Water, solute transport, and interfacial electric phenomena in soils

Electric charges of soil particles affect soil structure, permeability, soil erosion, nutrient, and contaminant transport. Those related studies have been reviewed here. An ion transport delays with the increase of adsorption caused by electrostatic attraction. When the electric charge is pH dependent, the ion transport is also affected by pH. A saturated hydraulic conductivity decreases when the soil structure changes by swelling and dispersion. The dispersed soil particles clog the necks of macropores and the swelling decreases macropores. They cause the decrease of saturated hydraulic conductivity. Such structure changes are also related to the electric charge. The development of electrical diffuse double layer and their overlap between the charged soil particles cause swelling and dispersion. The dispersion and the decrease of hydraulic conductivity are strongly affected by the state of ion adsofrption. When the ion adsorbs in the electrical diffuse double layer and the electrical repulsive force between neighbor soil particles becomes larger, the soil disperses and the hydraulic conductivity decreases. On the other hand, when the ion adsorbs strongly just on the soil surface, such as the case of a sulfate on an allophanic andosol, the electrical diffuse double layer does not develop and the soil flocculates and the hydraulic conductivity does not decrease, rather increases. The screening effect by high electrolyte concentration that damps an electric field near the charged soil surface is observed in an anionic surfactant adsorption on the negatively charged soil. When the electrolyte concentration is higher, the anionic surfactant adsorption increases due to the decrease of electric field. The surfactant adsorbs by hydrophobic interaction and the electrostatic force is repulsive in this case. A clay membrane clearly shows salt sieving because ions cannot enter into the membrane where the electrical diffuse double layers near the clay surfaces overlap. [GRAPHICS] .