Movement of ions in soil, II. Ion exchange and dissolution

Abstract

A one-dimensional equilibrium model for the movement of ionic species in a saturated soil is presented. Physical processes considered include mass flow, ion exchange, dissolution and precipitation. It is assumed that ionic concentrations in the soil can be described with step functions. The model assumes that a precipitate initially present in the soil dissolves upon leaching the soil with a solution not containing the ionic species that make up the precipitate. Unlike the situation in the absence of a precipitate, ion exchange would take place at two separate interfaces, moving at different velocities through the soil. At the faster penetrating interface, the precipitation would take place in conjunction with ion-exchange. At the slower penetrating interface, the precipitate initially present plus the secondarily formed precipitate would dissolve and ion exchange proceed. The composition of the transition zone between the two interfaces is calculated for step functions, from the conditions of mass conservation and electroneutrality, a linear ionexchange equation and a solubility product.