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 = (0.07) = V

10. Eo = (0.07) = V
CH2O = 0.01 M, O2 = 0.2 atm CO2 = 1.0 atm

11. The negative DG 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!

12. Biologically Mediated reactions
pe(w)
13.8
11.3
7.2
5.8
4.4
-3.9
-3.3
-4.1
-8.2

13. Conditional pe  pe’
2H+ + 2e-  H2 peo = 0

14. pe’ = pe +1/4Log[H+]4
pe’ = pe +1/2Log[H+]2

15. Sequence of Microbially Mediated redox processes

16. pe’ = pe +1/4Log[H+]4
NO3-
CH2O
SO42-
pe’ = pe +1/2Log[H+]2

17. Redox / Nutrient Cycling
PO43-
Surface Water
Bottom Water
Fe(OH)2PO42-
Sediment
Al(III)!

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

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

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

22. Hug, EAWAG News