ION-TRANSPORT IN AN ALLOPHANIC ANDISOL UNDER THE INFLUENCE OF VARIABLE CHARGE

Because the density of variable charge in a soil is influenced by the solution concentration and the pH, exchanging ion transport in the soil is also affected by them. In this study, the effect of variable charge on exchanging ion transport in an allophanic Andisol (Typic Hydrudand) that has negative and positive variable charges was investigated experimentally. Miscible-displacement experiments were conducted under saturated and unsaturated (-1.96 kPa of soil water pressure) conditions. The cases studied here involved ion exchange under a constant total concentration condition and a constant pH condition. A SrBr2 solution was applied to the soil column, which was initially saturated with a CaCl2 solution. The miscible-displacement experiments were carried out at different total concentration (0.001-0.1 mol(c) L-1) and pH (4.2-7.65). Bromine flowed out slowly at lower pH due to the large anion-exchange capacity (AEC). Strontium discharged slowly at higher pH due to the large cation-exchange capacity (CEC). The counterion penetrated slowly at a lower concentration because the amount of the counterion contained in a unit of solution volume was small. Observed breakthrough curves (BTCs) agreed with those expected from measured exchange isotherms. Based on the relationship between the Sr BTC and the Br BTC, no notable differences were observed between the BTCs under saturated conditions and those under unsaturated conditions. The influence of the solution concentration and the pH cannot be ignored when considering ion transport in an allophanic Andisol.