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Natural Systems and Redox Reactions

Explore the role of natural systems in redox reactions and the importance of oxygen as an energy source. Learn about the oxidation/reduction of organic materials and the impact of oxygen depletion on sediment and water environments.

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Natural Systems and Redox Reactions

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  1. Review

  2. Review II

  3. WHAT ABOUT NATURAL SYSTEMS? • Oxygen would like to be reduced. • If we can combine oxygen with a compound that wants to be oxidized we have an energy source!

  4. What wants to be oxidized?

  5. What about Natural systems? • This is a general equation for the oxidation/reduction of organic material

  6. Relative to activity series

  7. Using the two half reactions • Eo = 1.227 + (0.07) = 1.297 V

  8. Eo = 1.227 + (0.07) = 1.297 V CH2O = 0.01 M, O2 = 0.2 atm CO2 = 1.0 atm

  9. The negative G indicates that this reaction is spontaneous! • What happens when we run out of oxygen? • Other oxidants will take the place of oxygen. • Most sediments are anoxic!

  10. Biologically Mediated reactions

  11. Sequence of Microbially Mediated redox processes http://www.chem1.com/acad/webtext/pdf/c3redox.pdf

  12. Redox / Nutrient Cycling Surface Water Bottom Water Sediment

  13. Oxygen Demanding Substances • CH2O + O2 = CO2 + H2O • NH4+ + 2 O2 = 2H+ + NO3- + H2O • 4Fe2+ + O2 + 10H2O = 4Fe(OH)3 (s) + 8H+ • 2SO32- + O2 = 2SO42-

  14. Biological Oxygen Demand • Quantity of oxygen utilized by a suitable aquatic microorganism in five days. • Why five days?

  15. Chemical Oxygen Demand (COD) • oxygen utilized by a specific chemical titration. • 3CH2O + 16H+ + 2 Cr2O72- = 4 Cr3+ + 3CO2 + 11 H2O

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