ION EXCHANGE IN AGGREGATED POROUS MEDIA DURING MISCIBLE DISPLACEMENT

The transport and retention of ('45)Ca, ('36)Cl, and ('3)H(,2)O through a saturated aggregated oxisol was studied under different conditions of pH, concentration, aggregate size and flux; using miscible displacement techniques.;The experimental data were analyzed using the Diffusion-Dispersion model, the Mobile-Immobile water model of van Genuchten and Wierenga (1976) and the 2-site Equilibrium and Kinetic model of Selim, et al. (1976). The model of van Genuchten and Wierenga (1976) gave a quantitative explanation of the early appearance of the tracers in the effluent and the tailing in large aggregate fractions and at the large fluxes. In some experiments up to 45% immobile water was determined. Thus neglecting intra-aggregate diffusion resulted in a serious disagreement between observed and calculated effluent curves when the Diffusion-Dispersion model was used. The model of Selim, et al. (1976) also described the ('45)Ca and ('36)Cl experimental data well. The first-order reversible exchange reaction used in the model emphasized that under some conditions the exchange process can be time-dependent. The kinetics of exchange was more pronounced in large aggregates, at large fluxes and in small solution concentrations. The experimental data demonstrated that the Mobile-Immobile water model and the 2-site Equilibrium and Kinetic model, are essentially equivalent in describing ion exchange in an aggregated porous medium. The interaction between physical and chemical properties of the porous medium in affecting solute transport, was exemplified in the study. The shapes of breakthrough curves were found to be a function of the ionic species, pH, concentration, flow velocity and aggregate size used. The technique of triple labeling was successfully used to separate the interference of one isotope from the others. Using three isotopes in a pulse and three mathematical models facilitated greatly the interpretation of the data.;Changing the soil pH and that of the soil solution from 4 to 9 increased the adsorption of ('45)Ca. The adsorption of ('36)Cl increased as the pH was decreased from 7 to 4. Tritium adsorption was not that affected by changes in pH. A decrease in solution concentration from 0.1N to 0.001N increased the retention of ('36)Cl at pH 4 and ('45)Ca at any pH. However, an increase in concentration from 0.001N to 0.1N shifted the ('3)H(,2)O effluent curves to the right. Most experiments showed ('3)H(,2) adsorption. An increase in aggregate size from 0.5-1.0 mm to 2.0-4.7 mm, affected the movement of all three tracers. The breakthrough curves appeared earlier and showed more tailing as the aggregate sizes increased. This trend was more pronounced as the flux increased from 0.25 cm/hr to 3.4 cm/hr. ('45)Ca and ('36)Cl were more strongly adsorbed in the small aggregate fractions.