Processes Involved in Sequential Reduction

The free energy change for a particular redox reaction varies with pe, pH, and the concentrations of reductants and oxidants according to Equation (4.26)  

In this equation, the value of (Red) is a function of the nature of the reductant, its solubility, the crystallinity of solid phases containing it, effects of solubilizing agents, transport limitations, and other factors. Likewise the value of (Ox) is a function of various factors. As discussed in the previous chapter, most redox reactions are very slow and the prevailing conditions are therefore sensitive to catalysis. Three types of catalysis are involved  

Abiotic catalysis is generally less important than biotic but may be important. Examples are Mn(lll,IV) and Fe(III) reduction by microbial metabolites, and Fe(ll) oxidation which is catalysed by sorption onto soil particles. 

Abiontic, involving free extracellular enzymes or solubilizing agents, enzymes bound to soil surfaces, enzymes within dead or non-proliferating cells, or enzymes associated with dead cell fragments. Extracellular enzymes are important in the initial stages of organic matter oxidation, in which polysaccharides and proteins are hydrolysed to soluble compounds that can be absorbed by microbial cells and further oxidized in biotic processes. 

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