Résumé
The contribution of isothermal titration calorimetry is discussed to elucidate the mechanism of competitive adsorption of free Ca2+, Sr2+, Ba2+, and Cd2+ cations onto 4A-type zeolite from aqueous solutions at 298 K. The individual adsorption isotherms for these cations and the related enthalpy of displacement curves were compared between single-metal and two-metal systems. Separate dilution runs carried out under the same experimental conditions provided evidence that the dilution phenomenon made no significant contribution to the total cumulative enthalpy effect. In the light of the observations made in the present study, it is postulated that divalent cations exchanged with extra-framework sodium in different crystallographic locations within the zeolite structure. The binding affinity of individual cations for the zeolite surface was established to decrease in the following order: Sr ≈ Ba > Cd > Ca, in line with the increasing endothermic character of the displacement process. The experimental ΔdplHcum values in two-metal systems were also compared with the theoretical ones obtained by summing up the individual metal contributions as measured in appropriate single-metal systems. In the case of the Sr(II) + Cd(II) system, the competition became noticeable above a total amount adsorbed of 1 mmol g−1, as indicated by great differences between the experimental and theoretical enthalpy values. For the Sr(II) + Ba(II) and Sr(II) + Ca(II) systems, the sequence of site occupation was found to be modified as a result of competition. The effect was particularly pronounced for strontium which gave more exothermic contribution to the total enthalpy of displacement.