ION-TRANSPORT IN SOIL WITH ION-EXCHANGE REACTION - EFFECT OF DISTRIBUTION RATIO

Exchanging ionic-solute transport through soil columns is significantly affected by ion exchange reaction. The breakthrough curve (BTC) for the input ion based on time or pore volume shifts to the right as the ion-exchange capacity increases or the input ion concentration decreases. The shapes of the BTCs with different mean breakthrough times are easily compared when the BTCs are normalized by the mean breakthrough times. The influence of the distribution ratio, D(g), the ratio of the amount of ions adsorbed in the soil to that in the solution in a unit volume of the soil, on normalized BTCs with nonlinear exchange isotherms was evaluated. The cases studied here involved ion exchange in a binary system at a constant total concentration under steady-state water-flux conditions. The BTCs were calculated and interpreted using a normalized, one-dimensional advective-dispersive equation. It was determined that the effect of the nonlinear isotherm on the BTC increases as D(g) increases. It was also shown that, when D(g) is much greater than unity, an increase in D(g) has no effect on the BTC. These results were confirmed with experimental data on actual soil columns of an allophanic Andisol. When the BTCs had a similar nonlinear exchange isotherm and Peclet number, the deviations among them were easily interpreted by the D(g) values.