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

This equation controls atmospheric oxygen If not driven to right by primary

production, all O2 would be consumed

CO2 + N + P + other = Corganic + O2

Photosynthesis occurs until essential nutrients are depleted

Various nutrients may be limiting: N, P, Fe…

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 no free electrons, a corresponding half reaction must consume them

Terminal electron acceptors – TEAs

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+ + 2H2O

SO42- + 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 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

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

Oxidized and reduced C

Methanogenesis

Oxidized to reduced C

CH3COOH = CH4 + CO2

CO2 + 4H2 = CH4 + 2H2O

8e-

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

Dep

th

in

sedim

en

t MnO2/Mn2+

FeOOH/Fe2+

Sediment-water interface

Oxygen depleted

Nitrate depleted

N, P, CO2 (alkalinity) increase

Mn2+ increase

Fe2+ increase

SO42- 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

Eastern equatorial Atlantic:

Slow sed ratelow OC

contentCoastal salt marsh

High 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 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

Oxygen consumed,pe rapidly decreases

Occurs in water with no NO3

- or Fe(III)

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 Ice forms

Warming in spring to 4º C is maximum density 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

High productivity, O2 consumed

O2 more soluble in cold water

Oligotrophic

Eutrophic

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

Pycnocline = halocline + thermocline

High OC input upwelling system

Low OC input

Photic zone – OC production

Bottom configuration also important Silled basins

Cariaco Basin – Venezuela Sanich Inlet – B.C. Santa Barbara Basin - California

NO3, Fe, Mn, SO4 reduction

Stratified – little mixing

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