Solute electrical charge effects on molecular diffusion coefficients in unsaturated soils

The ratio of the diffusion coefficient of a solute in soil to the diffusion coefficient in bulk water (D_r) has been shown to be proportional to the volumetric water content of the soil (θ) to the power n, and studies of the dependency of D_r on θ have shown that n is approximately 2. This n value can be explained physically using a simple tortuosity model. However, n values of approximately 4 have been found in two soils (sandy and loamy) with low water contents, and these high values have been attributed to an electrically diffuse double layer formed in the soil water near charged solid surfaces, restricting ion diffusion within it. We examined the effect of electrical charge of solute on the molecular diffusion coefficient at low water content in soils with n values of approximately 4. We measured the molecular diffusion coefficient of the solute in the soil (D_p) for electrolytes (NaCl and CaCl_{2</ing>), which may be affected by soil surface charge, and a non-electrolyte (glycerin; C3}H_5<\inf>[OH]3), which should not be affected by the charge, in the two soils over a wide range of water contents. The experimental results showed that D_r for all of the solutes were approximately the same, even at low water contents, indicating that the observed sharp decline of D_r with the decrease in θ is these soils should not be attributed to the electric charge on the particle surface, but to other mechanisms such as drastic change in the geometrical distribution of pore liquid water due to the progress in dehydration.