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Processes of horizon development

Processes of horizon development. Redox processes Environmental Pedology. What is an oxidant?. What is an oxidant? Oxidants oxidize other compounds and in the process they accept electrons and are converted into reductants Oxidants electron acceptors

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Processes of horizon development

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  1. Processes of horizon development Redox processes Environmental Pedology

  2. What is an oxidant?

  3. What is an oxidant? Oxidants oxidize other compounds and in the process they accept electrons and are converted into reductants Oxidants electron acceptors Oxidant + e- Reductant

  4. What is a reductant?

  5. What is a reductant? • Reductants reduce other compounds and in the process they give away their electrons • Reductants electron donors • Reductant Oxidant + e-

  6. Name some oxidants • O2 • NO3- • Mn4+ • Fe3+ • SO42- • CO2

  7. Name some reductants • OH- • NO-2, NO, N2O, N2 • Mn2+ • Fe2+ • SO2, S2- • C

  8. Cell potential for a redox reaction can be expressed as (Nernst Equation): a Oxidant + b H+ + ne c Reductant Eh=Eo – RT ln [Reductant]c nF [Oxidant]a [H+]b Eh=Eo - 0.059 log [Reductant]c n [Oxidant]a [H+]b

  9. Eh - Electrode potential (volts) • Eo - standard electrode potential (volts) • R - Gas constant • T - Temperature (K) • F - Faraday’s constant • n - Number of electrons involved • [ ] - Concentration of the reductants and oxidants (mol/L).

  10. Fe3+ +e- Fe2+ Write the Nernst equation for the reaction • [Oxidant] = x mol/L • [Reductant] = y mol/L • Eo of Fe3+/Fe2+ = 0.77v • E = 0.77 – 0.059 log (y/x)

  11. Name an oxidation and reduction reaction that occurs in nature • Photosynthesis (Reduction) 6CO2 + 6H2O + energy C6H12O6 + 6O2 • Respiration (Oxidation) C6H12O6 + 6O2 6CO2 + 6H2O + energy

  12. Oxidation of organic matter • What happens when oxygen present is consumed? • Is there no more oxidation of organic matter? oxidant CO2 + H2O + energy Organic Matter

  13. Other electron acceptors are utilized in the order • NO3- • Mn4+ • Fe3+ • SO42- • CO2 But the energy yield decreases

  14. O2 + 4H+ 4e- 2H2O     • Eh about +350v • 2NO3 + 12H+ +12e- N2 +6H2O   • Eh about +220v • MnO2 + 4H+ 2e- Mn+2 + 2H2O       • Eh about +200v

  15. Fe(OH)3 + 3H+ + e- Fe 2+ + 3H2O  • Eh about +120v • SO42- + 10H+ + 8e- H2S + 4H2O       • Eh about -150v • CO2 + 8H+ 8e- CH4 +H2O • Eh about -250v

  16. Importance of studying oxidation – reduction processes in pedology • Influences soil color • Redoximorphic features • Organic matter content • Influences the forms of N2, Mn, Fe and S species

  17. Assignment: • Write the Nernst equations of the equations given in slides 14 and 15

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