Exchange isotherm nonpreference

Fig. 5.2. Nonpreference exchange isotherms calculated with Eq. 5.16b for three values of the total moles of adsorptive charge, Q (Eq. 5.7b).

Using a number of ARK values, as 1 —> 0, to cover the entire exchange isotherm, equivalent fractional loads for K+ can be estimated. A plot of (K+)/[(K+) + (2Ca2+)] vs. ExK/CEC will produce a curvilinear line representing the nonpreference isotherm. Upon introducing values in the K+/[K+ + 2Ca2+] term such that... 

FIGURE 5.7 Homovalent K+-Na+ exchange on a Brucedale subsoil (Bond 1995) at I = 0.05 M. (a) Measured exchange isotherms open symbols—K+ mole fraction, closed symbols—Na+ mole fraction. Dashed line nonpreference isotherm, (b) In Ky plot to determine the equilibrium constant The dotted lines are extrapolations and the stars represent extrapolated points. 

Figure 5.10 shows Ca-K exchange isotherms for two horizons in an acidic soil from Guinea, Nigeria (Agbenin and Yakubu 2006). This is mainly a kaolinitic soil, with the oxide fraction being dominated by iron oxides. The dashed lines show the nonpreference isotherms it can be seen that the surface horizon shows a small preference for potassium, whereas the subsurface shows a nearly nonpreferential behavior. 

FIGURE 5.13 Ca-Fe(II) exchange isotherms (plotted as amount sorbed) on montmoril-lonite, a layered clay mineral (see Chapter 8). ( ) Ca + sorbed (0) Fe(II) sorbed lines nonpreference isotherms. (With kind permission from Springer Science+Business Media Aquat. Geochem., Charlet, L., C. Tournassat, Fe(II)-Na(I)-Ca(II) cation exchange on montmorillon-ite in chloride medium Evidence for preferential clay adsorption of chloride - metal ion pairs in seawater, 11, 2005, 115-137.)... 

The results (Bond 1995) are shown in Figure 5.8a. Here the Na+ isotherm only is plotted, as an equivalent fraction in the solid and as a function of both mole fraction and equivalent fraction in solution phase the second way is often found in the literature. Obtaining the nonpreference isotherm is a bit more complex here, starting from the equilibrium expression for reaction 5.56 in terms of the Gaines-Thomas selectivity coefficient, because the exchanger phase activity coefficients are not known. 

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