Redox State and Biologic Activity

We have already mentioned that photosynthesis and other biochemical processes are the main causes of disequilibrium in aqueous solutions. The conversion of luminous energy into chemical energy (formation of stable covalent bonds) involves local lowering of the redox state. For instance, the conversion of carbon dioxide into glucose  

Let us now briefly review the effects of biologic activity on the various systems. 

A process of bacterial degradation of carbon compounds can be conceived essentially as a conversion from a C02-predominant to a CH4-predominant system (Stumm and Morgan, 1981). The production of organic methane by microbial activity is a typical example of such conversion. Organic matter is first transformed into organic acids, which are then decomposed into acetic acid, gaseous hydrogen, and CO2, and finally recombined as CH4  

The sulfate — sulfide reduction requires quite low Eh conditions (figure 8.21D). The process takes place through enzymatic mediation (enzymes oxidize organic matter at the Eh of interest). At low pH, the reducing process may result in the formation of native sulfur, as an intermediate step of the process 

As already mentioned, the Eh-pH extension of the field of native sulfur depends on the total sulfur concentration in the system. With solute molality lower than approximately 10 moles/kg, the field of native sulfur disappears. 

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