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MISCIBLE DISPLACEMENT OF INORGANIC CATIONS IN A DISCRETE HOMOIONIC EXCHANGE MEDIUM

We conducted equilibrium adsorption and miscible displacement experiments to study the effect of the presence of Ca2+ as a competing cation on the adsorption and transport of Na+ or Li+. The exchanger was homoionic with respect to the competing cation. Exchange isotherms were determined for Na+ and Li+ in Ca(NO3)2 solutions with 0.05 M, 0.02 M, and 0.005 M on a Ca-saturated synthetic exchanger. The exchange isotherms were approximately linear over the limited range of Li+ on Na+ concentrations (0 to 200 μg/ml) studied for each Ca(NO3)2 concentration. Na+ and Li+ adsorption decreased with increasing concentrations of Ca2+ in the equilibrium solution. The effect of varying Ca2+ concentrations on the elution of Na+ or Li+ “pulses” was studied, using miscible displacement experiments under steady flow conditions in water-saturated laboratory columns packed with the exchanger. Elution curves were obtained for “pulse inputs” of solutions containing Na+ in 0.05 M and 0.005 M Ca(NO3)2 and Li+ in 0.05 M, 0.02 M, and 0.005 M Ca(NO3)2. Tritiated water (HTO) was introduced with all tracer “pulses” to evaluate hydrodynamic dispersion. In each case, tracer-free solutions of Ca(NO3)2 at the same concentration as in the “pulse” were used for elution. An analytic solution to the convective-dispersive mass transport equation for a reactive solute (linear exchange isotherm) was used to describe the measured elution curves. The breakthrough of Na+ on Li+ occurred earlier, as the concentration of Ca2+ in the tracer solution was increased. The elution curves for Na+ in 0.005 M Ca(NO3)2 and for Li+ in 0.02 M and 0.005 M Ca(NO3)2 were not described completely by the analytical solution. It was demonstrated that the discrepancies were a consequence of the changing Ca2+ concentrations in the percolating solution due to the metathetical nature of the ion exchange process. It was shown that ion exchange isotherms for a cation in aqueous contact with a homoionic exchanger should properly be represented as a function of the concentrations in the equilibrium solution for both the adsorbing cation and the cation initially saturating the exchanger.