Calculated Changes in pe, pH and Fe During Soil Reduction

Consider an idealized soil containing ferric hydroxide and readily decomposable organic matter. The following conditions hold  

Following flooding, O2 dissolved in the soil solution is consumed according to Reaction (4.27). There is no pH change, the CO2 pressure being constant, and pe is poised by the O2-H2O couple  

Once pe falls sufficiently, precipitation of Fc3(OH)8 commences and Fe3(OH)g is formed at the expense of Fe(OH)3 according to the reaction 

Hence for a given generation of Fe + in Fe(OH)3 reduction, and for a specified initial soil CEC and concentration of non-carbonate anions in the soil solution ([X ]l), we have five unknowns the soil pH and the concentrations of Fe + and M2+ in the soil solid and solution and these can be found from the following five equations  

These equations can be solved simultaneously with Equations (4.33)-(4.35) to obtain values of pe, pH, [O2] and [Fe +] over the course of reduction. Eigure 4.7 shows results for realistic flooded soil conditions, expressed in terms of the amounts of CH2O oxidized in the different reactions. Eigure 4.7(a) gives results in the absence of pH and cation buffering by the soil Eigure 4.7(b)-(d) gives results for different values of hs. CEC and [X Jl. 

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