electron acceptor, pe = about -3
Oxygen consumed,pe rapidly decreases
There could also be solid phases controlling redox conditions
Stepwise lowering of pe as various terminal electron acceptors are depleted
Lakes Vertical stratification
Epilimnion – warm low density water, well mixed from wind
Metalimnion (thermocline) – rapid decrease in T with depth
Hypolimnion – uniformly cold water at base of lake
Stable – little mixing between hypolimnion and epilimnion
Amount of nutrient in lake determines type Oligotrophic – low supply of nutrients,
water oxygenated at all depth Eutrophic – high supply of nutrients,
hypolimnion can be anaerobic
Cooling T in fall Surface water reaches 4ºC – most dense
Causes breakdown of epilimnion – Fall turnover Metalimnion breaks down Wind mixes column
At T < 4º C, stably stratified Ice forms
Warming in spring to 4º C is maximum density Spring turnover
Monomictic – once a year turnover Dimictic – twice a year turnover
High productivity, O2 consumed
O2 more soluble in cold water
Oligotrophic
Eutrophic
Oxygen content (redox conditions) depend on turnover Oxygen in hypolimnion decreases as
organic matter falls from photic zone and is oxidized
The amount of oxygen used depends on production in photic zone
Production depends on nutrients, usually phosphate
Pollution convert oligotrophic lakes to eutrophic ones (e.g. Lake Apopka, Florida)
Difficult to reverse process Nutrients (P) buried in sediments
because adsorbed to Fe-oxides When buried Fe-oxides reduced and
form Fe(II) and Fe-carbonates and sulfides
Released P returns to lake
Ocean Oceanic turnover
Continuous – Broecker’s “conveyer belt” Nutrient distribution controlled by decay
in water column and circulation/upwelling
Oxygen profiles controlled by settling organic matter from photic zone Rate of input of organic matter controls
oxygen minimum zone
Broecker’s Conveyor Belt
Bottom configuration also important Silled basins
Cariaco Basin – Venezuela Sanich Inlet – B.C. Santa Barbara Basin - California
Little deep water circulation Oxygen rapidly depleted May go to sulfate reduction in water
column Sediment affected
Black (sulfides) Laminated (no bioturbation)
Ground Water Difficult to generalize about controls
on redox reactions
Multiple controls Oxygen content of recharge water
“Point recharge” – sinkholes, fractures well oxygenated
“diffuse recharge” – low oxygen, consumed by organic matter
Distribution of reactive C Aquifers vary in amount of organic
carbon Quality of carbon variable – usually
refractory Refractory because
Old subject to heat
Distribution of redox buffers Aquifers may have large amounts of Mn
and Fe oxides
Circulation of groundwater Flow rates, transit times, residence
times Longer residence times generally mean
lower pe