Simulating diffusion within soil aggregates: a simple model for cubic and other regularly shaped aggregates

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Summary: Diffusion within a cube was simulated by a model which divides the cube into concentric volumes and computes by Fick's first law the diffusive flux between adjacent volumes and between the outermost volume and the external solution. The model can also be applied directly to a sphere and, with minor modifications, to other regular shapes. It was tested in two forms: (a) with the distances between the interfaces of adjacent concentric volumes made equal, and (b) with the volumes themselves equal, by using it to simulate the uptake of solute by a sphere from a stirred solution of limited volume. The output from the latter form of the model agreed the more closely with values obtained from a solution of the diffusion equations in radial co‐ordinates, and this form was used subsequently. The model was used successfully to simulate the diffusion of bromide from cubes of chalk and chloride‐36 from porous ceramic spheres, and its flexibility was illustrated by its ability to simulate experiments in which aliquots of the external solution were removed for assay.

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