Diffusion processes in water saturated spherical soil aggregates

Abstract

The release curves of D₂O, K⁺, and Cl¯ from saturated spherical aggregates of four soils, presaturated with a 10 mol m⁻³ KCL and 10 kg m⁻³ D₂O mixed solution, to a 10 mol m⁻³ NH₄NO₃ solution were analyzed to examine the diffusion processes in soil aggregates. Aggregates were taken from the weathering crust of a mudstone (MDR), the AB horizon of a Brown Lowland soil (BLS), the B horizon of a Brown Andosol (ADO), and the C horizon of a Vol-canogenous Regosol (PUM). The apparent diffusion coefficients of the solutes for each sample were obtained by fitting the spherical diffusion model to the release curves. Although the release rates of solutes from the 2-4 mm aggregates were higher than those from the 6-8 mm aggregates, almost the same diffusion coefficients (D) were obtained for both aggregate sizes.

The values of the tortuosity factor (f) which was estimated as D/D ₀ for D ₂O(D ₀ is the diffusion coefficient in free water) ranged from 0.25 to 0.51. Higher values were found in samples with a larger porosity. The D/D ₀ values for K⁺ in all the samples were one hundredth to half of the f values due to the cation adsorption effect. Equilibrium coefficients for K⁺ adsorption calculated from the D/D ₀ of K⁺ and f agreed well with those obtained independently as a product of the bulk density of aggregates and the slope of K⁺ desorption isotherms.

The D/D ₀ values for Cl¯ in ADO and PUM derived from volcanogenous materials were lower than the f values owing to the anion adsorption effect. The equilibrium coefficients for anion adsorption were estimated at 0.65 for ADO and 0.17 for PUM. On the other hand, in MDR and BLS the D/D ₀ values were higher than the f values, indicating the presence of the anion exclusion effect. Anion exclusional water volume was estimated at 16% for MDR and 17% for BLS. The values could be explained by the model which determines the anion exclusional water volume as the volume of pores with a radius smaller than the anion exclusional distance from the pore wall.

Key Words:

• anion exclusion
• diffusion
• ion adsorption
• soil aggregate
• tortuosity factor