Download Hydrogeochemistry

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

Reaction that converts CO2 plus nutrients
(N, P, other micronutrients) to organic
matter and oxygen
CO2 + N + P + other = Corganic + O2

This equation controls atmospheric oxygen

If not driven to right by primary production,
all O2 would be consumed


Photosynthesis occurs until essential
nutrients are depleted
Various nutrients may be limiting:

N, P, Fe…
Redfield Ratio

Organic matter is approximately constant
composition
C106H263O110N16P1

Redfield ratio is thus 106C:16N:1P (molar
ratio)

More complex reaction better reflection of
photosynthesis
106CO2 + 16NO3- + HPO42- + 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:
Corg + 2H2O = CO2 + 4H+ + 4e-


Because no free electrons, a
corresponding half reaction must consume
them
Terminal electron acceptors – TEAs

For example – reduction of oxygen to
water:
O2 + 4H+ + 4e- = 2H2O

Here oxygen is the terminal electron
acceptor.

There are multiple terminal electron
acceptors:
2NO3- + 12H+ + 10e- = N2 + 6H2O
FeOOH + 3H+ + e- = Fe2+ + 2H2O
SO42- + 10H+ + 8e- = H2S + 4H2O
Terminal
electron
acceptor
controlled by
microbes and
by
concentration
of acceptor
MnO2
/Mn2+
Rare
FeOOH/Fe2+
Decreasing
amount of
energy derived
per mole of
electrons
transferred
Nitrate Reduction

Denitrification (dissimilatory nitrate
reduction)
7e
5Corganic + 4NO3- + 4H+ = 2N2 + 5CO2 + 2H20

Final product is molecular nitrogen

Conversion of nutrient to inert gas

Other nitrate reduction pathways

Reduction to nitrite:
2e-

Corg + 2NO3- = CO2 + 2NO2Reduction to ammonia
2Corg + NO3- + H2O + H+ = 2CO2 + NH3
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 Ni and Fe catalysts
utilize CH4 to generate needed H2
NH3 oxidized to NO3 and NO2
Prior to this fertilizers required


mining fixed N (guano)
N fixing plants (legumes)
Ferric iron (and Mn) reduction
Corg + 4Fe(OH)3 + 8H+ = CO2 + 4Fe2+ + 10H2O
e

Common in groundwater where metal
oxides concentrated. Rare in surface
water
Fe2+ commonly precipitates as carbonate
or sulfide depending on solution chemistry
Sulfate reduction
Corg + SO42- + 2H2O = H2S + 2HCO38e-



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 HCOOAcetate CH3COOLactate C3H5O3





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
CH3COOH = CH4 + CO2


Oxidized and reduced C
Methanogenesis
CO2 + 4H2 = CH4 + 2H2O
8e
Oxidized to reduced C


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
Sediments


The range of reactions are very common
in marine sediments
Controls


Amount of organic matter
Sedimentation rate – controls diffusion
Depth in
sediment
Sediment-water interface
Depth variations
depend on:
(1) Sedimentation
rate
(2) Diffusion rate
(3) Amount of
electron
acceptor
(4) Amount of
organic carbon
Oxygen depleted
Nitrate depleted
MnO2/Mn2+
N, P, CO2
(alkalinity)
increase
Mn2+ increase
FeOOH/Fe2+
SO42- decrease
Fe2+ increase
Sulfide increase
Methane increase
Eastern equatorial
Atlantic:
Slow sed rate
low OC content
Coastal salt marsh
High sed rate
high 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 SO42- (no nitrate, metals etc).


With oxygen present, pe remains fairly
constant at around 13
In oceans, once oxygen reduced, sulfate
becomes terminal electron acceptor, pe =
about -3
Occurs in water
with no NO3- or
Fe(III)
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
Generic Lake:
 May have multiple
metalimnions
 Depends on depth
of lake

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


Warming in spring to 4º C is maximum
density



Ice forms
Spring turnover
Monomictic – once a year turnover
Dimictic – twice a year turnover

Oxygen content (redox conditions)
depends 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
O2 more soluble in
cold water
Oligotrophic
Eutrophic
High productivity, O2 consumed





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
Fe2+ 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
Photic zone – OC
production
Pycnocline =
halocline +
thermocline
High OC input
upwelling system
Low OC input


Bottom configuration also important
Silled basins



Cariaco Basin – Venezuela
Sanich Inlet – B.C.
Santa Barbara Basin - California
Stratified – little
mixing
NO3, Fe, Mn, SO4
reduction

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