Ion Diffusion

The microregional transport of ions under an externally applied electric field has been discussed previously. When ions are distributed heterogeneously in soil on a macroscopic scale, because of the presence of concentration gradient (i.e., the difference in chemical potential), ions tend to migrate from a site of high concentration to a site of low concentration. Such a phenomenon is called ion diffusion. The diffusion rate of various ions in a soil is related to the nature of the ions and the interaction among them and is also affected by the chemical processes in the soil, such as adsorption, desorption, and repulsion. For variable charge soils carrying both positive and negative surface charges, the factors that affect ion diffusion are rather complex. In the present chapter, after treatment of basic principles of ion diffusion, the characteristic features of ion diffusion in variable charge soils will be discussed, with the emphasis on diffusion of anions because this is one of the important means for elucidating the characteristics of variable charge soils. In a solution, if the ion concentration in point A is higher than that in point B, under static conditions, the number of ions moving from point A to point B will be larger than that moving in the opposite direction due to the random thermal motion of ions. In order to express the net ion flux J within an unit time interval through an unit area, Fick introduced the first diffusion law: . . . J = –D dC/dX . . . (9-1) where dC/dX is the concentration gradient. The negative sign in the equation denotes that the flux is from high concentration to low concentration; that is, the direction of the flux is opposite to that of the concentration gradient. D is called the diffusion coefficient. It can be seen from the equation that the diffusion coefficient is the flux passing through an unit cross-sectional area within a unit time interval under a unit concentration gradient. D is the most important parameter in ion diffusion. Fick’s first diffusion law is applicable to both homogeneous and heterogeneous medium such as soil.