COUPLED DIFFUSION AND OXIDATION OF FERROUS IRON IN SOILS .2. A MODEL OF THE DIFFUSION AND REACTION OF O-2, FE-2+, H+ AND HCO3- IN SOILS AND A SENSITIVITY ANALYSIS OF THE MODEL

Kinetic equations are developed for a system in which a column of reduced soil is exposed to oxygen at one end. The equations are combined in a simulation model in which they are solved by finite‐difference methods. The model predicts the consequent diffusion of oxygen into the column; the diffusion of ferrous iron towards the oxidation zone; the rate of formation and concentration profile of the ferric hydroxide formed; and the diffusion by acid‐base transfer of the acidity produced in the oxidation reaction. A sensitivity analysis of the model, in which runs were made for a wide range of input parameters, showed that for most combinations of parameters, in water‐saturated soil, substantial amounts of iron are transferred towards the air‐exposed surface, leading to a well‐defined zone of ferric hydroxide accumulation. The profile of total iron in this zone is often banded. The pH in the zone falls by at least two units. A small amount of air‐filled pore space increases the depth of the oxidation front dramatically. The model indicates that coupled iron oxidation and diffusion reactions, which are very widespread in natural soils, may be understood quantitatively. Copyright © 1990, Wiley Blackwell. All rights reserved