To get a better understanding of the mechanisms of selective retention of cations by clay minerals, we have studied the physicochemical state and the location of immobilized cations. The approach developed is based on the concomitant study of the exchange isotherms and the compensating cations/clay structure interactions, using far infrared (FIR) spectroscopy. For that, cations are used as spectroscopic probes to characterize the selective sites.
Exchanged K, Rb-, K, Cs-, K, NH4- and NH4, Cs-Llano vermiculite samples were prepared. The exchange isotherms illustrate the higher selectivity of Llano vermiculite in the following order: K+ < Rb+ < Cs+. Desorption experiments show that a 2 N MgCl2 solution can extract a small fraction of immobilized Cs+. The concomitant analyses of the exchange phenomena at microscopic (X-ray diffraction) and molecular (far and middle IR spectroscopy) levels show that: (1) only ∼65% of the interlamellar hydrated Mg2+ of Llano vermiculite is exchanged; (2) cations are randomly distributed in the interlamellar spaces; and (3) retention is strongly related to the distance between compensating cations and oxygen atoms of the ditrigonal cavity.
The low hydration energy of selectively retained cations induces strong cation/clay interactions, which give FIR absorption bands of compensating cations. The FIR absorption bands of smaller cations shift towards lower wavenumbers when the proportion of the larger cations increases, whereas the wavenumber of the larger ones is constant. This reproducible scenario shows that larger cations act as wedges and expand layers, thus increasing the distance between the smaller cations and the layers. Calculation of the distances dM-O inner and dM-O outer shows that selectively-retained cations are six-coordinated in these dehydrated systems. The decrease of the difference between dM-O outer and dM-O inner from K+ to Cs+ may explain the observed selectivity of Llano vermiculite.
