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Redox of Natural Waters

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.

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Redox of Natural Waters

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  1. 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

  2. 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

  3. Photosynthesis occurs until essential nutrients are depleted • Various nutrients may be limiting: • N, P, Fe…

  4. Redfield Ratio • Organic matter is approximately constant composition • Redfield ratio is thus 106C:16N:1P (molar ratio) C106H263O110N16P1

  5. More complex reaction better reflection of photosynthesis 106CO2 + 16NO3- + HPO42- + 122H20 + 18H+ + trace elements = C106H263O110N16P1 + 138O2

  6. 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

  7. Reverse reaction (remineralization: respiration/decay) equally important • Products include • Nitrate • Phosphate • CO2 – decrease pH • Much respiration results from microbes (bacteria, archea etc).

  8. 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-

  9. For example – reduction of oxygen to water: • Here oxygen is the terminal electron acceptor. O2 + 4H+ + 4e- = 2H2O

  10. There are multiple terminal electron acceptors: 2NO3- + 12H+ + 10e- = N2 + 6H2O FeOOH + 3H+ + e- = Fe2+ + 2H2O SO42- + 10H+ + 8e- = H2S + 4H2O

  11. Terminal electron acceptor controlled by microbes and by concentration of acceptor Decreasing amount of energy derived per mole of electrons transferred MnO2/Mn2+ Rare FeOOH/Fe2+

  12. Nitrate Reduction • Denitrification (dissimilatory nitrate reduction) • Final product is molecular nitrogen • Conversion of nutrient to inert gas 7e- 5Corganic + 4NO3- + 4H+ = 2N2 + 5CO2 + 2H20

  13. Other nitrate reduction pathways • Reduction to nitrite: • Reduction to ammonia 2e- Corg + 2NO3- = CO2 + 2NO2- 2Corg + NO3- + H2O + H+ = 2CO2 + NH3 10e-

  14. 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)

  15. 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)

  16. 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-

  17. Sulfate reduction Corg + SO42- + 2H2O = H2S + 2HCO3- • 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 8e-

  18. 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

  19. Fermentation and methanogenesis • Breakdown of complex carbohydrates to simpler molecules • Products can be used by sulfate reducing bacteria • Don’t require terminal electron acceptors

  20. Fermentation • Oxidized and reduced C • Methanogenesis • Oxidized to reduced C CH3COOH = CH4 + CO2 CO2 + 4H2 = CH4 + 2H2O 8e-

  21. 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

  22. Sediments • The range of reactions are very common in marine sediments • Controls • Amount of organic matter • Sedimentation rate – controls diffusion

  23. Sediment-water interface Oxygen depleted N, P, CO2 (alkalinity) increase Depth in sediment Nitrate depleted MnO2/Mn2+ Mn2+ increase Depth variations depend on: Sedimentation rate Diffusion rate Amount of electron acceptor Amount of organic carbon Fe2+ increase FeOOH/Fe2+ Sulfide increase SO42- decrease Methane increase

  24. Eastern equatorial Atlantic: Slow sed rate low OC content Coastal salt marsh High sed rate high OC content

  25. Example IRL

  26. 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).

  27. With oxygen present, pe remains fairly constant at around 13 • In oceans, once oxygen reduced, sulfate becomes terminal electron acceptor, pe = about -3

  28. Occurs in water with no NO3- or Fe(III) Oxygen consumed, pe rapidly decreases

  29. There could also be solid phases controlling redox conditions Stepwise lowering of pe as various terminal electron acceptors are depleted

  30. 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

  31. Generic Lake: • May have multiple metalimnions • Depends on depth of lake

  32. 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

  33. Cooling T in fall • Surface water reaches 4ºC – most dense • Causes breakdown of epilimnion – Fall turnover • Metalimnion breaks down • Wind mixes column

  34. 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

  35. 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

  36. O2 more soluble in cold water Oligotrophic Eutrophic High productivity, O2 consumed

  37. 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

  38. 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

  39. Broecker’s Conveyor Belt

  40. Photic zone – OC production Pycnocline = halocline + thermocline High OC input upwelling system Low OC input

  41. 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

  42. Little deep water circulation • Oxygen rapidly depleted • May go to sulfate reduction in water column • Sediment affected • Black (sulfides) • Laminated (no bioturbation)

  43. Ground Water • Difficult to generalize about controls on redox reactions

  44. Multiple controls • Oxygen content of recharge water • “Point recharge” – sinkholes, fractures well oxygenated • “diffuse recharge” – low oxygen, consumed by organic matter

  45. Distribution of reactive C • Aquifers vary in amount of organic carbon • Quality of carbon variable – usually refractory • Refractory because • Old • subject to heat

  46. Distribution of redox buffers • Aquifers may have large amounts of Mn and Fe oxides

  47. Circulation of groundwater • Flow rates, transit times, residence times • Longer residence times generally mean lower pe

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