Saturated hydraulic conductivity of a volcanic ash soil affected by repulsive potential energy in a multivalent anionic system

Introduction:Acid rain has been a serious problem around the world. The adverse effects of acid rain on soils must be assessed. Because soils have pH-dependent charges, the solution pH affects saturated hydraulic conductivity (K) [1–7]. A volcanic ash soil whose predominant clay minerals are allophane and imogolite contains a substantial pH-dependent charge [8–10]. Its K decreases when a higher- or lower-pH solution percolates [3–6]. When dilute nitrate percolates in the soil, K decreases due to the swelling and dispersion of the soil. On the other hand, when dilute sulfate percolates, K does not decrease; under this condition, the soil neither disperses nor swells [6].Although K is supposed to decrease in response to an increase in electric repulsive force among soil particles, the relationship between K and repulsive potential energy has not been evaluated. The relationship between rheological properties and the potential energy of the interaction between sodium montmorillonite particles was discussed [11]. A numerical solution of the equation for swelling pressure in mixed systems with nonsymmetrical electrolytes was proposed [12]. However, research into repulsive potential energy in multivalent counterion systems has been rare.In this paper, repulsive potential energies for a volcanic ash soil (allophanic Andisol), which is characterized by a number of pH-dependent charges, were evaluated in an NO3–SO4 system at pH’s 3 and 4. In order to determine the mechanism of the change in K of the soil during dilute acid leaching, the relationship between K and the repulsive potential energies was evaluated. Because the acidity in rain is caused mainly by HNO3 and H2SO4, mixed dilute solutions of these acids were used in the experiment