Review

Review II

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!

What wants to be oxidized?

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

Relative to activity series

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

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

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!

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

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

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

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

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

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

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

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

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

Hug, EAWAG News