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 E o = 1.227 + (0.07) = 1.297 V

8. CH 2 O = 0.01 M, O 2 = 0.2 atm CO 2 = 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 Stumm and Morgan 1996

12. Redox / Nutrient Cycling Sediment Bottom Water Surface Water

13. Oxygen Demanding Substances CH 2 O + O 2 = CO 2 + H 2 O NH 4 + + 2 O 2 = 2H+ + NO 3 - + H 2 O 4Fe 2+ + O 2 + 10H 2 O = 4Fe(OH) 3 (s) + 8H + 2SO 3 2- + O 2 = 2SO 4 2-

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. 3CH 2 O + 16H + + 2 Cr 2 O 7 2- = 4 Cr 3+ + 3CO 2 + 11 H 2 O