Simultaneous Diffusion and Oxidation in Soil

Kirk et al. (1990b) and Kirk and Solivas (1994) used the above understanding of oxidation kinetics to develop a model of soil oxygenation. The model allows for the diffnsion of O2 into the soil, the diffnsion of Fe + towards the oxidizing surface, the rate of formation and concentration profile of the Fe(OH)3 formed, and the diffusion by acid-base transfer of the acidity formed H3O+ diffusing away from the zone of acidification and HCOs (derived from CO2) towards it. The principal equations are as follows, expressed in planar geometry so as to be able to test the predictions against experimentally measnred reactant profiles. 

Applying Equation (4.39) to the structured-soil system, and ignoring the slow oxidation of Fe + in solution, gives 

By contrast if the soil dries to any extent resulting in partially air-filled pores, the penetration of O2 increases dramatically for an air-space of just 1 % of total 

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